Grid Impacts And Solutions Of Renewables At High Penetration Levels

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Integrating intermittent renewable energy resources can be a challenge. This White Paper provide some guidance on renewable energy integration issues.

Integrating intermittent renewable energy resources can be a challenge. This White Paper provide some guidance on renewable energy integration issues.

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  • 1. Grid Impacts and Solutions of Renewables at High Penetration Levels Contact: Dr. Johan Enslin, PrEng Vice President – Sustainable Energy Quanta Technology, 4020 Westchase Blvd., Suite 300 Raleigh, NC 27607 JEnslin@Quanta-Technology.com 919-334-3037 (Office) 919-303-1574 (Mobile)
  • 2. CONFIDENTIAL/PROPRIETARY: This document contains trade secrets and/or proprietary, commercial, or financial information not generally available to the public. It is considered privileged and proprietary to the Offeror, and is submitted by Quanta Technology Services, Inc. in confidence with the understanding that its contents are specifically exempted from disclosure under the Freedom of Information Act [5 USC Section 552 (b) (4)] and shall not be disclosed by the recipient [whether it be Government (local, state, federal, or foreign), private industry, or non-profit organization] and shall not be duplicated, used, or disclosed, in whole or in part, for any purpose except to the extent in which portions of the information contained in this document are required to permit evaluation of this document, without the expressed written consent of the Offeror. If a contract is awarded to this Offeror as a result of, or in connection with, the submission of this data, the right to duplicate, use, or disclose the data is granted to the extent provided in the contract. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 2 of 62
  • 3. Executive Summary In 2002, California established its Renewable Portfolio Standard Program, with the goal of increasing the percentage of renewable energy in the state's electricity mix to 20% by 2017. On November 17, 2008, Governor Arnold Schwarzenegger signed Executive Order S-14-08 requiring that California utilities reach the 33% renewables goal by 2020. Achievement of a 33% by 2020 RPS would reduce generation from non-renewable resources by 11% in 2020. This is currently the most aggressive Renewable Energy Portfolio (RPS) standard proposed by any of the US states. Other state governments have similar, although at lower penetration levels, but also aggressive RPS allocations. As electric utilities prepare to meet their state’s renewable portfolio standard, for example 33% by 2020 in California, and to comply with Global Warming Solutions Act of 2006 (AB32), it becomes evident that US utilities must adapt its planning and operations in order to maintain the high levels of service and reliability. The state initiatives require integration of significantly higher levels of renewable energy, such as wind and solar, which exhibit intermittent generation patterns. Due to the geographic location of renewable resources the majority of the expected new renewable generation additions will be connected via one or two utility’s transmission systems. This presents unique challenges to these utilities as the level of planned intermittent renewable generation in relation to their installed system capacity reaches unprecedented and disproportionate levels as compared to other utilities in the state. Entities, such as CEC, NERC, CAISO, NYSERDA, SPP, CPUC, etc., have initiated and funded several studies on the integration of large levels of renewable energy and most of these studies concluded that with 10 - 15% intermitted renewable energy penetration levels, traditional planning and operational practices will be sufficient. However, once a utility exceeds the 20% penetration levels of renewable resources, it requires a dramatic change in planning and operational practices. These studies support continuing transmission and renewable integration planning studies, and recommend that demonstration projects installations should be conducted by the different power utilities. The USA, and especially California, has a different set of electric system characteristics than in Europe, but there is no experience or research in Europe that would lead us to think that it is technically impossible to achieve 20% - 30 % intermitted penetration levels at most US utilities. Long transmission distances between generation resources and load centers characterize the network in the US and especially Grid Impacts and Solutions of Renewables at High Penetration Levels Page 3 of 62
  • 4. in the WECC region. There are areas now in Europe that are highly penetrated with intermittent renewable, especially wind generation, at levels of around 30 – 40%. Large scale wind and solar generation will affect the physical operation of the grid. The areas of focus include frequency regulation, load profile following and broader power balancing. The variability of wind and solar regimes across resource areas, the lack of correlation between wind and solar generation volatility and load volatility, and the size and location of the wind plants relative to the system in most US states suggest that impacts on regulation and load profile requirements resource smoothing will be large at above 20% penetration levels. The European experience taught us that there are consequences of integrating these levels of wind resources on network stability that have to be addressed as wind resources reach substantial levels of penetration. A list of the major issue categories follows: • New and in-depth focus on system planning. Steady-state and dynamic considerations are crucial. • Accurate resource and load forecasting becomes highly valuable and important. • Voltage support. Managing reactive power compensation is critical to grid stability. This also includes dynamic reactive power requirements of intermittent resources. • Evolving operating and power balancing requirements. Sensitivity to existing generator ramp- rates to balance large scale wind and solar generation, providing regulation and minimizing start- stop operations for load following generators. • Increased requirements on ancillary services. Faster ramp rates and a larger percentage of regulation services will be required which can be supplied by responsive storage facilities. • Equipment selection. Variable Speed Generation (VSG) turbines and advanced solar inverters have the added advantage of independent regulation of active and reactive power. This technology is essential for large-scale renewable generation. • Strong interconnections. Several large energy pump-storage plants are available in Switzerland that is used for balancing power. Larger regional control areas make this possible. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 4 of 62
  • 5. Technical renewable integration issues should not delay efforts to reach the renewable integration goals. However, focus has increased on planning and research to understand the needs of the system, for example, research on energy storage options. Studies and actual operating experience indicate that it is easier to integrate wind and solar energy into a power system where other generators are available to provide balancing power and precise load- following capabilities. The greater the number of wind turbines and solar farms operating in a given area, the less their aggregate production is variable. High penetration of intermittent resources (greater than 20% of generation meeting load) affects the network in the following ways: • Thermal and contingency analysis • Short circuit • Transient and voltage stability • Electromagnetic transients • Protection • Power leveling and energy balancing • Power Quality The largest barrier to renewable integration in the USA is sufficient transmission facilities and associated cost-allocation in the region to access the renewable resources and connecting these resources to load centers. Other key barriers include environmental pressure and technical interconnection issues such as forecasting, dispatchability, low capacity factors and intermittency impacts on the regulation services of renewable resources. In the US, the sources of the major renewable resources are remote from the load centers in California and the Midwest states. This results in the need for addition of new major transmission facilities across the country. Wind and solar renewable energy resources normally have Capacity Factors between 20 – 35%, compared to higher than 90% with traditional nuclear and coal generation. These low capacity factors place an even higher burden on an already scarce transmission capacity. Identification, permitting, cost-allocation, approval, coordination with other stakeholders, engineering and construction of these new transmission facilities are major barriers, costly and time consuming. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 5 of 62
  • 6. There are numerous environmental issues associated with renewable integration. In addition to the environmental issues associated with transmission line construction, there are the impacts to birds from Although energy production using renewable resources is pollution free, wind and solar plants need to be balanced with fast ramping regulation services like peaker generator or hydro generation plants. Existing regulation generation is too slow and is polluting much more during ramping regulation service. The increased requirements in regulation services counteract the emissions savings from these renewable resources. Currently the frequency regulation requirement at the CAISO is around 1% of peak load dispatch, or about 350 MW. This is currently mainly supplied by peaker generating plants and result in higher emission levels. It has been calculated that around 2% regulation would be required for integrating 20% wind and solar resources by 2010 and 4% to integrate 33% renewables by 2020. With the integration of wind and solar generation the output of fossil fuel-plant needs to be adjusted frequently, to cope with fluctuations in output. Some power stations will be operated below their maximum output to facilitate this, and extra system balancing reserves will be needed. Efficiency may be reduced as a result with an adverse effect on the emissions. At high penetrations (above 20%) wind and solar energy may need to be 'spilled' or curtailed because the grid cannot always utilize the excess energy. To integrate high penetration levels like 33% intermittent renewable resources by 2020 in California, several planning and operational solutions should be followed. There is no silver bullet but requires a combined effort on three major levels: • Generation mix to utilize different complementary resources. • Advanced transmission facilities, including fast responsive energy storage, FACTS, HVDC, WAMPAC, etc. • Demand response, including distributed resources on the distribution feeders, distributed energy storage, SmartGrid, Plug-in Hybrid Vehicles (PHEV), Demand Side Management (DSM), etc. The purpose of increased transmission planning is to identify complete and preferred transmission plans and facilities to integrate these high levels of renewables. The clear goal would be develop a staged Grid Impacts and Solutions of Renewables at High Penetration Levels Page 6 of 62
