July 2019 © 2019 International Bank for Reconstruction and Development / The World Bank 1818 H Street NW Washington DC 20433 Telephone: 202-473-1000 Internet: www.worldbank.org This work is a product of the staff of The World Bank with external contributions. The findings, interpretations, and conclusions expressed in this work do not necessarily reflect the views of The World Bank, its Board of Executive Directors, or the governments they represent. The World Bank does not guarantee the accuracy of the data included in this work. The boundaries, colors, denominations, and other information shown on any map in this work do not imply any judgment on the part of The World Bank concerning the legal status of any territory or the endorsement or acceptance of such boundaries. Rights and Permissions The material in this work is subject to copyright. Because The World Bank encourages dissemination of its knowledge, this work may be reproduced, in whole or in part, for noncommercial purposes as long as full attribution to this work is given. Any queries on rights and licenses, including subsidiary rights, should be addressed to World Bank Publications, The World Bank Group, 1818 H Street NW, Washington, DC 20433, USA; fax: 202-522-2625; e-mail: pubrights@worldbank.org. Furthermore, the ESMAP Program Manager would appreciate receiving a copy of the publication that uses this publication for its source sent in care of the address above or to esmap@worldbank.org. Cover Image A system in University of British Columbia (UBC) with pyranometer and pyrgeometer for meteorological observations related to solar insolation. UBC Climate Station. © UBC Micrometeorology/Flickr. CC BY 2.0. Attribution Please cite the work as follows: ESMAP (Energy Sector Management Assistance Program). 2019. “Studies for Grid Connection of VRE Generation Plants.” ESMAP Technical Guide, World Bank, Washington, DC. Acknowledgments ― The financial and technical support by the Energy Sector Management Assistance Program (ESMAP) is gratefully acknowledged. ESMAP―a global knowledge and technical assistance program administered by the World Bank―assists low- and middle-income countries to increase their know-how and institutional capacity to achieve environmentally sustainable energy solutions for poverty reduction and economic growth. ESMAP is funded by Australia, Austria, Canada, Denmark, the European Commission, Finland, France, Germany, Iceland, Italy, Japan, Lithuania, Luxembourg, the Netherlands, Norway, the Rockefeller Foundation, Sweden, Switzerland, the United Kingdom, and the World Bank. References to vendors, products, and services in this report do not constitute or imply the endorsement, recommendation, or approval of the World Bank. 2 | ESMAP.ORG TECHNICAL GUIDES ON VRE GRID INTEGRATION: PREFACE .................................................................. v ACKNOWLEDGEMENTS ..................................................................................................................... ix EXECUTIVE SUMMARY ....................................................................................................................... x THE INTERCONNECTION PROCESS .................................................................................................................... xi POWER SYSTEM STUDIES REQUIRED BEFORE INTEGRATING VARIABLE RENEWABLE ENERGY INTO THE POWER GRID ..... xii CONCLUSIONS AND RECOMMENDATIONS ........................................................................................................xiii 1| INTRODUCTION ......................................................................................................................1 2| PROCEDURE FOR GRID CONNECTION OF VRE GENERATION PLANTS .........................................3 OVERVIEW OF THE INTERCONNECTION PROCESS ................................................................................................. 3 Preliminary Feasibility Studies and Design........................................................................................... 3 The Interconnection Process and Agreement ...................................................................................... 6 Control and Testing .............................................................................................................................. 6 FULL INTERCONNECTION STUDIES ..................................................................................................................... 8 Checklist for Full Integration Studies ................................................................................................. 10 Checklist for Final Design and Compliance ........................................................................................ 12 GOOD UTILITY PRACTICE AND ESSENTIAL STUDIES FOR INTERCONNECTION ............................................................ 14 Reliability and Steady-State Analysis ................................................................................................. 14 Short-Circuit and Breaker Duty Review.............................................................................................. 15 Dynamic and Transient Stability Analysis........................................................................................... 16 Thermal Capabilities of Transmission Facilities.................................................................................. 17 Voltage Regulation ............................................................................................................................. 17 Voltage Limits ..................................................................................................................................... 18 Reactive Power Capability .................................................................................................................. 18 Transmission Capacity and Load at Risk ............................................................................................. 19 GRID INTEGRATION STANDARDS FOR VARIABLE RENEWABLE ENERGY OPERATIONS ................................................ 20 3| SAMPLE SCENARIOS .............................................................................................................. 22 INTEGRATING A VERY LARGE WIND FARM INTO THE TRANSMISSION GRID............................................................. 22 Steady-State Analysis ......................................................................................................................... 22 Short-Circuit Analysis ......................................................................................................................... 22 Stability Analysis ................................................................................................................................. 23 Subsynchronous Resonance ............................................................................................................... 23 STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| i Summary of Findings from Power System Studies ............................................................................ 23 INTEGRATING A MEDIUM-SIZE WIND FARM INTO THE TRANSMISSION GRID .......................................................... 24 INTEGRATING A SMALL SOLAR PLANT INTO THE DISTRIBUTION GRID .................................................................... 24 4| CONCLUSIONS ...................................................................................................................... 26 APPENDIX A: INFORMATION REQUIRED FOR INTERCONNECTION STUDIES ........................................ 27 APPENDIX B: EXAMPLE OF INTERCONNECTION PROCEDURE USED BY GRID OPERATOR ..................... 30 APPENDIX C: DETAILED DESCRIPTION OF POWER SYSTEM STUDIES ................................................... 31 Power-Flow Studies ............................................................................................................................ 32 Short-Circuit Studies........................................................................................................................... 34 Electromechanical Transient Studies ................................................................................................. 34 Transient/Dynamic Stability Studies .................................................................................................. 34 Frequency Stability Studies ................................................................................................................ 36 Electromagnetic Transient Studies..................................................................................................... 36 Power-Quality Studies ........................................................................................................................ 37 Voltage-Distortion Studies ................................................................................................................. 37 Harmonics Studies .............................................................................................................................. 37 Flicker Studies .................................................................................................................................... 38 Coordination Studies .......................................................................................................................... 39 Grounding Studies .............................................................................................................................. 40 Arc Flash Studies ................................................................................................................................ 41 GLOSSARY ....................................................................................................................................... 43 REFERENCES .................................................................................................................................... 45 LIST OF STANDARDS ......................................................................................................................... 46 ii | ESMAP.ORG Box 2.1 Geospatial planning for preliminary site identification ................................................................... 4 Figure 1 Simplified generation interconnection process ............................................................................ xii Box Figure 2.1.1 Zoning Study in Vietnam through Geospatial Analysis ...................................................... 5 Figure B.1 Procedure for interconnecting variable renewable energy used by grid operator in ERCOT ... 30 Table 1.1 Typical voltage level selection and capacity of variable renewable energy ................................. 2 Table 2.1 Roles of resource entity and grid operator in integrating variable renewable energy into the grid ................................................................................................................................................................ 7 Table 2.2 Factors constraining interconnection of variable renewable energy transmission to the grid .. 19 Table C.1 Most frequently conducted types of power studies ................................................................... 31 Table C.2 Study requirements for transmission- and distribution-level grid interconnection ................... 48 STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| iii ESMAP Energy Sector Management Assistance Program FIS full interconnection studies GSEP Global Sustainable Electricity Partnership HSR high-speed reclosing IEEE Institute of Electrical and Electronics Engineers kV kilovolt LTC load tap changer MVAr mega volt amp (reactive) MW megawatt POI point of interconnection PV photovoltaic SCADA supervisory control and data acquisition SCR short-circuit ratio SSO subsynchronous oscillation SSR subsynchronous resonance STATCOM static synchronous compensator SVC static VAR compensator TCC time current curve V volt VAR volt-ampere reactive VRE variable renewable energy All dollar figures denote U.S. dollars unless otherwise noted iv | ESMAP.ORG Over the past ten years, the cost of technology for variable renewable energy (VRE) such as wind and solar energy, has declined considerably, providing a cost-effective and sustainable means of meeting electricity demand in developing and middle-income countries. Taking advantage of variable sources of energy requires significant expansion and modernization of electrical grids and implementation of VRE- specific technologies, processes and requirements to gradually transition power systems into “VRE- friendly” grids that will significantly reduce integration costs in the long term. The need for technical assistance on VRE integration is greatest in countries with limited capacity to tackle technical and regulatory challenges. To meet this growing demand, the Energy Sector Management Assistance Program (ESMAP) of the World Bank has prepared a set of technical guides that can help World Bank staff and clients understand some of the essential requirements and available technical and regulatory measures to integrate large shares of VRE into power grids without compromising the adequacy, reliability or affordability of electricity. The technical guides have been developed as a joint initiative between ESMAP’s Variable Renewable Energy (VRE) Grid Integration Support Program and the Global Sustainable Electricity Partnership (GSEP). The Global Sustainable Electricity Partnership is a not-for- profit international organization comprising the leading companies in the global electricity sector who promote sustainable energy development through electricity sector projects and human capacity- building activities in developing nations worldwide. It is projected for the next five years that annual worldwide addition of solar and wind energy will continue to grow and is likely to at least double compared to their current share in power systems. Modern renewable energy generation technologies provide a strong alternative for grid electrification in locations where renewable resources are abundant and are starting to become the least-cost option in many of the client countries thanks to rapid decline in prices. For this, many emerging economies have started to adopt policies to encourage the development of the industry to realize the benefits that renewable power generation can have for their energy supply and on the local environment. Solar and wind installations can be built relatively quickly, which presents a major incentive in rapidly-growing, emerging markets with urgent need for power and also tackle the realization of climate change commitments. The key challenging issue, however, is the intermittent nature of solar and wind power, which increases the complexity of overall grid operations. The grid operators have to manage variability of the energy resource, reliability of grid operations and least-cost optimal performance. The fast penetration of renewable energy, and especially, a high level of their penetration into the power grid requires an adapted power system planning, better forecasting methods, introduces challenges in grid management, imposes stringent requirements for VRE integration into the grid, and necessitates standardization and structured process for the conducting studies to ensure compliance with the grid code requirements. The basic grid support services are becoming now relevant to all generators, including VREs, which are connected to medium and lower voltage levels. The modern electricity industry is restructuring with two major trends: significant increase of renewable energy and deregulation providing consumers with energy purchasing options of highly reliable delivery. However, deregulation, open energy access, and cogeneration are creating scenarios of transmission congestion and forced outages. Restructuring envisions the transmission grid as flexible, reliable, and open to all exchanges no matter where the suppliers and consumers of energy are located. The modernization of STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| v the grid requires the increased power quality, system stability, and increased transfer capacity of the transmission. New approaches to Power System Operation and Control are gaining the development momentum for overload relief and efficient and reliable operation. High-voltage direct current (HVDC) and Flexible Alternating Current Transmissions Systems (FACTS) technologies appear especially effective for improvement of grid operations and management. The proliferation of smarter infrastructure, enabling participation of increasing amounts of demand in activities also help mitigate the variability of renewable generation along with technological advances of renewable and complementary technologies like batteries allow renewable generators themselves to effectively contribute to maintaining reliability. A variety of emerging end-use technologies like electrical vehicles, heat pumps, and smart and efficient buildings enable greater flexibility in power systems and lead to higher demand for wind and solar. These technologies help to enable even greater usage of VRE resources, but at the same time, they bring additional challenges of overall grid operations, which require new approaches to system operation and planning to ensure that the new trends contribute to clean, reliable and affordable power systems. A shorter dispatch cycles in combination with more accurate shorter-term forecasts of renewable generation can be used to reduce forecast variations from renewable generators and result in reduced ancillary service requirement. A look-ahead unit commitment and stochastic unit commitment can effectively deal with uncertainty. Wind farm can be tasked to provide frequency response, inertial response, and regulation if they meet eligibility requirements. Storage technologies are beginning to be gradually deployed or included in provision of ancillary services. Frequency regulation market, which awards quick-start and fast responding resources including batteries, has been attracting an increasing amount of battery storage and new ways of using storage. There are also ongoing innovations combining variable renewable production with measures aiming to make demand more responsive. The benefits and effectiveness of new emerging trends are well recognized, but there are yet to reach full maturity and become standardized. The focus of the technical guides is primarily on the industry proven technologies and methodologies, which have already been established, widely adopted, and continue to proliferate in electrical utilities. However, the discussion of some new VRE related technologies that have already started influencing the utility landscape (e.g. dynamic energy storage, implementation of superconducting materials in fault current limiting devices, advanced forecasting methodologies, wind farm synthetic inertia and regulation response) are selectively included in the technical guide material where appropriate. The information presented in the technical guides is compiled from various sources of information to serve as a high-level guidance and quick reference for the World Bank personnel on electrical power system projects involving implementation of VRE along with associated technologies and analysis. The technical guides are comprised of the following four sets of sub-documents, which are identified as the subjects of prime technical interest for VRE implementation: • Grid Integration Requirements for Variable Renewable Energy • Compensation Devices to Support Grid Integration of Variable Renewable Energy • Studies for Grid Connection of Variable Renewable Energy Generation Plants • Using Forecasting Systems to Reduce Cost and Improve the Dispatch of Variable Renewable Energy vi | ESMAP.ORG “Grid Integration Requirements for Variable Renewable Energy” document presents a general overview of VRE technology along with some recommendations for VRE technical specifications, applicable standards, and essential testing. The main focus of the document presents a detailed outline of the essential requirements of VRE power plants integration into power grid. The different levels of VRE penetration in the grid determine different technical requirements for VRE integration. However, some of the requirements are fundamental and need to be respected for a VRE integration in any power system, e.g. regulation and automatic response to grid events, power quality, protection system, forecasting and analysis. The basic and advanced VRE integration requirements are discussed in detail in this document in order to provide a guiding reference for VRE projects regardless of the grid code’s maturity. All essential requirements in the grid are summarized