Energy Storage & Reliability

Foreword

Foreword

This professional reference is one of a series Energy Compliance, Inc. publishes for registered entities and the people who run their compliance programs.

I’ve spent more than thirty years on every side of the bulk electric system. I’ve operated control centers as a Reliability Coordinator, Transmission Operator, and Power System Operator. I’ve audited grid facilities and signed off on findings as a senior compliance auditor. I’ve worked enforcement matters from inside the regulator’s process. For the last several years I’ve advised registered entities directly through the firm I founded.

The entities that do reliability well share a common habit. They take the standards seriously without confusing them with reliability itself. They know that a NERC Reliability Standard is a floor, not a ceiling. They know that compliance is something an auditor evaluates, but reliability is something a system either delivers or doesn’t. They prepare for audits by building programs that survive real questions, not binders that look thick.

That’s the perspective these references try to share. Each one focuses on a single topic. A standard family, an operational function, a regulatory framework, or an emerging industry challenge. Each one walks through how the topic actually works.

These references are written for the compliance manager who wants to understand the system, not just memorize requirements. For the legal counsel who has to brief a board honestly. For the senior operator who’s been told that compliance and reliability are the same thing and suspects they aren’t. And for the new compliance hire who got handed a binder and told good luck.

These references aren’t marketing material disguised as content. They’re the result of three decades of doing this work and watching it succeed and fail. I’ve written them in the same voice I use in a control room or in front of a Regional Entity audit team. Direct, evidence-grounded, honest about what the standards do and do not require.

Energy Compliance exists because most of the consulting offered to registered entities today is structured for billable hours rather than for outcomes. Every engagement is led by one senior practitioner. We don’t bring five people to a meeting that needs one. We automate the work that should be automated. We apply senior judgment to the work that requires it. If that approach matches what you’re looking for in a compliance partner, the back of this reference has our contact information.

If not, the reference still belongs to you. Take what’s useful. Apply it well. And remember the only test that ultimately matters: when the system needs to perform, does it?

Rob Smith, Founder, Energy Compliance, Inc.

EC-WP-503 Energy Storage and Grid Reliability

Chapter 1

Energy Storage as a Reliability Resource Class

Battery Energy Storage Systems (BESS) provide a new and different level of resilience to the bulk electric system that does not align cleanly with traditional reliability definitions. The BES provides vast amounts of energy storage that does not generate primary energy (like thermal, hydro, or wind power), nor is it a consumptive load (like pumps or heaters). Similarly, the energy storage resource cannot provide passive support to the transmission system in the same way that lines, transformers, and switches operate in a manner that is independent of the state of the component. Instead, battery energy storage systems act as resources that are state dependent and operate through power electronic interfaces. Thus, the reliability contribution of battery energy storage systems is highly dependent on system configuration, control strategy, and operational conditions. From a reliability perspective, the main difference among resources (with the possible exception of hydro) is the Energy Storage (ES) ability to operate two different modes (i.e. more than one operating state per time period) as opposed to the mono-mode-of operation of most conventional energy resources. An Energy Storage System (ESS) can be functioning at one time in the manner of load and at another time as generation. While the time periods may vary depending upon circumstances, experience with Batteries (as an energy storage technology that is becoming increasingly common as part of innovative resources including flexible loads and microgrids1) indicates they can switch from charge (acting like a load to the Grid) to discharge (then acting like generation) and back again more than a dozen times during an eight-hour period. This alternating load and generation capability complicates a resource’s inclusion within the classical models or methodologies that are used for determining a resource’s availability or determining its ability to act as required when required. When an ESS is in the discharge mode it can effectively function like conventional generation as the current supplied to the Grid meets the needs of consumers. However, at the other extreme, while an ESS can function like any load when it is in the charge mode (since all loads create current as they consume energy), certain of its charged states can be very noncharacteristic, occurring during critical times when Grid stability is of high concern. The traditional reliability tools have been developed for relatively stable type of assets. In conventional power systems, generators were known for their capability within physical limits of the machines, and loads were relatively uncontrollable in aggregate. Demand response was introduced later as an exception. Energy storage systems challenge the traditional assumptions by being highly controllable in the control space (i.e. very quick response in injected and absorbed real power), but limited in the energy domain. The ability to respond fast does

not ensure high energy capability and reliability analysis needs to take into account both steady state and time limited capabilities. The BESSs are recognized by a new set of features that are reflected in their distinctive operation and integration modes. The first aspect is the exclusive use of power electronics instead of synchronous machines. This leads to new characteristics in relation to inertia, short circuit current and voltage control; while the system can be controlled to replicate some of the synchronous generation features, the corresponding effects are dependent on the control strategy implemented, as opposed to being natively provided by the electromagnetic interaction between the latter’s fields and armature. Consequently, the dynamic effects observed during the occurrence of a power disturbance are not always consistent with the nominal features listed in the corresponding technical data sheets. Energy storage has its own set of challenges related to aggregation and control. While battery storage systems can be very small compared to conventional generation capacity, when large numbers are connected to the grid through centralized control or other market mechanisms, they can pose a significant challenge to grid reliability. Reliability challenges related to energy storage aggregation include visibility, control, and correlation. With large numbers of similar energy storage resources connected to the grid in parallel, they all react to the same grid event, potentially having a large impact on the frequency or power flow. Battery energy storage system (BESS) flexibility is widely discussed as a reliability benefit, but it is important to recognize that flexibility alone is not sufficient for contributing to reliability. For flexibility to actually support reliability, it must be appropriately utilized given the real-time conditions of the power system, the operational perspective of utilities and grid operators, and the overall reliability performance goals of the system. A storage resource that is dispatched primarily for economic benefit may have very little capacity remaining in case of a grid emergency or event. Careful consideration must be given to how the resource is scheduled, reserved, and prioritized when performing reliability assessments of BESS. The analysis of energy storage as a resource raises a number of planning adequacy, deliverability and performance assumptions. The existing resource adequacy metrics were designed for conventional resources that have predictable output profiles. Battery energy storage systems have characteristics that are significantly different and therefore pose unique challenges: â€" A significant number of assumptions are based on the prior charging and cycling of the resource â€" Various capacity and duration constraints exist, such as the maximum number of charge/discharge cycles allowed to the battery as well as the minimum/maximum state of charge of the battery. The impact of these storage-related metrics and assumptions on energy storage’s ability to provide reliable supply during extended system stress conditions (i.e., multi hour or multi day events) remains to be seen. The following highlights that Battery Energy Storage Systems (BESS) are a class of reliability resources with materially different characteristics than traditional resources. Understanding these differences and their implications for treatment under reliability initiatives, operations, and planning is critical background material for the rest of the chapter. Some of the chapter’s key points are highlighted below. The reliability functions that energy storage can perform and its associated interactions within a power system are the focus of this chapter. End of Chapter Summary Battery Energy Storage Systems (BESS) provide a different type of reliability resource