  • 7. transmission expansion plan, facilities and storage options to integrate this potential level renewable penetration levels. Most of the models for these advanced wind and solar facilities have not been fully developed yet and need to be validated. The generator models for wind and solar generation technologies need to be upgraded and validated to include short circuit models, dynamic variance models like clouding and short- term wind fluctuations. The European experience with high levels of intermittent resources up to 80% penetration levels does not transfer fully due to the difference in US grid design and load density. The integration of renewable energy at this scale will have significant impact, especially if the addition of energy storage devices (central and distributed) and FACTS devices utilized to counterbalance the influence of the intermittent generation sources. Utilities and ISOs in the US should conduct RD&D projects and commence studies to fulfill its obligation to accurately and reliably forecast the impacts on future system integrated resource planning. Due to the long lead time for some of the proposed technology solutions, it is recommended that utilities engage these challenges sooner versus later. If technical challenges manifest, a timely solution cannot be implemented if studies, demonstration installations and field tests still have to be conducted. Additionally, utilities should study all conceivable options that may severely affect transmission system integrity and stability. Otherwise, utilities may experience unintended consequences due to unforeseen technical issues resulting from high penetrations of new renewable energy sources. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 7 of 62
  • 8. Table of Contents: Executive Summary ...................................................................................................................................... 3  1  Introduction ......................................................................................................................................... 10  1.1  Greenhouse Emissions from CA Generation Mix ...................................................................... 10  1.2  Distributed and Centralized Renewable Integration ................................................................... 12  1.3  Developing Carbon Markets ....................................................................................................... 13  1.4  Short summary of other recent studies ........................................................................................ 13  1.5  International trends and penetration levels ................................................................................. 14  1.6  Relevance to California Renewable Portfolio ............................................................................. 16  1.7  Introduction to grid integration issues ........................................................................................ 17  2  Role Players and Incentives ................................................................................................................ 18  2.1  Roles and Coordination............................................................................................................... 18  2.2  Barriers to entry .......................................................................................................................... 19  2.3  American Recovery and Reinvestment Act 2009 ....................................................................... 20  2.4  CA RETI ..................................................................................................................................... 21  2.5  CAISO efforts and program ........................................................................................................ 21  3  California Renewable Energy Portfolio .............................................................................................. 21  3.1  RPS progress and goals for California ........................................................................................ 22  4  Defining Renewable Integration Impacts ........................................................................................... 23  4.1  Capacity Factor ........................................................................................................................... 23  4.2  Energy and Capacity Credits....................................................................................................... 23  4.3  REC Certification........................................................................................................................ 25  4.4  Low-Voltage Ride-Through (LVRT).......................................................................................... 26  4.5  IEEE 1547 DER Interconnection and Power Quality ................................................................. 28  5  Renewable Energy Resources and Characteristics ............................................................................. 29  5.1  Wind Energy ............................................................................................................................... 29  5.1.1  California wind resources and developable capacity (geographical map) .......................... 29  5.1.2  CA Wind Resource Profiles ................................................................................................ 30  5.1.3  Fluctuating issues (short term, medium term)..................................................................... 31  5.1.4  Wind turbine technologies and characteristics (Type 1 – 4) ............................................... 32  5.1.5  Wind turbine technologies and integration issues ............................................................... 33  5.2  Solar Energy................................................................................................................................ 34  5.2.1  California solar resources and developable capacity (geographical map) .......................... 34  5.2.2  Solar technologies ............................................................................................................... 35  5.2.3  Solar Generation Profiles and Capacity Factors in California ............................................ 37  5.2.4  Solar Array Power Fluctuating Issues ................................................................................. 38  5.3  Other ........................................................................................................................................... 40  5.3.1  Biomass ............................................................................................................................... 40  5.3.2  Geothermal .......................................................................................................................... 40  5.3.3  Wave Energy ....................................................................................................................... 40  6  Summary of System and T&D Impacts .............................................................................................. 41  Grid Impacts and Solutions of Renewables at High Penetration Levels Page 8 of 62
  • 9. 7  Potential Solutions for Integrating Large Scale Renewable (I.e. 33% by 2020 in CA) ...................... 42  7.1  Transmission and distribution planning studies .......................................................................... 43  7.2  Developing increased capability to integrate hybrid energy mix................................................ 44  7.3  Frequency regulation and fast ramping energy storage .............................................................. 44  7.4  Increased regional control areas .................................................................................................. 46  7.5  Improved interconnection facilities ............................................................................................ 46  7.6  FACTS devices integrated with energy storage .......................................................................... 46  7.7  Integrated control using WAMPAC with FACTS and PMUs .................................................... 47  7.8  HVDC interconnection options ................................................................................................... 48  7.9  Resource forecasting ................................................................................................................... 50  7.10  Load forecasting.......................................................................................................................... 50  7.11  Increase Capacity Credits and Firm Capacity from Renewables ................................................ 50  7.12  Large Scale Energy Storage ........................................................................................................ 51  7.13  Distributed Energy Storage (DES) .............................................................................................. 52  8  Summary and Recommendations........................................................................................................ 54  9  References and Links .......................................................................................................................... 55  Grid Impacts and Solutions of Renewables at High Penetration Levels Page 9 of 62
  • 10. 1 Introduction 1.1 Greenhouse Emissions from CA Generation Mix In 2006 California passed legislation in Assembly Bill (AB) 32 also known as the Global Warming Solutions Act of 2006. This law requires a reduction of greenhouse gas (GHG) emissions in the state of California to 1990 levels by 2020. The lead agency responsible for implementing AB 32 is the California Air Resources Board (CARB), so CARB developed a list of early actions to reduce greenhouse gas emissions. Many of the recommendations and actions are described in a Scoping Plan. This plan calls for reducing greenhouse gas emissions to 1990 levels by cutting approximately 30 percent from business-as-usual emission levels projected for 2020, or about 15 percent from today’s levels. On a per-capita basis, that means reducing annual emissions of 14 tons of carbon dioxide equivalent for every man, woman and child in California down to about 10 tons per person by 2020. Figure 1 : California Greenhouse Gas Emissions [47] The Scoping Plan states that significant progress can be made toward the 2020 goal relying on existing technologies and improving the efficiency of energy use. Other solutions involve Grid Impacts and Solutions of Renewables at High Penetration Levels Page 10 of 62
  • 11. improving our state’s infrastructure, transitioning to cleaner and more secure sources of energy, and adopting 21st century land use planning and development practices. The recommendations in the Scoping Plan were shaped by input and advice from CARB’s partners on the Climate Action Team, as well as the Environmental Justice Advisory Committee (EJAC), the Economic and Technology Advancement Advisory Committee (ETAAC), and the Market Advisory Committee (MAC). Figure 2: California Greenhouse Gas Emissions in 2020 and Recommended Reduction Measures [48] Key elements of California’s recommendations for reducing its greenhouse gas emissions to 1990 levels by 2020 include: • Expanding and strengthening existing energy efficiency programs as well as building and appliance standards; • Achieving a statewide renewables energy mix of 33 percent; • Developing a California cap-and-trade program that links with other Western Climate Initiative partner programs to create a regional market system; • Establishing targets for transportation-related greenhouse gas emissions for regions throughout California, and pursuing policies and incentives to achieve those targets; Grid Impacts and Solutions of Renewables at High Penetration Levels Page 11 of 62
