Inverter-Based Resources (IBR) in the Modern Grid

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.

ENERGY COMPLIANCE, INC. EC-WP-011 · APR 2026 Generation technologies and integration reference

RESOURCES SERIES · EC-WP-011

Inverter-Based Resources What changes when the synchronous machine is no longer the dominant grid resource, and what reliability standards now demand.

Chapter 1

The Emergence of Inverter-Based Resources in the Modern Grid

Disclaimer and Notice This publication is an independent educational resource developed by Energy Compliance, Inc.

The North American Electric Reliability Corporation (NERC), the Federal Energy Regulatory Commission (FERC), and any NERC Regional Entities are not affiliated with, do not sponsor, and do not endorse this publication or its contents.

This publication is not intended to constitute official staff guidance, interpretation or policy from NERC or FERC or any regulatory commission, nor shall it be deemed to establish, change or alter in any way the requirements of applicable laws, regulations, or NERC Reliability Standards.

This publication is provided for general information and educational purposes only. It may not be used or referenced as conclusions in legal or regulatory proceedings. It does not offer guidance on implementing specific regulations or recommend measures to prevent or mitigate cyberattacks. Likewise, it does not contain engineering advice, operational instructions or compliance recommendations.

This standard does not apply or impose specific requirements for ensuring reliable grid operations or effective cybersecurity measures. Achieving compliance with NERC Reliability Standards is not a direct or guarantee indicator of grid reliability or effective measures to prevent or mitigate Cybersecurity threats. Achieving compliance with NERC Reliability Standards is contingent upon a range of technical, management and external factors that are not provided in this document.

Energy Compliance, Inc. assumes no responsibility or liability for errors, omissions, or outcomes resulting from the use of this publication.

Unless otherwise specified, NERC and related terms are trademarks of their respective owners and are used here only in a descriptive sense.

The North American bulk electric system has undergone significant structural change in recent years. For many decades the system was designed, operated and reliability planning based on a synchronous bulk power plant connected by rotating machine impedance. This resulted in inherent system characteristics including system inertia, short circuit levels and voltage regulation.

Conventionally, synchronous machines (such as steam turbines, hydro-turbines, and permanent magnet synchronous generators) provide the interface between generation and the bulk power system. With the increasing adoption of inverter-based resources, this interface has changed. Instead of a synchronous machine, the interface is provided by a power electronic inverter that converts the DC or AC (of varying frequency) input into a grid frequency and phase-matched AC output. This new interface brings with it a host of new system behaviors that differ from the traditional synchronous machine interface. As such, conventional practices, reliability standards and planning models need modernization.

Inverter-based resources have proliferated over the past decade due to a multitude of factors including rapid progress in power electronics, decreasing costs of wind and solar equipment, policy changes and shifts in the power market. Large wind farms and solar photovoltaic projects, battery energy storage systems and a host of distributed energy resources including diesel, micro-hydro, fuel cells and others comprise an increasingly large portion of installed capacity at the utility level across North America. within certain balancing authority areas and transmission planning regions, inverter-based resources are rapidly becoming the largest source of new generation added to the grid.

The impact of increased penetration of inverter-based resources on system reliability has become a topic of great interest and scrutiny, with NERC, the Regional Entities and utilities exploring the implications of the changed behavior of these systems in great detail. There have been numerous NERC Technical Reports and Reliability Assessments published by NERC and the Regions over the years addressing issues relating to frequency response, voltage and protection and restoration. Many of these analyses support the conclusion that inverter-based resources, in and of themselves, do not lower reliability. Rather than negatively impacting reliability, inverter-based resources simply require accommodation in existing reliability rules and practices.

One of the hallmarks of the inverter-based resource (IBR) technology that is transforming conventional power systems is that the control function replaces what used to be electrical and mechanical inertia. In synchronous systems, frequency and voltage behavior are direct consequences of the physical properties of rotating machinery. In

IBRs, the frequency and voltage behavior is governed by digital control algorithms that are designed to sense the grid state and to receive directives from the control systems to the power electronics switches in the inverter. The consequences to power system reliability are multifaceted.

NERC technical studies have identified several instances where the operation of inverter-based resources (IBRs) played a role in unexpected grid performance during disturbances. These unexpected outcomes involved momentary shutdown, tripping, or other control interactions that were not fully contemplated by planning and operational personnel. These field experiences reinforce the need for understanding how large numbers of inverters will act together to meet the challenges and uncertainties of high penetration grid operations, as opposed to focusing on the behavior of single resources.

Inverter-based resources can provide reliability services traditionally provided by conventional synchronous generation. These resources can provide frequency response, voltage and reactive control if proper controls are implemented. NERC and the industry agree that proper interconnection requirements, performance criteria and Reliability Standards must be in place to enable such capabilities.

The advent of inverter-based resources has modified and extended the scope and focus of standards development. In addition to inclusion in various standards relating to modeling, protection coordination, and planning analysis, NERC has addressed performance of inverters in certain standards and policy statements. In addition, the NERC Reliability Guidelines and Reference Materials include documents developed to achieve common understanding and initial uniformity of treatment of inverter effects in particular areas without imposing specific engineering criteria for designs which are in conformity with such understandings.

The transition to an inverter-based grid is not happening uniformly across the bulk power system. Factors such as regional resource mixes, transmission networks, markets and interconnection processes all vary and can impact the level of exposure to inverter reliability challenges. While there is a clear need for region-specific analysis to support the transition, there is also a need for an integrated continental reliability framework. NERC recognizes this important balance of interests.

Understanding inverter-based resources in the grid goes far beyond simply understanding the power electronics that are involved. It is essential to couple that technology with an understanding of established reliability principles, with how inverter

based resources are modeled and accounted for in planning and operations analyses, and how grid operations are changing in order to ensure reliable bulk power system operations.

Comparison of SG Interface to Inverter-Based Resource (IBR) Interface to the BES Fig-001

Chapter one established why inverter-based resources have become a focal point of discussion in the North American power delivery reliability community. The subsequent chapters in this book will provide further background by describing the technical aspects and system-level impacts associated with inverter-based resources, and then reviewing reliability criteria and standards related to these new resource types in an objective and technical manner.

End-of-Chapter Summary

Inverter-based resources represent a paradigm shift in the manner in which resources connect to the bulk power system. Operation and planning of the grid with significant amounts of inverter-based resources is changing in many ways when compared with the traditional resources of the past.

Understanding these changes, developing analysis tools to manage these changes, and ensuring that procedures and standards are updated as necessary to properly manage the new connection points is critical to shaping the future policy and procedures for managing a high penetration of inverter-based resources. While inverter-based resources are by no means a reliability impairment per se, it is critical to understand and manage their new set of parameters to ensure that the Grid operates in a reliable fashion.

FROM THE FIELD

Inverter-based resources changed the structural composition of the BES. The standards are catching up, and the ones that don't catch up will be revised under enforcement.

Synchronous machines defined the grid for a century. Inverters are redefining it inside two decades. The reliability framework hasn't moved at that speed before.

The rotational machine era is ending. The power electronics era is beginning. Reliability has to be true in both.