and are characterized by two operation modes, power electronic conversion, a limited duration of energy supply and high levels of controllability. These characteristics do not align with the established reliability theory assumptions for generation, loads and transmission/distribution equipment. It is therefore important to acknowledge BESS as a separate resource category, in order to fully assess their impact on power systems and to ensure that their reliability performance can be adequately evaluated within established reliability methodologies.

FROM THE FIELD

A 100 MW battery isn't a 100 MW generator with a battery attached. It's a different resource class with different obligations, and the framework is still catching up.

Storage is the only resource on the grid that consumes its own primary fuel and produces it minutes later. The reliability framework was not designed around that profile.

Calling a BESS a "generator" is convenient. It is also wrong, often enough that the wrongness has shown up in audit findings.

Chapter 2

System Balancing and Frequency Response Considerations

The control of system balance and frequency is a fundamental reliability function of the bulk electric system. This is accomplished through real-time matching of supply and demand at various time scales from seconds to hours. Battery energy storage systems (BESS) interact with these functions in a different manner than conventional resources due to the unique characteristics of their response capabilities and energy storage duration. Battery energy storage systems (BESS) can provide fast changes in real power output in response to frequency deviations or dispatch signals. Therefore, when properly designed, configured and enabled, these resources can provide primary and/or secondary frequency control responses. From a reliability perspective, fast frequency response is most critical in the seconds following a disturbance when the frequency of the system is at its most unstable and therefore the most in need of control. Frequency response capability must be considered in conjunction with sustainability. Unlike a synchronous generator where the ability to sustain the output is determined by the available fuel supply and mechanical capacity, battery energy storage systems have limitations related to state of charge and discharge duration. Hence, a storage resource can deliver strong frequency response in the beginning of the disturbance but cannot maintain the frequency response if the frequency deviation does not return to nominal value within the allowed discharge duration. This consideration is more relevant to large contingency events as well as prolonged periods of system stress. Battery charging dynamics pose a new reliability challenge. Under low frequency conditions, a charged battery adds a small amount of incremental load, which can potentially cause further disruption to grid stability unless appropriately managed. Conversely, battery charging occurs during high frequency conditions in the grid when stabilization of the frequency is desired. Reliability planning for grid scale battery storage must therefore consider the effects of battery discharge and their charging dynamics. Factors that need consideration are when the battery is charging, as well as the flexibility that exists in terms of any required suppression or

adjustment of the charging current to stabilize the grid frequency during unstable conditions. This also includes battery energy storage systems (BESS) and automatic generation control (AGC). Energy storage resources participating in regulation services may experience a high cycle count. Managing the state of charge of the resources is a concern, as it can impact their availability. Again, the reliability concern is not related to providing the regulation service, but rather to the impact on being able to respond to

unforeseen events while still participating in the regulation service. Battery energy storage system (BESS) frequency response characteristics are set by control settings and operational instructions given by grid operators. In contrast to the inherent, physical characteristics of synchronous generators, storage system responses can be different from one BESS to another and are therefore a matter of assumption when modeling them in reliability analyses. Aggregation effects are also significant in the context of frequency response. A large number of energy storage systems (ESS) responding to the frequency deviation in a short time can cause large frequency changes. While the coordinated response can help in speeding up the frequency recovery, poorly coordinated or uniform tuning of control variables in ESS could lead to oscillations or other undesirable interactions. Battery Energy Storage Systems (BESS) can potentially and practically influence power system balance and frequency regulation, but only in a conditional and dependant manner. Consequently, reliability studies should account for both the positive and negative contributions of energy storage systems (ES) for this purpose, in terms of duration, charging characteristics and related synchro-nous behavior. End of Chapter Summary Battery energy storage systems BESs interact with system balancing and frequency control by providing fast dynamic control signal that is distinct from traditional power plants. BESs can provide the primary frequency response reserve in the short term but, owing to their limited energy, they require dedicated analysis to address reliability aspects. Interconnection requires a set of assumptions regarding the dynamic characteristics of BESs and their resource base and operational sustainability in addition to co-ordination with other control functions.

FROM THE FIELD

Balancing the system around storage is different from balancing it around thermal. Storage responds in milliseconds. Most operating practices were written for resources that respond in minutes.

The fast response is not all upside. A storage resource that depletes mid-event becomes the new contingency the BA has to plan around. Frequency response from storage is real, measurable, and increasingly relied upon. It is also conditional on state of charge. The conditionality is what the standards are still working through.

Chapter 3

Voltage Support and Reactive Power Behavior

Voltage control is a basic reliability function ensuring that normal operating conditions are maintained in the bulk electric system. Achieving adequate voltage control of the power system demands the integrated management of reactive power, voltage levels and circuit configurations, under both normal and fault conditions. Battery Energy Storage Systems (BESS) interact with voltage control via the power electronics, the characteristics of which are quite different from those of conventional synchronous machines. Batteries or battery energy storage systems (BESS) are typically connected through an inverter that can provide reactive current within a defined reactive current range. When properly configured and authorized, energy storage resources can help manage grid voltage. From a reliability perspective, the ability to provide reactive current through an inverter gives utility engineers more options for managing grid voltage, especially in areas with little or no synchronous generation. The reactive power capability of battery energy storage systems is limited by inverter ratings, thermal considerations and control priorities. Active power dispatch through charging or discharging can also further diminish the reactive power reserve. Therefore, reliability analysis must account for the interplay of real power management and voltage regulation, particularly during post-contingency scenarios where both functions may be in high demand. The dynamic response behavior of inverter-based resources during voltage disturbances is quite different to that of synchronous machines. Synchronous machines behave according to known physical characteristics (e.g. rotor impedance) that are in turn controlled by their excitation systems. Inverter-based storage resources behave instead in accordance with the operational control strategies that they are implemented with such as voltage ride through (VRT), reactive current injection or grid output reduction based on operational set points or protection. These differences may lead to significantly different system-wide dynamics. Fault conditions introduce a number of new considerations. The amount of fault current contributed by the inverter based resources is typically much lower than that contributed by synchronous generation. A lower fault current level may affect the operation of protection relays and fault detection. The reliability impact is greater as the majority of

