Functional Requirements for Electric Energy Storage Applications on the Power System Grid

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1 Functional Requirements for Electric Energy Storage Applications on the Power System Grid

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3 Functional Requirements for Electric Energy Storage Applications on the Power System Grid Technical Update, May 2011 EPRI Project Manager W. Steeley ELECTRIC POWER RESEARCH INSTITUTE 3420 Hillview Avenue, Palo Alto, California PO Box 10412, Palo Alto, California USA

4 DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. REFERENCE HEREIN TO ANY SPECIFIC COMMERCIAL PRODUCT, PROCESS, OR SERVICE BY ITS TRADE NAME, TRADEMARK, MANUFACTURER, OR OTHERWISE, DOES NOT NECESSARILY CONSTITUTE OR IMPLY ITS ENDORSEMENT, RECOMMENDATION, OR FAVORING BY EPRI. THE FOLLOWING ORGANIZATIONS, UNDER CONTRACT TO EPRI, PREPARED THIS REPORT: Norris Energy Consulting Company Technology Transition Corporation Electric Power Research Institute (EPRI) This is an EPRI Technical Update report. A Technical Update report is intended as an informal report of continuing research, a meeting, or a topical study. It is not a final EPRI technical report. NOTE For further information about EPRI, call the EPRI Customer Assistance Center at or askepri@epri.com. Electric Power Research Institute, EPRI, and TOGETHERSHAPING THE FUTURE OF ELECTRICITY are registered service marks of the Electric Power Research Institute, Inc. Copyright 2011 Electric Power Research Institute, Inc. All rights reserved.

5 ACKNOWLEDGMENTS The following organizations, under contract to the Electric Power Research Institute (EPRI), prepared this report: Norris Energy Consulting Co. 123 Clear Creek Ct. Martinez, CA Principal Investigator B. Norris Technology Transition Corporation 1211 Connecticut Ave NW, Suite 600 Washington, DC Principal Investigators J. Serfass E. Wagner EPRI 3420 Hillview Ave. Palo Alto, CA Principal Investigators H. Kamath A. Maitra W. Steeley A. Tuohy This report describes research sponsored by EPRI. EPRI acknowledges the work group leaders, who provided valuable guidance and leadership to this project. Our gratitude goes out to the following: Eva Gardow, FirstEnergy Service Company Mike Grant, Duke Energy Tom Walker, American Electric Power Dale Bradshaw, National Rural Electric Cooperative Association s Cooperative Research Network George Gurlaskie, Progress Energy Florida We would also like to thank the many unnamed individuals who participated in numerous conference calls and webinars and provided countless questions and comments that helped refine this report. This publication is a corporate document that should be cited in the literature in the following manner: Functional Requirements for Electric Energy Storage Applications on the Power System Grid. EPRI, Palo Alto, CA: iii

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7 PRODUCT DESCRIPTION This report describes functional requirements of energy storage connected to the power grid for several applications. The applications of interest include grid management at the substation and on the distribution system and storage to integrate larger scale variable renewable energy installations. The requirements developed in this project provide a common basis for manufacturers and utilities to consider the general needs of storage in these applications. They also provide a basis for utilities to develop storage equipment specifications in specific locations with specific grid, load, environmental and other characteristics. Results and Findings This report provides functional requirements for three key energy storage applications: substation-based storage, distributed energy storage systems, and energy storage to integrate renewables. Energy storage to integrate renewables is divided into three subcategories: solar photovoltaic ramping support, wind ramping support, and load- and resource-shifting applications. The requirements for each application include details on the use cases and operating modes, power output and duration, system ratings and effectiveness, physical requirements, communications and data flow, and operational and safety issues. Challenges and Objectives Energy storage is receiving increasing attention from utility engineers and regulators for its potential to solve a variety of technical challenges in the management of electric power. However, utilities, vendors, and regulators must develop a common ground on which to base their understanding of energy storage application requirements. This report presents a set of functional requirements for energy storage systems connected to the electric power system to be used in specific ways (use cases and operating modes). By steering procurement and development efforts consistent with these requirements, utilities and developers can work with a common understanding to develop the most effective storage solutions to utility problems. Application, Value, and Use This report will provide significant value to energy storage manufacturers and system integrators by communicating grid requirements for each of the energy storage applications. The report will help utility system planners who are considering the role that energy storage can have on the management of the grid and the temporal differences between economic energy supply and customer energy needs. The report provides generalized requirements for storage systems, independent of the technologies of interest, suitable for helping engineering and procurement personnel develop the more detailed specifications required in a procurement action. This report also will help power system operators communicate with the developers of large-scale wind and solar farms and also the developers of energy storage systems, so that they can each bring their value to the operation of the electric grid and, therefore, bring value to the electric consumer. Recommendations for further study have been included in the report. Many of the recommendations relate to the need for data and information to assist in development of storage operating modes and value propositions. v