Chapter 2

Fundamental Characteristics of Inverter-Based Resources

Inverter-based resources are characterized by their method of connection to the grid rather than the energy source itself. Regardless of whether the energy source is wind, or solar, or energy storage or other forms, inverter-based resources always include power electronic equipment in the form of the inverter used to synchronize the real and imaginary components of current and voltage to grid specifications. In contrast to the more familiar characteristics of synchronous machines, inverterbased resources utilize fundamentally different design and operating principles. In essence, an inverter is a controlled AC or voltage source. Unlike a synchronous generator that produces AC due to mechanical rotation of the rotor, an inverter produces AC by switching the semiconductor switches in accordance with a pre-determined control strategy. The control strategy in a resource inverter dictates how the resource tracks changes in voltage, frequency and phase angle. One of the defining characteristics of inverters has significant implications for the behaviour of the grid when inverters make up a large portion of its generating capacity. The lack of true synchronism between an inverter and the grid means that there is no direct connection between the grid and physical rotating inertia in the form of synchronised rotating mass. With a synchronous machine, the stored energy of rotation provides shaft inertia that resists rapid changes in frequency because the machine is physically rotating at a frequency synchronised to the grid frequency. Any dynamic response that an inverter is required to emulate using control techniques is completely different in nature to true inertia. The former is derived from the ability to observe, analyse and process the required signals in order to generate the requisite control response. The latter is an inherently passive physical effect based on rotating mass that does not require electronic or other form of processing in order to effect a change in response. NERC reports have frequently discussed this distinction in relation to system-wide frequency response issues, which are exacerbated by increased levels of inverter-based resources in the grid. Inverterbased resources can be characterized based on the control approach applied. In

grid-following inverters, the power electronic stack replicates the voltage associated with the grid current (i.e., it synchronizes to a given grid voltage waveform and converts the corresponding current order based on the characteristics of the grid). In general, such inverters are supposed to operate in a scenario in which a robust grid has been established by other generation resources. In grid-forming inverters, control establishes the grid voltage and frequency. The term is generally associated with high

penetrations of resource in systems with little or no synchronous generation. Notably, most communications from NERC and industry on the subject caution that these are only intellectual constructs of resource functionality and that operating inverters may combine some characteristics of each category. In addition to providing greater flexibility and efficiency in terms of power delivery and consumption, modern inverter-based resources exhibit different operational characteristics during disturbances. The means by which voltage irregularities, frequency deviations and protective relay operations are sensed and responded to can differ between the old and new systems. Various NERC disturbance analyses have uncovered numerous instances where the coordinated control activities of inverter-based resources at many locations have produced system-wide consequences that were not evident from the response of the individual resource itself. Scaling up the modeling and performance verification of inverter-based resources is key to fully understanding these complex systems and their impact on the grid. Short circuit current contribution is also materialy different for inverterbased resources. Conventional thinking and many established protection relays rely on synchronous generators providing a large short circuit current for an extended period following a fault. Typically, the high short circuit current level supports the associated protective relay operating functions. While inverter-based resources can also provide high short circuit currents following a fault, in most cases, these currents are limited to the capacity of the power electronic semiconductor switches and by design of protection control functions such as fault current limiter and damping control. These effects have been recognized and addressed appropriately in all related NERC planning and protection reference materials. Reactive power and voltage control capabilities of inverter-based resources are identified by control settings rather than by inherent machine characteristics. Many inverter-based resources can change reactive output on a fast basis within designated ranges and have the potential to provide voltage support. NERC recognizes that the value of these capabilities depends on the specific needs of the interconnection, the control and coordination functions required of the transmission operator and the operating conditions of the transmission system. One other consideration is control system complexity. All inverter-based resources require multiple layers of control to maintain stability – from phase lock loop (PLL) and current regulators to voltage regulators and relay protection. These interactions not only with other control systems on the resource but also with the grid pose uncertain and sometimes non-intuitive system-wide dynamics. NERC wants to see these complexities more accurately captured and represented in planning and simulation tools to enable more reliable analyses. Not all inverters are created equal Either by design or a byproduct of other design choices, there is significant variation in the characteristics of inverter-based resources across technology, vendor and operating conditions including voltage level and region. Much of the recent technical discussion on how inverter-based resources affect the power grid has centered on the need to consider aggregate and regional effects of these technologies and systems, as opposed to a single, generic model of an inverter. Understanding these characteristics is important for analyzing the impact of inverter-based resources (IBR) on bulk electric system reliability. These characteristics play a role in the way frequency response, voltage and protection and restoration

activities are characterized and behave. As the resource mix of the future begins to take shape, the way we analyze, plan and operate the grid to ensure reliability is also evolving, and an understanding of these characteristics will be important to those changes. [FIGURE: High-level control architecture of a grid following inverter-based resource]

End-of-Chapter Summary

Inverter-based resources (IBRs) such as PV inverters and synchronous condensors behave differently to synchronous generation in that they do not have the same power electronic circuit, control, or mechanical characteristics to provide inertia or provide fault current. Instead, the dynamic performance of IBRs in steady state and during disturbances is determined by the control algorithms implemented in their digital computing circuits. This unique combination of characteristics both presents challenges and opportunities for ensuring reliable operation of the power system. The modeling, analysis, and verification of IBRs are therefore crucial to ensure reliable operation of power systems, especially when high penetration levels are reached.

FROM THE FIELD

An inverter-based resource is defined by its connection, not its source. Solar, wind, and storage all behave like IBR because of the inverter, not because of the fuel.

The grid sees what the inverter sends. If the inverter is programmed conservatively, the resource looks weaker than it is. If it's programmed aggressively, the resource can destabilize what it was supposed to support.

The IBR control settings are now reliability artifacts. The settings that left the factory are no longer just engineering choices; they are compliance choices.

Chapter 3

System Frequency Behavior and Inverter-Based Resources

The system frequency is controlled so that at all times the real power supplied equals the real power demanded in the bulk power system. Frequency control has been based on the inertial response of the machines and the governor response of the synchronous machines for the last few decades. With the rapid increase in the number of inverter-based resources (IBRs), NERC and utilities have been studying the new frequency control characteristics of the system, as pointed out by the ERCOT Real Time Operating Task Force. In a power system with many synchronous machines, changes in frequency following an instability are dampened by the inertia of the rotating plant mass. The generators use their energy stored as rotation to help to slow the rate of fall in frequency during a power shortfall, or to damp out the rate of rise in frequency during a power surplus. The frequency regulation is a fast and continuous response based on shaft movement without the need for measurement or control signal. The primary frequency regulation involves automatic speed governor responses based on the change in frequency and related to the power input to the generator. Inverter-based resources do not natively possess this physical inertia. The grid frequency is detached from the output power of an inverter-based resource via the inverter conversion interface. The inverter then responds based on real-time control algorithms implemented within its electronic controls which analyze system frequency and other grid characteristics to actuate corrective control actions. Such fundamental differences have been cited in several NERC reports related to frequency phenomena in high penetrations of inverterbased resources. Many inverter-based resources can provide frequency response through centralized control through the use of operating points, within their physical and operational limitations. Adjustments in real power output are made in response to deviations in grid frequency as defined by specific control parameters. These adjustments are not physics-driven and are instead the result of specific control settings, protocols and performance criteria established through agreed norms, grid operations and control procedures defined at the regional level. The