in the checklist table and can be used in course of VRE’s project planning, implementation, and connection to the grid. The compliance with the technical requirements and grid code where applicable is validated through extensive series of interconnection studies such as steady state analysis, short-circuit and circuit breaker duty review, dynamic stability, and facility studies. “Compensation Devices to Support Grid Integration of Variable Renewable Energy” document provides an overview of FACTS and other compensation devices along with the essential characteristics describing industry need, applicable standards, functionality, applications, and recommendations for minimal technical specification. The main objective of the document is to discuss all available FACTS technologies with the underlying concept of independent control of active and reactive power flows, the essential differences and benefits of FACTS devices, and industry applications. Classification and comparison of performance factors are analyzed in detail and summarized to orient the reader in the wide spectrum of FACTS devices, and their effects on the power system. The applications of FACTS devices are associated with the following essential technical enhancements: System Capacity, System Reliability, Power Quality, System Controllability. Environmental benefits of FACTS are obtained through the deferral of the construction of much more expensive transmission lines and better utilization of existing system assets. “Studies for Grid Connection of Variable Renewable Energy Generation Plants” discusses the power system studies requirements for the stable grid integration of renewable energy plants. These requirements differ depending on the size of generation, the location of the connection, and whether it is transmission or distribution system. The main purpose of screening studies involved in the interconnection process is a successful integration of the VRE into the grid. Power system planning for interconnection of new variable generation resources ensures that there are sufficient energy resources and evacuation capacity to interconnect new supply, and that demand requirements are met in a reliable and efficient manner. Also, the studies verify that adequate reserves and necessary system resources exist to reliably serve demand under credible contingencies such as the loss of a generating unit, a transformer, or a transmission facility. “Using Forecasting Systems to Reduce Cost and Improve the Dispatch of Variable Renewable Energy” document discusses the need and benefit of forecasting capabilities and how it is becoming more relevant to both system operators and large-scale VRE generators. Forecasting solar or wind generation over a timeframe of days, hours and minutes before real time power system operations can reduce balancing costs, minimize VRE curtailment levels, improve system reliability and ultimately increase the penetration of VRE sources in the energy mix. The main objective is to focus primarily on the types of forecasting methods and how physical and statistical models are used for developing short- to long-term forecasts. Good forecast helps to reduce the gap between contracted supply of power and actual provision of power, reducing imbalance costs for the generator. Essentially, an effective forecasting system helps move the entire power system closer to a fully merit-order dispatch system, with reduced STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| vii uncertainty and costs around variable generation supply. Technological advances in weather forecasting, which, together with better data on historical performance of renewable energy, allow significantly improved forecasting accuracy of renewable generation, which results in a more efficient utilization of VRE. viii | ESMAP.ORG The Technical Guides on VRE grid integration is a joint initiative by the Energy Sector Management Assistance Program (ESMAP) of the World Bank and the Global Sustainable Electricity Partnership (GSEP). GSEP is a not-for-profit international organization made up of the leading companies in the global electricity sector that promotes sustainable energy development through electricity sector projects and capacity-building activities in developing countries. This Technical Guide is part of ESMAP’s Variable Renewable Energy (VRE) Grid Integration Support Program. This global program aims to help World Bank client countries scale up VRE in a cost-effective and sustainable manner by providing technical assistance; capacity building; and knowledge products for the development and implementation of planning, regulatory, market, and operational best practices in VRE integration. This document on “Studies for Grid Connection of Variable Renewable Energy Generation Plants” was written by a team comprising Silvia Martinez Romero (Task Team Leader and Senior Energy Specialist, ESMAP); Chong Suk Song (Energy Specialist, ESMAP); Martin Schroeder (former Energy Specialist, ESMAP); Kiamran Rajdabli, Chris Edward Jackson, Fabian Koehrer, and Sam Wheeler (Consultants, World Bank); and external experts Innocent Kamwa (Hydro-Quebec), Fernando Viollaz Garófalo (Enel Green Power), and Paulo Cesar Fernandez (Electrobas). The team is grateful to the GSEP Secretariat and its members Hydro-Québec and Enel for their invaluable contributions to the first drafts of this note. Luis Calzado (Senior Project Advisor at GSEP) provided significant insights and recommendations. The team also wishes to thank peer reviewers Manuel Jose Millan Sanchez (Senior Engineer, World Bank); Fernando de Sisternes (Energy Specialist, World Bank); and Xavier Remi Daudey (Consultant, World Bank), who provided valuable comments and insights at various stages of this work, including at the Decision Meeting, chaired by Rohit Khanna (Practice Manager, ESMAP). It offers special thanks to Kiamran Rajdabli (Consultant, World Bank), who made valuable contributions to this work, and to Gunjan Gautam (Operations Officer, World Bank), who provided highly appreciated comments. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| ix Variable renewable energy (VRE) is becoming a significant contributor to global electricity generation. The last decade saw a dramatic increase in the number of solar- and wind-powered plants, and VRE growth is projected to continue to grow in the coming years. Favorable meteorological and terrain conditions for building new VRE power plants exist in many countries, where VRE could easily meet 25– 30 percent of annual electricity demand. The high penetration of renewable energy increases the operational challenges for the power grid and calls for stricter requirements for VRE integration in a modern power system. The introduction of variability from renewable sources creates a new set of technical requirements for these systems. The main objective of integrating any new source of energy is to maintain the reliability of power system within the system operation constraints, which are generally defined in grid codes. These constraints must not be violated. The proliferation of large wind farms and solar power plants requires identification of the power system reinforcements needed to address violations, such as transmission line congestion and voltage instability. These reinforcements include the upgrading or addition of transmission lines, the addition of compensation devices to provide grid support, and operational adjustments to provide the primary and secondary reserves required for the reliable operation of the grid. This Technical Guide describes the power system studies that need to be conducted to ensure the stable integration of utility-scale VRE plants into the grid (mainly the transmission grid). The requirements differ depending on the volume of generation, the physical location of the connection, and whether the plant is to be integrated into a transmission or a distribution system. In some countries, interconnecting distributed VRE generation of less than 10MW at voltages below 60kV requires only a simplified interconnection review rather than a full interconnection study. All types of transmission generation with capacity of 10MW or more (that is, utility-scale generation resources) or an aggregate capacity of 10MW or more at the same point of transmission interconnection usually undergo a thorough interconnection review process, including full interconnection studies, to ensure the power system’s overall operational reliability and smooth integration of VRE into the grid. The document helps readers understand the requirements of the interconnection process that need to be followed by the VRE resource entity and the coordination of its efforts with the grid operator for successful integration of the VRE into the grid. It summarizes the studies typically carried out to ensure safe and reliable operations of the VRE plant and the grid, with a focus on the full integration studies performed by the grid operator. The studies are an essential part of the interconnection process. They identify any additional equipment that may be necessary to maintain the quality and stability of the electricity supply.1 1 A separate Technical Guide (“Technical Guide on Compensation Devices to Support Grid Integration of Variable Renewable Energy” [ESMAP 2018a]) addresses this equipment, which may include (but not be limited to) shunt capacitors/reactor banks, static VAR compensators (SVCs), static synchronous compensators (STATCOMs), shunt reactors, harmonic filters, and energy storage systems. x | ESMAP.ORG The Interconnection Process2 The integration of VRE into the power grid is highly dependent on the electricity transmission and/or distribution infrastructure. VRE design starts with a technical, social, economic, and feasibility analysis, which results in the selection of the site, the preliminary plant design, and a general grid integration assessment. A comprehensive, multistep process analysis is then initiated with the electrical utility to ensure grid integration of the VRE plant. The process involves multiple administration tasks, technical studies, design analysis, and sometimes adjustments to comply with the power grid’s operational requirements. It depends on countries’ specific regulations, but it always involves rigorously controlled reviews and approvals to ensure the quality of new VRE installations and compliance with the grid code. The process can be broadly divided into the three major stages: 1. VRE preliminary studies, design, and application for interconnection 2. Reviews, full interconnection studies, and technical data collection 3. Resource registration, commissioning, testing, and certification for operations. Figure 1 shows the detailed stages. 2 The terminology “interconnection” is often used interchangeably with “connection”. “interconnection” is the generalized term that includes all requirements/considerations used to reflect the interaction between any generation resource/grid asset and the power system. “Connection” is generally used specifically for the physical connection of the generation resource/grid asset. In this report, “interconnection” will be used for the general interconnection process and “connection” will be used for the connection of the VRE plants. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| xi FIGURE 1 Simplified generation interconnection process Power System Studies Required before Integrating Variable Renewable Energy into the Power Grid Power system studies allow the grid operator to ensure that connecting a new VRE power plant will not compromise the quality, security, or reliability of the grid. They assess whether grid upgrades and/or the installation of new equipment are necessary. These studies must be performed before the grid operator allows the VRE plant to be connected to the transmission or distribution grid. Power system studies also assess whether the VRE plant interconnection has an effect on the load-supplying capability of the transmission or distribution grid, exposes any grid equipment to operating conditions above the specified withstand requirements (generally defined in the grid code), and guarantees the safety of grid operator and VRE personnel working at the VRE connection. xii | ESMAP.ORG Several kinds of power system studies must be carried out before a VRE power plant can be connected to the transmission or distribution grid. Technical studies help ensure the viable connection of renewable resources from different perspectives, including the capacity for delivering the newly generated energy to consumers; the reliability of plant operations; the impact on the grid, safety, and power quality; and many other factors. Power system studies can be grouped into two main categories: preliminary feasibility studies and design, and control and testing. They are carried out at different stages of the interconnection process. The grid operator usually determines which studies are required and either conducts them or requires the resource entity to do so to prove the viability of interconnection. Conclusions and Recommendations Integrating VRE into the grid poses a variety of challenges for the stable and reliable operation of the power system. Addressing them requires not only adjustments in the operation of the power system but also system-wide analysis of generation and transmission system planning to identify the required reinforcements/adjustments. This analysis is needed to ensure that power system reliability is maintained and that the essential requirements for reliable system operation generally defined in grid codes are met after the connection of VRE resource. Conducting a thorough power system analysis of the network with the proposed new VRE power plants is fundamental to ensuring the viability of the VRE plants working in parallel with other generation and regulation resources and helps establish necessary grid upgrades and/or the installation of new equipment. Interconnection studies must include at least power-flow, short-circuit, and electromechanical (power system dynamic simulation) studies. Additional studies may be needed depending on the requirements defined by the grid code, the grid operator, or regulatory agencies; the characteristics of the plant; and the voltage level. This document describes the interconnection process and the place of interconnection studies in that process. It provides checklists for conducting studies and acceptance criteria for essential interconnection studies and presents case analyses to illustrate how to apply and interpret the results of VRE interconnection studies. Countries that lack well-developed regulations and standards can rely on the references and good business practices of grid operators and electrical utilities described in this document. The approaches described may not be fully replicable, but they can be adapted to some extent for use in developing countries. For power grids that are still working on establishing strong business rules and procedures to facilitate the integration of VRE resources, it is important to strike the right balance between international practices for integration and the realities of power system development in a particular country. Setting up overly demanding requirements for VRE integration in the absence of a grid code can undermine the cost-efficient development of the power grid. The requirements for VRE compliance and integration should be reasonable from a cost perspective, and the technologies proposed to meet the requirements must be available. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| xiii The share of renewable sources in the global power supply is rising, as a result of rapid technological progress that has reduced the cost of solar and wind power, the ambitious national commitments made by many countries to meet international agreements, and the preference of many national governments for clean energy. Integrating these renewable sources into the grid poses a variety of challenges stemming from the uncontrollable variability of renewable resource. Addressing them requires a systemwide analysis of generation and transmission system planning, to ensure that power system reliability is maintained when variable renewable energy (VRE) sources are integrated into the power system. Assessing the ability of the system to maintain a reliable operation of the grid generally requires three types of studies (IRENA 2017): • Generation expansion planning is the national and/or regional sector-level planning process that projects how the power system will grow. It is based on assumptions about future loads and the size of the required and available investment in generating capacity additions to meet these load projections. The results yield a high-level view of what the least-cost generation mix should be in the medium to long term (20 years or more). • Dispatch studies analyze the optimal use of all power plants in the most economical way. They verify the adequacy of the generation mix to address the net load variability in the power system, including variations in load and VRE generation, and define the operation requirements for the generation fleet for a given period. Grid operators use these studies to schedule generation assets. Policy and regulatory entities can use results from dispatch studies to inform policy and regulation during the planning process. For example, dispatch studies can inform the need for new ancillary services as a result of the addition of VRE in the generation mix and the added variability associated. Provision of specific ancillary services, such as a rapid primary reserve to accommodate faster and higher-frequency deviations in the grid from larger VRE shares, would require adjustments in regulations. Dispatch studies generally cover a period of a few representative weeks in each season of each year studied in the generation expansion plan. • Power system studies have two purposes: At the system level, they verify whether the least- cost generation expansion plan is technically feasible and identify the transmission infrastructure needed to implement the generation plan in the medium to long term. At the plant level, over the short term—before network reinforcements are introduced, for example— detailed studies are required to verify the feasibility of new VRE generation plants to ensure stable operation of the grid. These studies are based on technical network analysis that considers system constraints, such as transmission line congestion, system voltage operational levels, and system stability, at the point in time when the generation is connected to the power system at the point of interconnection (POI). Power system studies help determine the VRE system’s compatibility with the desired grid interconnection point in the power system. They reveal the viability of the interconnection and identify required changes to the grid infrastructure. In a worst case, they may indicate that a proposed interconnection is not viable and requires complete redesign. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 1 The requirements to connect a VRE plant to a power system depend on the renewable technology, the connection requirements,3 geographical conditions, and grid attributes. The size of the plant is typically a key determinant of the connection level. The typical connection to a transmission system is 69kV or higher.4 These connections are usually for large utility-scale photovoltaic (PV) or wind farms and other renewable energy generation plant of more than 20MW. The typical connection to a distribution system is 5–46kV. These connections are common for small to medium-size PV/wind farms and other renewable energy generation plants in the range of 1–20MW. Grid connection voltage is a fundamental and crucial feature of any VRE-to-grid integration. Table 1.1 presents typical voltages along with typical VRE plant sizes, based on the German grid. TABLE 1.1 Typical voltage level selection and capacity of variable renewable energy International Electrotechnical VRE capacity Grid voltage Grid voltage level Commission (IEC) definition of voltage level 100kW 480V–11kV Industrial to distribution; may not Low require power system studies 10MW 11–30kV Distribution Medium 50MW 33–110kV Transmission Medium 100MW 110kV and up Transmission High 200MW 220kV and up Transmission High/extra high Source: Adapted from Corfee et al. 2011. Installation capacity is one of the main factors that determine the connection voltage level. Flicker level, the POI, the distance from a grid substation, and many other factors can also affect the connection voltage level, which power system studies determine. The following sections describe VRE connection to the transmission grid. Given the large number of small and medium-size VRE plants being connected to the distribution network, some references to distribution connection are also made. 3 The connection requirements depend on the size of the VRE plant. In South Africa, for example, a unity power factor is allowed for VRE plants below 0.1 MVA; for plants above 0.1 MVA, the renewable power plant must be designed to operate according to a power factor characteristic curve, determined by the network service provider or system operator (NERSA 2016). 