the connected capacity is now composed of inverter-based resources, including energy storage systems. The local nature of the voltage support provided by Battery Energy Storage Systems (BESS) can be largely expected. Indeed, reactive power effects are weakened with electrical distance, and the primary effect on voltage of a storage unit is usually expected to be locally, near the connection point. This is to be

considered in conjunction with the coordination of the capabilities of storage, and with the planning of the electrical transmission network, which takes into account the possible modifications of the energy mix of the area in transformation. Operational visibility and control is a critical factor for the reliability contribution of storage related voltage support functions. Understanding the dynamic reactive power response of the storage resources under different operating scenarios and system conditions is necessary. In stressed system conditions lack of visibility to control modes and state of charge can further complicate effective voltage management. As covered in this chapter, battery energy storage systems (BESS) can provide a voltage support capability, though the reliability benefit of this capability depends on a range of factors including control methods, operating mode and time, and degree of coordination with real-time system needs. This capability should always be viewed as a complementary resource to other voltage control resources, and not a replacement. End of Chapter Summary Battery energy storage systems (BESS) interact with voltage control through the inverter based reactive power capabilities, which are quite different than those of synchronous machines. Thus, the voltage control contribution from storage is limited by inverter capabilities and operating conditions and settings. Therefore, reliability analyses of voltage control have to account for these BESS interactions, as well as the highly localized and programmable nature of voltage control from storage systems.

FROM THE FIELD

A storage resource provides voltage support through power electronics, not through machine inertia. The two are not equivalent and the protection scheme has to know the difference.

Reactive support from storage is dispatchable. That sounds like an advantage. It's also a control gap if the dispatch logic isn't tied to the operator's voltage strategy.

Voltage performance from storage looks fine in steady state. The reliability question is what happens during the disturbance, when the controls are doing the work the inertia used to do.

Chapter 4

Resource Adequacy and Duration Limitations

Resource adequacy relates to the adequacy of existing and planned conventional and non-conventional resources to supply system load, under various hypothetical scenarios such as peak load conditions, contingency events, and extended stress events. The Battery Energy Storage Systems (BESS) adequacy study is different because the capability of a BESS is not defined solely by its power rating, as storage has a limited energy capacity as well as limited charge and discharge capabilities. Battery energy storage systems (BESS) are a different breed of animal. Unlike traditional generation (which is typically evaluated based on installed capacity and forced outage rates) BESS has to be evaluated by instantaneous capability and long term reliability and performance. A storage resource rated to meet the peak of a short duration event may not be of much value during a longer duration system event if all of the energy storage has been depleted. This is an important consideration when considering the adequacy of energy storage resources. State of charge (SOC) is a key variable in adequacy assessments. The availability of a battery energy storage system at the time it is needed depends on past operating and scheduling decisions, market results, and charging opportunities. From a reliability perspective, unknown initial SOC adds to the uncertainty related to resource availability during unusual operating conditions or extended power outages. Duration capacity is particularly relevant during extended periods of high risk that last for hours or days, such as during extreme weather events, periods of low fuel supply and transmission constraints. Energy storage can play a role in the early stages of such events, but the resulting reduction in the duration capacity of the system may not adequately replace the capacity provided by the conventional generation or long-duration resources. Therefore, it is important to consider the relationship between storage duration capacity and the durations of high-risk events. Energy Storage Aggregation Relieves Some Duration Constraints, but Does Not Resolve Energy Constraints The aggregation of energy storage resources can provide some degree of flexibility against duration constraints; however, energy storage does not change the fundamental energy constraint. Battery resources can be synced across a portfolio of energy storage assets to provide a longer duration of service, which

can vary depending on system conditions, the availability of charging infrastructure and the level of coordination between resources. Reliability analyses examining the aggregation of energy storage require specific assumptions and validation. The modeling of battery energy storage systems in planning

studies is an area that is evolving. Historical models for evaluating potential loss of load and assessing resource adequacy are based on systems of relatively stable output capacity factors. Including storage in planning models requires additional variables and representations to account for the charge and discharge performance of storage technology, cycle life, and switch-off rules that vary by technology. The choice of specific modeling approach for energy storage can have a material impact on derived adequacy results. Batteries for Resource Adequacy - Conditional Contribution Battery Energy Storage Systems (BESS) can provide capacity resources and contribute to resource adequacy but their contribution to resource adequacy is always conditional. Shortterm versus long-term reliability evaluations are necessary to account for the different applications and the different characteristics of Battery Energy Storage Systems. End of Chapter Summary Battery energy storage systems impact resource adequacy by providing a short duration capacity response with limitations due to finite energy and charged reserves. In contrast to traditional generation, storage response is heavily constrained by its previous use and the magnitude of the event. Therefore, reliability assessments must be designed to account for these effects to appropriately value storage’s contribution to adequacy.

FROM THE FIELD

A storage resource is a power rating times a duration. Either side of that product alone tells you nothing useful for adequacy planning.

ELCC for storage is not a number. It's a function of the portfolio. The same battery accredits differently in MISO than in PJM, and that's not a calculation error.

Resource adequacy models built on thermal assumptions over-credit storage routinely. The next adequacy event will tell you how routinely.