timing and magnitude of inverter-based frequency response are different than those of traditional frequency response resources. The measurement delays, filtering and control execution times can result in fast and slow components that differ from the conventional frequency response characteristics. NERC’s analysis confirmed that the combined inverter frequency response has the potential to mitigate and stop frequency decline when properly coordinated. However, in the event of poor coordination or

inadequate consistency among resources, the inverter-based frequency response can also be a source of unexpected regulatory behavior. All of the disturbance events reviewed by NERC identified potential risks involving reduction of real power output or momentary cessation of real power injection by inverter based resources in response to frequency excursions and thereby contributing to the extent of frequency deviation. These events revealed a variety of potential vulnerabilities of inverter-based resources including potential sensitivity to protection settings and the need for careful design and coordination of the frequency ride-through requirements to match system needs. The Working Group has taken frequency ride-through into account in its reliability analyses and in its recommendations. One more aspect to consider is system strength and frequency stability at low inertia levels. As more non synchronous generation is brought online, the inertial energy supplied by the remaining synchronous generators is reduced. NERC and RTOs have studied this issue in various reports. The impact on power system protection and stability is less well understood. Higher frequency changes can create problems with existing systems that are designed to operate at lower frequency changes in a higher inertia system. Systems with lower inertia will have less energy stored to slow the rate of change of frequency following a major sudden loss of generation. With the advent of inverter-based resources, the system will need more frequency regulation to maintain stability. There is a critical distinction between frequency response capability and frequency response obligation. Requirements related to frequency response in NERC Reliability Standards apply to certain registered entities and the fact that inverter-based resources are subject to those requirements or not depends on their registration status and applicability. This document does not provide legal advice on any individual entity’s obligations under those standards – it is simply important to note that frequency performance of the system remains a shared responsibility and that the responsibility is still measured within a balancing authority and an interconnection. Recent Work The Federal Energy Regulatory Commission (FERC) issued several Orders for Reports on the effects of High penetration of Inverters in Power Systems. NERC fulfilled

these requirements and the final reports are located below. As a result of these Orders, NERC performed additional work on the impact of Inverter Based Resources (IBRs) on Underfrequency Load Shedding (UFLS). This work, illustrated in the graphic to the right, explores how changes in frequency behavior (e.g., steeper droops and non-linear drops) could potentially impact current UFLS protocols. The analysis further emphasizes the need for coordinated system studies and cross-disciplinary analysis between planning, operations, and protection activities. The ongoing debate about the role of inverter-based resources in frequency response is part of a larger trend away from reliance on inherent system characteristics and toward engineering solutions. NERC and others are engaged in a deliberative process to determine the appropriate focus for ensuring adequate frequency response given the evolving resource mix, using studies, reliability standards, and industrywide initiatives to inform those discussions. (Conceptual illustration showing the fundamental differences in frequency response to a contingency between synchronous and inverter-based systems)

End-of-Chapter Summary

Frequency response of the system has traditionally been based on the natural inertia and governor response of the synchronous machines. Inverter-based resources change this basis from a physical control to a control based on measured values, set points and communication. While NERC’s analysis has determined that inverter-based frequency support can be a valuable asset when properly utilized, ill coordinated control and protection can have negative impacts on frequency stability. This is a major reliability challenge that must be managed as penetration of inverter-based resources continues to grow.

FROM THE FIELD

Synchronous machines provide inertia automatically, as a byproduct of physics. Inverters provide it on purpose, if at all. The difference is whether frequency response is built in or programmed in.

A grid dominated by inverters can be stable, but stability is no longer a free byproduct. It has to be designed, configured, and verified.

Frequency events on inverter-heavy systems happen faster, with less warning, and propagate further. The standards are revising to that reality.

Chapter 4

Voltage Performance and Reactive Power Considerations

Voltage performance is a fundamental element of bulk electric system reliability and impacts a broad range of aspects of power system operation, including equipment performance, power quality and stability. Voltage regulation and reactive support have traditionally been supplied by synchronous machines, static var compensators and transmission system design. These established practices are challenged by advances in power systems that incorporate significant levels of inverterbased resources. Special considerations must be taken into account when addressing these new aspects of voltage control and reactive management in the power system. The synchronous generator always supplies reactive power as a function of the excitation and the state of the machine. The voltage response behavior is continuous and closely linked to the physical properties of the machine. Inverter-based resources supply or absorb reactive power by controlling the current output through modulating the individual current components as per the control strategy, thereby changing the voltage response behavior from a physics dominated behavior to a control dominated behavior and hence affecting coordination and reliability. NERC in April released technical assessments finding that inverter-based resources (IBRs) can quickly and precisely control their reactive response within certain boundaries. In fact, many IBRs can control reactive power more quickly than existing excitation systems, which can be stretched to varying lengths to provide voltage support in areas with high penetration of IBR technologies, according to the NERC analysis. The ability of an IBR to rapidly adjust its reactive output can be used to provide a form of voltage control, per NERC. It also noted that regions served by the high voltage transmission grid may need to comply with NERC regulations pertaining to voltage control under these circumstances. Voltage ridethrough (VRT) behavior during transient conditions is one of the key performance metrics for inverter-based resources (IBRs). The NERC recently released a report which includes the results of several studies of disturbances at which large numbers of IBRs rapidly or momentarily lost voltage synchronism and tripped in response to voltage declines that had not previously led to loss of generation in traditional sources. The report highlights the need for consistent VRT expectations and modelling practices for IBRs in transient stability studies. Reactive Power Capabilities for Inverter-Based Resources (IBRs) are fundamentally different from those of synchronous machines (SMs). Current limits rather than excitation limits restrict the performance of the inverter. There are inherent real power/reactive power trade-offs. NERC provides several guides/documents that underscore the need to account for the real power/reactive power trade-offs in planning and operation decisions, particularly at high real power

output, when reactive power capabilities may be heavily limited. System strength, often described as short circuit strength or available fault current, can impact voltage performance in high inverter penetration scenarios. Weak system conditions can affect the stability of inverter control and the ability of the system to provide adequate voltage regulation. NERC and RTO studies have analyzed scenarios where reduced system strength can impact voltage sensitivity and cause potential interactions between inverter-based resources. Increasingly, utilities are interested in coordinating the performance of several voltage regulating devices (VRs) at the same location. Many voltage regulation issues will arise in transmission voltages and voltages will be affected by many sources including generation, Static VAR Compensators (SVARCs), load characteristics and inverters. NERC Releases address specific voltage and reactive stability concerns arising from lack of coordination among control actions, such as voltage oscillations or inadequate control, caused by synchronized responses of inverterbased resources to the same voltage control signal. In addition to steady state analysis, the dynamic behavior of inverters during transients and voltage recovery have become important factors in ensuring that voltage performance requirements are met. The rate at which an inverter’s output recovers following a transient voltage event can significantly impact the overall system restoration and stability performance. Many of the NERC disturbance reports document the impact that the timing and sequencing of inverter restoration has had on the recovery of system voltages. Therefore, it is important to evaluate the dynamic performance of systems in order to fully understand how the system will respond to a variety of transient conditions. Like many other inverter-based resources issues, the effect of regional characteristics is very significant. The variation of transmission system configurations, load compositions and resource mix cause a wide range of voltage regulation issues. Numerous RTO and ISO technical reports emphasize the need for a region by region analysis, based on reliability criteria and standards, in order to assess the impact of inverter-based resources on the voltage level. A more representative comparison between synchronous generators and inverter-based resources (IBRs) is given in the following, by adding also a graphical representation of the reactive power capability envelope of an inverter-based resource.