8 EPRI Perspective This project is one element of the Electric Power Research Institute (EPRI) Energy Storage Program to accelerate the grid-readiness of energy storage systems by Other related projects include the analytics to support the business case for energy storage systems (see the EPRI report Electricity Energy Storage Technology Options: A White Paper Primer on Applications, Costs and Benefits [ ]) and research reports on testing, evaluation, and demonstration of storage systems for grid and application readiness. Widespread use of storage will require a coordinated effort by technology developers and utilities to ensure that systems are designed to adequately address utility needs. Utilities must understand the technical and cost characteristics of the various technologies being advanced by developers. System providers must offer energy storage systems that meet those requirements. It is that latter objective that has been the central motivation of this project. The project has brought together all the stakeholders including utilities, manufacturers, and system operators to document some of the basic requirements for storage owned and operated by utilities. The project operated under the guidelines that the functional requirements were to be technology neutral, that the economic justification of the applications would not be addressed, that the functional requirements were not to be detailed technical specifications, and finally, that not all storage applications envisioned could be included. This report focuses on the applications of high priority identified by the utility participants. All the included applications represent energy storage systems that would be controlled and operated by the utility and located on the utility side of the meter. To be most effective, the functional requirements that the participants have developed must be reviewed periodically to take advantage of utility and manufacturer experience being gained. Approach The project created a public, open source approach by including high-level energy storage stakeholders, including representatives from utilities, renewable energy project developers, equipment developers and manufacturers, regulators, independent system operators, power pools, and government and educational institutions. After a first draft of application requirements were developed, the project was opened up for public comments and received increasingly detailed input from stakeholders through public webinars, while application-specific work groups refined their individual functional requirements. Keywords Distributed energy storage system (DESS) Energy storage Functional requirements Renewables integration Substation-based storage vi

9 ABSTRACT Energy storage on the electric power system is becoming an increasingly important tool in managing the evolving transmission system and the integration of large-scale, intermittent solar and wind generation. Electric utilities are evaluating and deploying energy storage technologies to serve a variety of applications to address the challenges posed by these fundamental changes. The needs differ from utility to utility. This report, developed by the Electric Power Research Institute (EPRI), Norris Energy, and Technology Transition Corporation, with the support of more than 100 utilities and other stakeholders, provides functional requirements for three key energy storage applications: substation-based storage, distributed energy storage systems, and energy storage to integrate renewables, which itself is divided into three subcategories, each describing a different set of circumstances solar photovoltaic ramping support, wind ramping support, and load and resource shifting applications. The requirements for each application include details on the use cases and operating modes, power output and duration, system ratings and effectiveness, physical requirements, communications and data flow, and operational and safety issues. These sets of functional requirements present guidelines for utilities and equipment developers and manufacturers. Using this report as a basis for developing more detailed specifications allows both parties to better understand energy storage application needs. vii

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11 EXECUTIVE SUMMARY Project Description Energy storage is receiving increasing attention from utility engineers and regulators for its potential to address a variety of technical challenges in the management of electric power. Utilities face the need to economically serve uncertain peak load growth at substations, the desire to provide enhanced reliability and resilience in adaptive Smart Grids, and the need to use highly variable and unpredictable renewable energy sources. All these needs could potentially be addressed using storage technologies. Widespread use of storage will require the coordinated effort of technology developers and utilities to ensure that systems are designed to adequately address utility needs. On one hand, utilities need a better understanding of the technical and cost attributes of the various technologies being advanced by the developers. On the other hand, developers need a better understanding of the prospective uses and requirements of utilities. It is that latter objective that has been the central motivation of this project. The project has brought together representatives from all stakeholder groups including utilities, manufacturers, and system operators to document some of the basic requirements for storage owned and operated by utilities. The requirements vary depending on location transmission, substation, and distribution and the specific challenges addressed, such as peak load management or enhancing the flexibility of the bulk grid to enable the integration of large-scale renewables. In the process, a set of functional requirements for energy storage systems to be used in specific ways (use cases and operating modes) was developed. By steering procurement and development efforts consistent with these requirements, utilities and developers can work with a common understanding to develop the most effective storage solutions to utility problems. Rationale for Energy Storage The current and anticipated challenges facing the electric grid include the following: Variable renewable energy generation (primarily wind energy) is rapidly growing in its overall contribution to the resource mix, requiring a more flexible grid system that can accommodate its uncertain, partly dispatchable output. Variability in solar photovoltaic (PV) power output due to the diurnal solar cycle, passing clouds, and other events can lead to ramping effects and unpredictable load management at the system level. Likewise, variations in wind power output, whether short-term (seconds) or longer term (minutes), can affect distribution voltage (in those cases where wind is connected at the distribution level) and might create a requirement for additional regulation and ramping support at the system level. State renewable portfolio standards, which require that a specified minimum fraction of the electricity supplied in a state be generated from renewable energy sources, will likely result in substantial increases in the penetration of these sources on the grid in the coming 10 to 20 years. ix