  • 12. • Adopting and implementing measures pursuant to existing State laws and policies, including California’s clean car standards, goods movement measures, and the Low Carbon Fuel Standard; and • Creating targeted fees, including a public goods charge on water use, fees on high global warming potential gases, and a fee to fund the administrative costs of the State’s long term commitment to AB 32 implementation. 1.2 Distributed and Centralized Renewable Integration Figure 3: Centralized Vs. Distributed Generation [44] Local positioning avoids transmission and distribution losses, Generation adjacent to loads allows convenient use of heat energy, but conventional distribution systems need adequate protection in order to accommodate exchange of power. However, distribution planners must make provision for distributed generation dispersed across the system. Factors such as islanding, projection, safety, and system stability must be considered, especially if the anticipated generation is a significant portion of the circuit load. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 12 of 62
  • 13. 1.3 Developing Carbon Markets California passed the Global Warming Solutions Act of 2006 (AB32), committing the state to reduce carbon dioxide emissions to 1990 levels by 2020. AB32 gave the California Air Resources Board (CARB) the authority to impose an economy-wide cap on state-wide greenhouse gas emissions. CARB has a multi-track process which includes considering market mechanisms in its Scoping Plan, which was adopted in December 2008. Now that the Scoping Plan is adopted, CARB will embark on a 2-year effort to develop specific implementation policies [51]. Figure 4: Carbon Share [51] 1.4 Short summary of other recent studies Recently a number of publically available studies for integrating large levels of renewables in California, sponsored by CEC [10, 19], CAISO [34] and the CPUC [16, 57, 58], have been performed. Most of these studies address the importance of increased efforts in system planning, operational challenges with increased penetration levels and required transmission facilities, including energy storage. Most studies Grid Impacts and Solutions of Renewables at High Penetration Levels Page 13 of 62
  • 14. indicate that integrating fluctuating wind and solar up to levels of 20%, based on local regional loading, seems doable with traditional approaches and transmission upgrades. Integrating higher levels of renewable resources require a change in planning and operating practices. It is also recommended by most studies that a balanced generation portfolio mix need to be implemented in the future to balance the fluctuating renewable resources. The results and findings support continuing transmission and renewable integration studies and demonstration installations to be conducted by the different power utilities, CAISO, and the California Public Utilities Commission. . 1.5 International trends and penetration levels Worldwide wind capacity reaches 121,188 MW, out of which 27,261 MW were added in 2008. Wind energy continued its growth in 2008 at an increased rate of 29 %. All wind turbines installed by the end of 2008 worldwide are generating 260 TWh per annum, equaling more than 1.5 % of the global electricity consumption. The wind sector became a global job generator and has created 440,000 jobs worldwide. Table 1: Wind and solar installed penetration levels Region Peak Load Installed Wind Wind Installed Solar Solar MW (2007/8) MW (2008) Penetration MW (2007/8) Penetration Denmark 3,700 3,180 86% N/A N/A Germany 82,800 23,903 29% 1,328 1.6% Spain 43,400 16,754 39% 650 1.5% The 17,800 2,225 13% N/A N/A Netherlands India 107,010 9,645 9% 3 0.0002% China 409,000 12,210 3% 820 0.2% Continental 785,930 25,170 3% 226 0.003% USA Texas 62,188 4,296 7% N/A N/A New Mexico 1,500 497 33% N/A N/A Japan 182,690 1,880 1% 226 0.12% Grid Impacts and Solutions of Renewables at High Penetration Levels Page 14 of 62
  • 15. There are areas now in Europe that are very highly penetrated with intermittent renewable, especially wind generation with levels of around 30 – 40%. These include the Northern German transmission system (E.ON Netz and VE) and Denmark. In these areas more than 90% - 100% wind penetration is common under low loading conditions. The European experience showed us that during a relatively short period of just a few years, network reliability was largely maintained as the penetration developed with added efforts in system planning, large-scale storage utilization and improved system operations. The network operations have adjusted to accommodate the change in the generation mix. The European experience taught us that there are consequences of integrating these levels of wind resources on network stability that have to be addressed as wind resources reach substantial levels of penetration. A list of the major issue categories follows: 1. New and in-depth focus on system planning. Steady-state and dynamic considerations are crucial. 2. Accurate resource and load forecasting becomes highly valuable and important. 3. Voltage support. Managing reactive power compensation is critical to grid stability. This also includes dynamic reactive power requirements of intermittent resources. 4. Evolving operating and power balancing requirements. Sensitivity to existing generator ramp- rates to balance large scale wind and solar generation, providing regulation and minimizing start- stop operations for load following generators. 5. Equipment selection. Variable Speed Generation (VSG) turbines and advanced solar inverters have the added advantage of independent regulation of active and reactive power. This technology is preferred in large-scale renewable generation. 6. Strong interconnections. Several large energy pump-storage plants are available in Switzerland that are used for balancing power. Larger regional control areas make this possible. Technical renewable integration issues should not delay efforts to reach the renewable integration goals. However, focus has increased on planning and research to understand the needs of the system, for example, research is underway on energy storage options [7, 21, 26, 24]. In Europe several disturbances were contributed by these high levels of wind penetration affecting operations [22]. Penetration levels of solar generation are on a fast rise, with most of the Solar PV integration currently in Europe, see Figure 5. The USA market is very small compared to Germany, but it is the fastest growing area and we would see, similarly to wind power development, an even faster rate of solar integration in the next few years. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 15 of 62
  • 16. Figure 5: Solar PV Market 1.6 Relevance to California Renewable Portfolio The USA, and especially California, has a different set of electric system characteristics than in Europe, but there is no experience or research in Europe that would lead us to think that it is technically impossible or even very challenging, to achieve 20% wind penetration in California. Long transmission distances between generation resources and load centers characterize the network in the WECC. But with modest, adequate measures, experience and research regarding offshore wind integration in Europe suggest that the 20% level of penetration is indeed possible. Larger penetration levels in the 30 – 40% are however much more challenging, especially if limited large-scale energy storage is available. In Europe large scale pump-hydro storage is available in Switzerland and other regions which already showed to be very valuable in integrating large levels of wind energy [2,33,20,21]. Large scale wind and solar generation will affect the physical operation of the grid. The areas of focus include frequency regulation, load profile following and broader system planning. The variability of wind and solar regimes across resource areas, the lack of correlation between wind and solar generation volatility and load volatility, and the size and location of the wind plants relative to the system in California suggest that impacts on regulation and load profile following will be large at above 20% penetration levels. The reference list at the end of this report is a very valuable compilation of work done to date on these subjects. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 16 of 62
  • 17. A consequential issue under study by the California Wind Energy Collaborative activity is the cost of integrating wind into the grid. Another issue being addressed at the CPUC, ISO and WECC level, is the cost of network upgrades associated with wind and other new resources [32],[17],[18]. 1.7 Introduction to grid integration issues Wind turbine and solar inverter technology have advanced dramatically in the last 20 years. From a performance point of view, modern wind and solar power plants have much in common with conventional utility power plants, with the exception of variability in plant output. System operators can significantly reduce the uncertainty of wind and solar output by using improved wind and solar forecasts that incorporate meteorological data to predict the production. Such systems yield both hour-ahead and day-ahead forecasts to support real-time operations. Studies and actual operating experience indicate that it is easier to integrate wind and solar energy into a power system where other generators are available to provide balancing power and precise load-following capabilities. The greater the number of wind turbines and solar farms operating in a given area, the less their aggregate production is variable [45]. High penetration of intermittent resources (greater than 20% of generation meeting load) affects the network in the following ways: Power flow: We need to ensure that interconnected transmission and distribution lines are not over-dutied. Reactive power should be generated through out the network, not only at the interconnection point and should be compensated locally. Short circuit: Impact of additional generation sources to the short circuit current ratings of existing electrical equipment on the network should be determined. Transient stability: dynamic behavior of the system during contingencies, sudden load changes and disturbances. Voltage and angular stability during these system disturbances are important. In most cases, fast-acting reactive-power compensation equipment, including SVCs and STATCOMs, are included for improving the transient stability of the network. Electromagnetic transients: Ensure these fast operational switching transients have a detailed representation of the connected equipment, wind turbines, their controls and protections, the converters, and DC links. Protection: Investigate how unintentional islanding and reverse power flow may have a large impact on existing protection schemes, philosophy, and settings. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 17 of 62