End-of-Chapter Summary

While transmission systems with more inverter connected resources are trying to address increasing voltage challenges, the performance of the grid voltage is determined by the control strategies for reactive power rather than by the inherent characteristics of the rotating machines. While inverter connected resources have the ability to provide a high level of voltage control on very short time scales, the suitability to support the grid voltage in such scenarios is determined by the relevant operating margins, the grid topology, its reactance and the degree of control from other voltage control devices. The NERC Reliability Region released analysis in June 2015 that highlights several issues with inverter based resources and their impacts on system reliability including their inconsistent ride through of short

circuits and their lack of coordinated control. The paper notes in great detail the need for grid wide voltage performance analysis.

FROM THE FIELD

Reactive power from inverters is fast, programmable, and conditional. From synchronous machines, it was slower, automatic, and continuous. The reliability properties of the two are not interchangeable.

The voltage profile of an inverter-rich system depends on the firmware. Firmware updates are now reliability events.

A resource that can't ride through a voltage disturbance leaves the system in worse shape than a resource that doesn't exist. Disconnection during a disturbance is the failure mode the standards are working hardest to prevent.

Chapter 5

Protection Systems and Fault Response Implications

Fault protection provides the means to identify and isolate faults in the bulk electric system in an effort to prevent or minimize damage to affected equipment, prevent exposure to dangerous conditions, and maintain stability in the power system. Existing protection design practices and reliability analyses have been formulated based on the known characteristics of faults on synchronous based systems and passive network components. With the growing incorporation of inverter-based resources (IBR) into the power system, there is a need to revise these practices to account for new inverter fault behaviors and to quantify the associated impact on protection system operation and reliability. Fault current from Synchronous Generator (SG) during short circuits is generally several times its full load rating and remains available until the protection system clears. High and predictable fault current from the SG together with other sources (such as transmission lines) have for many years supported conventional protection methods that are largely based on current values and direction. Inverter based resources such as Inverters (IVs), Static Synchronous Compend (STATCOMs), Synchronous Condors (SCs) and Modular Multilevel Converters (MMCs) will likely provide much lower fault current as well as significantly reduced duration. In a recent series of NERC technical reports, it was discovered that most inverter-based resources will supply fault current near to name plate level, and in many instances, this current is cleared or reduced within a few cycles. This can pose significant challenges to protections which are normally designed with the assumption of much higher fault currents. These challenges are particularly pronounced at the system level with higher levels of inverter-based resources. Distance protection, overcurrent protection and directional elements can all be affected by the decrease in fault current due to the impact of inverter-based resources. NERC and industry wide studies show that it is possible to envision situations where increased inverter connected generation can result in longer times to detect faults or a decrease in protection zone security. As the mix of resources in the grid changes, the impact of these changes on protection needs to be assessed. Inverter control strategies during faults are another factor that affects the behavior of protections. Some inverter controls during faults protect the inverter itself by reducing current and transitioning into a protection mode. Although inverter protections are required to prevent damage to the inverter and the elements it is connected to, NERC performed analyses and determined that the aggregate impact of inverter responses during faults could affect the nature of faults in ways that may not be anticipated by existing protection relay theories. Ground fault response is one more. Although the ground fault detection and response of inverter-based

resources is different from that of synchronous machines, these differences are generally noticed at lower voltages. In addition, NERC guides dealing with the groundings and fault contributions for protection coordination studies of inverter-based resources are more applicable at lower voltages. Coordination of protection system also involves recovery of system after the fault has been cleared. The timing of turn-on or restoration of output of inverters after fault has been cleared plays significant role in stabilization of the system and speedy recovery of voltage. NERC disturbance analysis reports have highlighted examples of system disturbances that are heavily influenced by the timing of recovery of inverters, indicating that protection function alone cannot determine the protection performance of modern power systems, since the control of inverters also plays vital role. Standards dealing with protection system maintenance, coordination and misoperation analysis analyses continue to be technology neutral. Nevertheless, NERC Staff provided some background information and key findings of their work on potential effects of inverter-based resources (IBRs) on these standards, in an effort to raise industry awareness, and without prejudice to any final determinations that may be made by Regional Electrics Reliability Councils (RERCs) or FERC. This document does not constitute an interpretation of the specific standards but it is intended to serve as a cautionary note to remind that the analysis of protection system performance may have to take into account the new fault response characteristics of the power system. Much like other reliability domains, the extent to which inverters impact protection relays will vary depending on a particular regional area and system configuration. This includes transmission system conditions, concentration of inverters at the substation location, and protection relay settings. RTO and ISO work has highlighted the need to update relaying protection studies to utilize more actual measurements of inverter behavior during fault conditions. Protection and fault response to inverter-based resources is an important factor in assuring reliable operation of the bulk power system. Inverters in

the power system is an important relay function tied to overall reliability of the system. [FIGURE: Comparison of fault current contribution from synchronous generators and inverter-based resources during a transmission fault]

End-of-Chapter Summary

The existing protection systems have been designed on the assumption that the faults on the power system would be evidenced by high and sustained fault currents supplied by the synchronous resources. The limited and controlled nature of fault currents supplied by inverter-based resources may affect the ability of protection systems to detect faults reliably, maintain sufficient clearance times, achieve required levels of selectivity, or maintain proper coordination. The reliability analysis performed by NERC emphasizes the need for accurate inverter model dynamic simulations to assess protection system impacts on power system reliability. The study underscores the importance of considering accurate

dynamic models of inverter-based resources and system topography to assess their impact on fault response behavior. And this will be more significant, as the increased adoption of inverter-based resources in the power system makes fault response behavior an important aspect of power system reliability.

FROM THE FIELD

Protection schemes were calibrated to synchronous fault current. Inverter fault current is lower, faster to peak, and shorter in duration. Same algorithm, different input — different outcome.

A protection misoperation on an inverter feeder isn't a setting error. It's a category error. The settings were correct for a system that's no longer there.

The protection engineer who hasn't reset their assumptions for IBR is the engineer whose findings will arrive in the next audit cycle.