12 Bidirectional power flow created by distributed energy resources presents a challenge for distribution systems with voltage regulation and protection schemes originally designed for one-way power flow. Smart Grid designs call for additional distribution automation and sophistication, such as islanding and self-healing designs aimed at improving user reliability. Limited transmission capacity threatens to force existing clean-energy wind resources to be curtailed during peak production times, but expansion of transmission capacity presents regulatory and environmental challenges. Utilities seek new ways to extend the useful life of existing capital assets to defer investment in capital upgrades, maintaining reliable power at a reasonable cost to users while accounting for uncertainty in load growth. Energy storage has the potential to address all these concerns. Objective and Scope This project sought to develop a set of functional requirements for energy storage systems to be used in specific ways (use cases and operating modes). The goal is for utilities and developers to work with a common understanding to develop the most effective storage solutions to utility problems. The project scope adhered to the following guidelines: The process and this report were based entirely on utility requirements for specific applications and not on the capabilities of underlying storage technologies. Although certain energy storage technologies might operate better in one application or area than another, it is not the purpose of this project to evaluate the suitability of specific storage technologies. Economic analysis of the applications was not performed. The process and this report address only high-level functional requirements and are not intended to serve as complete technical specifications for procurement. The ultimate list of requirements, including specific standards and other utility-specific requirements, will have to be specified by the utilities through its normal procurement process. Not all storage applications were included. Other variations, such as storage for the exclusive purpose of frequency regulation or seasonal energy storage, are possible, but they are not included in the scope of this project. This report focuses on a few high-priority applications identified by the utility participants. Energy storage at the customer side of the meter is not considered in this project. The project focuses, instead, on opportunities in utility system management. Approach and Methodology This project was based on stakeholder review of draft application requirements and stakeholder advice on how to refine the requirements to produce this report. The project began with a draft application requirement document. Following internal reviews, the Electric Power Research Institute (EPRI) conducted a series of webinars and work group calls with stakeholders to obtain their comments and suggestions for the developing document. Over the course of this project, more than 100 companies have participated in varying capacities (see Appendix D). This report is the result of six work group conference calls, nine webinar meetings, and off-line inputs received directly from the participants. x

13 Requirements The project focused on three essential energy storage applications that serve different functions on the electric grid. These applications vary in categories such as location on the electric grid, capacity, operating modes and use cases, and the required duration of energy storage. These applications are not intended to represent a comprehensive list of all energy storage applications but rather focus on the applications of primary interest to the participating utilities. An overview of the three key systems is shown in the following two tables. Table 1 is an overview of the first two applications, substation-based storage and distributed energy storage systems (DESS). Table 2 describes the third application, storage to integrate renewables, with its three sub-cases. Table 1 Matrix for Substation-Based Storage and Distributed Energy Storage Systems Applications Application Substation-Based Storage (see Section 2) 1 20 MW 2 6 hours Includes both stationary and transportable Distributed Energy Storage System (DESS) (see Section 3) kw (individual unit rating) Single-phase (25 75 kw) Three-phase (up to 200 kw) 2 4 hours Use Cases / Operating Modes (See Note) Peak load management Frequency regulation Capacity market (regional transmission organization [RTO] or independent system operator [ISO]) Voltage regulation/reactive power support Peak load management Backup power/islanded grid operation Voltage regulation/reactive power support Frequency regulation (may require aggregation) Capacity market (RTO/ISO) Interconnection Point Distribution voltage (4kV 34kV) Substation or feeder Secondary (customer) voltage Utility side of meter May operate as island Note: Use cases are listed in order of decreasing priority. Products need not meet all use cases. Notes Peak load management is controlled using substation/feeder real-time load signals. Frequency regulation controlled using signals from ISO. Although not currently in effect, a capacity market controlled using signals from ISO is a future option. Peak load management is controlled using substation/feeder real-time load signals. Frequency regulation controlled using signals from ISO. May be used in capacity markets. Reactive power dispatch based on local voltage. If only frequency regulation is desired, duration may be as low as 15 minutes. xi