Chapter 5

Protection Systems and Fault Response Implications

Protection systems are based on detecting unusual operating conditions and disconnect faults to ensure reliability of the power system and to prevent damage to individual components. The behavior of these systems is based on predefined knowledge of the electrical behavior of the components during such unusual operating conditions, i.e. the behavior of the fault current including its amplitude and direction. With respect to reliability aspects, Battery Energy Storage Systems (BESS) have to be taken into account since control and switching elements such as power electronic devices and switchgear can be a source of malfunction, and faults currents from the storage system itself are very small. Synchronous generators deliver fault current in magnitude and shape that is based on their design and size. Inverter-based battery energy storage systems typically provide controlled and limited fault current based on design, control strategy and protection settings, rather than electromagnetic characteristics of the system. So fault current levels and characteristics in grids with high penetration of energy storage systems are expected to be different than in the past. Low fault current from the grid can potentially cause issues for protection functions that are current based or rely on directional elements. Many of the newer protection functions have taken into account the inverter based resources. Legacy protection schemes were not designed to account for such low fault current levels. This issue is more relevant to the reliability of storage systems as they replace or support traditional synchronous sources. Battery Energy Storage System Behaviour During Faults This includes ride through and protection requirements, and how the inverter control behaviour such as reducing the output current, injecting reactive current and switching off the output in response to low voltage and exceeding protection levels impacts the post fault voltage recovery and system stability. Rounding out reliability analysis of energy storage systems this includes how the system is expected to behave during and following a fault occurrence. Interactions between storage resource protection and system protection must be managed appropriately. Inappropriate timing between inverter protection features and transmission/distribution protection relays can lead to unwanted tripping or fault clearance delays. Although this document does

not directly address protection design or settings, reliability implications of protection coordination should not be ignored. Aggregation effects again play a role in fault response. A large number of inverter based resources reacting in a similar way to a fault may result in effects on system behavior that would not have been present when analyzing a single facility. Examples: voltage effects from simultaneous

reduction of output from a large number of energy storage resources impacting system power flows during recovery from faults. This chapter examines the impact of Battery Energy Storage Systems (BESS) on how assumptions of fault response are challenged, not only through their contribution to fault current in a controlled manner but also through their protection response being configurable. Reliability analyses have to be adapted to consider BESSes in a system wide context without adopting assumptions that BESSes are identical to synchronous generation. End of Chapter Summary Battery Energy Storage Systems (BESS) impact protection systems by potentially limiting or severely restricting fault current contribution in a controlled manner depending on inverter performance. This can differ substantially from that of synchronous generation and can impact the ability to identify faults, achieve proper protection relaying coordination, as well as post fault system recovery. These impacts must be considered in reliability analyses as more systems incorporate energy storage.

FROM THE FIELD

Protection schemes were designed around the predictable behavior of synchronous fault current. Storage faults look different, and "different" is what causes misoperations.

A protection setting that worked for the synchronous era can underperform on a storage feeder. PRC findings on inverter-coupled storage are no longer rare.

If your protection engineer has not specifically studied storage fault behavior, your protection system is not yet properly coordinated for it. That's not a criticism of the engineer; it's the state of the practice.

Chapter 6

Operational Visibility, Control, and Coordination

Operational visibility and controllability are fundamental to reliable bulk power system operations. They provide essential information needed for system planning and operations under normal and disturbance conditions. Battery Energy Storage Systems (BESS) have introduced a number of reliability issues related to operational visibility and controllability, largely because operational state, availability, and the ability to respond to commands can change frequently, especially based on control strategies and current state of charge. Beyond just showing whether a battery energy storage system (BESS) is on line or off line, visibility of real time status is needed to a greater degree than generally realized. From reliability perspective, the key parameters needed for monitoring and visibility are state of charge, charge or discharge mode, active and reactive power ratings, and control mode. Without this visibility, system operators may incorrectly assess the availability and capacity of BESS during periods of peak system demand. Battery energy storage system control designs vary widely. Some energy storage resources are directly dispatched by grid operator, others are dispatched through market-based and automated and aggregative controls. The level of direct operator control over storage resources impacts their ability to reliably provide fault current following system event. Nondirectly controlled storage resources will inherently respond to design considerations for the dispatch regime they are designed for – whether that be price-based market signals or automated controls – and not to reliability requirements as they may arise in the event of a system fault. Scalability of resource coordination is a crucial issue as the scale of energy storage systems increases with their aggregation for economical and efficient participation in energy markets. Scalability may introduce visibility losses on individual behaviors and lead to unknown operational effects on local areas. The paper emphasizes the importance of control and response of the aggregated battery fleet on reliability issues related to dynamic operating regimes of power systems that storage resources can impact very quickly in a highly coordinated manner. State of charge management presents a different type of coordination challenge. Decisions made many hours in advance may be based on estimated customer loading hours ahead and may be based on assumptions as to

when the energy storage will be charged and/or discharged. In some cases, the fact that energy storage resources have been fully charged and discharged prior to hours of high peak margin may severely limit their availability to provide real time support in the event of a reliability event. Therefore, the relationship between normal operating conditions and emergency conditions must also be addressed

within the reliability framework. The Communication and Data Quality aspect also plays a role on the reliability contribution of battery energy storage systems. Delays, inaccuracies or loss of telemetry data can impact the confidence of operators in managing their resources and it can make decision making more complex, especially for high rate of change events. This aspect will be becoming more relevant as storage resources are expected to engage in real time operations more frequently. Chapter highlights The reliability value of Battery Energy Storage Systems (BESS) is heavily dependent on operational visibility and interaction. The reliability value of high capability resources is limited if their state of operation is not well understood by the system operator at the time of real-time operational need. End of Chapter Summary Visibility and control over operations of Battery Energy Storage Systems (BESS) is required to ensure reliable integration of these resources. Information on the state of charge, mode of control, level of aggregation and communication performance will all have a bearing on whether BES can be relied upon during grid events. Reliability studies need to account for the coordinating functions of the grid, in addition to the inherent capabilities of the resources.

FROM THE FIELD

The control center sees what the SCADA tells it. If state of charge isn't telemetered, the operator is dispatching a resource whose remaining capability is unknown.

A storage asset that the operator can't see in real time is a contingency the operator can't plan for.

Visibility into storage is not a nice-to-have. It is the precondition for coordinated reliability operation. Without it, the resource is a black box dispatched on faith.