4 Grid voltage levels vary widely across countries (and sometimes even within large countries). Typical voltage levels around the world are 5–46kV for distribution, 35–69kV for subtransmission, and 110–750kV or higher for transmission. It is important to verify the voltages and grid levels used by the local grid operators before interconnecting a VRE. 2 | ESMAP.ORG Overview of the Interconnection Process Two main parties are involved in connecting a VRE plant to the network. The grid operator (an independent system operator, market operator, or large electrical utility) is the institution that oversees grid operations. The resource entity (an independent power producer or electrical utility) is the party that owns the generation resource and intends to commercially operate the power plant. Generation resource interconnection encompasses multiple tasks, including analysis, design, reviews, approvals, modeling, data gathering, testing, and administrative actions, which are standardized in the business procedures of the grid operator. (For the information required to conduct this analysis, see the checklist in Appendix A; for the business procedure used by a grid operator in the United States, see Appendix B.) The essential tasks of bringing a VRE resource online include the following (Figure 2.1): • analysis of competitive renewable energy zones and site selection • engineering design of the power plant • feasibility studies and preliminary power system studies • application for interconnection with initial resource data submission • reviews and technical data collection • screening study • full interconnection studies (FIS) • preparation of resource data for planning models • interconnection agreement • protocol compliance review • metering and modelling data exchange • generation resource registration • approvals for energization and synchronization • reactive capability testing and approval • approval to begin commercial operations. Preliminary Feasibility Studies and Design Site selection and VRE plant design are usually performed by an electrical utility or resource entity (plant owner). The feasibility study (which includes social, renewable energy intensity, regulatory, and economic analyses) establish the need, the benefits, and the justifications for proceeding with construction of a VRE plant. Feasibility or access studies determine whether the interconnection access to the grid at the point in question is possible, practical, and economical. The traditional approach to site selection and feasibility analysis involves transmission system–level interconnection studies, which usually examine power flow, short circuits, voltage harmonic distortion, and transient electromechanical stability (with detailed or simplified/general models for the plant to be connected). Feasibility studies have four main purposes: 1. They verify compliance with grid codes. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 3 2. They determine the viability of the interconnection from a power-flow, power factor, and voltage compatibility standpoint (this determination leads to a “go” or “no go” decision about the viability of the interconnection site selection). 3. They allow land leases/purchases and construction permits to proceed on both sides of the meter. 4. They allow material and equipment needs for the site to be verified and modified (if needed) and procurement to begin (some material, such as transmission-level transformers, can have a one-year or longer lead time). The planning studies associated with the feasibility analysis determine the viability of an interconnection at a specific point on a grid. They are used to (a) determine whether the transmission and distribution lines near the point of interconnection (POI) can absorb the proposed plant’s generation and (b) estimate the impacts of that generation on the grid. These studies include power-flow, short-circuit, and electromechanical transient stability studies. Transmission and distribution power-flow and short-circuit studies are almost always required. The connection of the VRE generation plant may “pass through” a distribution-level connection to get to a transmission-level connection. In such a case, the required studies should account for the transition through the distribution voltage and the transmission voltage system as well. A full-blown distribution system set of studies may not be needed. Instead, software models should include the distribution-level transformers and other equipment in the model. BOX 2.1 Geospatial planning for preliminary site identification Geospatial analysis is an emerging new methodology for analyzing the most effective geographical placement and economical connection of VRE plants. It identifies cost-effective site locations for constructing renewable energy power plants using multiple criteria for integration, including the terrain, the renewable energy intensity, road transportation infrastructure, transmission facilities, and other relevant parameters. Evaluation of multiple criteria for solar PV projects allows different measures affecting VRE construction to be overlaid, yielding GIS representations of quasi-optimal zones for VRE power plant sites, as shown in Box Figure 2.1.1. The geospatial approach yields a simplified solution that can potentially expedite a multisite selection process, identifying renewable energy projects with the highest energy yields. It is not a substitute for preliminary VRE feasibility studies and design, but it can be helpful for preprocessing global information about a country’s VRE potential and constraints and identifying potential zones before embarking on a detailed analysis of plant construction and integration into the grid. It is particularly useful in countries where VRE initiatives are at the early stages and analysis of multiple projects is required. Geospatial planning requires analysis of the following inputs: ● distance from the closest line and substation ● distance from the closest road ● exclusion of land unsuitable for renewable energy development ● intensity of the renewable energy source (irradiance or wind speed). The first step is to exclude all areas where VRE development would not be feasible. The next step is to assess the levelized cost of electricity (LCOE) for the generation type (solar or wind), based on the region’s resource intensity. The third step is to conduct a high-level assessment of the extra cost of 4 | ESMAP.ORG transmission lines and substations from the possible POIs to the existing grid (this analysis does not include any extra infrastructure costs associated with interconnecting these plans, an estimate that is available only from detailed power system studies). Using these criteria, the total LCOE is evaluated to provide an initial screening of the quasi-optimal site locations. These sites must be further assessed for feasibility of VRE connection at the POIs identified. Power system studies and assessment of the interconnection process identify whether the locations selected are feasible for the connection of the VRE plants. That determination depends on whether the closest line and substation have enough evacuation capacity for the locations and whether the grid at that location is strong enough to host renewables without any reinforcements. The studies also reveal the additional costs for transmission infrastructure reinforcements, such compensation devices. BOX FIGURE 2.1.1 Zoning study in Vietnam through geospatial analysis Source: World Bank (2018). STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 5 The Interconnection Process and Agreement After performing preliminary feasibility studies and design, the resource entity initiates an integration process by submitting an application to the grid operator for VRE grid connection. The grid operator conducts a screening study and issues a notice to proceed. It then starts a detailed analysis of interconnection, using a standardized FIS process. Once the FIS are complete, the grid operator and the resource entity usually reach an interconnection agreement. Design amendments and transmission infrastructure reinforcements are sometimes required to ensure compliance with grid requirements. The interconnection agreement is the main formal outcome of the FIS. After it is signed, resource interconnection proceeds. However, the provision of additional modeling information to the grid operator, plant design adjustments, and real-time telemetry are usually required before commissioning, testing, and physical connection begin. Control and Testing After an interconnection agreement is signed, the interconnecting entity provides generator data sufficient to update the planning cases to the grid operator, based on agreed data formats and/or templates. The grid operator models the new generation resource in planning base cases, which are used for future power system analysis that account for new generation. The proposed generation is also checked for protocol compliance, to ensure that operational standards established in the grid code (such as power quality, harmonics, and voltage support capability) are maintained. Power system studies are conducted again toward the end of construction and precommissioning. These studies provide deeper insights into the performance, operation, and safety aspects of the VRE plant and investigate protective device coordination, controls, arc flash, and other issues. Grid connection codes specify the minimum requirements the generator systems must fulfill in order to be allowed to connect to the public electricity system. The grid code defines sets of essential technical features that need to be built into the VRE and plays an important role in balancing between meeting the requirements of secure system operation and rapidly developing generation in the region with a high share of available renewables. The grid operator requires installation of telemetry points, in accordance with the existing supervisory control and data acquisition (SCADA) plan for the grid. Real-time measurements from the VRE power plant must be transmitted to the grid operator for monitoring. If necessary, control points may have to be established to execute plant control remotely. Telemetry usually includes at least the following real- time measurements for all modeled equipment: • generation resource gross and net generation MW and MVAr • transmission line MW and MVAr flows for each modelled line to the POI • status of all modelled breakers and switches • load MW and MVAr for all modelled loads • reactor and capacitor MVAr • transformer high-side and low-side voltages (kV) • high-side transformer MW and MVAr flows for each modelled transformer. Metering information from the plant is usually provided to the market operator for settlements in the electricity market environment. The grid operator reviews the generation resource’s commissioning plan and approves the request to commission the POI, begin initial synchronization, and start commercial operation. 6 | ESMAP.ORG For the resource entity to declare itself commercial, it must demonstrate that it can satisfy the operational requirements established by the grid operator. For example, one of the important operational requirements for VRE is reactive power support to maintain the voltage profile established by the grid operator. All utility-scale generation resources connected to the transmission network are expected to provide some voltage support (on some grids, voltage support is expected only for resources with individual or aggregated gross capacity of 20MVA and above). The reactive power requirements may be met through a combination of the generation resource’s unit reactive limit (that is, dynamic lead/lag power factor), switchable static VAR-capable devices, dynamic VAR-capable devices, and other voltage support services (ESMAP 2019a). The generation resource usually complies with the following reactive power requirement: maximum leading (underexcited) power factor of 0.95 and lagging (overexcited) power factor of 0.95, determined at the generating unit’s net power output supplied to the system operator and measured at the POI. In many U.S. and European electrical utilities, the reactive power requirements for wind-powered generation resources apply at all MW output levels at or above 10 percent of the nameplate capacity. When wind generation is operating below 10 percent of nameplate capacity and is unable to support voltage at the POI, the system operator may require disconnecting the wind generation resource from the power system. TABLE 2.1 Roles of resource entity and grid operator in integrating variable renewable energy into the grid Resource Grid Task entity operator Analysis of competitive renewable energy zones and site  selection Engineering design of power plant and construction Feasibility studies and preliminary power system studies  Application for interconnection with initial resource data  submission Reviews and technical data collection  Screening study  Full interconnection studies  Subsynchronous oscillation study  Economic study  Preparation of resource data for planning models   Interconnection agreement   Protocol compliance review  Metering and modelling data exchange   Resource registration  Approvals for energization and synchronization  Reactive capability testing and approval   Approval to begin commercial operations  Table 2.1 summarizes the division of roles and responsibilities for a typical business process flow of VRE integration in well-established grid operations in developed countries. This example can be used as a benchmark. Responsibilities for VRE integration in developing countries may be different, however, STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 7 because the grid operator may need to take a more active role in selecting the best sites for VRE construction, performing or assisting the resource entity in performing feasibility studies, and testing and commissioning VRE. For a very large VRE power plants (1,000 MVA or higher), the grid operator may play an even larger role, given the significant impact of the power injection into the grid. Full Interconnection Studies Planning is a critical component of all power systems to indicate the resources and assets needed to efficiently supply electricity to consumers. It is especially important where levels of VRE penetration are high. The power system must provide the required flexibility in responding to expected and unexpected sharp and rapid changes in grid demand and corresponding generation supply from conventional and VRE plants. Coordinated, integrated planning helps decision makers anticipate how the VRE plant will affect grid and power system operations and identify the options that can maintain the balance between demand and supply and minimize the cost of supply. Numerous technical studies are generally required to determine the viability of connecting a VRE plant to the grid.5 These studies examine the impact of interconnection on the grid, identify possible effects on both sides of the POI, and highlight issues that could affect the performance and operation of the VRE plant. This Technical Guide identifies and describes these power system studies. It does not address studies that need to be performed for system planning, civil or mechanical engineering, or land use purposes. Detailed integration studies define the characteristics of the equipment to be installed. The VRE plant owner conducts a power plant protection study. It shows the interactions and indicates the necessity of reassessing and resetting the grid protections, including protections related to fast auto-reclosing schemes for overhead transmission lines, which should be approved by the grid operator supervising the POI. The studies are performed by or under the supervision of the grid operator. The resource entity must perform this step to obtain permission for grid interconnection. When a resource entity proposes a location or set of locations for connecting its facility to a grid, the grid operator conducts initial power system studies to determine whether the proposed points of interconnection are viable from its perspective. These FIS also help define the required equipment configurations, specifications, and technical compatibility and identify the best interconnection solution. They seek to ensure the following: • the reliability of the overall power system • the security of the power supply • the stability of the power system and the VRE plants connected to it • preservation of high-quality service • protection of VRE and grid power system equipment • the safety of VRE plant and grid personnel. The FIS consist of the following mandatory studies: • steady-state and transfer analysis • short-circuit and breaker duty review 5 If grid operator determines that the VRE plant connection will not have a significant effect on the grid, power system studies may not be necessary. 8 | ESMAP.ORG • dynamic and transient stability analysis • facility study. Two additional studies are sometimes required: • A subsynchronous oscillation (SSO) study may be required based upon the results of an initial screening study. A detailed study is performed by the grid operator if it is determined, through visual evaluation of the proposed resource’s location, that any set of several (usually five or six) transmission element outages can make the proposed generation POI in a series to a series- compensated transmission line.6 • An economic study is required if the estimated cost of transmission improvements exceeds a certain threshold set by utility regulations (such as $30 million). The main objectives of the power system studies are described below. Appendix C provides detailed description of all studies. • Steady-state analysis identifies transmission facilities that may have a limiting impact on the output of VRE generation. The analysis is performed using power-flow calculations for various transmission contingencies (that is, outages of transmission elements that are electrically close to the new power plant). The objective of the study is to prove that the new VRE power plant meets the transmission criteria and that no single contingency will create transmission congestion after the plant is connected to the grid. If power-flow analysis of contingencies detects thermal overloads or voltage violations in the transmission network, additional measures and/or VRE facility improvements will need to be proposed to remove congestion. Measures may include reactive power compensation/regulation, adjustment of power plant settings, transmission network reinforcement, or other measures. • A short-circuit study analyzes the maximum available fault current in the system and the maximum level of current the electrical equipment (mainly circuit breakers) should be able to withstand during faults. A violation of the short-circuit requirement for any equipment identified in the study will require either replacement of the equipment or facility improvements to prevent short-circuit violations. Short-circuit modelling is also used to study the POI for the stiffness of the grid through a metric called the short-circuit ratio (see Appendix C for details). Dynamic and transient stability analysis calculates the VRE’s response to disturbances and examines whether the system would be able to cope with them. The objective is to ensure that connecting new VRE to the grid will not decrease the overall system’s ability to maintain synchronous operations of all equipment during disturbances. The system is generally assessed for the case of disconnection of the largest generation in the system in the scenario in which VRE penetration is highest (mid-day for systems with high levels of solar PV generation) to assess whether the system has enough primary frequency reserve to perform without resulting in a system blackout or other type of failure. If there are stability criteria violations, the VRE power plant must be enhanced through installation of additional equipment and frequency 6 Subsynchronous oscillation occurs when one or more resonant frequency coincides with a natural resonant frequency of the electrical system with long radial transmission network and series capacitors creating a sustained, cyclic exchange of energy between the generator and the electrical system. This definition includes any system condition that provides the opportunity for an exchange of energy (energy oscillation) at a given subsynchronous frequency of the system. This exchange of energy can lead to severe equipment damage. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 9 control improvements. Considering the fluctuating nature of VRE, ramping studies, including frequency stability studies, are sometimes included for very large VRE integration to estimate the adequacy of reserve requirements and gradient limitations. • A facilities study estimates the cost of interconnecting VRE to the grid. It encompasses design descriptions, construction milestones, and detailed cost analysis for all direct interconnection−related transmission and substation facilities. • A subsynchronous oscillation (SSO) study examines the subsynchronous resonance conditions in a series-compensated transmission system. Under fault and other disturbance condition, line reactance and series capacitor may come in resonance and cause a current with subsynchronous frequency to flow through the generator stator. • An economic study is performed when the connection of VRE plant requires significant improvements/reinforcements to transmission facilities and the estimated cost of transmission improvements exceeds a predefined threshold. A cost-benefit economic analysis is usually performed to justify integration of the plant. The main objective of this guide is to describe the FIS performed by the grid operator. After obtaining the grid operator’s approval in principle, the resource entity proceeds to the detailed design phase of the project, in which equipment model numbers and the electrical, civil, and mechanical aspects of the project are finalized. These issues are beyond the scope of this document. They include the following: • finalization of the sites’ physical size (footprint) • finalization of cable sizes, conduit sizes, circuit breakers, fuses, switchgears, and inverter ratings/types • sizing and finalization of grounding, station service power, site lighting, and other auxiliary systems (in some cases, the more detailed aspects are performed and completed at the final stage • execution and completion of civil engineering design, such as geological, seismic, drainage, soil compaction, concrete pads, and related design • design and procurement of mechanical systems, such as humidification, dehumidification, and air conditioning • procurement of material and equipment • revision of the preliminary analyses of load flow, short circuits, voltage harmonic distortion, and transient electromechanical stability • execution of electromagnetic transient studies for defining/revising the requirements for fault withstand, which are necessary to assess the voltage-withstand requirements of equipment under faulted and/or switching conditions and to assess whether adjustments or reconfigurations are necessary when using fast auto-reclosing schemes for overhead transmission lines near AC/DC converter stations used in the VRE plants • assessment of subsynchronous resonance of thermal generating units that are near the POI and AC/DC converter devices that are to be used at the connection point to the grid. Checklist for Full Integration Studies The VRE integration process captures some common generic tasks that must be performed for successful integration based on the practices of utilities in the United States. All grid operators have their own set of protocol rules/requirements based on the grid code and internal procedures that serve as a guidance for new resource integration. These procedures may vary significantly from one grid operator to another, but they usually share the common objectives of ensuring the reliability of the grid after interconnection of a new generating resource. 10 | ESMAP.ORG The following checklist captures the essential components of the FIS process. It represents a practical but simplified example of the actions usually taken by the engineer responsible for executing the FIS. The checklist contains logical step-by-step actions that can be used as a general reference for conducting FIS in countries that do not have a well-developed grid code and internal procedures/protocols for resource integration. 1. Select the study base case (which must include both steady-state and dynamic models) for the year of interconnection. Perform the following modifications: a. Add projects in the vicinity with recently signed interconnection agreements that were not included in the base case. b. Increase nearby dispatchable thermal generation to 100 percent output and nearby wind to a predefined percentage to reflect a realistic maximum (for example, 80 percent of output in the interior, 95 percent of output in coastal areas). Identify “nondispatchable” units (VRE and nuclear plants). c. If the slack bus is in the study vicinity, change it to a distant generator. d. If the VRE integration project is in a special climate zone that may be affected by nearby wind patterns, factor weather patterns into the analysis to account for the worst-case weather scenario. A second case for study (for example, a high wind case) may need to be created for a comprehensive analysis. e. Add the proposed generating resource to the study case, with capacity indicated by the interconnection request and set the minimum and maximum reactive power limits to fixed quantities corresponding to 0.95 power factor capability. 2. Perform steady-state contingency analysis (based on single [N –1] and sometimes double [N–2] outages) to confirm the reliability of operations without power system security violations after the addition of the new generating resource. It is assumed that a predefined list of applicable contingencies exists for interconnection studies. If the contingency analysis identifies power system security violations, compile a list with all equipment identified as overloaded and determine what transmission improvements will be required to avoid security violations after integration of the generating resource. Follow these steps: a. Examine the base case and N–1 (some utilities may require N–2 for special areas and network topology configurations) contingency limitations for the proposed generators. Identify the maximum transfer limit, limiting contingency and equipment for each scenario. b. If MVA flows on transmission equipment are greater than 100 percent of the equipment rating with the studied new generator resource injecting at full capacity, identify and model appropriate transmission upgrades or additions as necessary to allow energy injection without MVA limit violations. Apply equipment rating upgrades using industry standard available conductor and facility ratings. Rerun the study to confirm the overloads are resolved. c. Note the list of applied upgrades or reconfigurations in the report. 3. If the steady-state analysis identifies consistent voltage violations or serious degradation of voltage profile, it may be necessary to perform additional voltage analysis (such as a voltage stability study to ensure sufficient stability margin) or determine necessary local voltage support, such as shunts, capacitors, or Flexible AC Transmission System (FACTS) devices. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 11 4. Perform short-circuit analysis and determine whether breakers close to the plants have sufficient fault duty to clear a local fault.7 If fault duty is insufficient, propose breaker replacements. 5. To measure the strength of the area, calculate the short-circuit ratio (SCR), a metric that represents the voltage stiffness of the grid at the POI.8 The lower the SCR, the less stiff the POI, which means that some compensation devices are needed to strengthen the system. 6. Perform dynamic and transient analysis to determine whether the grid remains stable and the VRE resource remains online under typical fault conditions (for example, three-phase and single- phase-to-ground of key transmission lines, and disconnection of largest generator in the system). It is assumed that a predefined list of applicable dynamic contingencies exists for interconnection studies. 7. Estimate the cost of the transmission improvements required for interconnection. Determine whether an economic study is required and perform it if needed. 8. If there are stability violations or concerns, determine what transmission improvements are required to avoid stability problems after integration of the generating resource. 9. Review the network topology and determine the number of outages that can make the proposed generator connect in a series to a series-compensated transmission line. Create a scenario in which the connection of the POI is radial with that of series compensation, determine if an SSO study is required, and perform it if needed. Checklist for Final Design and Compliance Once all individual studies in the FIS have been reviewed and approved, the FIS are deemed complete. The resource entity and the grid operator can then prepare the integration agreement within a reasonable time frame (for example, six to eight months), followed by the resource registration. The integration agreement takes into account the findings of the FIS and the identified transmission facility and power plant adjustments, which are summarized as integration requirements. The integration agreement provides the green light to proceed with resource integration. However, the grid operator also requires protocol compliance of essential requirements of resource operation, which may correspond to some integration requirements already identified in the FIS. The protocol is highly dependent on the rules established in the grid code. The final design studies need to be performed to ensure successful VRE commissioning and acceptance. The detailed studies, which are associated with the end of construction and the pre-commissioning of the plant, achieve the following: 1. Allow for the final design, testing, and commissioning of VRE generation equipment (usually on a statistical basis); control systems of the VRE plant; and interconnection to the grid. 2. Lead to coordination and protection system studies, which determine the protective relay and circuit-breaker settings and fuse and arrester ratings. 7 The fault duty of the circuit breaker is the maximum short-circuit current capacity the device can withstand. 8 The SCR assesses the grid strength at the POI. 12 | ESMAP.ORG 3. Determine code compliance of the site’s electrical infrastructure and assess safety aspects, such as grounding and lightning protection, of the VRE plant. 4. Allow metering monitoring, communications, supervisory control and data acquisition (SCADA), and similar systems as well as related procurement to proceed (analysis of this item is beyond the scope of this note). 5. Allow for final procurement of the interconnection infrastructure material and equipment. 6. Allow for commissioning of the VRE site upon completion of construction. 7. Evaluate the final power factor and generation on/off ramp rate. The final acceptance studies finetune VRE performance for frequency control, energy storage and discharge, and system response times. In some cases, flicker, harmonics, and arc flash studies may be needed. These studies tend to be related to the “inside” performance or safety of the VRE system and do not normally affect the transmission system. They focus on the site and interconnection itself rather than the wider grid. These studies are often performed just before commissioning, to finalize controllers and protective device settings, such as relays and circuit breakers. Field tests could be required to validate control and protection settings. The results require approval from the grid operator, so that bulk system definition on the transmission system can begin. Studies related to transmission interconnections are usually more complex and time consuming than distribution system studies. The components of protocol compliance analysis depend on the VRE penetration level in the grid. Verification of some technical requirements (for example, protection, power quality, power reduction during over-frequency) is always needed. Other VRE-specific technical requirements (for example, adjustable reactive power, active power management, LVRT, active power gradient limitation, reduced output operation mode for reserve provision, synthetic inertia, stand-alone frequency/voltage control, full integration into general voltage control scheme) are verified depending on the VRE share in the grid and its participation in the regulation. The resource entity must comply with the protocol requirements established by the grid operator to ensure the safety, reliability, and power quality of the grid. The detailed analysis of protocol compliance and final VRE acceptance studies is beyond the scope of this Guide. Given the importance of the protocol for the operational acceptance of VRE plants, the following example (which does not represent a comprehensive list) of a protocol compliance checklist is provided as reference guide: 1. Confirm that the design will be able to meet all reactive power requirements (for example, produce ±0.95 power factor) at all MW levels between low and high operation capability limits of the active power output of the plant. 2. At the POI (the high side of the main transformer), dynamically provide voltage regulation that is able to respond to changes in the voltage profile. The generation resource needs to provide documentation to prove reactive support compliance with the grid operator’s requirements. 3. Confirm that the unit will dynamically provide voltage regulation, which may be met by the performance of the generators by installing additional reactive equipment behind the POI or by a combination of generator performance and additional equipment behind the POI. 4. Confirm that the unit can respond to changes in the voltage profile. 5. Confirm that the reactive capability is based on the ability to deliver to the high side of the step- up transformer. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 13 6. Confirm that the unit will provide frequency response. Generation resources that have capacity available to either increase output or decrease output in real-time must provide primary frequency response, which may make use of that available capacity. 7. Confirm that the breaker interruption capability of the main high-side breaker is suitable to fault duty. 8. Confirm that the power quality (e.g. harmonics, flicker, voltage distortion, current sags) of the output from the VRE power plant meets the grid operator’s requirements. (Details on various power-quality general requirements are provided in the technical guide on Compensation Devices to Support Grid Integration of Variable Renewable Energy [ESMAP 2018a]. See also Appendix C.) 9. Confirm that transient voltage ride-through is compliant with the operating guides and the grid operator’s requirements. All generation resources must meet the operating guide requirements with wind-powered resources or intermittent renewable resources meeting the nine-cycle voltage ride-through requirements.9 (Details on ride-through definitions and general requirements are provided the technical guide on Compensation Devices to Support Grid Integration of Variable Renewable Energy (ESMAP 2018a).) Good Utility Practice and Essential Studies for Interconnection Conducting elaborate interconnection studies based on strict business procedures and grid code rules is not always possible in countries that have not fully developed a mature grid code. Therefore, it is important to establish essential requirements for the connection of VRE plants based on the mandatory studies that must be performed: steady-state and transfer analysis, short-circuit and breaker duty review, and dynamic and transient stability analysis. The results of these studies encompass power flows, voltage profiles, short-circuit currents, transient processes, and oscillations for various states and conditions (that is, steady-state contingencies/outages, faults, dynamic disturbances) of the power system. The objective of the verification is to ensure continued reliable operations. If power system security violations are detected, recommendations must be made for reinforcing the transmission facility (by adding electrical equipment, building new transmission lines, or reconductoring lines, for example) or making adjustments at the interconnecting power plant to reduce the negative impact on the grid. The following descriptions are a compilation of guidelines based on several U.S. utilities’ practices. They are applicable to interconnection studies and power system reliability in general. Reliability and Steady-State Analysis The N–1 contingency analysis study usually investigates “more probable contingencies” for losses of the following: • any single critical transmission line • any single transformer • any bus section • any double circuit line of one mile or longer 9 The most common solution for preventing voltage collapse is to require low-voltage ride-through capabilities from the VRE power plants in the grid code. Such capabilities can be supplied by modern wind turbines and solar PV inverters as well as by external reactive power devices. 14 | ESMAP.ORG • any tie breaker • any generating unit • a critical transmission line or auto-transformer when any generating unit is unavailable. If any of these failures occurs, an interconnection study must conclude that (a) all transmission facility loadings are within their emergency ratings and all voltages are within their emergency limits and (b) facility loadings can be returned to their normal limits within two hours. “Less probable” contingency analysis cases usually encompass the following situations: • loss of any combination of related facilities, a critical transmission line when a 345kV auto- transformer is out of service, or a generating unit when another generating unit is out of service • sudden outage of any multicircuit transmission line at a time when any other single circuit is out of service • sudden outage of any single or double-circuit transmission tower line at a time when two generating units are out of service, for maintenance or economics • sudden outage of any generating unit at a time when any two other generating units are out of service for maintenance or economics • sudden outage of all generating units at any plant • sudden outage of all transmission lines on the same right-of-way • sudden outage of any transmission station, including all generating capacity associated with such a station • sudden dropping of a large load or a major load center • any other credible contingent scenario that might lead to cascading outages. If any of these contingencies occurs, an interconnection study must conclude that neither uncontrolled islanding nor uncontrolled loss of large amounts of load will result. Short-Circuit and Breaker Duty Review All circuit breakers at interconnected stations must withstand the short-circuit currents calculated in the short-circuit analysis. The analyzed faults are bolted symmetrical values for normal system conditions.10 Future increases in fault currents are possible. It Is the resource entity’s responsibility to upgrade its equipment and/or protective equipment coordination when necessary. Protection and coordination studies do not form an integral part of short-circuit analysis. However, given the importance and pertinence of relay settings to fault analysis and circuit-breaker current interruption duty, system operators verify that the resource entity provides utility-grade relays for protection of the transmission system. Relay operation for any of the following functions must initiate immediate separation of the generation from the transmission system: • Frequency: Detect under-frequency and over-frequency operation. • Overvoltage: Detect overvoltage operation. • Undervoltage. Detect undervoltage operation. • Ground fault detector: Detect a circuit ground on the transmission system. • Phase fault detector: Detect phase-to-phase faults on the transmission system. 10 A bolted fault is a short-circuit fault with no fault resistance. Bolted faults deliver the highest possible fault current for a given location. They are used to select equipment that can withstand the highest possible fault. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 15 • Transfer trip receiver: Provide tripping logic to the generation owner for isolation of the generation upon opening of the supply circuits. • Directional power: Detect the loss of the primary source under all system conditions. The relay must be sensitive enough to detect transformer-magnetizing current supplied by the generation. The relays are sometimes verified by the system operator when the short-circuit analysis is reviewed. Dynamic and Transient Stability Analysis A stability interconnection study covers the entire range of power system dynamics, from “first-swing” transient stability to longer-term oscillatory and steady-state stability. The dynamic study analysis is an essential complement to the steady-state contingency analysis. Power plant transient stability is an important consideration, because loss of synchronism of a generating unit or an entire generating plant can lead to equipment damage and severe power system transient swings, which compound the disturbance by causing the tripping of the unstable generators and possibly other equipment. Simulating system contingencies affecting power plant stability involves the analysis of various types of fault and network conditions using the transient stability performance criteria. Steady-state and oscillatory stability performance problems may be initiated by a wide variety of contingencies or operating conditions on the transmission network. For interconnection studies, a predefined set of stability disturbance criteria is analyzed that includes the following: • permanent single line-to-ground fault with one-phase breaker failure; fault is cleared by primary/backup breaker • permanent single line-to-ground fault with three-phase fault with unsuccessful high-speed reclosing (HSR); fault is cleared by primary/backup breakers • three-phase line opening without fault • temporary single line-to-ground fault with successful HSR • temporary three -phase fault with successful HSR. Analysis of more severe disturbances is sometimes performed to evaluate the strength of the transmission system and the potential for cascading outages. Examples of such analysis include common-failure mode disturbances, such as double-circuit tower faults or bus faults that result in the outage of multiple facilities at a location. Validation criteria for dynamic stability in an interconnection study can include the following: • The system is transiently stable and post-contingency oscillations are positively damped with a damping margin of at least 3 percent. For all cases, transient stability is maintained, with all oscillations stabilized in less than 10 seconds. • The generation resource is able to ride through faults (except faults in which protective action trips the generation resource). All generators in the grid are able to withstand all contingencies. • The voltage levels return to normal for all cases following the fault clearance, and all bus voltages recover to a minimum of 0.7 per unit after 2.5 seconds (except where protective action isolates that bus). • No transmission element trips (except elements either directly connected or designed to trip as a consequence of that fault). 16 | ESMAP.ORG The main objective of stability analysis in the interconnection study is to ascertain that the new VRE has no adverse impact on the grid (or remedial amendments are identified). The actual VRE dynamic performance characteristics are likely to be analyzed separately as part of VRE operational and design testing. The following sections describe the reliability criteria needed to assess the results of steady-state studies. Thermal Capabilities of Transmission Facilities 1. Transmission lines • Existing transmission lines were designed to meet operating standards that were in effect at the time the line was built. The National Electric Safety Code (NESC) specifies the safety rules for overhead lines. They include the maximum conductor temperature that maintains acceptable overhead line clearance while having acceptable tensile strength for the overhead lines. • Thermal capabilities are assigned on a line-by-line basis, with constraints applied based on design limits. • The circuit thermal capabilities should be reduced to the rating of the substation terminal equipment if the ratings of that equipment are lower than the conductor ratings. In general, substation terminal equipment should be sized to match or exceed conductor ratings. • The emergency rating for transmission lines is for two hours. 2. Autotransformers • The normal rating for autotransformers should be the top nameplate rating, including the effects of forced cooling equipment if it is available. • The emergency rating for autotransformers should be 110 percent of its nameplate rating for the first two hours of emergency operation and 100 percent thereafter. Such ratings may be increased on a case-by-case basis following detailed evaluation of the transformer’s manufacturer test results. 3. Disconnect switches • The normal and emergency rating should be 100 percent of nameplate rating. 4. Wave traps • The emergency rating should be 110 percent of nameplate rating. 5. Current transformers • The normal rating should be 1.5 times the primary current rating of the current transformer. The two-hour emergency rating should be 10 percent greater than the normal rating. 6. Circuit breakers • The normal and emergency rating should be 100 percent of the nameplate rating. Voltage Regulation • Generators are generally scheduled to hold higher than nominal generator voltage during peak load periods, to stabilize transmission system voltages. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 17 • Capacitor banks, reactors, and load tap changer (LTC) autotransformers are used in transmission substations, to hold voltage levels within acceptable ranges during normal and emergency conditions.11 • System conditions must be controlled to prevent excessive load tap changes. • Dynamic reactive resources, synchronous condensers, stored energy devices, and series compensation should be used as appropriate. Voltage Limits • Transmission voltages should not exceed 105 percent or fall below 95 percent of the nominal voltages shown above during normal operation of the system, to avoid equipment damage or voltage collapse in the worst cases of very low voltage. • Transmission voltages during emergencies should not exceed equipment overexcitation ratings.12 • Transmission voltages during emergencies should not result in customer voltages exceeding or falling below prescribed limits at substations on the transmission system. • Transmission voltage should not exceed 105 percent or fall below 90 percent of nominal voltage shown above during emergencies. The low limit can be lower if voltage-regulating equipment maintains voltage to customers within prescribed limits at distribution substations without causing voltage problems at nearby loads. • Voltage flicker on the transmission system (caused by motor starting, capacitor or reactor switching, furnace loads, or other intermittent or varying real and reactive loads) will be dictated by the sensitivity of the load or loads being served (for more information on flickers, see IEEE Std. 141-1993, IEEE Std. 1453-2015, and IEC 61000-4-15). Reactive Power Capability Reactive power resources are provided in amounts that are sufficient to control system voltage under normal and contingency conditions, including the dynamic period following system disturbances. Each operating company is responsible for providing or arranging for the provision of reactive power that can supply both its own reactive power load and any reactive power losses associated with service to its transmission service load, whether such losses are incurred on its own system or the facilities of others. 11 An LTC is a device that adjusts the voltage in transformers in order to maintain the voltage within predefined limits. Automatic voltage adjustment schemes may result in excessive tap changes because of the fluctuation of the VRE plant outputs nearby. 12 “Overexcitation of a generator or any transformer connected to the generator terminals will occur whenever the ratio of the voltage to frequency (V/Hz) applied to the terminals of the equipment exceeds 105 percent (generator base) for a generator and 105 percent (transformer base) at full load, 0.8 power factor or 110 percent at no load at the secondary terminals for a transformer, as per IEEE C37.102-2006. Overexcitation causes saturation of the magnetic core of the generator or connected transformers; stray flux may be induced in nonlaminated components that are not designed to carry flux. Excessive flux may also cause excessive eddy currents in the generator laminations that result in excessive voltages between laminations. This may cause severe overheating in the generator or transformer and eventual breakdown in insulation.” (IEEE 2006) 18 | ESMAP.ORG The power factor for each transmission operating company and its major subareas is maintained as follows: 1. The overall system power factor range should be maintained with a high lagging value (for example, more than 98 percent). It is calculated from the net MW and MVAr flows on the high side of the generator step-up transformer and at the interconnections. A high power factor (for example, 97 percent or above) should also be maintained on the generator side of the step-up transformer. Power factor standards may vary across grids. 2. Leading power factor on generators will normally be used only for off-peak, low-load situations for limited amounts of time, to reduce the likelihood of generator instability. Transmission Capacity and Load at Risk • The transmission capacity of individual power transmission lines is planned with the objective of having generation economically scheduled for all load levels, assuming that all lines are in service. The analysis needs to consider the amount and cost of transmission losses as well as future loading of the lines. • With one line out of service, no generation curtailment should be necessary. • The minimum transmission capacity of transmission lines from/to a major transmission substation/plant needs to be planned considering the total MVA substation/plant rating, based on the assumption that the largest incoming line is out of service. The minimum transmission line capacity to be selected will allow sufficient operational margin to ensure that full power from/to the substation/plant can be securely delivered. TABLE 2.2 Factors constraining connection of variable renewable energy transmission to the grid Type of constraint Factor Grid code • Voltage output minimum and maximum levels • VRE plant harmonic distortion output at POI • VRE flicker levels at POI • VRE plant frequency levels at POI • Need to meet UL 1741 SA inverter testing and certification for PV inverters (the standard by which inverters are certified for connection to the grid, particularly with respect to grid voltage and frequency. Grid operator and • Up and down ramp generation rates site • Fault clearing times • Flicker levels • VRE penetration levels • Circuit loading • VRE maximum capacity at a POI • Coordination settings for protective devices STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 19 Grid Integration Standards for Variable Renewable Energy Operations Regulators, grid codes, and grid operators’ requirements constrain VRE plant construction and integration into the grid. The resource entity must read the grid operator’s connection requirement documents to determine the extent of the constraints it will face in interconnecting. Because of the complexity of a connection, constraints are usually scattered throughout the documents. Transmission connections usually have far more constraints than distribution connections, because of the larger number of stakeholders, capacity, and scale of the connection equipment. Constraints come in two main forms, as summarized in Table 2.2. Many other constraints can be found at any given VRE connection. Constraints are key features in a number of studies. Their values (or value ranges) can be built into software models and used throughout an interconnection project. The main purpose of power system studies is to ensure that the connection of the VRE plants complies with the grid operation levels defined in national or regional grid codes. Where countries do not have grid codes or national grid standards, the resource entity can use the references and standards at the end of this Technical Guide. A separate Technical Guide (ESMAP 2019b) discusses the minimal essential requirements for VRE grid integration and identifies the International Electrotechnical Commission (IEC) and Institute of Electrical and Electronics Engineers (IEEE) standards for connecting VRE plants. Most advanced countries have robust bulk systems with a certain level of VRE penetration. Their rigorous standards and established grid codes may need to be modified to reflect the development stage of VREs in a particular country. Working with the local grid operator and sharing this kind of information is essential. It is possible to adopt some key elements of the standards and grid codes of developed countries. Extracting the set of “must be implemented” technical requirements is not easy, however, as standards are still evolving and being modified for VRE integration. Even in developed countries, the detailed operational rules at each grid connection site are frequently decided through mutual agreement between the grid operator and the resource entity based on the guidelines established in the standards. These issues notwithstanding, generation plants must implement standards on the following three measures: • permissible operating ranges of voltage and frequency • operating range for active power and power-frequency response required for over- frequency of the system • fault-ride-through (FRT) requirements for VRE plants and provision of reactive current during fault-ride through period “Strongly recommended” connection requirements include the following: • reactive power control by VRE plants (inverter-based control) • provision of dynamic grid support wherever applicable (particularly applicable for wind generation, which can provide synthetic inertia by detecting the grid frequency). In Germany, Bundesverband der Energie und Wasserwirtschaft (BDEW) published technical guidelines for distribution grids (BDEW 2008). They emphasize that generating plants supplying medium-voltage (1kV–35kV) networks that fall under the distribution network should make a contribution to network support. “Dynamic network support” requires that generating plants not disconnect from the network 20 | ESMAP.ORG immediately in the event of failures. “Steady-state voltage control” requires that they make a contribution to voltage stability in the network during normal network operation. Most states in the United States have adopted IEEE 1547 as the primary standard for distributed resource interconnection or use IEEE 1547 in the development of their own interconnection. This national uniformity has made the interconnection process faster and easier and has encouraged the integration of many new distributed resource projects. The IEEE 1547 standard is being followed by additional standards, recommended practices, and guidelines, making up the IEEE 1547 Series of Standards under SCC21, the IEEE Standards Coordinating Committee that oversees the development of standards for fuel cells, PV, dispersed generation, and energy storage and coordinates efforts in these fields among the various IEEE societies and other affected organizations to ensure that all standards are consistent and reflect the views of all applicable disciplines. One of the most recent updates is the major revision of the IEEE 1547 standard made in April 2018, which requires distributed energy resources to provide grid-supportive functionalities. IEEE Standard 1547-2018 requires frequency regulation (also referred to as frequency droop or frequency-watt) and improved power-quality requirements by default. However, voltage-regulating functions (e.g. reactive power functions, such as power factor, volt-var, watt-var, and constant var) are not required by default but are highly recommended. In distribution systems, to make the most of the standard and prepare for higher distributed energy resources (DER) penetration in the future, it is recommended that utilities consider the widespread use of voltage regulation at an early stage, which can be done through smart inverters for PV generation (IREC 2018). STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 21 This section examines three sample scenarios: integrating a very large wind farm into the transmission grid, integrating a medium-size wind farm into the transmission grid, and integrating a small solar plant into the distribution grid. For each scenario, it shows the studies that must be conducted and what they reveal. The examples are fictitious but based on industry experience. Integrating a Very Large Wind Farm into the Transmission Grid In the first scenario, a power resource entity asks to interconnect a 1,000MW wind farm to the transmission system. The project will be split into five 200MW phases, with a project duration of five years. The existing 345kV transmission lines are located about 20 miles from the proposed wind farm. A new 345kV substation and double-circuit 345kV transmission line will be constructed to connect the power plant to the grid. Steady-State Analysis The most recent power-flow case is used to determine the impact of the 1,000MW of new VRE generation on the transmission system. Single-circuit contingency analysis is performed, and the upgrades detailed below are added to avoid violations. The large amount of generation also mandates that a double-circuit contingency analysis be performed. The critical contingencies are identified in the studied area with transmission line loadings for all cases. The generation facility will be connected to a new 345kV substation as a cut into the existing 345kV transmission line—that is, a new substation will be established as a tap from the existing transmission line for the connection of the new generation resource. Each phase of the wind farm construction will require additional system improvements. Phase 1 will require the construction of three new 345kV substations and a 20-mile 345V transmission line with a double circuit. This transmission line will connect the new substations located at the wind farm facility to the new 345kV substation cut into the existing 345kV transmission line. Because of high voltages during off-peak periods, 2–100MVAr 345kV shunt reactors will be installed at the new substation. Phase 2 will require the construction of a second independent 30-mile 345kV transmission line to complete the 345kV transmission loop into the area. No improvements are planned for Phase 3 and 4. Phase 5 will require the installation of one +/–200MVAr static var compensator (SVC) and two 100MVAr 345kV shunt capacitor banks at the new 345kV substation. With the new 345kV transmission line completed, the power system will be capable of supporting the full 1,000 MW of renewable generation. The existing 345 kV transmission line is also series compensated. Therefore, two 100 MVAr shunt reactors will need to be installed at the existing 345kV substations (based on the stability analysis and subsynchronous resonance studies described below). Short-Circuit Analysis The short-circuit analysis is modified to add the 1,000MW generation at the new 345kV bus along with the new 345 KV substations and lines. The study uses lumped modeling of the 2.4MW wind turbines in groups for the 34.5kV collector sites. Each collector site is connected to the 345kV with a 345/34.5kV Wye Delta main transformer bank connected to the new 345kV bus. 22 | ESMAP.ORG There is no significant increase in fault current that would require the replacement of any equipment on the transmission system. The fault current at the new 345kV bus is 20 kA for a three-phase fault and 15 kA for a single line-to-ground fault, which is below the limits. Stability Analysis A stability study is undertaken to evaluate the dynamic performance of the proposed wind farm in response to transmission disturbances and to check the adequacy of dynamic reactive compensation in correcting any instability and maintaining acceptable system voltage during the post-disturbance transient period. Three phase-to-ground faults with primary clearing and phase-to-ground faults with delayed clearing resulting in the tripping of nearby transmission lines are considered. The wind farm power factor at the 345kV POI is assumed to be close to unity. Three shunt capacitor banks at the 34.5kV collector, reactive compensation in the form of 345kV shunt capacitor banks at 345kV substations, and 200MVAr SVC are modeled in the dynamic case, based on the additions required by the steady-state analysis. Dynamic modeling of the 2.4MW wind turbines is presented in the aggregated several groups. Capacitor bank switching time delays of 30 cycles were modeled at 345kV with a six- cycle switch closing/opening time. These delay times were observed to generally coordinate with the wind turbine undervoltage