14 Table 2 Matrix for Energy Storage to Integrate Renewables Application Use Cases/Operating Modes Interconnection Point Notes Solar PV Ramping Support (see Section 4) Power up to several megawatts (TBD by utility site) 1 second to 20 min (TBD by utility) Accommodate rapid power swings that would otherwise create disturbances on systems where highpenetration levels of solar PV systems are found Distribution voltage (4kV 34kV) Better manage the variability of solar active power output. Reactive power controlled based on local voltage. (This function is accomplished by the power electronics accompanying the storage.) Wind Ramping Support (see Section 4) MW 2 15 minutes Life equivalent to 10,000 full energy cycles Accommodate variable wind farm output so that ramp rates (MW/min) are kept within a desirable range Provide net load ramping support for the grid at large Maintain local transmission and distribution system voltage Provide frequency regulation Provide low-voltage ridethrough (LVRT) for wind farm (if required) Store energy generated during off-peak demand periods to serve loads during peak demand periods Participate in capacity markets as dispatchable energy and reserves Provide ancillary services Distribution voltage (4kV 34kV) Transmission voltage (>34kV) If local benefits for control area or a specific distribution feeder are required (as for a single wind farm), storage system must be co-located with the variable generation. If bulk power system benefits are desired, storage system can be located anywhere on the grid. LVRT support is built-in to most modern wind turbines. Load or Resource Shifting (see Section 4) Kilowatts to many hundreds of megawatts 2 10 hours Distribution voltage (4kV 34kV) Transmission voltage (>34kV) May be directly coupled and sized to local renewable resource or sized and operated independently. May also serve to reduce ramp rates from variable wind farm output and dampen solar PV ramping. Substation based storage applications (see Section 2) provide primarily peak load management and frequency regulation services at the substation, could be either stationary or portable, and provide power in the range of 1 20 MW for 2 6 hours. Distributed energy storage systems (DESSs) (see Section 3) and include applications near the customer for management of small groups of loads. The primary uses of these systems are peak load management, backup power, and regulation services. The systems would be rated in the range of kw for 2 4 hours of discharge duration and would be located on the utility side of the meter. xii

15 Energy storage to integrate renewables (see Section 4) addresses issues that arise due to increasing amounts of renewables on the grid. This application is divided into the following three categories, each with a different set of circumstances. Solar PV ramping support applications, which provide power for short periods of time (a few seconds up to 20 minutes) to mitigate power swings on the system. Wind ramping support applications, which can improve the flexibility of the transmission and distribution system to accommodate rapid changes in power generation from wind farms by providing buffer capacity for short periods of time. Load and resource shifting applications, in which energy is shifted from peak generation hours to peak load hours. These storage systems will require up to 10 hours of energy. They could be deployed in a wide range of power ratings, from a few kilowatts to many hundreds of megawatts (potentially even gigawatts) depending on where they are connected to the power delivery network. Systems could be designed to address other applications, as well, to obtain a better return on the upfront capital investment. Recommendations The following additional work is required to further define the functional requirements for these applications (see Section 5): Collection and sharing of data. The application of the functional requirements presented in this report would be enhanced by detailed operating data on wind and solar generation and other system characteristics that will impact the specifications for equipment being applied. It is also important that the information sharing in this project be continued among utilities, storage equipment suppliers, wind and solar developers, and related institutions so that the equipment being developed and purchased continues to improve in its applicability to grid management issues and opportunities. Utilities and others are encouraged to share their data, challenges, issues, and case studies for inclusion in possible revisions of this report. System sizing for solar PV ramping application. Utilities that use storage to accommodate solar PV ramping events must determine the power and energy requirements for the storage system based on local conditions. The means for determining optimal sizing is not yet well understood. The factors might include local meteorological characteristics, the local installed capacity of solar PV, the relative spacing of the individual solar PV systems, the existing voltage regulation available, and the local distribution system (such as wire sizes). In addition, solar PV models have only recently been integrated into distribution load flow models. Additional work needs to be done in this area. The degree to which storage is necessary to accommodate variability is similarly unclear for all the renewable applications described in this report. Solar PV control algorithms. Although the reactive power capabilities that might be included in a grid-connected energy storage system can be used to regulate voltage, it is not clear how to dispatch the active power capabilities for this purpose. The details of the dispatch model must be developed. For example, storage can be used to charge and discharge to limit the rate of change of power on the line at the point of storage interconnection. The algorithm might be dependent on local distribution impedance. Wind plant LVRT. The application of using storage for LVRT for wind plants is not yet well understood. Most modern wind plants provide LVRT as a matter of course, obviating the xiii