Chapter 6

Modeling, Studies, and System Analysis Considerations

The topic of modeling in order to accurately simulate a bulk electric system in order to determine reliability has been a frequent discussion in power and energy circles. Reliability analysis models are utilized in planning studies as well as in operational analysis. Aspects of system behavior for a variety of conditions including normal operating conditions, contingencies and extreme weather occur often in reliability models and simulations. In recent years, NERC and other Industry organizations have increased discussions related to modeling issues involving increasing levels of inverter-based resources (IBRs). Existing power system models have been developed with the synchronous machine as the foundation and corresponding characteristics. Inertia, exciter, governor and protection models are typically tied to the synchronous generator. New inverter based resources come with a different set of modeling challenges and require new approaches since their actions are defined by their control algorithms, not by the natural laws of the mechanical elements of the conventional rotating machine. For inverter-based resource models, the NERC technical reports emphasize the need to include control features such as phase-locked loop (PLL) behavior, current limit, voltage control, and frequency response logic. While generic and simplified models may suffice for some analyses, they can be insufficient to accurately capture the behavior between inverters during disturbances. In high penetration scenarios, even small inaccuracies in the inverter model can lead to questionable results and potentially unexpected system behavior. Inverter modeling is pertinent to both steady-state and dynamic analyses. In power flow, inverters are often modeled as controlled PV buses where the inverter injects real and reactive power within specified bounds. While this is suitable for some analyses, it does not preserve the dynamic behavior of inverters during transient events. NERC suggested that dynamic simulations are necessary to complement steady-state power flow studies to verify the correct operation of inverter control strategies. Dynamic models of inverters are often considered to be some of the toughest to develop, not in terms of the technology involved, but because of the

sometimes commercial-in-confidence nature of control implementations that can differ from one vendor to another. Many of the NERC and RTO reliability reports document limitations to grid planning and operation caused by lack of disclosure of the proprietary aspects of new technologies. Another, perhaps more positive, development is the increasing interest in developing a small number of model structures and procedures for validating dynamic models of inverter-based resources. Model validation and

verification are a key component of system analysis. The NERC has emphasized that models should be validated against actual system performance data as often as possible, particularly after significant disturbances. Any discrepancies between model performance and actual system performance have recently been shown, with issues associated with inverter-based resources being a prime example of the need for future model refinement work. As discussed earlier, modeling an aggregate of many inverter based resources is not straightforward. The aggregate behavior of all the resources may not be simply the sum of the behavior of individual resources. This phenomenon is particularly common when resources are controlling their output based on the same control signal (e.g., voltage or current). NERC has conducted several studies confirming the occurrence of this phenomenon where the aggregate behavior of many inverter-based resources is distinctly different than the behavior of any single resource within that group. In addition to stability, system restoration and blackstart modeling of power systems has become an important consideration. Inverter-based resource behaviors during power system energization, islanding and restoration are very different from those of traditional synchronous machines and NERC is performing studies related to inverter controls under low voltage, low frequency and other conditions encountered during the restoration process. The recent public RTO and ISO reports concerning transmission planning models recommend updating the modeling practices to reflect the changing resource mix. The planning criteria, study assumptions and validation methods and procedures need to be re-examined and updated to address the significant changes brought about by increased inverter-based resources. This work is done in conjunction with NERC reliability assessments, guidance documents and industry wide efforts. The adequacy of modeling of inverter-based resources for operations planning is a means to an end – the end being the ability to make reliable resource decisions. Through accurate system modeling, system planners and operations personnel are better able to assess and manage risk, develop alternatives to

address potential risks to the grid and remain in sync with changing reliability requirements of the evolving grid. FIGURE Example structure of a dynamic model for an inverter-based resource used in system studies.

End-of-Chapter Summary

Inverter-based resources require significantly different modeling approaches than synchronous machines. Their behavior is heavily controlled, affecting static and dynamic modeling. Studies completed by NERC have confirmed that the appropriate modeling, validation, and modeling aggregation of inverter-based resources must be understood in order to produce accurate system reliability studies. Given the increasing share of inverter-based resources, advanced modeling techniques will be needed to ensure that future modeling approaches support adequate reliability planning and operation activities.

Chapter 7

Operational Visibility and Real- Time Considerations

Reliable operation of the bulk electric system requires accurate situational awareness and timely operator response. Control centers operate in real time based on a broad range of data, including measurements and remote line readings, as well as analytical tools that monitor system performance and aid in restoring stability following a disturbance. New challenges to situational awareness and near real-time reliability monitoring have arisen with the integration of inverter-based resources. Under current operating conditions, system operators rely on familiar information such as real-time generation output, frequency, voltage levels and power flows from synchronized conventional resources like hydro, gas, coal and nuclear. These system resources exhibit well-understood and broadly similar dynamic responses to changes on the grid. New inverter-based resources such as wind and solar power may respond quickly and in a manner that is controlled by individual producers. As a result, the established grid system dynamics based on synchrophasor measurements could change dramatically with the increased penetration of such inverter-based resources. NERC has identified visibility gaps associated with inverter-based resources in several technical assessments. These gaps may include lack of real-time, high-resolution telemetry, or delayed or poor-quality status reports, or an inability to understand the operational details of control modes and states. In many situations, a lack of visibility may preclude the synchrophasorbased monitoring of the inverter-based resources’ contributions to system behavior during significant events. The granularity of Telemetry is one of the many factors contributing to operational visibility. The real and reactive power output, voltage control status, frequency response modes and protection states of inverters determine how they interact with the grid and are highlighted in the various publications of the NERC and the RTOs. These highlight the growing importance of largescale measurement in real time to ensure informed decisions can be made. Voltage Reactivity of Inverter-based Resources Determining the appropriate voltage reactive control actions for an operational response is an important consideration. For conventional resources the voltage reactive behavior is established from first principles. For example, the reactive behavior of hyro generators, and other conventional synchronous generators is well understood and can be determined with a high degree of reliability. With respect to inverter-based resources, the voltage reactive response is controlled by the inverter operating logic, and therefore the response from a similar resource may differ significantly under the same system conditions due to a wide range of possible control settings and firmware versions, operating restrictions, etc. The NERC team has studied this aspect in some detail, and has

determined that its understanding of the expected actions by all resources impacted by a system disturbance may be compromised by the uncertainty as to how all the inverter-based resources (which are often numerous in size) will behave. A wide range of frequent, real-time frequency and voltage deviations can trigger innumerable short cycle events involving reactive control actions or uncontrolled cessation or current limiting of operation in inverter-based resources which do not necessarily show up on conventional SCADA systems. Hence momentary shutdowns or current limiting caused for example by a short loss of synchronization or by over current may not have a visible trigger point and accordingly their relationship to the mass of frequency and voltage deviations, the triggering of reactive compensators or failure of certain resources to operate are inherently obscure and can obscure analysis of the actual cause of the power grid event, which can in turn interfere with the implementation of required mitigation measures. Operational coordination between transmission operators, balancing authorities and generators is becoming an increasingly important issue in high penetration of inverters. Understanding the capabilities, limitations and response of inverters will help improve coordination and system operations. NERC has identified coordination and information sharing as key operational practices to ensure reliable transmission operations in a changing resource mix. Alarm management and operator training are other consequences of increased penetration of inverter-based resources. Changes in operating procedures, faster system behavior and differing power system response require modification to existing training programs and simulation models. The public RTO and ISO discussions addressed the necessity of training operators to deal with inverter-based resources related alarms and carrying out corresponding contingency preparedness drills. Operations Challenges Continue to Surface related to Restoration/Shut-down activities and System Reconfiguration Commissioned work related to system reconfiguration for certain inverter-based resources, and restoration and system reconfiguration activities are still surfacing operational challenges associated with voltage recovery and islanding and resynchronization of inverter-based resources that are not the same as with

traditional synchronous generation. Some of these issues have been surfacing in relation to NERC reliability restoration activities, where an understanding of inverter-based resource behavior that can affect restoration timing and stability issues related to limited visibility to inverter status following a significant event impacting restoration and recovery activities. Operational visibility is a technical and nontechnical challenge. Issues range from data availability and communication protocols to inverter operation understanding. As more inverter-based resources are interconnected to the grid, NERC and industry is pushing for greater visibility to improve situational awareness necessary to ensure reliable BES operation. [FIGURE: Example control room visualization highlighting inverter-based resource telemetry and system conditions]

End-of-Chapter Summary

Achieving operational visibility is essential for maintaining reliable real-time management of the Bulk Power System. The control-driven nature and fast response of inverter-based resources add complexity to visibility and predictability challenges. NERC surveys have highlighted existing challenges related to telemetry, situational awareness, and operator knowledge that can impact real-time operational decisions. Visibility, coordination, and operator training is still needed to address growing inverter levels.