protection so as to avoid tripping the wind farm during or after the contingency events simulated in this study. Stability simulations were run with all transmission elements in service. The results showed that 345kV three-phase faults were 3 cycles in duration and that 345kV single phase-to-ground faults with delayed clearing were 10 cycles in duration. In each case, the faulted line was removed from service, with no automatic reclosing simulated. Different scenarios were simulated (for example, a three-phase fault at the new 345kV substation and a trip of new 345kV transmission line from the new substation and a phase-to-ground fault and trip of the new line from the new 345kV substation). No transient stability problems were identified in any of the simulations. The main performance issue is the steady-state post-contingency voltage drop. The type and quantity of reactive compensation necessary to maintain acceptable post-contingency voltage levels in the vicinity of the proposed wind farm were identified and summarized. Several scenarios cause post-contingency quasi-steady-state voltage regulation challenges because of the lengthy connection resulting from the combined loss of two lines. These difficulties can be addressed by adjusting capacitor bank switching time delays at the new and existing substations. Subsynchronous Resonance The proposed wind farm will be connected to the 345kV transmission line, which is series compensated at one end. The wind farm could potentially be radially fed through the series-compensated 345kV transmission for loss of one of the existing 345kV transmission line. The series compensation could have potential subsynchronous resonance (SSR) interaction with the wind farm generators tied to the new substation. Two issues are identified: (a) SSR on wind turbines and (b) resonance between the 34.5kV collector cable charging and converter controls. The turbine manufacturer should determine whether the turbines are susceptible to any damaging interaction caused by the close proximity of the series- compensated 345kV transmission line. Additional study by the turbine manufacturer is required. Summary of Findings from Power System Studies The system studies indication that addition of the 1,000MW of new generation is above the maximum the existing 345kV transmission lines can support and meet the electrical utility and grid operator’s planning criteria. Therefore, this project will require construction of new 345kV substations and 345kV STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 23 transmission lines with two circuits that connect the wind farm facility and the new 345kV wind farm substation. A substantial dynamic reactive support will also be required in terms of new capacitor banks, reactors, and an SVC. The cost of improvements to transmission system facilities is significant (about $150 million). Additional economic analysis needs to be performed to justify construction of the wind farm facility. Integrating a Medium-Size Wind Farm into the Transmission Grid The connection of a 200MW wind farm to the transmission grid is planned in a rural area with a local transmission utility. The grid operator connection is a 34.5kV multiple-transformer connection to a 345kV transmission line. The wind farm output of 600V AC feeds into several 600V/34.5kV transformers that simultaneously feed the transmission line through a large distribution/transmission substation. This large connection is located in a rural location but is within 70 miles of a major city that could use additional electrical power. Power-flow, short-circuit, and transient studies were performed to acceptable generation injection levels at various locations on the transmission grid. The results defined how much power the grid can absorb from the wind farm on an hourly, daily, and seasonal basis and still maintain proper grid operation locally and on the wider regional grid. The initial site, which was located at a large transmission substation, was found to be a good location for the connection. The power-flow, short- circuit, and transient/stability studies demonstrated that the 345kV grid was able to easily absorb and transmit all of the proposed wind farm’s capacity. Because the site was in a high lightning zone, transient stability and frequency studies were performed. The results showed that lightning levels in the area were not intense enough to affect the transmission grid at the connection but that some frequency oscillations do occur. It was decided that a battery- based frequency support system was needed. Given frequency variations above 62 Hz and below 58 Hz, the battery frequency support device would be able to keep the frequency levels at 59.5–60.5 Hz, in compliance with regional grid codes. This frequency stability support system was added to the project scope as a result of the studies. Without this additional frequency study, the wind farm would not have performed as needed or desired. The frequency stability support system was an expensive addition to the project, but it was considered necessary based on the scope and cost of the project. Design, protection, and control studies verified local code compliance and arc flash safety for the grid operator and VRE staff. The substation control room had 480V controls that required national electric code, national electric safety code, and arc flash code compliance studies. Depending on the VRE’s system complexity; layout; and the presence of switchgear and/or a motor control center for air conditioning, cooling and heating fans, and related equipment, it may be necessary to perform an arc flash study in the VRE control building. Arc flash studies are not universally required, but they are common in North America, as they address a serious safety issue. Integrating a Small Solar Plant into the Distribution Grid A 5MW PV farm is connected to a local rural utility’s distribution grid. The local grid operator connection is a 12.47kV, three-phase, grid-operator distribution line. The PV farm produces power at 480V DC and uses an inverter to convert DC to 600V AC. The 600V AC is then converted to 12.47kV at the grid– operator distribution interface transformer. The power-flow study yielded MW levels at various locations on the distribution power grid. The results defined how much power the local grid could absorb from the PV farm and still maintain proper grid 24 | ESMAP.ORG operation. The system must maintain adequate voltage, power factor, frequency, and other parameters despite the injection of the VRE power. A power-flow study is almost always required and necessary, regardless of the type of connection. In this case, it revealed that the power factor in the grid would be severely affected by the size of the VRE plant being considered and that the 5MW rating proposed needed to be reduced to 3.5MW. The study also verified that alternate locations on the distribution line impose the same PV farm capacity constraint and that an alternative distribution feeder connection may be desirable. The PV developer decided to continue examining the initial location for compatibility. Once the power-flow study was completed and analyzed, its results were used to prepare a short-circuit study, which determined both grid operator and PV farm short-circuit levels for the electrical equipment (circuit breakers, fuses, lightning arresters) and bus bar ratings in the switchgear being considered for the VRE site. This information is essential to ensure personal safety and protect equipment. Like power- flow studies, short-circuit studies are almost always required. The short-circuit study was used to conduct a coordination study that provided protective relay rating and setting data as well as information on the timing of the VRE’s main circuit breaker and fuse operation in the event of faults on the distribution system. This site is located in a high-lightning, high- wind zone. Because of its location, the utility required additional electromagnetic transient studies. A large irrigation pumping water load is on the grid–operator distribution feeder. Surrounding feeders and the source ramp rate and power factor aspects of the power-flow study were examined in more detail. Depending on the complexity of the VRE system and the presence of switchgear and motor control centers at the POI, it may be necessary to perform other studies and tests. At this site, the coordination study for the VRE was conducted, outside the normal order. Other studies, such as a lightning protection system control study and an arc flash study, were also performed, because of conditions at the plant site. This site had a significant switchgear set-up that required a small arc flash study. Arc flash studies are not universally required, but they are common in North America. In this case, because fast-moving cloud cover could cause flicker, a flicker study, including a grid operator LTC operations study, was conducted.13 A voltage/power ramp rate study was also performed. 13 The LTC operations study was essential because of the radial characteristics of the distribution network at the point at which the VRE plant would be connected. It informs how the VRE plant (especially an inverter-connected plant) should provide reactive power compensation to minimize voltage fluctuation. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 25 This document describes the main tasks and components of the complex process grid operators undertake to integrate VRE. It describes the studies that must be conducted before VRE is interconnected to the grid to ensure that renewable sources do not adversely affect the power grid. Grid codes and grid operator interconnection requirements vary across countries, but some power system studies must be conducted for all interconnections. These studies assess whether the VRE plant connection reduces the transmission or load-serving capacity, determine whether the interconnection exposes any grid equipment to operating conditions above the specified withstand requirements, help guarantee the safety of grid operator and VRE personnel working at the VRE connection, and reveal whether grid upgrades and/or the installation of new equipment is necessary. This document identifies good practices for conducting interconnection studies, which countries that do not have mature grid codes can apply. It describes the minimal criteria for grid acceptance of a new VRE generation resource. Meeting only these criteria does not provide a comprehensive analysis, but it identifies the viability of the interconnection and power transmission system measures that can make the connection successful. Use of these criteria can be recommended in some countries with underdeveloped grid codes. 26 | ESMAP.ORG This appendix provides a checklist of site information the resource entity needs to obtain, with the assistance of the grid operator (this information will vary depending on the VRE resource). The resource entity shares the completed checklist with the grid operator, providing both parties with a common reference. This checklist is not comprehensive. It is intended only to initiate a project. 1. Address of facilities: Plant will be located in the municipality of _________, in the administrative region of ___________. Coordinates: Latitude: ___ Longitude: ___ 2. Intended date of commissioning: 3. Installed power: 4. Maximum injected power at POI: 5. Electrical and mechanical systems: • Type: • Number: • Brand: • Model: • Nominal power: • Nominal voltage: • Rated power factor: • Turbine type: 6. Generator type (synchronous/asynchronous): 7. Speed regulator: 8. Voltage regulator (model and parameters): 9. Stabilizer (model and parameters): 10. Transformer at source (wind turbine): 11. Number (of wind turbines): 12. Nominal power: 13. Nominal voltage: 14. Impedance: 15. Winding: 16. Number and voltage ratio of taps (off-load): 17. Regulating range: 18. Winding material: 19. Cooling: 20. Fuses (type, amp rating): 21. Arresters: 22. Collecting network: 23. Number of circuits: 24. Voltage: 25. Approximate total length: 26. Dimensions of underground cables: 27. Interconnections circuit breaker/fuse protection: 28. Type: STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 27 29. Voltage rating: 30. Current rating: 31. Circuit-breaker frame size: 32. Trip settings: 33. Protection relay-brand/model: • Under-/over-voltage: • Overcurrent: • Under-/over-frequency: • Other: 34. Interface step-up transformer • Number: • Nominal power: • Nominal voltage: • Impedance: • Winding: • Winding material (Cu, Al, Ni, etc.): • Cooling: • Fuses (type, amp rating): • Grounding: • Number of taps: • Regulation range: • Number and voltage ratio of taps (off-load): 35. Tap under load with automatic regulation: 36. Neutral reactance/resistance in neutral of step-up transformer (if needed) 37. Equipment for reactive power support (if needed) • Number: • Type (capacitor bank/SVC/STATCOM/) • Nominal power: • Nominal voltage: 38. Switching station (distribution substation only) 28 | ESMAP.ORG • Main breakers: • Configuration/lay-out: • Number: • Grounding: STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 29 Figure B.1 Procedure for interconnecting variable renewable energy used by grid operator in ERCOT Source: Adapted from ERCOT (2019). 30 | ESMAP.ORG Grid operators have extensive data inventories of the grid assets in transmission and distribution grids, including detailed specifications, ratings, connection information, locations, and other details. Sophisticated software models often include this information and can perform all studies, with the addition of information on the VRE plant. It is therefore essential that the resource entity include a detailed list of equipment in its application for the interconnection. This list should include specifications, one-line diagrams, and equipment model details. Appendix A provides a template of the information that should be provided. The grid operator provides the resource entity with the study results and implications. If the interconnection does not pose any grid-related issues, the interconnection is approved and can proceed. If problems or concerns are identified, the grid and resource entity work together to resolve them. Where they cannot be resolved, the VRE point of interconnection is relocated or cancelled. Table C.1 describes 10 power system studies performed for VRE design and integration into the power grid. TABLE C.1 Most frequently conducted types of power studies Type of study Description Preliminary feasibility studies Power-flow studies Power-flow studies examine how power flows from point to point on a given grid. Directional power-flow capacity can be determined at different times of the day, seasonally, and under varying weather and system constraint conditions. Power-flow studies are usually required in all VRE installation cases. They are particularly important in transmission network interconnections. Short-circuit studies Short circuits can occur when part of the power system, such as a power line cable, comes into contact with the ground or another part of the system. In these cases, known as faults, electric current can change direction, sending power from surrounding power circuits into the fault. A line normally carrying a few hundred amperes can suddenly carry tens of thousands of amperes, putting extreme stress on equipment and possibly causing injuries or fatalities. Studies calculate fault levels. They are needed before almost all VRE installations. Transient studies Power system transients can occur as a result of various system (electromechanical disturbances, such as system faults, loss of power generation, frequency transients, instability from high-voltage line switching, equipment failures, current electromagnetic in-rush, lightning, and other causes. Transient stability studies can define transients, voltage the locations and possible damage from these events. They can also stability, and frequency identify wider weaknesses in the power system. These studies are stability) particularly useful on older grids, in severe weather environments, and in cases where the grid system is upgraded or requires upgrades to STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 31 improve system reliability and stability. These studies are required when issues are known to exist or are expected as a result of the increase in the penetration of these renewable sources. Design studies Power-quality studies Power-quality studies are used to determine power-quality problems, including voltage and current sags (dips), harmonic distortions, power factor swings, flicker, and related problems. Voltage-distortion studies Voltage sags and notches can severely affect the quality and reliability of power to and from the grid. Voltage studies define such problems and allow mitigation strategies to be implemented. Harmonic studies Harmonic studies determine the level of harmonic distortion under different system conditions and locations. Harmonic distortion is extremely detrimental to microchips, affecting computers, controllers, and modern manufacturing processes. Flicker studies Fluctuations that induce a flickering effect on lighting systems have become a much wider problem as a result of fast-moving cloud cover over PV systems, which can damage transformers on the grid. Flicker can also be induced by large industrial processes and flows into VRE systems, which can damage them. Historically, the problem has been caused by steel mills, shipyards, and other large industrial plants. Control and testing studies Coordination studies Coordination studies are used to determine the trip settings and timing of tripping for circuit breakers, fuses, and arresters used at the VRE site. Grounding and lightning Grounding and lightning protection studies can be very simple or protection studies extremely complex, depending on the site soil conditions. Grounding studies are used to determine the required grounding system, system- to-earth ohmic values, grounding material needs, and material patterns and locations. Grounding is an essential safety requirement that is usually required in all VRE installations. Grounding studies can occur at any stage of a VRE project. Lightning protection studies are a subset of grounding studies. Arc flash studies Arc flash studies determine whether energy can harm personnel working on energized electrical equipment. Many jurisdictions require these studies. Power-Flow Studies Power-flow studies are used to define the scenarios of load and generation across the life cycle of the VRE plant. The results determine whether the interconnection location is acceptable. Power-flow studies demonstrate that the VRE plant will not overload the grid equipment, impose any restriction or limitation on the grid's transmission or distribution capacity, or allow voltages outside the allowable ranges required by grid operator or grid code rules. Many power-flow models also examine the reactive power capability of the grid and how reactive power is affected by the VRE at the POI and the 32 | ESMAP.ORG surrounding grid. The power output capacity of the proposed VRE plant is usually downsized or upsized based on the reactive power situation determined by a power-flow model. If reactive power requires support at the POI, mitigation mechanisms (such as capacitor banks, static VAR compensators, STATCOM, shunt reactors, and power plant controllers) may be prescribed. It is also necessary to identify transmission facilities that limit VRE output for various contingency situations in the grid. N–1 contingency analysis is frequently performed to demonstrate that existing or planned transmission capacity in the area will meet the electrical utility’s transmission criteria during various outages if the proposed resources are installed. If there is insufficient transmission capacity to interconnect the proposed VRE without congestion, grid reinforcement may be considered. In high VRE penetration situations, it is important to take into account the balance of the grid operator’s conventional generation compared with connected VRE generation. The greater the penetration of VRE, the less stable the grid can become. Power system studies can help determine the needed versus actual balance of VRE and grid-based generation and identify possible corrections where imbalance is found. Studies can also be used to investigate sudden variations of generation in a geographical area (such as sudden lack of sunlight from cloud cover or variations in wind velocity). In a simple case, power-flow studies can provide snapshots of the grid’s dynamic performance in a very quick and simple way. The results can lead to further investigation of more complex and dynamic evaluations, if needed. In some transmission- or generation-constrained corridors, power-flow studies can help balance conventional generation with VRE resources and ensure that the maximum and minimum set-point ramps of conventional units (mainly coal-fired, pumped storage, or gas turbines) will follow any on/offloading of sharp variations in renewable generation in most operating scenarios. After proper modeling of the load conditions, power-flow studies can determine what load levels would lead to voltage instability. Many software tools calculate this maximum possible load condition. If not enough voltage support is encountered at the POI, additional reactive power or energy storage can provide voltage support in the form of shunt capacitor banks, static VAR compensation, or STATCOM (ESMAP 2019b). Additional mitigation studies may be required if the studies reveal power-quality or transients issues. Some utilities control reactive power on the grid themselves; others require the resource entity to correct reactive power deficits or surpluses on the VRE side of the meter. The power-flow model is often used to determine the state of the reactive power. To avoid or minimize the need for such equipment, the reactive capability of VRE plants could be increased by adjusting the operating voltage on plant’s equipment. Continuous voltage control at the point of common coupling can be achieved by using a dedicated voltage regulator as part of a power plant controller. This new product helps grid operator predict the VRE plant’s performance. Its advanced algorithm, subsecond response time, and efficient communications system permit precise control of the active and reactive power delivered to the grid. Such features are often included in both static power-flow studies and dynamic simulation models. Depending on the grid code in effect, the grid could require injection or absorption of reactive power even without the availability of primary resources. Wind plants can meet this requirement by using full- converter wind turbines; PV plants can use smart inverters. For existing plants, this requirement implies the installation of static VAR compensation or STATCOM devices. New smart inverters can be used to provide reactive power, harmonic correction, flicker control, and a host of other functions in these situations. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 33 Short-Circuit Studies Short-circuit studies are almost always required for transmission and distribution interconnections. Short circuits (faults) occur when energized electrical apparatus such as cables, bus bars, or other equipment lose electrical isolation and touch each other or the ground. In the worst case, rotating loads, such as motors and generators, push electric power into the short circuit. During a fault, the direction of power flow can change and multiple sources of power can inappropriately feed into the fault. Extremely high levels of electric current flowing into a fault can damage electrical infrastructure and injure or kill workers. Short-circuit studies are also used to calculate short-circuit power flow at the POI and at selected points on the grid that represent locations where faults would be more problematic for grid operator. The short-circuit situation depends on the number of VRE plants and other heavy industries and the number of transmission or distribution circuits connected to the same power line or substation. VRE plants can contribute to short circuits in several ways. Wind machines are rotating generators and can actively feed into a fault. Older wind farms with fixed-speed induction generators normally contribute moderate fault current levels. Full-scale converter or PV plants equipped with grid-side inverters normally contribute very low or negligible levels of short-circuit current, because PV systems are not rotating generators. Depending on the specification of the inverter used, some short-circuit current can be present. Current controllers on inverter facilities can reduce output to cancel the fault- current contribution from VRE plants in half a cycle, preventing damage to the existing grid’s equipment by not allowing specified limits to be exceeded once the VRE plant is integrated. Short-circuit studies can also be used to calculate other parameters, including the following: • the increase in short-circuit power and verification of the withstand capability • the requirements for short-circuit current ratios of existing equipment • the short-circuit ratio and voltage increase after loss of power generation. In some cases, a short-circuit study is used to provide baseline data for a coordination study (discussed below), to find the rating and settings of existing/proposed protective devices, circuit breakers, fuses, lightning arresters, and power cable sizes. Short-circuit calculations on parts of the grid are also important for power-quality, coordination, and arc flash evaluations, because short-circuit levels can drastically affect these study results. Electromechanical Transient Studies Electromechanical transient studies assess the ability of the power system to return back to its steady state following a disturbance. Traditional AC-based power systems are based on generators that are rotating electromechanical machines. These generators are rotated using an external force, such as steam, water pressure, or wind. The external force spins magnets inside wire coils, which generate electricity. Synchronous machines of this type create inertia, which stores rotational mechanical energy, helping maintain transient stability in these rotating machines. The lower equivalent inertia found in conventional generation plants plus the high penetration of VRE plants on a transmission line or distribution feeder can create grid instability that requires transient stability and dynamic analysis. Transient/Dynamic Stability Studies Transient/dynamic stability studies can address a variety of issues, including (but not limited to) the following: 34 | ESMAP.ORG • fast-response inverter functionality and response times • low-voltage ride-through levels and response times • reactive power levels and correction • frequency levels and excursions • fault-clearing levels and response times • fast auto-reclosing on transmission lines. In some cases, transient software models can help evaluate and solve transient issues. In other cases, complex and expensive software models are needed. It is common for electromechanical transients to require field, factory, or laboratory testing and the use of extensive analysis using software models. The fast response of grid inverter–type renewable plant is a helpful way of mitigating transient stability problems. Fast-response inverters allow rapid reduction of active power from VRE plants without loss of voltage control. Depending on the ability to provide mechanical inertia power from existing conventional active power sources, successful shutdown of the VRE plant requires rapid response and inverter controls. This need, along with the inverter specifications, must be understood at the early design stages. Monitoring feature requirements and communication delays also need to be considered. The low-voltage ride-through feature acts as a mitigation tool for low inertia. It is activated when local voltage falls below a given preset limit. Performance from the electromechanical transient point of view is rarely an issue with VRE plants, because of their inherent ability to modulate and/or reduce the active power, depending on the specified inverter controllers, particularly in the case of smart inverters. Modal analysis could be required to identify whether the new plant modifies system modes of oscillation. In some cases, reactive power issues can be treated as a transient analysis problem. Some grid operators control reactive power and power factor themselves, using equipment on their side of the meter. Others require that the customer base, as well as the resource entity, provide grid operator–preapproved reactive power for the benefit of the grid. Some power-flow studies evaluate reactive power and associated power factor, which some transient study models do as well. Transient stability studies can also assess how VRE plants will react during large frequency deviations, which can occur in the event of load rejection (sudden load loss condition), loss of generation events, or changes in the topology of the transmission grid. If the VRE plant shuts down as a result, the consequences of such additional loss of generation can be predicted and avoided. Transient stability studies can thus demonstrate that the system and VRE plants remain stable without generation tripping or identify situations where stability will not be reached. In situations where VREs produce high power flows—for example, short-circuit faults or line tripping— the topology of transmission lines may affect electromechanical stability. Implementation of fast auto- reclosing on such transmission or distribution lines, or a revision of the auto-reclosing scheme, may be necessary. In extreme cases, upgrades of transmission lines should be considered, in order to evaluate necessary changes to the grid topology to accommodate VRE interconnection. The resource entity should be aware that grid operators sometimes have to refurbish transmission or distribution lines associated with a VRE interconnection as a way of addressing these kinds of problems. A grid operator or grid code requirements may require that the VRE remain in service following remote faults of long duration. After fault clearance, voltage restoration should be achieved within a prespecified period of time. The transient study can be used to simulate this case and find the situation STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 35 representing stability after shutting down the VRE plant when submitted to severe voltage conditions. Otherwise, special protection schemes must be implemented to avoid the drastic loss of generation in the grid. The grid operator provides the specification for minimum operation times during low-voltage events. These requirements are specified in grid codes as design criteria in the case of low voltage ride through situations. Both wind and solar plants can dampen the dynamic electromechanical oscillations that can occur across the interconnection, by properly setting the control devices intrinsic in such plants, such as circuit breakers, fuses, and relays. Electromechanical transient and coordination studies determine these settings. Frequency Stability Studies VRE plants are not usually intended to regulate frequency. However, new smart inverters can provide some assistance in providing this function, if the grid operator allows them to do so. Frequency stability studies identify frequency excursions and out-of-bounds situations. Smart inverters and battery-based energy storage are proven ways to help maintain frequency stability. Some grid codes require VRE plants to be equipped with all equipment necessary to participate in primary frequency control, providing reserve for both under- and over-frequency conditions. These features need to be modeled and considered for midterm transient stability analysis. High-frequency ride-through is a very effective resource for VRE plants that could help the system in an over-frequency situation. PV systems using smart inverters and wind or PV systems using battery-based frequency support systems are available for frequency stability support. Resource entities often perform studies, for which they must acquire the modeling data themselves. They analyze the results and provide the study and reports to the grid operator. These studies involve the following activities: • identification of the needs for particular studies • gathering of raw data and data entry into a software model • creation of the study model in the software • field testing as needed • interpretation of study results • revisions of the model • preparation of a study report or presentation. Electromagnetic Transient Studies Electromagnetic transient studies are used mainly for VRE plant system or equipment design issues inside the VRE plant. In some cases, they can also be used on the grid operator’s side of the meter. Electromagnetic transient studies that are commonly performed for VRE interconnections examine the following issues: • circuit-breaker transient recovery voltage • transient overvoltage from capacitor bank switching (for power factor compensation or voltage control) • power transformer in-rush currents during their energization procedure (and their undesirable consequences) • energization of the transmission line and cable • transient voltage withstands requirements during load rejections 36 | ESMAP.ORG • fast auto-reclosing schemes of overhead transmission lines • subsynchronous resonances • shunt reactor switching transient overvoltage • other specific cases, depending on the proposed configuration of the VRE plants and integrated power equipment. These issues are usually subsets of the features within the electromagnetic transient software models; they are beyond the scope of this Guide. The manufacturers of specific VRE components and equipment provide the specifications, test results, and data for these types of equipment. That information can then be used as an input into a software model and the results used in a specific study. Some studies may require field testing of a specific component or system. Power-Quality Studies The introduction of microchips and digital controls has elevated the importance of power-quality analysis, as relatively small voltage and current distortions may damage and disable digital electronic devices. Power-quality studies are heavily dependent on the results of short-circuit and electromechanical and electromagnetic transient studies. Power-quality problems may include voltage and current waveform distortions, harmonics, power factor, flicker, and other issues. Power-quality studies have become more sophisticated, as new software models can perform deeper levels of calculations to help identify and solve these problems. Many IEC and IEEE standards have evolved to define the necessary limits of acceptable power quality. Identification of power-quality problems, the use of software, and standards-based limits for such problems are all parts of a typical power-quality study. Power factor issues are sometimes treated as a separate category of power system problems, but they are normally associated with power-quality issues. Power-quality studies often require field tests and data gathering to provide the raw data for software models. A wide range of portable, handheld, test instruments allows fast, accurate data gathering and logging. These instruments are usually multifunction devices that can cover voltage, current, harmonics, and other functions. Many test instruments can download recorded data directly into software models or data storage devices. Voltage-Distortion Studies Voltage sags are the most common type of power-quality problem. Nearly every analysis model examines them. A host of issues—including equipment failures, single phasing of three-phase circuits, loss of the generation source or load, and many others—can cause voltage sags. Solutions include low- voltage ride-through control, battery energy storage, and reactive power control. Voltage problems can be simulated by applying the required wave shape at the point of interconnection in the model. Depending on the cause of the distortion, voltage problems can often be field tested using handheld equipment or, in more severe cases, permanently installed logging equipment. The cost for this equipment varies widely. Testing results can easily be entered into software models for analysis. Harmonics Studies Harmonic distortions can cause serious problems in power systems, because they may damage microchips, affecting computers, controls, relays, and meters, as well as industrial processes. Harmonic distortion can pass from the VRE to the grid and from the grid to the VRE originating on either side of STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 37 the meter. In both cases the distortions are undesirable. Studies and testing are often performed to determine whether harmonic distortion exists on a power system in any form. The grid operator can penalize the resource entity for harmonics that originate on the VRE side of the meter; in some severe cases, the VRE can be shut down if excessive distortion is detected. In some cases, it is difficult to determine whether a voltage or current waveform distortion is from harmonics or sagging. Studies can help determine the cause. Modeling and testing for harmonic distortion is typically done at the POI. Harmonic data gathering can often be performed using handheld harmonic analyzers. Harmonic analyzers also have logging capabilities, allowing the results to be viewed over time. Results from some analyzers can be downloaded directly onto spreadsheets for more detailed analysis. Depending on the results of harmonic studies, further analysis/assessment of harmonic penetration into the grid may be necessary. An example is a case where frequency response impedance for each harmonic current is injected from VRE sources, which could cause unacceptable voltage wave distortions at other points on the grid rather than the POI. In this situation, frequency time domain analysis could be conducted using the harmonic software model to help locate these points. This kind of study is also important when capacitor banks are added to increase the reactive power capability. These reactors can transform amplification of the harmonic currents at a certain harmonic frequency. This behavior could overload the capacitor banks and damage their cell units. In some cases, harmonics can also be transferred into a VRE from the grid, damaging the VRE plant. resource entity should take into consideration such a possibility when performing grid studies. Properly rated harmonic filters are often used when voltage distortion limits are exceeded. Wind farms integrated to power grids sometimes cause harmonic distortion above code limits, which necessitate installation of harmonic filters at the POI. Harmonic filters are typically combined capacitors and inductors, which can eliminate or cancel out harmonics that have been identified in a harmonic distortion study. The harmonic filters must be properly selected to reduce the voltage distortions to accepted values. Flicker Studies Flicker is an electrical system phenomenon that was common in the early 1900s, when light bulbs flickered in intensity because of rapid changes in voltage levels from large industrial activity that affected the transmission or distribution line. The problem recently resurfaced because fast-moving clouds over PV banks cause the voltage levels of PV cells to inject flicker onto the grid. Flicker limits vary across utilities. Flicker that exceeds these limits can result in financial penalties or even a shutdown of the VRE plant by the grid operator. Mitigation of flicker is the responsibility of the resource entity and can be expensive. Flicker can also be transmitted to a VRE plant from other grid customers, potentially causing damage and control problems for VRE resource entity. Steel plants, electric transportation, shipyards, and other very large loads that have rapidly changing voltage levels can produce flicker. Some power-flow models include a flicker function, but most do not. Depending on the cause, origin, and severity of the problem, flicker studies can be performed at any stage of an interconnection. The possibility of flicker can be revealed in a power-flow study, but in some cases the problem does not reveal itself until the VRE plant has been in operation for some time. Capacitor bank switching can also cause flicker. Flicker from this source is usually mitigated when the correct number of capacitance or reactance stages is selected and switched into the transmission or 38 | ESMAP.ORG distribution circuit. Transient switching overvoltage as a result of capacitor bank energization can also cause flicker. It can be mitigated by using automated switching or pre-insertion resistors along with proper circuit breakers. Flicker can cause excessive operation of transformer load tap changers. A grid operator may require examination of any possible PV-generated flicker on its wider transmission or distribution system for this reason. Recent harmonization of flicker codes and standards in studies conducted in Europe and the United States have created very complex flicker testing and analysis procedures that many countries are slowly implementing. Detailed studies in some complex cases may require many months of field testing. Software models for flicker testing used to be rare; they are becoming more common, as this problem is becoming more apparent