16 need for storage in this application. The operating modes and design requirements, if any, must be better defined. Solar PV and wind duty cycles. For both the solar PV ramping and wind ramping applications, the required cycle life is not yet well understood. On a given day, the cycle requirements are expected to be dependent on the number of generators in the fleet (more generators reduce aggregate variability), their physical placement, local meteorological conditions, the local power system design, and the timing of generator output relative to loads. Additional work is necessary to model storage operation needed to maintain stability and, thereby, define cycling requirements. Communication protocol details. The details of communication protocols for all the applications described in this report must be further delineated. Definition and standardization of these protocols will simplify the process for both suppliers and utilities. Defining functional requirements for other energy storage applications. This report describes three key storage applications, excluding a number of additional applications. Further work is needed to define functional requirements for commercial and industrial (C&I) power quality, C&I power reliability, C&I energy management, residential energy management, and residential backup. xiv

17 CONTENTS 1 INTRODUCTION Current Developments Affecting the Electricity Grid Potential Uses for Electric Energy Storage Electric Energy Storage Options and Technology Neutrality Defining Functional Requirements What is a Functional Requirement Versus a Technical Specification? EPRI s Role in Energy Storage This Project The Basics Objective and Scope Approach and Process SUBSTATION-BASED STORAGE Overview Description of Application Use Cases and Operating Modes Performance Ratings System Definition Auxiliary Loads System Rating Practices Storage System Effectiveness Storage Efficiency Performance Curve Physical Characteristics Size Transportation Standards Rigging and Harnessing Status Lights and Alarms Environmental Conditions Electrical Interface Standards Disconnect Breaker Contactor Communications, Control, and Data Management Communications Method Communications Protocol Integrated Interface Operational Data Event-Triggered Data xv

18 Data Access Installation and Maintenance Safety DISTRIBUTED ENERGY STORAGE SYSTEMS UTILITY PAD MOUNT Overview Description of Application Use Cases and Operating Modes System Rating Practices System Definition Auxiliary Loads Operation As a Current Source or Voltage Source System Rating Practices System Effectiveness Standby Efficiency Storage Efficiency Speed of Response Performance Curve Physical Characteristics Size Transportation Standards Rigging and Harnessing Status Lights and Alarms Environmental Conditions Electrical Interface Standards Disconnect Breaker Communications, Control, and Data Management Communications Method Communications Protocol Integrated Interface Operational Data Event-Triggered Data Data Access Installation and Maintenance Safety ENERGY STORAGE TO INTEGRATE RENEWABLES Overview Solar Photovoltaic Ramping Support Overview Use Cases and Operating Modes xvi

19 Performance Ratings System Effectiveness Physical Characteristics Electrical Interface Communications, Control, and Data Management Installation and Maintenance Safety Wind Ramping Support Overview Use Cases and Operating Modes Performance Ratings System Effectiveness Physical Characteristics Electrical Interface Communications, Control, and Data Management Data Access Installation and Maintenance Safety Load and Resource Shifting Overview Use Cases and Operating Modes Performance Ratings System Effectiveness Physical Characteristics Electrical Interface Communications, Control, and Data Management Installation and Maintenance Safety RECOMMENDATIONS FOR FUTURE WORK Collection and Sharing of Data System Sizing for Solar Photovoltaic Ramping Application Solar Photovoltaic Control Algorithms Wind Plant Low-Voltage Ride-Through Solar Photovoltaic and Wind Duty Cycles Communication Protocol Details Defining Functional Requirements for Other Energy Storage Applications REFERENCES A APPLICABLE CODES AND STANDARDS... A-1 B APPLICABLE PARAMETERS FOR SPECIFICATIONS... B-1 xvii

20 C ABBREVIATIONS, ACRONYMS, AND TERMINOLOGY... C-1 D PARTICIPANTS... D-1 xviii

21 LIST OF FIGURES Figure 1-1 Overview of Energy Storage Use Cases Figure 1-2 Overview of Siting of Energy Storage Applications Figure 2-1 Block Diagram of Substation Based Storage Applications Figure 3-1 Block Diagram of Distributed Energy Storage System Applications Figure 4-1 Block Diagram of Solar Photovoltaic Ramping Support Applications Figure 4-2 Block Diagram of Ramping Applications Figure 4-3 Block Diagram of Load and Resource Shifting Applications xix

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23 LIST OF TABLES Table 1-1 Matrix for Substation-Based Storage and Distributed Energy Storage Systems Applications Table 1-2 Matrix for Energy Storage to Integrate Renewables xxi