Chapter 7

Planning Assumptions and System Modeling Considerations

Planning studies are fundamental to a number of reliability decisions related to transmission development, resource adequacy and system performance under contingency conditions. The incorporation of battery energy storage systems (BESS) into planning models is challenging due to their conditional nature, time dependency and the potential for operational controls that are not fully captured in existing planning models and frameworks. One of the many things that we must consider when designing a Battery Energy Storage System (BESS) is how we treat the charge and discharge characteristics of the battery. This is unlike traditional generation where the operation of the plant does not need to take into account the previous state of operation. When designing a BESS we need to have an opinion on when the battery is charging and when it is discharging and therefore how it will transition between states. These assumptions can have a material impact on the outcome of the analysis, particularly if the battery is being stressed for an extended period of time or if the analysis is being performed for a number of potential contingencies. Duration modeling is another important aspect of battery energy storage system (BESS) modeling. Battery energy storage systems are commonly characterized by a rating of the power that can be supplied or withdrawn, along with a discharge duration. From a reliability modeling perspective, the duration assumed for such storage systems has a material impact on the assessment of reliability, especially in studies related to supply adequacy and contingency. If the duration is assumed to be too short, the opportunity for storage to provide reliable supply during long duration events may be overstated. Conversely, assuming a more conservative duration for storage can result in it being understated in peak conditions. Another important aspect to consider is the interaction between energy storage capacity and transmission constraints. The charge and discharge behavior of energy storage is quite different from the behavior of existing generation units. planning models may incorrectly assess system conditions due to the fact that the impact of energy storage on transmission flows is often reversible.

During discharge, energy storage can reduce congestion, but charging can sometimes increase congestion to the same extent as the original congestion that was reduced by discharge. Modeling of inverter-based behavior is also needed. The stability and short circuit models we have always relied on have been developed primarily for synchronous machines and do not directly apply to power electronicbased resources with a very different control response and fault behavior and varied

interactions with other inverter-based resources, particularly in more heavilypenetrated storage systems. Despite advancements in forecasting methodologies, there are a considerable number of uncertain parameters that will have to be addressed during battery energy storage systems (BESS) planning. Future operation practices, market design and control strategies may undergo several changes during the life span of BESS projects. As such, reliability planning activities need to strive to achieve a level of tradeoff between the level of granularity of information required for assessment of future potential for battery participation, the number of uncertainties that are formally considered, and the amount of judgment exercised during analysis. Of particular importance will be ensuring that any number of assumptions made are fully characterized and acknowledged as such, with any resulting analyses carried out with full acknowledgement of the basis and limitations for any findings reached. This chapter highlights the potential for significant variability in determining the reliability contribution of battery energy storage systems within planning models and studies. This variability underscores the importance of model assumptions and limitations and the need for transparent communication in order to validate planning results and ensure that they accurately depict system reliability and capacity. End of Chapter Summary Battery energy storage systems pose significant challenges in reliability planning, which are largely attributed to the conditional nature of their operation, their operating limits in terms of both the time required to charge and their ability to reverse power flow. Assumptions related to charge characteristics, operational duration, inverter dynamics and uncertainty must be carefully considered in order to ensure valid results in all analyses that include storage. Consideration of these assumptions ensures that reliability analyses utilizing storage include only information for which the validity of the analyses can be established.

FROM THE FIELD

Planning models that don't represent storage controls accurately will produce planning results that don't match operations. The mismatch shows up the day the model assumed something the controls didn't actually do.

Generic storage models are improving, but generic isn't accurate. Project-specific models matter for reliability studies, especially as storage scales.

The hardest planning question for storage isn't capability. It's correlation: when the system needs storage most, will all the storage in the portfolio be available? The model has to ask that question.

Chapter 8

Event Analysis and Disturbance Performance

Event analysis is a key reliability analysis for understanding events on the power system. Understanding how disturbances occur and how resources perform is crucial for understanding the risk to the system. Battery energy storage systems (BESS) pose challenges related to event analysis that are different from those associated with the expected behaviour of synchronous generation and static loads. Battery energy storage system (BESS) dynamic response to disturbances is dictated by control strategies, protection settings and operational mode at the time of the event. Unlike traditional power generating resources where the dynamic response is dictated by physical laws, energy storage systems can instantaneously and/or limit their response or even come out of control in the event of changes in set points. Event evolution can be affected even if the energy storage is not the source of the initial disturbance. The state of charge (SOC) of a battery (or energy storage resource) at the time of an event is an important parameter for the analysis of various EV EV events including fires, explosions and electrical hazards. A fully charged energy storage resource will react or perform differently than one that is partially charged or fully discharged. Understanding the pre event operating conditions of a battery is critical for analyzing its performance at the time of an event and for evaluating whether its reaction is expected given its known design and performance characteristics. Many disturbance performance standards and criteria were developed with synchronous machines in mind. Given the growing role of inverter based resources such as battery energy storage systems, it is increasingly important that event analysis considers the potential for different ride through behaviors, different reactive current responses, and different recovery characteristics following a disturbance. Incorrectly assuming inverter based behaviors can lead to incorrect conclusions about an event, and potentially misinterpretation of important related data. Similar to voltage instability, aggregation affects should be borne in mind when analyzing power system disturbances. Several storage resources controlled in a similar way can synchronously respond to the disturbance and hence affect the system in a way that the impact of any single storage resource appears as minor. The event analysis in this case should be performed at the system

level. Data quality, particularly in terms of amount and granularity, can greatly affect the analysis of storage related phenomena. Fast dynamics in inverter behaviour and control actions are difficult to capture in the usual monitoring frameworks. Lack of insight in inverter and aggregated resource control

is a fundamental barrier to getting a detailed understanding of the phenomena of interest. The analysis of Battery Energy Storage Systems (BESS) is discussed in this chapter as being a sensitive topic during the analysis of disturbances. The characterisable programmable and state dependent nature of the BESS requires more detailed investigation than traditional components and it highlights the need for a more comprehensive examination of the control and switch logic, including aspects related to BESS aggregation and pre-event conditions in order to fully understand the effects on reliability performance. End of Chapter Summary Battery energy storage systems have a significant impact on system events in a dynamic manner, primarily through their ability to quickly and as needed, based on operating mode and other control settings, engage and respond to events. Event analysis for these systems, including consideration of the effect of aggregation and other potential data limitations, is required to properly examine the dynamic behavior of batteries during major disturbances and to determine their effect on the power system.

FROM THE FIELD

A storage resource that trips during a disturbance leaves the system without the contingency reserve it was counted on for. That's not a routine forced outage. It's a reliability event.

Disturbance analysis on storage requires data the asset owner has and the operator usually doesn't. Coordination on data sharing is now part of the EOP-004 conversation.

If the post-event analysis can't reconstruct what the storage did and why, the lesson is incomplete and the next event will be the same.