FROM THE FIELD

The operator runs the system on what the SCADA reports. If IBR telemetry is incomplete, the operator is running on partial information, in real time, with consequences.

Real-time visibility into IBR is increasingly a registration condition, not a courtesy. The Region expects it. The standards are codifying it.

An IBR fleet that the operator can't see, can't dispatch, and can't curtail in real time is a contingency. The system has to be planned around the contingency it can't see.

Chapter 8

System Strength, Stability, and Inverter Interactions

System strength refers to the ability of the BES to withstand large disturbances and provide stable voltage and frequency conditions. This term has historically been closely linked with the presence of synchronous machines in the BES, which provide inertia, fault current and voltage margins via direct mechanical interaction with the power grid. These characteristics are changing with the increasing penetration of inverter-based resources. In systems with a high level of synchronous generation, the voltages tend to be strong and stable. Fault currents are high and the excitation systems of the generators provide high levels of control on the voltage. In systems with high levels of inverter-based resources the short circuit levels may be lower and the voltage stiffness may be lower, especially in areas of the transmission system that are weak electrically. These are some of the factors that NERC identified as contributing to increased stability issues. Information obtained from the grid is utilized to regulate the power output of the inverter-based resources. In weak grid conditions, the distorted and unstable nature of voltage and frequency can impact the performance of the inverter. According to the NERC, “interactions between inverters and weak grids have led to oscillations, slow recovery, and unexpected shutdowns. This notes that the impact of inverters on the power grid is heavily dependent upon the condition of the system (strength) and other parameters. Control interactions among multiple inverter based resources (IBRs) is also a factor affecting stability. When many inverter systems respond to the same control signal they can either exaggerate system oscillations or dampen them out. NERC analysis has determined that non-coordinated controls can even lead to unstable control oscillations, a reduction in stability margins and potential losses of stability within the transmission system, despite the fact that individual IBRs are in compliance with the regulations established by each transmission region grid reliability organization (RTO). This topic is associated with system strength.”Grid following" is a term associated with grid following inverters. These are grid tied devices that utilize a voltage reference from the grid. They are not always stable on weak grids. “Grid forming” is also a term that is associated with the performance of modern inverters. Again, we are dealing with devices that are tied to the grid and associated with new concepts of grid modernization, where in addition to following the grid voltage and frequency, they form and provide their own grid with a new associated voltage and frequency. The reliability of these new inverter performances is a subject that is addressed by the North American Electric Reliability Corporation (NERC). NERC indicates that these performance characteristics are evolving, and are subject to a full evaluation of their effects on a power system, considering the

operating conditions at the time of their application and their coordination with other system protections and controls. Small-signal and transient stability analysis of power systems are becoming more and more affected by the presence of inverters. For many years, stability studies relied heavily on the rotor angle dynamics and the excitation control system of synchronous generators. Nowadays, with an increasing number of inverters connected to the grid, sources of instability can be due to interactions between inverter control loops, phase locked loop dynamics, current limit, etc. NERC is also requiring the adaptation of stability studies to these new changes. The reliability implications associated with system strength are another consideration in the transmission planning and resource siting processes. Low system strength conditions can occur in areas with weak transmission connections or high penetrations of inverter-based resources. The effect of low system strength on transmission needs and potential solutions for reinforcement (such as additional transmission, synchronous condensers, etc.) has been discussed in various technical publications from Public RTOs and ISOs. System strength is not an on/off switch but rather a spectrum of conditions defined by voltage level, location in time and on the grid and by available resources. This is a core conclusion of recent NERC assessments of grid stability. System strength can vary by time and place. It can’t be treated as a constant. The ability of a system to provide reliable grid operation in the age of inverter-based resources is a critical aspect and affects many aspects of power system planning, operation and protection. It is an interface point between these individual fields and shows clearly the interrelation between reliability challenges in today’s power system. Fig. 3: Illustration of strong versus weak grid conditions and associated inverter control interactions.

End-of-Chapter Summary

System stability and adequacy have classically been supported by the high performance characteristics of synchronous generation. The new challenges for systems with increasing amount of inverter-based resources mean changes to the system stability and adequacy definitions of System Strength and System Stability. Poor system conditions impact on inverter performance and voltage ride through. The NERC Reliability Task Force Stability Subgroup research provided examples that highlight the impacts of new control interactions, decreased short circuit levels and new stability phenomenon. This research has highlighted the need for a more holistic system level approach that recognizes the changes to traditional systems strengths and considers the new reliability challenges to ensure Adequate System Strength and continue to support reliable supplies.

FROM THE FIELD

System strength used to be a byproduct of how many synchronous machines were online. Now it's a planning constraint, and IBR penetration limits are how the constraint shows up in real decisions.

Two well-designed inverters in close proximity can interact in ways that don't appear in either of their individual studies. The interaction is the new reliability question, and most studies don't yet ask it.

Chapter 9

Disturbance Events and Reliability Lessons Learned

Disturbances provide valuable insight into the operational performance of the bulk electric system during stressed conditions. Analyzing real disturbances provides reliability stakeholders and industry with a first-hand view of the discrepancies between the planned and real-world performance of the system. In response to the growing presence of inverter-based resources in the bulk power system, NERC has made high-priority reliability efforts to gain insight into the participation of these resources during disturbances and associated reliability impacts. The integration of inverter-based resources onto the power grid has introduced a number of complex issues related to grid stability and disturbance response, including reports from the National Electric Reliability Organization (NERC) of events where inverter based resources have reacted to real voltage and/or frequency shifts in ways that have led to loss of generation that exceeded the margin of grid stability as understood by planning and operational staff. Several events have been reported where a large number of inverter-based resources instantaneously or almost instantaneously switched off or tripped in response to routine grid voltage and/or frequency fluctuations; again, none of these events were associated with any component failure but were related to the operational control strategies and/or protection schemes implemented in these resources. Another common theme in NERC’s analyses is the phenomenon of the “whole being greater than the sum of its parts” that defines the behavior of inverter-based resources at the aggregate level. While each generation and transmission resource operates and responds within established limits individually, the combined impact of many such resources can still pose significant risks to the power system. These effects are not always revealed in one-toone analyses focused on individual resources. As described in NERC’s reports, they emerge only when the system is examined as a whole. These analyses have also begun to reveal the extent to which model assumptions influenced the observed behavior of inverters during the disturbances. Initial models created prior to the disturbances turned out not to accurately capture the dynamic response of inverter controls. The results of detailed post-event modelling, with the benefit of hindsight and with access to more detailed measured data, has confirmed this and points to