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25 1 INTRODUCTION Current Developments Affecting the Electricity Grid The electric power grid is quickly evolving into a smarter, more sophisticated delivery system that incorporates new renewable, distributed generation, end-use, and communications and control systems. The changes will provide many benefits, such as the ability to respond to public policy goals, increase the diversity of generation options, and provide consumers with more choices; however, the challenges are several, including the following: Variable renewable energy generation with limited dispatchability is rapidly growing in its overall contribution to the resource mix. Variability in solar photovoltaic (PV) power output due to the diurnal solar cycle, passing clouds, and other events can lead to ramping events and unpredictable load management at the system level. Likewise, variations in wind power output, whether short term (seconds) or longer term (minutes), may affect distribution voltage (in those cases in which wind is connected at the distribution level) and might create a requirement for additional regulation and ramping support at the system level. State renewable portfolio standards, which require that a specified minimum fraction of the electricity supplied in a state be generated from renewable energy sources, will likely result in substantial increases in the penetration of these sources on the grid in the coming years. Bidirectional power flow created by distributed energy resources presents a challenge for distribution systems with voltage regulation and protection schemes originally designed for one-way power flow. Smart Grid designs call for additional distribution automation and sophistication, such as islanding and self-healing designs aimed at improving user reliability. Limited transmission capacity threatens to force existing clean energy wind resources to be curtailed during peak production times, but expansion of new transmission capacity presents regulatory and environmental challenges. Utilities seek new ways to extend the useful life of existing capital assets to defer investment in capital upgrades and maintain reliable power at a reasonable cost to users while accounting for uncertainty in load growth. Potential Uses for Electric Energy Storage In principle, all these issues can be addressed with appropriately designed grid-connected storage systems. Storage can be sized from the kilowatt range up to thousands of megawatts. It can be designed to discharge from subcycle durations up to many days. Storage can be controlled locally or remotely and can be designed for extremely fast reaction in response to control signals. Storage can both absorb and inject active power and can be coupled with power electronics that can absorb and inject reactive power. 1-1

26 Depending on the utility requirements, storage systems can provide voltage and frequency regulation, load and resource shifting, ramping, and dispatchability. They can be designed for the needs of distribution and/or transmission systems and for single-purpose or multi-purpose operation (see Figure 1-1). Power Rating (MW) System Stability VAR Support Power Quality Temporary Power Interruptions Spinning Reserve Frequency Regulation Load Leveling Ramping Energy Arbitrage Renewables - Wind - Solar Peak Peak Shaving Shaving T&D Deferral and T&D Deferral Transmission Conjunction Management Remote Island Applications Village Power Applications Cycle 10 Cycle 15 Second 15 Minutes 1 Hour 5 Hour Energy Discharge Time (Axis Not To Scale) Source: Electric Power Research Institute Figure 1-1 Overview of Energy Storage Use Cases Electric Energy Storage Options and Technology Neutrality A wide range of technologies that have the potential to meet these application needs have been or are being developed, including the following: Electrical. Capacitors, supercapacitors, and superconducting magnetic energy storage systems Electrochemical. Battery systems, flow batteries, and hydrogen with fuel cells Mechanical. Pumped hydroelectric energy storage, compressed air energy storage, flywheel energy storage, and hydraulic accumulators 1-2

27 The process and this report were based entirely on utility requirements for specific applications, and not on the capabilities of underlying storage technologies. Although certain energy storage technologies might operate better in one application or area than another, it is not the purpose of this project to evaluate the suitability of specific storage technologies. To the extent possible, the descriptions of the functional requirements in this report are applicable across all possible technologies. However, on occasion, there are references to requirements that are expected to be relevant only to specific technologies. For example, the capacity of electrochemical batteries might degrade over time, so the rating of systems must account for this possibility, and this is addressed. Such references are provided merely for comparability and clarity; they are not intended to represent recommendations about the suitability of a given storage option. Figure 1-2 illustrates possible siting options for energy storage applications. Source: Electric Power Research Institute Figure 1-2 Overview of Siting of Energy Storage Applications Defining Functional Requirements What is a Functional Requirement Versus a Technical Specification? This report is the result of a consensus process among participating utilities (with input from developers, independent system operators, government and private institutions) as a means of communicating basic requirements and uses for storage as they pertain to identified needs. It is not intended to serve as a procurement specification. For developers, the report will provide insight into the needs and basic system requirements that their products will be called on to 1-3