  • 18. Power leveling and energy balancing: Due to the fluctuating and uncontrollable nature of wind power as well as the uncorrelated generation from wind and load, wind power generation has to be balanced with other fast controllable generation sources. These include gas, hydro, or renewable power generating sources, as well as short and long-term energy storage, to smooth out fluctuating power from wind generators and increase the overall reliability and efficiency of the system. The costs associated with capital, operations, maintenance and generator stop-start cycles have to be taken into account as well. Power Quality: Fluctuations in the wind power and the associated power transport (AC or DC), have direct consequences to the power quality. As a result, large voltage fluctuations may result in voltage variations outside the regulation limits, as well as violations on flicker and other power quality standards. 2 Role Players and Incentives The roles of various organizations for renewable integration and often overlap. 2.1 Roles and Coordination Department of Energy (DOE): The role of the DOE is to encourage increased grid investment, increase regional transmission planning and remover regulatory barriers at the wholesale level to address the development of renewable energy. Federal Energy Regulatory Commission (FERC): The role of the FERC regarding renewable integration is to encourage grid investment by implementing various forms of incentive rates. North American Electric Reliability Corporation (NERC): The role of NERC is to develop standards for power system operations, monitor and enforce compliance to those standards, assess resource adequacy. Western Electricity Coordinating Council (WECC): The WECC is a voluntary organization which has a role in the coordination and promotion of electric system reliability. It provides a forum for resolving transmission access disputes and for coordinating the operating and planning activities of its member organizations. California Energy Commission (CEC): The CEC has several roles associated with renewable integration. They include forecasting future energy needs, promoting energy efficiency, supporting public interest energy research, supporting renewable energy by providing market support to existing, Grid Impacts and Solutions of Renewables at High Penetration Levels Page 18 of 62
  • 19. new and emerging renewable technology, providing incentives for small wind and full cell electricity systems, providing incentives or solar electricity systems in new homes and implementing the State’s Alternative Renewable Fuel an Vehicle Technology Program. The CEC also performs other activities as directed by the legislature, such as managing the Renewable Energy Transmission Initiative (RETI), a statewide initiative to help identify the transmission projects needed to accommodate renewable energy goals, support future energy policy, and facilitate transmission corridor designation and transmission and generation siting and permitting. California Public Utility Commission (CPUC): In conjunction with the CEC, the CPUC implements the Renewable Portfolio Standards (RPS) program. The CPUC's responsibilities include determining annual procurement targets and enforcing compliance, reviewing and approving each IOU's renewable energy procurement plan, reviewing IOU contracts for RPS-eligible energy, establishing the standard terms and conditions used by IOUs in their contracts for eligible renewable energy and calculating market price referents (MPRs) for non-renewable energy that serve as benchmarks for the price of renewable energy. California Independent System Operator (CAISO): The role of the CAISO is to operate the California transmission grid reliability and efficiently, facilitate effective markets and promote infrastructure development. It ensures policy makers understand the electric grid and how integrating new generation changes power deliveries. CAISO also plays the role as a stimulator and forum for innovation. Utilities: The specific role of the utility depends upon the utility is a regulated investor owned utility or not. But generally the role of the utility is to support the federal, state and regulatory requirements. They are to seek renewable resources to meet or exceed the Renewable Portfolio Standards, implement tariffs, integrate new technologies and propose and construct infrastructure, 2.2 Barriers to entry The largest barrier to renewable integration in the CAISO is sufficient transmission facilities and associated cost-allocation in the region to access the renewable resources and connecting these resources to load centers. Other key barriers include environmental pressure and technical issues such as forecasting, dispatchability and intermittency of renewable resources. In California, the sources of the major renewable resources are remote from the load centers. This results in the need for addition of new major transmission facilities across the state. Identification, permitting, cost-allocation, approval, coordination with other stakeholders, engineering and construction of these new transmission facilities is costly and time consuming. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 19 of 62
  • 20. There are numerous environmental issues associated with renewable integration. In addition to the environmental issues associated with transmission line construction, there are the impacts to birds from wind generation and to the desert tortoise from solar plants. There are still a large number of technical issues to over come, such as generation and load forecasting, dispatchability, interconnection process and intermittency. These issues require a large emphasis on research, planning, studies and demonstration installations in the next few years to overcome the technical and transmission hurdles to get to the proposed goal of 33% RPS in California by 2020. Especially lacking is complete, overall forward looking renewable energy impact planning. Currently most utilities and the CAISO are extremely occupied with individual interconnection projects and do not plan for a complete system impact. 2.3 American Recovery and Reinvestment Act 2009 Figure 6: American Recovery and Reinvestment Act 2009 The Stimulus Package categorized in the figure above and with a total value of $ 787 B, has a lot of funding for shovel-ready renewable projects. Many US states are positioning themselves to become the recipients of federal funding for renewable energy and transmission upgrades. Most of this funding will come from the $ 43 B Energy set-aside with additional tax relief for renewable projects, asset owners and developer. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 20 of 62
  • 21. 2.4 CA RETI Vast quantities of renewable energy resources are available in the USA, but it lacks a modern interstate transmission grid to deliver this renewable energy to load centers in highly populated areas. The Obama administration is challenging the industry to achieve up to 25% renewable energy consumption by 2025. This will require an integrated effort from local, state, and federal government and significant new investment in transmission infrastructure. The Renewable Energy Transmission Initiative (RETI) is a statewide initiative in California to help identify the transmission projects needed to accommodate the renewable energy goals. It is expected to invest $60 billion over 20 year period to achieve 20% renewable penetration levels by 2030. Both private sectors and federal government will have to invest to achieve this goal. 2.5 CAISO efforts and program Long term energy efficiency goals for California are to make all new homes and businesses energy efficient by 2020. For a combined approach by all the utilities in the CAISO territory, an expected energy savings of 3,521 GWh per annum is expected by 2012. These expected savings will increase the renewable energy portfolio and ease some balancing power requirements. An additional focus is also to increase the Distributed Renewable Generation portfolio with small, local power generators, such as residential solar systems and small wind power. New homes built after 2020 may not need to rely on power from the state's electricity grid. The plan calls for applying the same standard to commercial buildings by 2030. To increase the amount of power from renewables the so-called "feed-in tariff”, which has worked very well in Europe, may also be considered for the CAISO territory [68]. Feed-in tariff is an incentive structure to encourage the adoption of renewable energy through government legislation. The regional or national electricity utilities are obligated to buy renewable electricity (electricity generated from renewable sources, such as solar Photovoltaics, wind power, biomass, hydropower and geothermal power) at above market rates set by the government. 3 California Renewable Energy Portfolio Most states have now a portfolio standard in place or is in the process to have it in place. On national level the National Renewable Electricity Standard (RES) is under development with the expected outcome in the next few months. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 21 of 62
  • 22. 3.1 RPS progress and goals for California In 2002, California established its Renewable Portfolio Standard Program, with the goal of increasing the percentage of renewable energy in the state's electricity mix to 20% by 2017. On November 17, 2008, Governor Arnold Schwarzenegger signed Executive Order S-14-08 requiring that California utilities reach the 33% renewables goal by 2020 [35]. Achievement of a 33% by 2020 RPS would reduce generation from non-renewable resources by 11% in 2020 as shown below. Fossil procurement undertaken today must be oriented towards enabling ever-increasing levels of renewable generation [57]. Figure 7 : Resource Mix for Statewide 33% RPS [57] Grid Impacts and Solutions of Renewables at High Penetration Levels Page 22 of 62
  • 23. 4 Defining Renewable Integration Impacts 4.1 Capacity Factor Capacity Factor (CF) is the ratio of the actual energy produced, to the hypothetical maximum possible, i.e. running full time at rated power. The plant Capacity Factor is a measure of grid and system utilization. Average Power (kW ) x 8760 h / yr Average Energy CF = = Rated Power (kW ) x 8760 h / yr Maximum Rated Energy Typical CFs for different generation technologies are shown below: Wind power: 20 – 40 % Hydro power (depending on size): 30 – 90 % Solar Photovoltaic (tracker & region): 12 – 25 % Nuclear CF: 60% - 100% (Avg. 92%) Thermal plants (e.g. large coal): 70 – 90 % Combined cycle gas plant: 60% Biomass thermal plant: 80% The low capacity factors for wind and solar generation emphasize the need for increased transmission facilities to server load centers. Technology improvements, but especially local energy storage can increase the capacity factor to closer to other recourses like thermal, large- scale hydro or nuclear power plants [45]. 4.2 Energy and Capacity Credits Dispatchability is the ability of a power plant to ramp up and down quickly to a desired level of output from a desired control signal. Typical required ramp-rates are in the order of a couple of minutes to 30 minutes for balancing intermitted resources like wind and solar. Energy storage is an important technical challenge that could enhance the dispatchability of renewables. Two important factors that have economic impact to integrating renewables are: Grid Impacts and Solutions of Renewables at High Penetration Levels Page 23 of 62