an urgent need for model validation and data sharing. Continued analysis of disturbance events highlights one more important trend and factor to consider in today’s electrical grid, involving inverter based resources and the associated protection systems. The reduced fault current from these new systems combined with the faster and more precise operational responses are altering fault detection

and the resulting restoration activities, observed in numerous disturbance events, as noted by NERC in relation to protection misoperations and misoperations that need more detailed analysis. Communication and coordination challenges have been experienced for the first time due to disturbance events. In most instances, the lack of real-time visibility into the inverter operating state made it difficult to manage and mitigate the immediate effects of the disturbance and to thoroughly investigate the root cause. NERC is focusing additional effort on improving telemetry, event reporting and data availability to enable utilities to rapidly and accurately determine the impact of inverter-related disturbances. Some impacts have been characterized by NERC as integration issues rather than problems with inverter technology. All can provide reliable service if their integration with other components of the power system is managed properly through synchronization of relays and protection settings, synchrophaser models, operating procedures, and reliability standards. Events that provide insight into the operation of the power system under disturbance conditions are valuable in enhancing knowledge of the system as it relates to voltage instability and dynamic performance. Lessons learned are being used in the revision of the associated guidance documents, in system reliability studies and in industry outreach. Much more will be done to confirm understanding of inverter characteristics and to prevent any unintended responses to system disturbances that are not yet fully understood. Disturbance analysis is an active field of research due to the ongoing high penetration of inverters in the power system. Every disturbance gives an opportunity to learn more about the behavior of inverters under stressed operating conditions of the power system. [FIGURE: Timeline of a representative disturbance event highlighting inverterbased resource response]

End-of-Chapter Summary

Fault Occurrences Provide Important Insights about the Performance of Inverter-Based Resources During Disturbances When several disturbance events occurred at a power plant and caused losses of power to the interconnected grid, initial studies and observations undertaken by NERC staff highlighted performance issues with respect to the aggregate response of inverter-based resources (IBRs) caused by inaccuracies in current models and lack of visibility of their operation which impacted generation restoration following these transient events and power losses, highlighting the need for continued system wide studies to investigate issues facing the grid that may be affected by IBRs, as well as more accurate models, greater visibility of IBR operation and greater synchrophasor visibility and integration of the IBRs into the system to ensure greater reliability in the bulk power system.

FROM THE FIELD

Every IBR disturbance event of the last decade has revealed a different gap. Read the post-event reports as a curriculum, not a punishment.

Blue Cut. Canyon 2. Odessa. The names are shorthand for control settings, ride-through profiles, and protection logic that didn't behave the way the planning model said they would.

Chapter 10

Reliability Frameworks and Oversight Considerations

The North American Bulk Electric System (BES) is governed by a reliability regime that defines performance criteria, provides for consistent regulatory frameworks and provides for ongoing improvement. The regime, enforced by NERC under the authority of FERC, was developed in a system with predominantly synchronous generation. With growing amounts of inverter-based resources on the BES it is important to assess how existing regimes function under these changing system conditions. NERC Reliability Standards were written to be technology neutral, specifying performance requirements rather than design, materials, or specific technological approaches that might change over time. While technology-neutral language does not preclude consideration of technology in a reliability assessment, NERC’s experience has been that it does not preclude consideration of technology as an input when analyzing the impact of resource types on reliability. Inverter-based resources present a number of characteristics that impact compliance with reliability standards, the analysis of performance and system reliability effects, and the assessment of system risks. Planning standards are an important component in analyzing the reliability impacts of inverter-based resources. Standards for system planning, including those for modeling, contingency analysis and transfer capacity are all based on analyzing system performance under a variety of assumed conditions. NERC has emphasized the need for accurate modeling of inverter performance, particularly in areas with high penetration of inverters. In addition to Reliability Standards that incorporate assumptions about how a system will respond and how it can be controlled, the establishment of operational standards for frequency performance, voltage control, and event response also occur at the system level despite individual unit behaviors being different. Inverter based resources generally impact these system-level characteristics as a group rather than as individual resources with specific operating requirements. Standards related to protection cover three main areas: coordination, maintenance and misoperation analysis. Increased penetration of inverter-based resources (IBRs) is changing the nature of

fault currents and their interaction with protection relays. NERC points out that it is important to consider the effects of IBRs in the protection relay studies. The impact is that system changes can affect protection relay operation, yet the underlying protection relay settings do not change. NERC Standards for Transmission Reliability also include guidance documents and reference materials developed to support compliance with the formal standards. These documents are not mandatory but rather provide

technical background, description of tools and practices, and illustrations of concepts and techniques that may be used in implementations of the requirements of the standards. The NERC Reliability Guidelines for InverterBased Resources provide background information on dynamic modeling practices, considerations for performance criteria and lessons learned from the analysis of disturbances. Standards compliance is only part of the equation for ensuring proper oversight of inverter-based resources. In addition to monitoring compliance with new standards, NERC has been engaged in reliability studies, technical meetings, and outreach in order to identify and better understand the risks and challenges posed by these new resources. In parallel, all the Public RTOs and ISOs have been engaging in technical forums to explore in greater detail their regional needs, challenges and lessons learned related to the integration of inverter-based resources. Reliability Monitoring and Resource Policies differ from each other in some ways. NERC and FERC do not determine what new power plants will be built or what technologies will be used. The Grid Reliability rules exist to insure that any resource that is connected to the high voltage transmission system (such as the grid that powers homes and commercial buildings) is connected in a way that supports grid reliability. This is a focus of discussion in several recent NERC publications related to inverter-based resources. In response to the evolving resource mix, reliability frameworks are undergoing refinement and increased detail through the provision of additional guidance and analysis rather than a re-write from first principles. The underlying principles of reliability have not changed and are not expected to. [FIGURE: Relationship between NERC Reliability Standards, guidance documents, and technical assessments]

End-of-Chapter Summary

Reliability standards for the bulk electric system are technology neutral but an understanding of how inverters affect the system operation is necessary for effective compliance with these standards. This document provides an update of developments in this

area noting the importance of reliability analysts having reasonable models of inverter performance, an overall systems view and informed procedures for exercising their discretions under the standards. Guidance, procedures and staff technical work to accommodate changes to new resources, while upholding the technology neutral basis for reliability standards and the enduring reliability goals, have been provided.

FROM THE FIELD

The reliability framework was developed for a system whose composition was stable. IBR has changed the composition faster than the framework can revise. The standards are now chasing the system.

Order 901 is not the final answer to IBR oversight. It's the beginning of one. Expect more cycles of registration, study requirements, and ride-through specifications.