28 deliver. For the utilities, the report will serve to facilitate the development of specific procurement documents based on the existing knowledge base for energy storage. Such a utility procurement document would include much more specific information that is not covered in this report. For example, although this document does reference a few key standards (see Appendix A), most utilities would likely reference additional requirements that are specific to their procurement practices (see Appendix B). Utilities might also elect to simplify the requirements, for example, by reducing the number of operating modes. In this report, performance requirements are suggested, subject to more precise determination by the buying utility. Accordingly, the verb form used in these functional requirements is, in many cases, should or may rather than shall or must. When used in a purchase specification, the verbs in these suggested requirements should be modified appropriately to declare a requirement. EPRI s Role in Energy Storage The Electric Power Research Institute (EPRI) and its member companies have done considerable research and analysis of the benefits of energy storage within the utility distribution system. Much of this progress will enable energy storage to be a fundamental component and benefit to the creation of the Smart Grid. The U. S. Department of Energy defines the Smart Grid as an automated, widely distributed energy delivery network, characterized by a two-way flow of electricity and information, that is capable of monitoring and responding to changes in everything from power plants to customer preferences to individual appliances. This advanced network will make it possible to lower the high cost of meeting peak demand and will support the incorporation of distributed and renewable energy sources [1]. Many utilities and other stakeholders believe that the development and implementation of energy storage at the distribution level is critical to creation of the Smart Grid. One utility that has taken a leadership role in the development and implementation of distributed energy storage specifications is American Electric Power (AEP). AEP has circulated for public comment a set of specifications defining a concept for distributed energy storage called community energy storage. In 2009, AEP and EPRI engaged in a stakeholder review process using these open source specifications as a starting point, developing a set of broader functional requirements based on stakeholder input. This input included a broad range of comments from potential users, manufacturers, and systems developers. Four webcasts were held, and many stakeholders contributed. The AEP community energy storage specifications are procurement documents; therefore, they contain many technical details specific to AEP s requirements [2]. The functional requirements in this report are intended to be more general so that they apply all utilities that have typical applications (that is, use cases and operating modes) for energy storage systems. This Project The Basics EPRI developed this project to bring together stakeholders in the development of stationary energy storage systems, solicit their input, and develop application requirements for energy 1-4

29 storage system solutions in transmission, substation, and distribution applications and for integration of large-scale renewables into power system operations. The project team believes that the application of energy storage to improve the flexibility of the grid to accommodate the penetration of renewable energy potentially has the greatest benefit to renewable energy project developers and power system operators alike. Energy storage can have value for specific customer installations, such as allowing a solar array to serve off-peak or nighttime loads. This project does not address customer-sited storage only utility-sited storage on the utility system although a system could be located near a customer facility and it could address the unique grid impact of a small group of customers. These functional requirements were developed through the consensus of utilities and other stakeholders that have participated in the process. Objective and Scope The objective of this project was to develop a set of functional requirements for energy storage systems to be used in specific ways (use cases and operating modes). The objective is for utilities and developers to work with a common understanding to develop the most effective storage solutions to utility problems by steering procurement and development efforts consistent with these requirements. The project scope consisted of adhering to the following set of guidelines. The process and this document were based entirely on utility requirements and not on the capabilities of underlying storage technologies. Although certain energy storage technologies might operate better in one application or area than another, it is not the purpose of this project to evaluate the suitability of specific storage technologies. Economic analysis of the applications was not performed. The process and this report address only high-level functional requirements and are not intended to serve as a complete technical specification for procurement. The ultimate list of requirements, including specific standards and other utility-specific requirements, will have to be specified by the utilities through its normal procurement process. Not all storage applications were included. Other variations, such as storage for the exclusive purpose of frequency regulation or seasonal energy storage, are possible, but they are not included in the scope of this project. This report focuses on a few high-priority applications identified by the utility participants. Energy storage at the customer side of the meter was not considered in this project. The project focuses, instead, on opportunities in utility system management. Tables 1-1 and 1-2 are provided to summarize how each application fits into the overall list of functional requirements being developed. These matrices are not intended to list all possible applications. Rather, they are intended to describe systems, their uses, and technical requirements as configured in each section. 1-5

30 Table 1-1 Matrix for Substation-Based Storage and Distributed Energy Storage Systems Applications Application Substation-Based Storage (see Section 2) 1 20 MW 2 6 hours Includes both stationary and transportable Distributed Energy Storage System (DESS) (see Section 3) kw (individual unit rating) Single-phase (25 75 kw) Three-phase (up to 200 kw) 2 4 hours Use Cases / Operating Modes (See Note) Peak load management Frequency regulation Capacity market (regional transmission organization [RTO] or independent system operator [ISO]) Voltage regulation/reactive power support Peak load management Backup power/islanded grid operation Voltage regulation/reactive power support Frequency regulation (may require aggregation) Capacity market (RTO or ISO) Interconnection Point Distribution voltage (4kV 34kV) Substation or feeder Secondary (customer) voltage Utility side of meter May operate as island Note: Use cases are listed in order of decreasing priority. Products need not meet all use cases. Notes Peak load management is controlled using substation/feeder real-time load signals. Frequency regulation controlled using signals from ISO. Although not currently in effect, a capacity market controlled using signals from ISO is a future option. Peak load management is controlled using substation/feeder real-time load signals. Frequency regulation controlled using signals from ISO. May be used in capacity markets. Reactive power dispatch based on local voltage. If only frequency regulation is desired, duration may be as low as 15 minutes. 1-6