Chapter 9

Reliability Boundaries and Evolving Frameworks

Reliability frameworks define the boundaries within which planners, operations personnel, and regulators perform their tasks. They are meant to be robust, technology neutral and dynamic. Battery energy storage systems (BESS) test these features for several reasons. Firstly, they do not fit easily into the traditional categories of generation, load or transmission. Secondly, their performance in each mode is highly dependent on the specific operational scenario and less on the inherent physical capabilities of the resource. This entry discusses one of the many reliability boundary questions related to battery energy storage systems (BESS). It revolves around determining the performance of batteries over various reliability functions. In power systems, a storage resource may satisfy one reliability function while negatively affecting other reliability functions at the same time, depending on the operating mode and system state, which indicates a multidimensional resource characteristics that cannot be handled by simple categorizations. Increased focus on system strength, stability and grid resilience, especially with respect to high penetration of inverter-based technologies, including energy storage technologies has motivated this work. This work does not propose a new set of reliability metrics, but rather illustrates that the conventional understanding of inertia, fault response and dynamic control will likely change with the evolving mix of grid resources. Battery Energy Storage Systems are considered in this context, both in relation to traditional reliability services, as well as their involvement in the evolving systembehaviour. Another boundary that needs to be considered is between capability and obligation. While battery energy storage may have the capability to provide certain reliability services, it is not necessarily assured that such services can be activated at the right time, as this will also depend on market and operating rules, as well as on contractual agreements. Reliability frameworks need to recognize the difference between the theoretically possible and the rule-bound required. Boundary questions for battery energy storage systems remain The jurisdictional and functional boundaries identified in prior Focusing Facts continue to be relevant. In particular, BESSs can be connected to the transmission or distribution networks, and can participate in wholesale markets. their control may

not necessarily reside with entities located within the traditional operational hierarchy of a utility. Thus, new visibility, coordination and accountability issues have been raised that extend beyond the individual asset and into the broader context of reliability governance. Reliability modeling is becoming more

complex and therefore more consistent and clearform expression is required. Any incremental changes to the modeling process, operating procedures or evaluation methods are required to be clearly justified with identified assumptions and supported by analysis of system behavior. Battery energy storage systems are one of the areas where the relationship between the underlying reliability theories and practices and the real world systems behavior and operating practices needs to be closely examined. Energy Storage is not about redefining reliability. Rather it is a matter of how the current concepts of reliability are implemented. What is needed is to understand the boundaries and underlying assumptions of reliability metrics, their interpretation, and how energy storage can be implemented to enhance reliability achievement rather than obscure it. End of Chapter Summary The fact that battery energy storage systems can act both as generation and load, in addition to their variable operation mode under different conditions, and that their operation is heavily influenced by the surrounding system, all pose significant challenges to conventional reliability concepts. Although existing reliability concepts are still applicable, there is a need for careful consideration to distinguish between must-run, discretionary and optional capability as well as for the understanding of multi-dimensional resources. Consistency and clarity is also important as reliability concepts are further refined to accommodate higher penetration of energy storage resources.

FROM THE FIELD

Storage doesn't fit cleanly as generation, load, or transmission. The framework's response has been to treat it as a hybrid, which works until the hybrid framing produces a gap.

The reliability framework is technology-neutral by design. Storage is a stress test of how neutral the design actually is. So far, the neutrality is holding, but the seams are showing.

Storage is the leading edge of how the framework will accommodate every emerging asset class. The lessons from storage will travel.

Chapter 10

Reliability Risk, Resilience, and System Dependency

Reliability risk assessment encompasses more than just meeting current conditions at the time of the assessment. It is about how the bulk electric system will perform under a wide range of potential stressing conditions, such as unusual, uncertain or emergency events. Battery energy storage systems affect reliability risk and system resilience in ways that are generally more related to factors of dependency, coordination and duration as opposed to isolation and the ability of individual resources to meet current regulatory performance standards. Battery Energy Storage System (BESS) reliability risk identifies a number of issues that must be considered to ensure reliable operations. For instance, storage resources are not always able to always provide energy at times when it is most needed, and have various dependencies that must be considered, such as: The availability of charging energy resources. Adequate access to the transmission system to evacuate the electrical energy. Effective communication of control signals through information technology resources, both for charging and discharging. Dependence on various external factors to enable stable and efficient operations. A particular concern is that storage resources and their associated infrastructure can be rendered ineffective for providing emergency supply reserves in the event of extended outages that occur, such as those caused by catastrophic weather or extensive transmission failures. Therefore, in assessing the ability of storage to operate when it is most needed, the operational readiness of the various associated resources must be considered, along with the storage resource itself. Battery energy storage systems can provide short term resilience during transient disruptions by offering almost real time support to mitigate the impact of the transient. While the short term resilience benefit of batteries provides quick support to the system to mitigate and stabilize the transient, the benefit of batteries on long-term resiliency is limited to the duration for which the batteries can supply energy. Once disruptions last longer than the battery duration, the system’s long-term resiliency is determined by other backup resources with sustained operational capabilities, by diversity of fuel supplies and by the order in which restoration is implemented. Hence batteries might only affect the initial phase of system restoration. Cyber and communication dependencies can also impact reliability risk. Battery energy storage systems utilize digital control, communication links and automation in order to ensure high availability, rapid response and maximum flexibility. While these features provide many benefits, such as real time monitoring, the loss of data or system availability can result in a loss of visibility and control during a disturbance thereby potentially reducing or eliminating the value of the storage asset to the grid during these critical times

and therefore it should be considered when evaluating reliability risk. Finally we also have Correlated performance. One aspect of system resilience is the fact that systems have the ability to mask variability or unavailability of resources from other parts of the system. Correlated performance could impact reliability of systems such as shared storage systems which use similar hardware, management systems or operational practices. While high levels of homogeneity in system design practices can bring benefits such as lower cost of acquisition, easier resource administration, better technology integration, improved application support, and higher resource utilization, they also may increase risks such as greater vulnerability to malware outbreaks and the triggering of the same control actions in response to different sources of disruption in different storage resources. Both Battery Systems and Charge Storage devices present a system dependency perspective, and the interaction with markets is an important issue. An Economic Dispatch that corresponds to periods of high demand for reliability purposes, ends up affecting the State of Charge (SoC) of the storage resources of the system. The incompatibility between market signals and system reliability could compromise the availability of storage resources when they are needed the most, and would therefore need to be fully automatically managed by the system to avoid overloading other resources. This chapter reminds the reader that BES will experience a change in reliability risk and resilience conditions due to energy storage system properties, not necessarily due to capabilities, but also due to the limitations and dependencies. In order to better understand how storage can contribute to reliable and resilient BES, the dependency aspects must be taken into consideration. End of Chapter Summary Battery Energy Storage Systems (BESS) can impact reliability risk and resilience through charged energy requirements, communication, control systems and synchronization. Short term reliability enhancement is possible with

storage but their impact is limited for extended outages due to duration and supporting infrastructure requirements. Risk assessments and analyses need to account for BESS dependencies.