  • 24. • Energy Credit The energy credit refers to the rating of a continuously operating conventional power plant that can be replaced with a renewable energy power plant in terms of the energy generated per year. This forms the basis of the Renewable Portfolio Standard (RPS) developed and implemented by several states. Furthermore Energy Credits can also be traded in terms Renewable Energy Certificates (RECs) or Green Tags. The upcoming and proposed “Cap-and-Trade” policy will also play in the field of Energy Credits. • Capacity Credit However the amount of conventional generation that could be ‘replaced’ by renewable generation, without making the system less reliable is a measure of the Capacity Credit associated with a specific renewable energy plant. Currently wind and solar plants normally are not eligible for capacity credits as a result of the anticipated intermittency. As the penetration levels grow, firm capacity credits have to be assigned to portions for these plants. Fast-track research and analysis is required to determine the best approach of assigning capacity credits for renewable energy resources. True backup power balancing generation cannot be developed for the complete intermitting renewable resources at the anticipated penetration levels of 30 – 40%. Based on load profiles, available spinning reserves, CAISO generation mix, available transmission capacity, resource diversity and complementary nature of the renewable resources, these wind and solar resources can be assigned firm capacity credits [70]. Table 2: Capacity Credit in California [70, 43] Resource Relative Capacity Credit Medium Gas 100.00% Biomass 97.8% Geothermal (constrained) 73.6% Geothermal (unconstrained) 102.3% Solar 56.6% Wind (Altamont) 26.0% Wind (San Gorgonio) 23.9% Wind (Tehachapi) 22.0% Above table gives the anticipated allocated capacity credit calculated for different regions and renewable resource. The percentages are based on estimated nameplate rating of the generator. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 24 of 62
  • 25. 4.3 REC Certification Renewable Energy Credits (RECs) also known as Green Tags or Tradable Renewable Certificates (TRCs) are tradable environmental commodities in the United States which represent proof that 1 MWh of electricity was generated from an eligible renewable energy resource. RECs are playing an increasing part in several important aspects of energy policy, including participation in net-metering, the California Solar Initiative, the Self-Generation Incentive Program, tariffs or standard contracts for utilities' purchase of RPS-eligible generation from certain facilities sized at or below 1.5 megawatts (MW), and voluntary programs reducing emissions of greenhouse gases (GHG). These certificates can be sold and traded and the owner of the REC can claim to have purchased renewable energy. While traditional carbon emissions trading programs promote low-carbon technologies by increasing the cost of emitting carbon, RECs can be an incentive for carbon- neutral renewable energy by providing a production subsidy to electricity generated from renewable sources. A certifying agency gives each REC a unique identification number to make sure it doesn’t get double-counted. The green energy is then fed into the electrical grid (by mandate), and the accompanying REC can then be sold on the open market. There are two main markets for renewable energy certificates in the United States – compliance markets and voluntary markets. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 25 of 62
  • 26. Figure 8: RECs Calculation Chart [46] 4.4 Low-Voltage Ride-Through (LVRT) New generation interconnection requirements have been adopted by FERC as part of the FERC Order 661, docket RM05-4-0000 NOPR, mainly for large wind and solar power facilities, larger than 20 MW. These provisions are updated and adopted as Appendix G to the LGIA [27]. FERC requires now also renewable energy plants to be able to provide sufficient dynamic voltage support and reactive power if the utility’s system impact study shows that it is needed to maintain system reliability. This implies that wind generators should have dynamic reactive capability for the entire power factor range, and that dynamic reactive power capability must be required in every instance. These orders do however put the burden on transmission providers to demonstrate the power factor requirements of their system up to +0.95 p.f. FERC’s interpretation of the voltage measurement point is at the high side of each collector substation, rather than the less defined interconnection point. The main concern is that any generator (also wind and solar) needs to have ride through fault clearing capability. This is documented in the so-called Low-Voltage Ride Through (LVRT) Grid Impacts and Solutions of Renewables at High Penetration Levels Page 26 of 62
  • 27. requirement. This graph indicates when the wind generator should stay online after a fault on the feeder. It should still stay online with up to zero remaining voltage after a fault and for at least 0.625 seconds. After that a gradual recovery slope is indicated for a total of 3 seconds. NERC is also in the process of finalizing a national Voltage Ride-Trough (VRT) Standard which is more stringent than the FERC LGIA for generators above 20 MVA. A summary of the proposed VRT requirement is described in Figure 9. Figure 9: Proposed NERC Voltage Ride-Through Capability Requirement (PRC-024-1) [71] WECC is currently using the FERC Order 661 LVRT Interconnection Standard, but will probably follow the new NERC VRT requirements. The WECC standard applies to units larger than 10 MW, versus 20 MW and greater for the FERC requirement. However a new WECC LVRT Standard is currently being finalized that is more stringent than the FERC standard and it requires that all generation should stay online even to a depression of zero (0) V for a duration of around 150 ms. The rest of the requirement is similar to the FERC order 661. For the purposes of this study, the FERC order 661 is assumed for conditions when Grid Impacts and Solutions of Renewables at High Penetration Levels Page 27 of 62
  • 28. the wind generation is suppose to be on-line. If a new more stringent WECC requirement comes into effect, the existing dynamic results will be more conservative in any case.This LVRT requirement implies that wind generation facilities need to stay connected through a fault scenario to provide post-fault voltage, active and reactive power support to the distribution or transmission network it is connected to. 4.5 IEEE 1547 DER Interconnection and Power Quality Large scale Distributed Renewable Generation are planned to form part of the renewable energy portfolio. The additional increased use of these Distributed Energy Resources (DER), including Storage and Plug-In Hybrid Vehicles, will result in bi-directional power flows, protection issues on utility distribution systems that were not designed to accommodate active generation and storage at the distribution level. The technologies and operational concepts to properly integrate distributed resources (DER) into the existing distribution feeders need to be addressed to avoid negative impacts on system reliability and safety. Through the last few years several standards and guidelines were introduced and updated to reduce these impacts. The IEEE-1547 [1] set of standards addresses in one document most of these issues. There are however concerns on some of the practical impacts of this standard on distribution feeder design and safety. These include islanding and Power Quality of high levels of inverter penetration at the distribution levels. Even though the different standards are met utilities are experiencing some Power Quality and voltage fluctuation issues [8]. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 28 of 62
  • 29. 5 Renewable Energy Resources and Characteristics 5.1 Wind Energy 5.1.1 California wind resources and developable capacity (geographical map) Figure 10: California Wind Resource Map [36] Existing utility-scale wind power generation facilities can be found in five major resource areas in California Grid Impacts and Solutions of Renewables at High Penetration Levels Page 29 of 62
  • 30. • Solano • Altamont • San Gorgonio • Tehachapi, and • Pacheco Three of these primary regions (Altamont, Tehachapi and San Gorgonio) account for nearly 95 percent of all commercial wind power generation in California, and approximately 11 percent of the world’s wind generated electricity (Source: Energy Commission WPRS 2001-2002). 5.1.2 CA Wind Resource Profiles 1st Quarter 3rd Quarter Figure 11: Typical Wind Generation Profiles and Capacity Factors Large variations in Capacity Factors are clearly shown in the figures above. The wind generation profile and load profile is shown below and it is clear that wind generation is mostly off-peak generation in California, providing a large unbalance concern for load following capabilities. This is placing a higher requirement in terms of balancing power. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 30 of 62
  • 31. Figure 12: : California Wind Generation [56] 5.1.3 Fluctuating issues (short term, medium term) Although energy production using wind resources is pollution free, wind plants need to be balanced with peaker power plants, especially fast ramping gas or hydro generation, as supply of wind is intermittent. With the integration of wind generation the output of fossil fuel-plant needs to be adjusted frequently, to cope with fluctuations in output. Some power stations will be operated below their maximum output to facilitate this, and extra system balancing reserves will be needed. Efficiency may be reduced as a result. At high penetrations (above 20%) wind energy may need to be 'spilled' or curtailed because the electricity system cannot always utilize [43]. Energy is not generated while components are being repaired or replaced. Although a single failure of a critical component stops production from only one turbine, such losses can mount up to significant sums of lost revenue [45]. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 31 of 62
  • 32. 5.1.4 Wind turbine technologies and characteristics (Type 1 – 4) • Type 1 – conventional induction generator: operate in a narrow speed range, and absorb reactive power from the grid during operation during operation. Most of the older wind farms use Type 1 wind turbines • Type 2 – wound rotor induction generator with variable rotor resistance: These machines have wider speed variation and tend to exhibit slower active power fluctuations than Type 1 machines, but have similar reactive power characteristics. • Type 3 – doubly-fed induction generator. With some LVRT and voltage support capability. • Type 4 – full converter interface (with or without gearbox). Capable of LVRT, local voltage support and independent P and Q regulation. Type 3 and Type 4 machines use essential power electronics for the provision of wider speed range and finer control of active power production. The power electronics also inherently provide the ability to produce or consume reactive power. It is largely controllable independent from the active power production. In this regard, these machines resemble conventional synchronous generators with excitation systems and automatic voltage regulators (AVR). Grid Impacts and Solutions of Renewables at High Penetration Levels Page 32 of 62
  • 33. 5.1.5 Wind turbine technologies and integration issues A summary of these wind turbine types and grid integration issues are listed below: Figure 13: Wind turbine technologies and integration issues
  • 34. 5.2 Solar Energy 5.2.1 California solar resources and developable capacity (geographical map) Figure 14: California Plat Panel Solar Resources [38, 41]