31 Table 1-2 Matrix for Energy Storage to Integrate Renewables Application Use Cases/Operating Modes Interconnection Point Notes Solar PV Ramping Support (see Section 4) Power up to several megawatts (TBD by utility site) 1 second to 20 min (TBD by utility) Accommodate rapid power swings that would otherwise create disturbances on systems where highpenetration levels of solar PV systems are found Distribution voltage (4kV 34kV) Better manage the variability of solar active power output. Reactive power controlled based on local voltage. (This function is accomplished by the power electronics accompanying the storage.) Wind Ramping Support (see Section 4) MW 2 15 minutes Life equivalent to 10,000 full energy cycles Accommodate variable wind farm output so that ramp rates (MW/min) are kept within design limits Provide net load ramping support for the grid at large Maintain local transmission and distribution system voltage Provide frequency regulation Distribution voltage (4kV 34kV) Transmission voltage (>34kV) If local benefits for control area or a specific distribution feeder are required (as for a single wind farm), storage system must be co-located with the variable generation. If bulk power system benefits are desired, storage system can be located anywhere on the grid. Provide low-voltage ridethrough (LVRT) for wind farm (if required) LVRT support is built-in to most modern wind turbines. Load or Resource Shifting (see Section 4) Kilowatts to many hundreds of megawatts 2 10 hours Store energy generated during off-peak demand periods to serve loads during peak demand periods Participate in capacity markets as dispatchable energy and reserves Provide ancillary services Distribution voltage (4kV 34kV) Transmission voltage (>34kV) May be directly coupled and sized to local renewable resource or sized and operated independently. May also serve to reduce ramp rates from variable wind farm output and dampen solar PV ramping. In many cases, the working groups avoided the tendency to list all possible uses and focused instead on the one or two most likely uses given the size, location, and other characteristics of the system. In addition, the working groups recognized that some pairs of uses are mutually exclusive. For example, capacity allocated to longer-duration load following might not be simultaneously allocated to instantaneous frequency regulation. In such cases, the use cases are listed in order of priority. 1-7

32 It is true, however, that some equipment intended to serve one of the applications might also provide benefits described in another application. For example, storage designed for substation support might also provide local utility assistance useful for handling short-term ramps from a big box store s solar system. Approach and Process This project began with EPRI drafting a strawman application requirement, followed by an internal review, and a webinar with EPRI members to provide initial review of the strawman document. A series of webinars were conducted for review of the document by increasing numbers of utility and non-utility stakeholders. Stakeholders were identified through either prior engagement in energy storage activities, a position of significance for the project, or project members who were already involved. Participants were encouraged to join one or more application work groups. Five work groups were formed to scrutinize the functional requirements of each of the applications in preparation for application-specific webinars. Throughout the project, participants were encouraged to submit comments, additions, and specific wording to the project group for inclusion. Over the course of this project, more than 100 companies have participated in varying capacities (see Appendix D). This report is the result of six work group conference calls, nine webinar meetings, and a number of off-line inputs received directly from utility companies, vendors, independent system operators, regulators, government agencies, and universities. 1-8

33 2 SUBSTATION-BASED STORAGE Overview Description of Application Substation-based storage systems provide utility-controlled energy storage for any or all of the following: Peak load management Frequency regulation and area control Generation capacity Reactive power support Critical load support during outage (islanding) Systems should generally have a maximum power rating of 1 20 MW (charging and discharging) and the ability to store 2 6 hours of energy for on-demand delivery to the power grid. For frequency regulation and capacity markets, systems may be able to provide energy for shorter time periods. For peak load management to provide substation grid support, the minimum practical storage size is believed be about 1 MW for 2 6 hours, for 15-kV class distribution systems. For 25-kV and 35-kV classes, the minimum practical size of a unit is probably 4 MW. Products can be modular. Systems would connect at distribution voltage at the substation or feeder. Systems can also serve the purpose of renewable integration (see Section 4). Systems include stationary units and transportable units. Stationary units are physical assets sited at the substation or distribution feeder, with a useful service live of about 15 years. Transportable units are physical assets that also have a service life of about 15 years but that may be easily relocated from site to site as utility needs change, typically on a seasonal basis. They may be installed with minimal site preparation in a period of approximately one day and removed in approximately one day, with standard utility equipment such as cranes and lifts. Scheduled maintenance is expected, including replacement of a storage device (such as energy storage). Replacement schedules and costs must be understood in the purchasing decision. Figure 2-1 illustrates a typical substation-based storage application. 2-1