FROM THE FIELD

Storage shifts the reliability risk profile. It removes some risks and creates new ones, and the entity that adds storage without re-running its risk assessment is operating with stale assumptions.

Resilience from storage is real. So is dependency. The grid that leans hardest on storage is the grid most exposed to a coordinated storage failure.

Asset owner risk and system risk are not the same risk. The asset owner sees revenue exposure. The system sees cascading exposure. Both views matter.

Chapter 11

Integration Trends and Reliability Implications

Battery Energy Storage Systems (BESS) continue to expand within the bulk electric system, both in terms of size, location and functional applications. Reliability implications associated with the deployment of such systems reach beyond the individual BESS project, reflecting the more fundamental transformations underway in the make-up, operations and planning of the transmission and distribution infrastructure of the power grid. Understanding these trends and patterns provides valuable context and perspective to evaluate the reliability significance of battery storage in the BES. Another trend we see is that batteries are being deployed in conjunction with many other resources, such as inverter-based generation. When these systems are co-located, they may not be modeled in the same way to power system planners and operators, and the effect of each element on the overall system behavior may not be clear. From a reliability perspective, understanding the availability and performance of such integrated resources under normal and disturbance conditions, and how they can be called upon to meet system needs is important. Battery Storage Trends affecting High Voltage Reliability As we look to the future of the power system, Battery Storage is becoming a ubiquitous component. More systems are incorporating battery storage at a multitude of voltage levels, including transmission connected, distribution connected and behind the meter. Although this publication primarily focuses on high voltage reliability issues, the addition of distribution connected battery storage can also influence high voltage transmission level reliability through aggregated effects. Therefore, Reliability studies will also need to account for the less visible affects of distributed resources such as battery storage which are not necessarily readily accounted for within the traditional transmission based high voltage reliability analysis tools. Market participation models for batteries are also an important consideration. We see storage providing services across multiple market segments including energy, capacity and ancillary services markets. While being able to serve multiple markets can bring flexibility and revenue opportunities to battery storage, it also adds complexity and uncertainty as to which specific reliability services (or sets of services) the resource is expected to provide at any given time, and how these simultaneous commitments may impact availability in the event of a surprise event. Geographic concentration of storage resources is another factor. Storage could be deployed in certain regions for economic, transmission, or regulatory reasons. Concentration of storage resources can have positive and negative effects. A coordinated response from storage in a concentrated region can provide reliability benefits at the local level. However, the potential for correlated ratings or outages could create reliability vulnerabilities at the same level. Storage

integration operations are evolving, and more experience with various storage technologies is being gained. Operation and planning assumptions and expectations are changing as more knowledge of their reliability and performance is gained. The changed operating practices may make past performance unreliable as a guide for future system reliability studies. Care must be taken when using past performance data in order to account for changes in how storage is integrated into the power system. Battery Energy Storage Systems Reliability Concepts and Challenges Chapter 5 emphasized that the reliability implications of battery energy storage systems are shaped by more than just the characteristics of the resources themselves. They are influenced by the ways in which those resources are aggregated, sited, used in markets, and operated on the grid. The chapter analyzed trends in those areas and concluded that battery energy storage systems system reliability planning will need to be undertaken on a holistic basis that accounts for all of these factors. End of Chapter Summary Battery Energy Storage Systems BESs integration trends such as co location aggregation and market participation including potential geographic concentration have material reliability implications that are more than just a function of individual component design. Visibility into coordination among systems and overall system behavior is changed. Reliability assessment must begin to account for new integration trends to fully understand their implications on system reliability. HAPTER 12 Summary of Reliability Considerations for Battery Energy Storage Systems Battery Energy Storage Systems (BESS) have become a material resource in today’s Bulk Electric System (BES) and interact with nearly all reliability functions. Batteries are used in storage systems as the storage medium and the systems allow the electrical energy in the battery to be controlled in output to meet power requirements. The increased reliance on storage technology is a function of changes in generation and operation, as well as participation in wholesale electricity markets. Thus, it is necessary to consider Energy Storage Systems in a reliability analysis using a system wide perspective and relying on the basic principles of reliability as with all other resources. The Battery Energy Storage Systems (BESS) publication examined the role of battery energy storage systems as a new resource class with significantly different behavior to other resources on the power system. The key distinguishing characteristics include operation in two modes, a limited duration of energy, power electronic conversion and high degree of automation with logic control. These characteristics impact the interrelationship of battery energy storage systems with power system functions related to balancing, frequency response, voltage control, protection and resource adequacy. One theme that was prevalent throughout this effort is conditionality. The reliability capacity of battery energy storage systems (BESS) is a conditional quantity as it varies based on a large number of variables such as state of charge, mode of operation, level and type of control, whether they are operating in isolation or in clusters, and the specifics of any operating instructions provided by grid management and utilities. Simply having capacity does not ensure that the capacity will be available or be used in the optimal manner during times of grid reliability stress. Planning assumptions about how an energy storage resource is charged and controlled during normal times, dispatched during transient periods and prioritized during significant disturbances can affect the validity of resulting planning studies, operational assumptions, as well as the resulting

conclusions associated with grid disturbance events. Battery energy storage systems (BESS) add reliability risk and additional considerations for the resilience of the power system. Charging energy is required for long term operation, as well as reliable communication links, and associated system hardware. The Battery Energy Storage Systems (BESS) will be able to provide rapid response and high levels of variability to short duration transient events; however, long term system stability and outages will be limited by the duration of the battery life and associated system dependences. The fact that Battery Energy Storage Systems (BESS) need to be integrated into existing reliability frameworks does not mean to say that existing reliability standards and practices need to be substantially changed. Rather it is a matter of disciplined application of those standards within well defined boundaries, assumptions and interpretations. The presence of storage will test the flexibility of categorizations in reliability analysis but, underlying them all, the fundamental reliability goals remain the same. While perhaps intuitive, it is important to reiterate in this chapter that battery energy storage systems (BESSs) are not reliability resources designed to be substituted for traditional resources in any particular manner, nor are they exotic systems that require a new reliability theory with new reliability rules to apply in order to provide reliability benefit. Instead, their successful application depends on an understanding of how