  • 35. Figure 15: Solar Resource for Concentrating Collectors [38] Figure 17 shows the gross solar potential of flat plate PV systems in California. The values in figure 17 represent average annual estimates and show that most of California has a relatively good solar resource that could be harnessed using PV systems. 5.2.2 Solar technologies The different solar generation technologies are normally distinguished between Photovoltaic (PV) and Concentrated Solar Power (CSP) power plants. Each of these main generation options has again different technologies like mono-crystalline, poly-crystalline, amorphous (Thin-film) technologies, etc. with their characteristics and impacts to the grid [40,41,42]. The 64-MW Solar one solar thermal electric plant built in Boulder City, Nevada went on-line in June 2007. Nine solar thermal electric plants with a capacity of 354 MW were constructed in California from 1985 to 1991 and continue to operate today [42]. These plants are the closest to traditional steam plants and have an additional option of thermal steam or molten salt storage. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 35 of 62
  • 36. Figure 16: : Cumulative U.S. PV Installations by Year [40] Most of the traditional PV solar plant installations (residential, commercial or large scale utility connected) are either mono-crystalline or poly-crystalline cells with a long performance and life-time history. After a rocky start in the 1990s, thin-film PV production has become more cost-effective with improved energy efficiency, degradation improvements and longer lifetimes. Currently very large 30 – 100 MW PV thin-film plants are planned and developed. Most PV technologies are only producing power with direct sunlight, but thin-film has additional advantages that it has some power production capability under direct, reflective and diffused light conditions. The solar market is now in a fast track development phase and we will see large impacts on grids due to solar thermal and PV generation. PV is also ideally suited for Distributed Generation, utilizing residential and commercial roof space. Customer-Side Distributed Generation (California Solar Initiative): California’s electric utility customers now receive upfront incentives when they install solar electric systems on homes, businesses and public sites under the California Solar Initiative. This portal connects users to all California Solar Initiative-related programs, including the affordable housing program, the research and development program, and the solar hot water pilot program [39]. Utility-Owned Distributed Solar PV Programs. On March 27, 2008, Southern California Edison (SCE) filed A.08-03-015, requesting CPUC approval of a plan to install 250 MWs (DC) of distributed rooftop solar PV at the distribution level in its service territory [39]. On July 11, 2008, San Diego Gas and Grid Impacts and Solutions of Renewables at High Penetration Levels Page 36 of 62
  • 37. Electric (SDG&E) filed A.08-07-017, seeking CPUC approval of its proposal to install 52 MWs (dc) of distributed solar PV systems with single-axis tracking at the distribution level [39]. Current projections show that annual U.S. grid-tied installations grew by 45 percent in 2007 over 2006 to nearly 150 MW-dc. The annual installed capacity has more than doubled since 2005. More than 12,700 sites connected Photovoltaics to the grid in 2007. California continues to dominate the U.S. market with 58 percent of the market [40]. 5.2.3 Solar Generation Profiles and Capacity Factors in California In Figure 17, the estimated monthly capacity factor (CF) for Concentrated Solar facilities is provided based on a load capacity of the CAISO of 50 GW. Solar electric power is a better matched, compared to wind power profiles, to the load profiles especially if a couple of hours of storage can be added or solar power can be wheeled over a solar time zone to California [55]. Figure 17: Estimated Solar Capacity Factor of in California with different storage and a solar multiple SM2 factors. [55]. The Solar Multiple (SM) is the ratio of actual solar array size to the minimum size required to run a turbine at full capacity at solar noon in midsummer. The three SM lines in the graph above indicate “Solar Multiple” figures, the ratio of the size of the solar field to the peak input of the turbine; larger solar multiples provide more hours of generation per year. The SM3 case is very close to the needs of the California. The electrical load requirements are shown in the dotted line. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 37 of 62
  • 38. Solar generation has the typical daily production levels with some seasonal variations. A typical daily production profile is shown in Figure 18. Figure 18: Daily output production of 64 MW Nevada Solar-1 Farm with Annual Production of 130 GWh @ Capacity Factor = 23% [Source:- Acciona] 5.2.4 Solar Array Power Fluctuating Issues Concentrated thermal solar production in the Mojave Desert region can be forecasted with relative good accuracy without large daily fluctuations. In other California regions where Photovoltaic (PV) flat-plate collectors are predominately used, can produce power production fluctuations with a sudden (seconds time-scale) loss of complete power output. With partial PV array clouding, large power fluctuations can also result at the output of the PV solar farm with large power quality impacts on distribution networks [8,72]. Some practical measurement data of the power output from utility scale PV solar farms are presented in Figure 19 and Figure 20. It is clear that these types of power variations on large scale penetration levels can produce several power quality and power balancing problems. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 38 of 62
  • 39. Figure 19: Daily real power output data over one full day in summer at 1 minute sampling frequency [72] Figure 20: Daily real power output data from individual arrays over ~4 days at 10 minute resolution [72] Grid Impacts and Solutions of Renewables at High Penetration Levels Page 39 of 62
  • 40. 5.3 Other 5.3.1 Biomass Californians create nearly than 2,900 pounds of household garbage and industrial waste each and every second; a total of 85.2 million tons of waste in 2005 (according to the California Integrated Waste Management Board). Of that, 43.2 million tons is recovered and recycled or used to make energy, but 42 million tons has to be disposed in landfills [15, 49]. In 2007, 6,236 GWh of electricity in homes and businesses was produced from biomass: burning forestry, agricultural, and urban biomass; converting methane-rich landfill gas to energy (LFGTE); and processing wastewater and dairy biogas into useful energy. Biomass power plants produced 2.2 percent of the total electricity in California in 2007, or about one-fifth of all the renewable energy. Biomass is based on traditional gas generation, which in most cases can be dispatchable, can be designed with fast ramp rates and used for balancing wind and solar power. 5.3.2 Geothermal Because of its location on the Pacific's "ring of fire" and because of tectonic plate conjunctions, California contains the largest amount of geothermal generating capacity in the United States. The majority of the geothermal generation facilities in California are located in The Geysers, north of San Francisco, in Sonoma County and in Imperial County. Geothermal plants form an important dispatchable baseload component of the generation mix. Geothermal generators produce currently about 5 % of the electricity consumed in California and there are over 600 active wells. The Center for Energy Efficiency and Renewable Technologies estimates that geothermal resources could provide 20% of the State's total electricity needs. The geothermal generation technologies most often used to produce electricity are traditional steam turbines and combined cycle power plants [15,50]. Geothermal is using mostly traditional steam generation, which in most cases can be dispatchable, can be designed with fast ramp rates and used for balancing wind and solar power. 5.3.3 Wave Energy Wave energy development is at the infancy level in California and would not make a large impact on the RPS in the next 10 years. Recently PG&E did however receive approval by the Grid Impacts and Solutions of Renewables at High Penetration Levels Page 40 of 62
  • 41. CPUC to move forward with the first commercial size wave energy project off the Mendo Coast in Humboldt County in the order of $ 6M [69]. Wave Energy has been studied and demonstrated in several installations on the French, South African, Portuguese, Spanish and British Island coasts in the 2 – 20 MW power range. This resource is however also intermitted in nature with required mitigation measures when implemented at large penetration levels. 6 Summary of System and T&D Impacts Intermittent renewable resources at high penetration levels, mainly wind and solar generation, have large impacts on system operations, transmission and distribution as described in the previous sections. A Summary of the main system impacts are listed below: • Thermal limits and contingencies • Transient stability • Voltage stability • Generator operating P-Q margins • Generation power balancing and ramp rates • Reactive power requirements - steady-state and dynamic. • Impacts on Special Protection Schemes (SPS) • Protection and negative power flows • Transmission congestion management • Power Quality and LVRT • Required ancillary services o Frequency regulation o Spinning reserve requirements • Distribution Expansion Plans o SmartGrid and AMI o Distribution feeder congestion o Sub-station Automation Grid Impacts and Solutions of Renewables at High Penetration Levels Page 41 of 62
  • 42. 7 Potential Solutions for Integrating Large Scale Renewable (I.e. 33% by 2020 in CA) To integrate 33% intermittent renewable resources by 2020 in California several solutions and planning approach should be followed. There is no silver bullet but requires a combined effort on three major levels: • Generation Mix to utilize different complementary resources • Advanced transmission facilities • Demand response The complete impact of the mentioned approaches is depicted in Figure 21. These approaches need to be studied in more detail and specific demonstration projects need to be integrated into the grid, demonstrating the features and advanced capabilities of these approaches. These demonstration projects will also help to maturing the new technologies. Generation Mix Advanced Transmission Demand Response Hybrid Energy Mix WAPAC and FACTS Wind forecasting Complementary Gen. Local FACTS Integration Off-peak loading Advanced Power HVDC Transmission Gen. - load match Electronics Energy Storage Price sensitive load Turbines with Capacity Credits Frequency Response independent P & Q Renewable Dispatch Responsive to Wind regulation. SmartGrid / AMI Congestion Management Larger control areas Voltage Support & LVRT Distributed Storage Fast ramping Ancillary Services PHEV Regulation capability Frequency Regulation DES Control Error Spinning reserves Strong interconnections Figure 21: Large Scale Renewables Integration Solutions and Approach Grid Impacts and Solutions of Renewables at High Penetration Levels Page 42 of 62