in power systems. Flicker study procedures are documented in IEC and IEEE codes (see IEEE 1453 in the list of standards). Final design studies use data that are already available to or at least accessible by the resource entity. They are usually performed and used by the resource entity and not shared with the grid operator. In some cases, such as grounding studies, the resource entity shares study results with the grid operator, as both operators benefit from them. These studies involve the following: • identification of the need for a particular study • field, factory, or laboratory testing (in some cases) • gathering of raw data and data entry into software models • running of the software model • interpretation of the results • revisions to the software model • preparation of a study document or presentation of the results and implications • sharing of the study with or presentation to the VRE or grid operator. Coordination Studies Coordination studies provide time versus current comparison curves, known as time current curves (TCCs). TCCs are used to coordinate the various protective devices, with one another and with the wider power system. These curves allow an operator to examine in detail how circuit breakers, fuses, and arrestors react to currents and faults. Multiple layers of circuit breakers and fuses are often present on a circuit. The TCC demonstrates the proper order of their tripping. It allows an operator to examine how circuit breakers, fuses, and arrestors react to overcurrent or faults. Circuit breakers sometimes trip in the wrong order. TCCs help operators determine how to reset them so that they trip in the proper sequence. TCCs also provide the correct ampere ratings, in order to prevent dangerous over-currents on VRE and grid equipment. Coordination studies can also be beneficial when breaker failure or anti-islanding situations arise. Coordination studies can require considerable data gathering. The grid operator may have to provide equipment specifications and locations. Coordination studies rarely involve the use of test equipment; they do involve gathering data on circuit breakers, fuses, motors, generators, transformers, cabling, and other site equipment and entering them into the appropriate software model. Discussions with equipment manufacturers may be needed to obtain equipment specifications and technical parameters, such as reactance values of motors, cable resistance and reactance, and transformer impedances. The software model will reveal the proper protective device settings and STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 39 identify mismatches between protective equipment. Short-circuit study results are used in coordination studies. Safe operation of an interconnection may require numerous iterations of the coordination study. Protective devices such as circuit breakers, fuses, lightning arresters, and relays are placed on electrical circuits in strategic locations to provide maximum protection of the power system. The devices trip, or open the circuit, at preset times and ampere values. Proper tripping time is essential. A coordination study provides insight into how the system’s protective devices will operate in real-world conditions. Multiple layers of circuit breakers and fuses are often present on an electrical circuit. The TCC demonstrates the proper order of their tripping. It also provides the correct ampere ratings, in order to prevent dangerous overcurrent on VRE and grid equipment. Coordination studies can also be beneficial when breaker failure or anti-islanding situations arise. Coordination studies use data from the short-circuit, electromechanical, and electromagnetic studies. Both grid and resource entity can perform coordination studies on their sides of the POI. Coordination studies used in support of VRE installations differ in scale from studies used in industrial plant or grid power system settings. Coordination studies of large industrial plants can require the collection and entry of the specifications of thousands of circuit breakers, fuses, arresters, transformers, and cables. In contrast, a VRE plant may have only a few such devices. A large wind farm, for example, may have a few hundred. A coordination study used in a VRE interconnection thus requires significantly less effort than an industrial or wider grid coordination study. Normally, the resource entity gathers its own plant data, then acquires the utilities’ circuit breaker, fuse, transformer, and cable data at the POI and performs the study. In some cases, grid operator asks for the results of the resource entity’s coordination study and then reruns the study with a larger subset of the grid’s equipment. In other cases, the VRE and grid operator perform the coordination study jointly. In very large VRE installations, data gathering for coordination studies can be time consuming, and the grid operator may have to provide the specifications and locations of the equipment. Coordination studies rarely involve the use of test equipment. Rather, they involve gathering site equipment data and inputting them in the appropriate software model. Both the grid operator and the resource entity perform coordination studies. Depending on the grid operator’s choice of software model, coordination studies may be performed at the same time as other studies or at a different time. The resource entity often performs the coordination study and then shares the results with the grid operator, which then performs its own coordination study. The results of the studies by the grid operator and resource entity are compared; occasionally, adjustments to circuit breaker and fuse settings are made to accommodate the study results. Coordination studies are usually the foundation for ARC flash studies (discussed below). Grounding Studies Grounding studies determine grounding system layouts. They often involve overhead lightning protection. Grounding systems provide a zero reference for system voltages and create a path to earth for stray voltages and currents, making the work environment safe and protecting equipment within both the VRE and grid systems. A common ground between the grid and the VRE sides of the interconnection maintains uniform voltage levels across levels of voltage transformation at interconnections. A proper grounding system shunts lightning and grid or VRE equipment faults to earth. Grounding studies are often based on IEEE or IEC standards (see IEEE 1547 in the list of standards). The resource entity normally shares the results with the grid operator. 40 | ESMAP.ORG Lack of a proper ground inside a VRE plant can lead to odd or fluctuating voltages on instrumentation, meters, and equipment. Testing and an associated study may be performed to identify cases where lack of a ground path is found and located, so that it can be corrected. Grounding systems are often analyzed with relatively simple testing instruments that yield immediate results. In more complicated grounding systems, simple instruments can be used intensively or more expensive grounding test equipment sets can be used. Field-gathered data are entered into the software. Proper grounding system performance is based on electric codes. Testing results can help verify compliance with the code. VRE substation interconnections may require studies that lead to mitigation, such as adding grounding rods, enhancing soil conductivity, or adding/upgrading grounding mats under the interconnection substations. Arc Flash Studies Arc flash can occur when a worker touches live electrical apparatus, causing an electrical arc to flash from the energized metal to the worker’s body. It is a major cause of serious injury and death of electrical workers around the world. To prevent it, many countries have created and enforce arc flash codes and standards. Arc flash studies are relatively new. They are performed inside a facility on switchgear, circuit breakers, fuses, and other infrastructure. The studies use data from a coordination study as their foundation. Field testing is rarely required. In VRE interconnection situations, arc flash studies are normally not exhaustive in terms of data gathering. They pertain only to the major pieces of electrical equipment, such as transformers, fuses, circuit breakers, and service panels. Computer models for arc flash models are readily available, but they require specialized training to use. In the simplest case, one-line drawings of the VRE plant are used to provide the data for input into the modeling software. Some coordination study software products combine coordination and arc flash study capabilities. Once the input data are entered into a software model, the results are used to create labels that are applied to the electrical equipment to indicate the level of protective equipment needed to work around the installation. Arc flash studies can be tedious where sites are old and have thousands of pieces of equipment. New VRE interconnections generally provide ready and easy access to equipment specifications and undamaged nameplates on the equipment. Table C.2 summarizes the recommended, optional, and site-dependent studies that should be conducted for new VRE plants. All studies should be based on agreements between the grid operator and the resource entity. The resource entity should provide essential information on VRE plant equipment specifications and models. The grid operator normally provides initial interconnection study results and many of the facilities study reports. VRE interconnection studies are complex and data intensive. The specific requirements are usually shared among the resource entity, the grid operator, and occasionally other grid regulatory or operating entities. Power-flow, short-circuit, and dynamic stability studies are mandatory for the VRE interconnection in the full interconnection study process performed by the grid operator. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 41 TABLE C.2 Study requirements for transmission- and distribution-level grid interconnection Requirement/stage Transmission level Distribution level Steady-state limits Power flow Required Required Short circuit and grounding Required Required Electromechanical transient Required Site dependent, optional Dynamic stability (response to transmission/distribution disturbances) Transient stability/fault ride through Required Required Frequency stability and ramping Required Site dependent, optional Small-signal stability (modal analysis) Required Optional Voltage stability Required Site dependent, optional Harmonics (grid to VRE) Site dependent Site dependent VRE ramp rates Required Site dependent Flicker (grid to VRE or VRE to grid) Site dependent Site dependent Protection Coordination (protective relays, remote tripping, Required Required breaker failure) Anti-islanding and reconnection Required Required Electromagnetics transients Transient overvoltage from capacitor bank Required Optional Power transformer inrush currents during energization Required Optional Transmission line and cable energization Required Optional Transient voltage withstand requirements during load Required Optional injections Subsynchronous resonances Site dependent, Optional optional Shunt reactor switching transient overvoltage Optional Optional Switching transients, power-flow compensation, Optional Optional voltage control Circuit-breaker transient recovery voltage Optional Optional Fast auto-reclosing schemes of overhead transmission Optional Optional and distribution lines Power quality Voltage sags or distortion Required Required Harmonics/emissions limits (voltage harmonic Site dependent Site dependent distortion from VRE to grid) Flicker/emissions limits (flicker, voltage sags) Site dependent Site dependent Power factor Site dependent Site dependent 42 | ESMAP.ORG Arc flash. Light and heat produced as part of an arc fault, a type of electrical explosion or discharge that results from a low-impedance connection through air to ground or another voltage phase in an electrical system. Also known as flashover. Energy storage. Technology or methodology that stores grid or VRE for reuse when the source cannot produce energy, such as at night for PV plants and during windless periods for wind farms. Energy storage systems take energy produced by the VRE or excess energy off the grid and store it in batteries or reservoirs, releasing the energy for use at a later time. Fault duty. The maximum short-circuit current that a circuit breaker can interrupt. Application of a breaker in a circuit with a prospective short-circuit current higher than the breaker's interrupting capacity rating may result in failure of the breaker to safely interrupt a fault and subsequent damage to other equipment. Grid code. Technical specifications that define the parameters an electricity-generating plant, consumer, or grid must meet to ensure safe and secure functioning of the electric system. The International Electrotechnical Commission (IEC); the Institute of Electrical and Electronic Engineers (IEEE); and many national, regional, and local regulatory entities produce grid codes. Grid operator. The owner, operator, or manager of electricity utility, grid, or network that resource entity interconnects with. As used here, the grid is the transmission and/or distribution system. Harmonic filter. Element used in power systems to eliminate the harmonic distortion caused by appliances such as inverters connected to a power system. Harmonic filters counteract voltage and current waveform distortion. High-speed reclosing. Automatic reclosing utilized on transmission systems to restore transmission elements to service following automatic circuit breaker tripping after fault on the line. The auto reclosing scheme may be high speed or time delayed, supervised or unsupervised. It can consist of one shot or multiple shots (reclosing attempts), depending on the attributes of the transmission system in the local area and the criticality of the line for transferring power across the system and/or supplying load. Load supplying capability. The maximum load that can be supplied in the transmission or distribution system with all lines and generators operating. Load tap changer. Automatic or manual device built on to a transformer that allows one side of the transformer’s voltage rating to be changed. Built-in voltage ranges allow operators to alter voltage at will within the range. Low-voltage ride-through. Capability of electric generators to stay connected in short periods of lower electric network voltage. Nameplate capacity. The full-load sustained output of a power plant, electrical generator or motor. Also known as rated capacity, nominal capacity, and installed capacity. Point of interconnection. Point in electrical system where the resource entity, multiple customers, or electrical loads may be connected. Should be accessible to both the utility and the customer for direct measurement. Also known as the point of common coupling. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 43 Power system reliability. Measure of the ability of a power system to deliver power to all points of utilization within acceptable standards and in the amounts desired. Resource entity. An entity that owns and controls a generation resource and complies with the grid operator’s rules for integration generation into the power grid. Shunt capacitor bank. Grid-scale version of a common capacitor, used primarily to improve power factor on the grid. Also improves voltage stability and reduces grid power losses. Improving the power factor increases the power transmission capability and control of the power flow. Shunt reactor. Device that provides inductance in a system that has too much capacitance. Most compact device commonly used for reactive power compensation in long high-voltage transmission lines and cable systems that can create high capacitance levels. Slack bus. Device used to balance active and reactive power while performing load flow studies for the power system. Also known as reference bus and swing bus. Static synchronous compensator (STATCOM). Device that continuously provides variable reactive power in response to voltage variations, supporting the stability of the grid. Functions with very limited need for harmonic filters, contributing to a small physical footprint. Faster acting than a static VAR compensator. Static VAR compensator (SVC). Set of packaged electrical devices for providing fast-acting reactive power on high-voltage electricity transmission networks. Part of the Flexible AC Transmission System (FACTS) device family that regulates voltage, power factor, and harmonics with the goal of stabilizing the system. Subsynchronous oscillations (resonance). Oscillations resulting from resonance condition that can exist in a power system especially for long-distance transmission systems with series-compensated lines, which can result in possible damage or failure of the generators involved. Wave trap (or line trap). Maintenance-free parallel resonant circuit mounted on a high-voltage AC transmission power lines to prevent the transmission of high frequency (40–1,000kHz) carrier signals of power line communications to unwanted destinations. Blocks the carrier signals so that it travels only in the specific path. 44 | ESMAP.ORG BDEW (Bundesverband der Energie und Wasserwirtschaft). 2008. Technical Guideline for Generating Plants Connected to the Medium-Voltage Network. Berlin: BDEW. https://electrical-engineering- portal.com/res2/Generating-Plants-Connected-to-the-Medium-Voltage-Network.pdf. Corfee, K., D. Korinek, W. Cassel, C. Hewicker, J. Zillmer, M.P. Morgado, H. Ziegler, N. Tong, D. Hawkins, and J. Cernadas. 2011. Lessons Learned from Electricity Markets in Germany and Spain. European Renewable Distributed Generation Infrastructure Study. Oakland, CA: KEMA, Inc. https://www.energy.ca.gov/2011publications/CEC-400-2011-011/CEC-400-2011-011.pdf. ERCOT (Electric Reliability Council of Texas). 2019. ERCOT Planning Guide. Austin, USA: ERCOT. Accessible via: http://ercot.com/content/wcm/libraries/177445/March_26__2019_Planning_Guide.pdf. ESMAP (Energy Sector Management Assistance Program). 2019a. “Compensation Devices to Support Grid Integration of Variable Renewable Energy.” ESMAP Guidance Note, Washington, DC: World Bank ———. 2019b. “Grid Integration Requirements for Variable Renewable Energy.” ESMAP Guidance Note, Washington DC: World Bank. IREC (International Renewable Energy Council). 2018. Smart Inverter Update: New IEEE 1547 Standards and State Implementation Efforts. https://irecusa.org/2018/07/smart-inverter-update-new-ieee- 1547-standards-and-state-implementation-efforts/. IRENA (International Renewable Energy Agency). 2017. Planning for the Renewable Future-Long-Term Modelling and Tools to Expand Variable Renewable Power in Emerging Economies. Abu Dhabi: IRENA. http://www.irena.org/- /media/Files/IRENA/Agency/Publication/2017/IRENA_Planning_for_the_Renewable_Future_2017.p df. NERSA (National Energy Regulator of South Africa). 2016. Grid Connection Code for Renewable Power Plants (RPPs) Connected to the Electricity Transmission System or the Distribution System in South Africa. South Africa: NERSA.http://www.nersa.org.za/ContentPage.aspx?PageId=685&PageName=Renewable%20Energy %20Grid%20Code. World Bank. 2018. “Vietnam: Achieving 12 GW of Solar PV Deployment by 2030” An Action Plan. World Bank: Washington, DC. STUDIES FOR GRID CONNECTION OF VARIABLE RENEWABLE ENERGY GENERATION PLANTS| 45 Barbados Light & Power Company, Limited. 2014. Grid Code. Interconnection Requirements at Voltages 24.9kV and Below. https://www.blpc.com.bb/images/brochures/GRID-C0DE-Rev1-May percent2020141.pdf. FERC (Federal Energy Regulatory Commission). 2003. Standard Large Generator Interconnection Procedures. FERC Order 2003-C. Washington, DC. http://www.ferc.gov/industries/electric/indus- act/gi/stnd-gen-asp. IEEE (Institute of Electrical and Electronic Engineers). 2006. IEEE C37.102-2006. IEEE Guide for AC Generator Protection. Piscataway, NJ. ———. 2015. IEEE 80-2013. IEEE Guide for Safety in AC Substation Grounding Series. Piscataway, NJ. ———. 2014a. IEEE 519-2014. IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems Series. Piscataway, NJ. ———. 2014b. IEEE 1453. IEEE Recommended Practice for Measurement and Limits of Voltage Fluctuations and Associated Light Flicker on AC Power Systems. Piscataway, NJ. ———. 2014c. IEEE 1547-2018. IEEE Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces. Piscataway, NJ. National Fire Protection Association. 2015. Standard for Electrical Safety in the Workplace. Quincy, MA. Underwriters Laboratory. n.d. UL 1741SA. Standard for Inverters, Converters, Controllers and Interconnection System Equipment for Use with Distributed Energy Resources. Northbrook, IL. 46 | ESMAP.ORG