their idiosyncratic properties impact and interact with traditional reliability concepts and system states. End of Chapter Summary Battery energy storage systems (BESS) can impact reliability of the Bulk Electric System (BES) in three stages: planning, real-time operations, and during disturbances. Any BESS impact is typically conditional, state dependent, and a function of the way the resource is integrated into the system and the level of interdependency with other components. Traditional reliability analyses and metrics can still be applied, but will require caution and clarification of assumptions and interpretations in order to effectively assess and understand the reliability value of energy storage resources.

Glossary

Glossary

Battery Energy Storage System (BESS): A Battery Energy Storage System is a facility capable of storing energy in electrochemical batteries and providing real and reactive power by means of power electronic devices. - Limited by energy duration and charging capacity. They typically operate in a state dependent fashion.

Bulk Electric System (BES) - Electric energy facilities and lines, including associated transmission and transformation equipment and control systems, which are needed to supply, transfer, and regulate electric energy for operation of the interconnected transmission system and certain parts of the transmission and generation sides of the interconnections with the generators. (Definition of North American Electric Reliability Corporation)

Frequency Response - The ability of the bulk electric system and its resources to respond to a change in frequency resulting from an imbalance between supply and demand. Typically refers to the ability of the grid to arrest and stabilize changes in system frequency following an imbalance between supply and demand.

Reactive Power - The unused or extra voltage supporting function of AC that enables utilities to maintain voltage on their transmission and distribution lines, and does not do any useful work on loads.

Resource Adequacy (RA) The ability of the bulk electric system to provide sufficient resource capacity to meet projected electricity demand and to provide reliability within a planning horizon considering forecasted outages, including consideration of system dynamic conditions.

State of Charge - This is a measure of the level of charge stored in a Battery Energy Storage System (BESS) at any given time, relative to its fully charged capacity.

Voltage Support refers to the Reactive Power and Voltage Regulating capabilities that are provided in order to maintain adequate voltage levels on the bulk transmission system during both normal and abnormal voltage conditions.

variety of functions including: Bulk Electric System (BES) operations, reliability coordination, regulatory compliance, and cybersecurity reliability. He has many years of practical experience with real-time transmission and reliability operations, including acting as a Reliability Coordinator, a Transmission Operator and a Power System Operator for several regional transmission organizations and utility control

centers. In addition to operational roles, Mr. Smith has engaged in regulatory and oversight activities including work as a senior compliance auditor and subject matter expert for NERC Reliability Standards. He has audit experience related to grids of varying complexity, involving evaluations of mitigation programs, self-reporting, and responses to ERO audits and risk-based oversight. He has also been involved in enforcement activities using risk-based oversight methods. Material reproduced here from Reliability Standards publications is provided for educational and professional use only. The author of this document is solely responsible for its content. The author does not represent NERC, FERC or any NERC Regional Entity.

About the Author

About the Author

Rob Smith is a senior electric industry professional with over thirty years of experience across every major function of the North American Bulk Electric System. His work spans reliability coordination, transmission operations, regulatory compliance, and cybersecurity reliability.

Rob has worked directly in real-time grid operations as a Reliability Coordinator, Transmission Operator, and Power System Operator within RTO/ISO and utility control center environments. He has also held senior regulatory and oversight roles, including senior compliance auditor and subject matter expert for NERC Reliability Standards. In those roles he audited grid facilities for compliance with applicable standards, evaluated the adequacy of mitigation actions, supported the development of violation notifications and settlements as part of FERC-directed enforcement actions, and participated in risk based oversight of utility mitigation activities.

Rob founded Energy Compliance, Inc. to bring senior, regulator-side compliance authority to registered entities directly, without the layered staffing, billable-hour overhead, and generalist advice typical of larger consulting firms. Every Energy Compliance engagement is led by Rob personally.

About Energy Compliance, Inc.

About Energy Compliance, Inc.

Energy Compliance, Inc. is an independent consulting and advisory firm focused exclusively on electric reliability, cybersecurity reliability, and regulatory compliance for organizations connected to the North American Bulk Electric System.

Our work supports registered entities, including Generator Owners and Operators, Transmission Owners and Operators, Reliability Coordinators, Balancing Authorities, and Distribution Providers. We work across NERC Reliability Standards, FERC orders, RTO/ISO market participation rules, Regional Entity oversight, and state regulatory frameworks.

We do this work differently than larger consulting firms. Engagements are led by a single senior practitioner with regulator-side experience. We don’t staff for billable hours. We staff for outcomes. Our deliverables are written to be operationally executable and audit-defensible, not to manufacture activity. Where automation can replace manual work, we build the automation. Where senior judgment is required, the senior is in the room.

Energy Compliance is not affiliated with, sponsored by, or endorsed by the North American Electric Reliability Corporation, the Federal Energy Regulatory Commission, or any Regional Entity.

Services Provided

Our services are written to be clearly defensible. Operationally executable in real time. Audit-defensible at compliance review. Every deliverable is structured for the auditor’s question, not the consultant’s binder.

Energy Compliance services include, but are not limited to:

• NERC reliability and compliance advisory support
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• Compliance program design and improvement
• Audit and enforcement support (non-advocacy)
• Mitigation planning and Self-Report development
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• Regulator-perspective program reviews

Each engagement is scoped to the entity’s role, function, and bulk system impact.

ENERGY COMPLIANCE PROFESSIONAL REFERENCE

Rigorous Compliance. Defensible Programs. Energy Compliance, Inc. partners with registered entities on the institutional and technical questions that define strong reliability and cybersecurity programs, from classification through audit through enforcement response.

N ERC CO MP LIANC E S ENIO R ADV ISO RY Program support, interpretation, and audit Direct engagement on complex reliability preparation. questions.

I ND USTRY ENGAGEMENT AUD IT D EFENSE Standards development and working-group Notice of Penalty response and settlement participation. posture.

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