  • 43. 7.1 Transmission and distribution planning studies The purpose of increased transmission planning is to identify complete and preferred transmission plans and facilities to integrate 15 - 18 GW, renewables of mainly wind and solar generation. The clear goal would be develop a staged transmission expansion plan, facilities and storage options to integrate this potential level renewable penetration levels. Most of the models for these advanced wind and solar facilities have not been fully developed yet and need to be validated. Model upgrades and refinements are required in stages from 2010 to 2020. Upgrades to the existing PSLF, CAPE and PSACD models are essential. The generator models for wind and solar generation technologies need to be upgraded and validated to include short circuit models, dynamic variance models like clouding and short-term wind fluctuations. Typical analyses to be performed include: i. Thermal studies ii. Contingency studies iii. Short-Circuit analysis iv. Voltage and angular stability v. Power Quality, including harmonics and flicker vi. Voltage ride-through analysis vii. Energy storage controllers Solutions and alternatives need to be investigated including: i. Transmission alternatives and proposed ROWs ii. FACTS devices with integrated control and PMUs (WAMPAC) iii. HVDC transmission lines and Back-to-Back DC interconnections. iv. Regional and inter-company interconnections v. Large power balancing and control areas. vi. Centralized and decentralized energy storage Grid Impacts and Solutions of Renewables at High Penetration Levels Page 43 of 62
  • 44. vii. Dynamic balancing of off-peak wind power, fast ramping hydro and FACTS devices. viii. Protective relay setting optimization in presence of high levels of Renewables. ix. Wide-Area-Monitoring-Protection and Control (WAMPAC) algorithms x. Testing hardware in the loop controllers for FACTS devices, wind and solar interface controllers and protection technologies. 7.2 Developing increased capability to integrate hybrid energy mix One of key solutions to integrate 33% renewables in California is to utilize the natural complimentary nature of renewable resources like wind and solar generation. Hybrid systems will maximize the use of renewables, resulting in a system with lower emissions than traditional fossil-fueled technologies. Hybrid renewable integration allows dispatchable generation while maximizing renewable resource usage. Some hybrid systems typically include redundant technologies and energy storage, which can simultaneously improve the quality and availability of power. It is proposed to develop models to determine the true firm capacity credits allocation for complementary resources like wind, solar and geothermal resources. 7.3 Frequency regulation and fast ramping energy storage Wind and solar generation are mostly intermittent in nature and do not provide load following characteristics and in most cases provide off-peak generation. On a daily basis wind power drops off rapidly while the load profile is still developing. This results in a high ramp rate of a few minutes for other generation taking over from the diminishing wind power production. Solar power production has the same characteristics, a bit later in the daily load profile. These transition phases provide problems for system operations and frequency regulation. Similar, but opposite ramping characteristics are experience when the wind power production rapidly increase with diminishing load profile retraction. With an increase in the expected wind and solar penetration levels more challenges are foreseen to perform adequate frequency regulation and generator control. Most of the older peaker generating plants are not capable of delivering the fast ramp rates or they do not have enough capacity in regulating power left for large renewable power balancing in the California region. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 44 of 62
  • 45. Currently the frequency regulation requirement at the CAISO is around 1% of peak load dispatch, or about 350 MW. This is currently mainly supplied by peaker generating plants and result in higher emission levels. It has been calculated that around 2% regulation would be required for integrating 20% wind and solar resources by 2010 and 4% to integrate 33% renewables by 2020 [86,87,88]. Fast ramping regulation will be more valuable and is currently considered. Only fast acting storage facilities will be capable of following 10 sec. / MW ramp rates that will be required for fast ramping regulation [73, 88]. The required capacity of fast ramping generation needs to be studied and validated. Furthermore new fast ramping storage technologies like flywheel and battery storage plants, bidding into the regulation market without emissions, need to be evaluated as shown in Figure 22. Short-term (1 – 15 min) Supply and Demand Curve Daily Supply and Demand Curve Frequency Excursions with Spinning Reserve Figure 22: Fast-ramping energy storage plants for regulation ancillary services [73] Unlike an equivalent thermal generator, the inertia of a wind generator may, or may not, contribute to the system spinning reserve, depending on the particular technology involved. A reduction in system inertia in periods of high wind generation may require more fast acting reserve to be deployed to maintain reliability standards [65]. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 45 of 62
  • 46. 7.4 Increased regional control areas One of the reasons why penetration levels could increase to relative high values in Europe is the larger control areas and the stronger interconnections between regions. Several RTOs are currently studying the system wide impacts and upgrades associated with large scale wind power. An example of progress in this are is the activities of Southwest Power Pool (SPP) [75]. 7.5 Improved interconnection facilities In utilizing large control areas for renewable power balancing, regional interconnection facilities need to be improved. Preferably these inter-connection points should also be controllable using Phase-shifting Transformers (PST) or Back-to-Back HVDC links. The power flow can then be controlled and congestion can be managed. Most of the original regional interconnections were not designed for large power transfers and remote power wheeling and are the bottlenecks for large renewable integration. 7.6 FACTS devices integrated with energy storage Recent advances in electric energy storage technologies provide an opportunity of using energy storage to address the wind and solar energy intermittency [11,88]. It is technically practical to apply electricity storage to wind and solar generation to improve availability and the capacity factor. Furthermore storage can increase the amount of renewable generation that may be installed on the grid without risking system voltage instability, to increase throughput of existing grid infrastructure, and to yield various ancillary benefits such as reduced system losses and improved power factor control. For example, improving availability can be done by storing the excess energy from wind farm when the wind turbine output is high and returning that energy to the grid when the wind turbine output drops off in the 15 – 30 minute timeframe making wind energy 1 hour ahead dispatchable to the ISO or utility. From the supply side perspective this is an availability enhancement tool. It provides spinning reserve and a firm source of supply. From the demand side it is a load shaping or leveling tool [11]. Another example for storage use is when power fluctuation occurs in the system, the energy storage can be used to level the power fluctuation by charging and Grid Impacts and Solutions of Renewables at High Penetration Levels Page 46 of 62
  • 47. discharging operation. Also, during a sag or fault the energy storage can be used to boost the stability margin by absorbing active power from the wind farm [12]. Similar to above examples, complementing wind with energy storage has been considered in many papers [13 - 21] for different improvements in wind energy integration. A variety of storage technologies are available which can be used with wind energy. Some of these storage technologies are ultra-capacitors, superconducting magnetic energy storage, flywheels, batteries, compressed air energy storage and hydro pumped storage. The basic issues with these storage technologies consist of the cost of these technologies, operation and maintenance requirements [9]. Amongst the storage technologies mentioned above, battery energy storage are currently used for wind power balancing in Japan and new Lithium based battery technology is proven to have low losses, high charge and discharge rates with relative low losses round-trip losses [64,17]. Moreover BESS is also shown to be cost-effective for use in power systems in regulation and fast power balancing applications. Therefore, BESS can be selected as a suitable choice for energy storage that will be complemented with wind and solar energy. The power conversion systems required for energy storage systems (ESS) are similar to the Voltage Source Converter (VSC) of a STATCOM like FACTS devices, thus an ESS can serve the dual purpose of providing voltage support and active power capabilities. In most applications, the cost of FACTS electronics system is a small portion of the cost of the ESS (less than 10%, [60]. A combined FACTS / ESS system thus has comparable cost and can provide the FACTS device with four quadrant control [63]. 7.7 Integrated control using WAMPAC with FACTS and PMUs Power-grid congestion issues and disturbances worldwide have emphasized the need to enhance power grids with Wide Area Monitoring, Protection, and Control (WAMPAC) systems as a cost- effective solution to improve grid planning, operation, maintenance, monitoring and energy trading. WAMPAC systems take advantage of the latest advances in sensing, communication, computing, visualization, and algorithmic techniques. Synchronized Measurement Technology Grid Impacts and Solutions of Renewables at High Penetration Levels Page 47 of 62
  • 48. (SMT), including Phasor Measurement Units (PMUs) and its applications are an important element and enabler of WAMPAC [83,84]. With the advent of highly precise time-synchronized measurement technology made possible by the availability of global positioning systems (GPS) satellites, advanced computer technology and applications, grid improvement can be achieved by Wide Area Monitoring, Protection and Control (WAMPAC) systems. New phasor measurement units (PMU) can now provide synchronized phasor measurements that eliminate the need to have several different types of protection, control and electric power system analysis for system-wide protection applications. Synchronized phasor measurement can provide improve regional power system disturbance analysis, protection and control capabilities for electrical system operators [85]. In the future these PMU facilities can be used in a combined with distributed FACTS devices to control and regulate power over large control regions. 7.8 HVDC interconnection options Back-to-Back: In HVDC back-to-back connections both the rectifier and the inverter are located in the same station and are normally used in order to create an asynchronous interconnection between two AC networks, which could have the same or different frequencies. Figure 23: Back-Back Converter Grid Impacts and Solutions of Renewables at High Penetration Levels Page 48 of 62
