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Concentrating Solar Power Impact on Grid Reliability Nicholas W. Miller and Slobodan Pajic GE Energy Consulting Kara Clark National Renewable Energy Laboratory NREL is a national laboratory of the U.S. Department of Energy Office of Energy Efficiency & Renewable Energy Operated by the Alliance for Sustainable Energy, LLC This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. Technical Report NREL/TP-5D00-70781 August 2018 Contract No. DE-AC36-08GO28308 Concentrating Solar Power Impact on Grid Reliability Nicholas W. Miller and Slobodan Pajic GE Energy Consulting Kara Clark National Renewable Energy Laboratory Prepared under Task No. SETP.10304.23.01.10 NREL is a national laboratory of the U.S. Department of Energy Office of Energy Efficiency & Renewable Energy Operated by the Alliance for Sustainable Energy, LLC This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. National Renewable Energy Laboratory Technical Report 15013 Denver West Parkway NREL/TP-5D00-70781 Golden, CO 80401 August 2018 303-275-3000 • www.nrel.gov Contract No. DE-AC36-08GO28308 NOTICE This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Solar Energy Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes. This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. U.S. Department of Energy (DOE) reports produced after 1991 and a growing number of pre-1991 documents are available free via www.OSTI.gov. Cover Photos by Dennis Schroeder: (left to right) NREL 26173, NREL 18302, NREL 19758, NREL 29642, NREL 19795. NREL prints on paper that contains recycled content. Acknowledgments This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Solar Energy Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The National Renewable Energy Laboratory and General Electric also thank the members of the technical review committee for their insightful comments and assistance. Participation in the committee does not imply agreement with the project findings. The committee included: • Mark Ahlstrom, WindLogics • Erik Ela, Electric Power Research Institute • Bradley Albert, APS • Tassos Golnas, U.S. Department of • William Anderson, Xcel Energy Energy • Jamie Austin, PacifiCorp • Irena Green, California Independent • Ron Belval, Tucson Electric Power System Operator • Ray Byrne, Sandia National • Rebecca Johnson, Western Area Laboratories Power Administration • Thomas Carr, Western Governors’ • Debra Lew, General Electric Association • Eddy Lim, Federal Energy • Kemal Celik, U.S. Department of Regulatory Commission Energy • Clyde Loutan, California • Malati Chaudhary, Public Service Independent System Operator Company of New Mexico • Kate Maracas, Western Grid Group • Bob Cummings, North American • Mark O’Malley, University College Electric Reliability Corporation Dublin • Donald Davies, Western Electricity • Brian Parsons, Independent Coordinating Council consultant • Joe Desmond, BrightSource • Henry Price, Independent consultant • Charles Diep, SolarReserve • Vikas Singhvi, Electric Power • Tom Duane, Public Service Research Institute Company of New Mexico • Charlie Smith, Utility Variable • Bob Easton, WAPA Generation Integration Group • Sara Eftekharnejad, Tucson Electric • Guohui Yuan, U.S. Department of Power Energy The National Renewable Energy Laboratory regrets any inadvertent omission of project participants and contributors. iii This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. Executive Summary This study examines the impact of concentrating solar power (CSP) on grid reliability by investigating the dynamic behavior of the Western Interconnection under conditions of high solar and wind generation. Reliability in this case refers to the somewhat narrow context of stability: transient stability and frequency response; and control stability, especially that associated with weak grids. The objectives of this study were to identify renewable energy penetration levels and mixes, severe disturbances, and load conditions where grid performance and reliability could be enhanced with CSP plants. Instantaneous penetrations of wind and solar—both photovoltaics (PV) and CSP—up to approximately 60% were considered. The focus is on situations in the Western Interconnection bulk power system during which variable renewable generation has displaced other (non-CSP) synchronous thermal generation under highly stressed, weak system conditions. Particular attention was given to impacts of frequency-responsive controls and synchronous generation characteristics. This is relatively new ground for the industry, and this investigation is not a substitute for detailed planning, but the risks illustrated can be analyzed and mitigated. Tools, data, and the current state-of-the-art interconnection and bulk power system stability studies, if used following good system engineering practices as systems are built out, will ensure continued reliability of power systems. Key Findings Grid Build-Out to Support Added Solar and Wind Changes Transient Stability Transmission added in solar-rich areas to avoid thermal and voltage violations, plus changes in dispatch and commitment (Section 2), have some effect on transient stability. The impacts are mixed, with some improvements and decreases (Section 5.2). No stability violations— noncompliance with Western Electricity Coordinating Council (WECC) criteria—were found for the primary fault-clearing cases tested (Section 5.2 and Section 10.2). As noted, good planning practice needs to be observed. WECC-wide system inertia dropped up to 27%–32% from earlier light load planning cases. The earlier cases had less wind and solar generation and included synchronous generation, which was retired in the final study cases (Section 4.1). The lower system inertia did not present any significant stability or frequency response challenges. We did not observe systemic issues related to frequency and transient stability resulting from the solar and wind build-out. That is, the overall behavior was similar in character to the present system. The system seems to better tolerate non design-basis north-south separation with the grid additions (Section 6.6). For the conditions studied, a simultaneous nonsynchronous penetration (SNSP) of approximately 70% (Section 4.3) did not have an adverse impact on system-wide transient stability (Section 5.2). Transient stability issues seemed to be rather localized (Section 10.2). iv This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. Primary Frequency Response from Concentrating Solar Power Helps Meet Frequency Response Obligation Because CSP uses a conventional synchronous steam turbine generator system to produce electricity, it always contributes inertia when running. Further, depending on the design and operation of the plant, it can provide primary frequency response (PFR) via governor action. It is by no means ensured that CSP plants will necessarily provide this service. Steam systems and turbines must be designed with this capability in mind for best economy. This report provides some discussion and concepts for possibly squeezing additional frequency response out of steam systems (Section 8.1). The discussion includes the concept of a triggered, open-loop control based on the accepted practice of fast-valving special protective systems (Section 8.3). PFR from CSP benefits frequency response, improves the system nadir, and helps the system (and regions thereof, such as California) meet their frequency response obligations (FRO) (Section 9.3). The contribution of synchronous inertia is observable, but it not very important for the conditions and cases examined. Tripping two of the Palo Verde Nuclear Generating Station units, for a loss of approximately 2,750 MW, is the design basis frequency event for WECC (Section 6.1.2). We have continued to use that event extensively in this study. Figure ES-1 shows three cases for that event run on the lighter load case (60% instantaneous wind and solar penetration for the U.S. WECC) that illustrate two separate points. The red trace shows the reference lighter load case. The CSP units are online contributing inertia, but there is no governor response. The blue trace shows the impact of enabling the governors on the CSP plants (per the model discussion in Section 3.2). As expected, both the frequency nadir and the settling frequency
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