  • 49. HVDC Long-Distance Transmission: Transport of bulk power over long distances. Especially useful for connecting renewable from remote locations to load centers. Figure 24: Monopole with metallic return path, ground return path, bipolar balanced operation The normal operating configuration of an HVDC link is in bipole operation. In the event of loss of one DC transmission line, the two converter poles can also be connected in parallel by using appropriate switches for polarity reversal in at least one station pole, thus enabling both poles to operate in the monopole mode. Normally this mode of operation is only valid under emergency or maintenance conditions, when one of the DC lines are out of service. HVDC links are excellent transmission facilities to integrate remote renewable generating resources like wind and solar farms. Lower line costs, smaller Right of Ways (ROW), les Electromagnetic Interference (EMC) and accurate power flow and transient characteristics make these links ideal for intermittent renewable resources [66,74,86]. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 49 of 62
  • 50. 7.9 Resource forecasting System operators can significantly reduce the uncertainty of renewable energy resources by using accurate wind and solar forecasts that incorporate meteorological data to predict wind production. Such systems yield both hour-ahead and day-ahead forecasts to support real-time operations. They also inform the scheduling and market decisions necessary for day-ahead planning. Forecasting allows operators to anticipate renewable generation levels and adjust the remainder of generation units accordingly. Advanced forecasting systems can also help warn the system operator if extreme wind events are likely so that the operator can implement a defensive system posture if needed. The operating impact with the largest cost is found in the unit-commitment time frame. 7.10 Load forecasting Load forecasting is an important element for economically efficient operation and for effective control of power systems. Also, more accurate load forecasting can avoid high generation cost, optimize regulation requirements, ramping and the spinning reserve capacity. End-use and econometric approaches are broadly used for medium- and long-term forecasting. A variety of methods, which include the so-called similar day approach, various regression models, time series, neural networks, expert systems, fuzzy logic, and statistical learning algorithms, are used for short-term forecasting. The California ISO Day-Ahead (DA) load forecast is calculated by utilizing neural-network forecasting software. The California ISO utilizes an Automated Load Forecast System (ALFS) to calculate its DA hourly forecast demand approximately 14 hours prior to the next operating day [79]. 7.11 Increase Capacity Credits and Firm Capacity from Renewables The key determinants of capacity credit are as follows: The greater the degree of correlation between demand peaks and intermittent output the greater the capacity credit. A higher level of average output over peak periods will tend to increase capacity credit. The range of intermittent outputs: Where demand and intermittent output are largely uncorrelated, a decrease in the range of intermittent output levels will tend to increase capacity credit because the variance decreases. More consistent wind regimes decrease variance and increase capacity credit [78]. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 50 of 62
  • 51. 7.12 Large Scale Energy Storage Utility scale applications are providing power balancing, arbitrage, load balancing, peak load shaving (2 and 4 h duration), improving reliability of: (i) systems with large amount of renewable energy, (ii) isolated systems and (iii) micro-grids, support for transport and automobile feeding, deferment of new generation and transmission construction in cases where up-gradation is needed due to the increase in peak demand [64]. Figure 25:Storage Power Requirements (ESA.org) Grid Impacts and Solutions of Renewables at High Penetration Levels Page 51 of 62
  • 52. Table 3: Different Storage Technologies and their development status [77] Key Storage Technology for Renewable Development Status and Application Integration Batteries Lead-acid batteries are commercially available and widely used. Research and Development is ongoing for advanced batteries. Flywheels Flywheels are commercially available as fast ramping regulation services and power quality applications. Significant research is also underway to develop new, larger flywheel products. Superconducting Magnetic Energy Storage Superconducting magnetic energy storage is commercially (SMES) available using superconductors in liquid helium. Superconductors in liquid nitrogen are in the development stage, and are mainly used for power quality applications in 1 – 5 second range. Pump Hydro Storage Pump storage are the key resource for large scale power balancing and peak-power saving with large renewable energy penetration levels. These facilities are well utilized all over the world to the 300 MW / 4 – 6 hour range. Compressed Air Storage Systems (CAES) CAES used in several large-scale storage applications are commercially available since the 1990s. Renewed interests at the 100 MW / 4 hour range are currently underway to replace pumped hydro schemes where they are not feasible. Research and development is ongoing for all areas of energy storage. There are advanced sodium/sulfur, natrium-salt, zinc/bromine, and lithium batteries nearing commercial readiness and offer promise for future utility application. Flywheels continue to be developed and improved, as well as integrated with other DER equipment such as engines and micro turbines [77]. 7.13 Distributed Energy Storage (DES) Energy storage technologies can deliver stored electricity to the electric grid or an end-user. They are used to improve power quality by correcting voltage sags, flicker, and surges, or correct for frequency imbalances. At high levels of distributed penetration, they can also be used for DSM applications. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 52 of 62
  • 53. Customers today have very few choices for commercially available DES options. One option being promoted is a smart energy management and storage device developed by Grid Point. Their device is installed at a customer premise serves as a system integration gateway for grid power, renewable energy (e.g., solar PV), plug-in hybrid vehicles and on-board battery storage. Utilizing their software platform, utilities can access and charge customer-owned storage to optimize their local distribution system. Customers and utilities alike can use the distributed storage as back-up service in case of emergencies and when continuity of service is an issue. o Plug-in Hybrid Vehicles Plug-in Hybrid Vehicles have potential as a quick-response, high-value service to balance fluctuations in load and distributed generation. Some experts predict that by connecting enough vehicles to the grid and transmitting power back and forth as needed, utilities could have large benefits [79]. Figure 26 : Plug-in Hybrid Vehicle Plug-in hybrid vehicles are high fuel economy vehicles which give mileage greater than 100 MPG and can be an excellent load for off-peak wind and solar generation resources. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 53 of 62
  • 54. Figure 27: Projected 2030 generating capacity (left), base generation (center), and new generation dispatched to meet demand for each PHEV recharging scenario (right) for WECC-CA [59]. In 2030, the emissions factors tend to decrease with a shift of demand to the evening, as more gas-fired generation and less coal is used. The unserved energy in the last scenario serves to drive up the average price and the PHEV cost significantly. 8 Summary and Recommendations The opportunities, challenges and solutions to integrate 33 % intermittent renewable resource in California is presented in this report. European experience with high levels of intermittent resources up to 80% penetration levels does not transfer fully to the California situation due to the difference in US grid design and load density. The integration of renewable energy at this scale will have significant impact, especially if the addition of energy storage devices (central or distributed) and FACTS devices utilized to counterbalance the influence of the intermittent generation sources. Utilities should conduct RD&D projects and commence studies to fulfill its obligation to accurately and reliably forecast the impacts on future system integrated resource planning. Due to the long lead time for some of the proposed technology solutions, it is recommended that utilities engage these challenges sooner versus later. If technical challenges manifest, a timely solution cannot be implemented if studies, demonstration installations and field tests still have to be conducted. Additionally, utilities should study all conceivable options that may severely affect transmission system integrity and stability. Otherwise, utilities may Grid Impacts and Solutions of Renewables at High Penetration Levels Page 54 of 62
  • 55. experience unintended consequences due to unforeseen technical issues resulting from high penetrations of new renewable energy sources. “I’d put my money on the sun and solar energy. What a source of Power! I hope we don’t have to wait until oil and coal run out before we tackle that” - Thomas Edison, 1931 9 References and Links 1. IEEE 1547-2003: Standard for Interconnecting Distributed Resources with Electric Power Systems. 2. M. Luther: “European Aspects of Wind Energy Integration into Liberalised Markets”, E.ON Netz GmbH, Euro-CASE Workshop, Düsseldorf, Germany, 14 November 2003. 3. Zasatzliche technische und organisatorishe Regeln fur den Netzanschluss von Windenergieanlagen innerhlb der Regelzone der E-ON Netz GmbH,” 12/2001. (E-ON Netz, LVRT Grid Code, www.eon-netz.com - Netzanschlußregeln) 4. Potential for Renewable Energy in the San Diego Region 2005 by SDG&E, San Diego State University, Tanaka R&C, and other entities 5. AWS Truewind: “Assessing the Availability and Economics of Additional Wind Energy Resources for SDG&E”, 2004. 6. Hirst, E: “Integrating Large Amounts of Wind Energy with a Small Electric-Power System” 2004 7. J.H.R. Enslin; C.P.J. Jansen; P. Bauer, “Integrated approach to network stability and wind energy technology for On-shore and Offshore Applications”, PCIM-2003 (Europe), Nuremberg, Germany, 20 – 22 May 2003. 8. J.H.R. Enslin; P.J.M. Heskes: “Harmonic interaction between a large number of distributed power inverters and the distribution network”, IEEE Transactions on Power Electronics, Vol. 19, No. 6, pp. 1586 - 1593, Nov. 2004. 9. J.H.R. Enslin; C.P.J. Jansen; P. Bauer: “In store for the future? Interconnection and energy storage for offshore wind farms”, Proceedings of Renewable Energy World, James & James Ltd, pp 104 – 113, Jan/Feb 2004. Grid Impacts and Solutions of Renewables at High Penetration Levels Page 55 of 62
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  • 62. 85. Edmund O. Schweitzer, David E. Whitehead, “Real-Time Power System Control Using Synchrophasors”, 61st Annual Conference for Protective Relay Engineers, pp 78-88, April 2008. 86. PSERC: U.S. Energy Infrastructure Investment: Long-Term Strategic Planning to Inform Policy Development, March 2009: http://www.pserc.org/ecow/get/publicatio/2009public/pserc_energy_modeling_white_paper_marc h_2009_adobe7.pdf 87. PSERC: U.S. Energy Infrastructure Investment: Large-Scale Integrated Smart Grid Solutions with High Penetration of Renewable Resources, Dispersed Generation, and Customer Participation, March 2009: http://www.pserc.org/ecow/get/publicatio/2009public/pserc_smart_grid_white_paper_march_200 9_adobe7.pdf 88. CAISO: Integration of Energy Storage Technology White Paper – Identification of Issues and proposed Solutions, May 22, 2008: http://www.caiso.com/1fd5/1fd56f931140.pdf Grid Impacts and Solutions of Renewables at High Penetration Levels Page 62 of 62