Chapter 11

Restoration, Blackstart, and Extreme Operating Conditions

One of the most complex reliability challenges facing the Bulk Power System (BPS) is restoration after a large number of customers are affected by power outages. The restoration of power to the BPS involves switching on parts of the transmission system, reenergizing power generation, and synchronizing the voltages and frequency in divided sections of the system when voltages and frequency are not within normal bounds. Historically, power system restoration practices have been built around the characteristics of synchronous machines. These practices are evolving to consider the impacts of increased inverter-based resources, as well as extreme operating scenarios. Blackstart resources are typically synchronous generators that can be started without access to external power and provide voltage and frequency references for restoration of transmission lines and other generation. A significant portion of the inverter-based resources available at any given time are not traditional blackstart resources. They require energization from an existing power system in order to operate. This is a major theme highlighted in the studies conducted by NERC. Early in a restoration, the system conditions are typically characterized by low short circuit capacity, low inertia, and large variations in voltage. Inverters operating in this environment may experience instability or violations of limit protection. Observations made during recent black start restoration efforts indicate that some inverter-based resources are simply left off, or operated in a reduced mode, until system conditions improve. These observations were documented during recent technical reviews conducted by NERC. The control strategies that determine how an inverter-based resource acts during the energization of the system are based on the measurement of volts, frequency, and phase angle. The performance of the phase-locked loops and the voltage sensing and control algorithms in the inverter-based resources may be different during restoration than during normal operation. Recent analyses by NERC have determined that the incorrect or incomplete representation of these restoration control strategies can impact the outcome of restoration studies and the basis for restoration assumptions and planning. Concepts related to grid forming inverters have been discussed more frequently in the context of grid restoration and micro-grids (islanded) operations. The purpose of these resources is to follow their own local grid voltage and frequency instead of tracking the grid voltage and frequency that is supplied by other resources. In several of the NERC publications it has been mentioned that although these capabilities may provide many benefits under different operational scenarios, the operation of these resources at the grid level will depend on how they are coordinated and operated within a large grid in a reliable manner, and in

conjunction with traditional restoration methods. NERC stability assessments indicated that restoration may need to be preformed differently to account for effects of inverters, low voltage phenomena and limited fault current. While primarily an evaluation rather than a prescriptive effort, these impacts may play a role in restoration following an outage. In addition, effects of extreme operating conditions such as low system inertia and high levels of non-synchronous generation during restoration may also be a consideration. Inverter-based resources can impact the latter stages of power system restoration, including synchronization and load pickup. The timing and the rate at which inverter-based resources are re-energized and ramped up can impact recovery of voltage and frequency stability. Several NERC disturbance reports have highlighted uncoordinated synchronization of inverter-based resources as an issue impacting the re-synchronization of the power system and restoration of load. Restoration is still a coordinated effort among the transmission operators, the balancing authorities and the generator operators. The NERC is still emphasizing the need to understand the capabilities and limitations of the resources to be able to plan and implement the restoration. This still includes understanding the inverters resources and capabilities as well as understanding where and when the inverters can self recover and where the inverters still need support from the conventional grid systems. Increased penetration of inverter technology requires more restoration studies and emergency preparedness exercises to account for actual system conditions. The public RTO/ISO technical reports have covered in several occasions the need to consider the behavior of inverter-based resources for credible and reliable restoration scenarios.

End-of-Chapter Summary

Synchronous generation characteristics have long been associated with system restoration and restoration operations under extreme conditions. New challenges are arising in inverter-based resources associated with dependence on the grid, restoration control behavior, and system recovery ruggedness. NERC has identified that modeling, system planning, and expectations will all be critical to the successful restoration of power following an event as the resource mix of the grid continues to change.

FROM THE FIELD

Blackstart on an IBR-heavy system is not a solved problem. It's an active research question.

The grid that came back from a black start in 1965 isn't the grid that has to come back today. Inverters need a reference voltage to start. Synchronous machines didn't.

A restoration plan that assumes synchronous blackstart units does is one that the next major outage will test. If the plan hasn't accounted for the inverter dependency, the restoration will be slower than the plan estimates.

Chapter 12

Future Reliability Outlook and Ongoing Industry Focus

The changes to the bulk power system are the result of ongoing trends and evolving technologies, energy markets, and resource developments. Inverter-based resources are now a permanent and increasingly significant part of the North American power system. Accordingly, NERC, FERC, and the Regional Entities, as well as utilities and other industry entities, have continued to focus on ensuring reliable grid performance. Recently we heard again from NERC that reliability assessments confirm that inverters in the power system are not an “emerging issue” – rather they are the new normal. Inverters are becoming a larger proportion of connected resources on the grid and the reliability of these resources will shift from being driven by the unique characteristics of an individual piece of equipment to being driven by the dynamics of the system as a whole. Increasingly reliability will require a system view, coordination across multiple teams and functions, and a common understanding between all stakeholders of what is happening in real time during normal and abnormal operations. We continue to work on several issues regarding performance standards for inverter-based resources in a manner that does not prescribe technology or design. NERC has Reliability Standards for outcomebased reliability objectives for the high voltage transmission system with technical basis and associated guidance, as opposed to performance standards based on detailed engineering. This is because technology and system conditions in the grid vary by region within the power grid interconnected system. Reliability efforts will continue to emphasize accuracy, reliability and quality of model and data. Items to watch are the new NERC Reliability Guidelines and RTO publications focusing on reliability and issues like model validation and disturbance analysis. In addition, the rapid pace of development in inverter controls suggests a need to consider how to ensure that their behavior is properly represented in models in order to maintain a valid basis for operational and planning decisions. NERC is seeing many new operational procedures being put in place in response to changing system conditions. Situational awareness, improved communication through better telemetry

and training are often discussed during NERC meetings and webinars. The grid cannot function reliably if the machinery is not operated in accordance with its changing capabilities and performance characteristics. All participants must have a sufficient understanding of how the interconnected system is behaving at any given time. Relay Protection and Substation Automation – Advances in protection and control schemes and the increasing role of inverters for dynamic voltage regulation continue.

Understanding their impacts on fault current, relay operation, and power system stability remains important. Recent studies and considerations point to new challenges to consider in relay protection performance. Such as postNERC directives to learn from disturbance events and apply relevant knowledge to reliability studies and guideline revisions. Restoration capability and resilience under extreme conditions are ongoing topics of discussion. The NERC Reliability Technical Notes regarding the role of inverter-based resources in restoration was reemphasized with the statement that inverters will only participate in restoration under certain conditions, with synchronous resources remaining the primary resource for blackstart and early restoration. Reliability conversations are shifting to discussion of how all resources will work together to provide system resilience. The electricity reliability standards, as they are developed and interpreted by NERC and FERC, address who has control of the bulk electric system, not what form of generation is used. The standards do not endorse, favor or reject any particular type of generation technology. Instead, the requirement is that any generating resource or technology that connects to the grid must do so in a manner that preserves reliability and supports Grid performance. Evidence of learned experience with inverter-based resources is building from real-time operation, distirbution level fault analyses, and collaborative discussions and research. This process indicates an advanced level of reliability culture with an enduring tradition of learning from evidence based analysis and shared responsibility to safely and reliably integrate new technologies rather than speculating about their impact on high voltage transmission systems. Understanding the role of inverter based resources in this broader reliability context is important for a wide range of stakeholders, including planners, operators, asset owners and policy-makers. As the power system continues to undergo dramatic change, ongoing focus on system-wide dynamics, vigilance and technical competence will be required to ensure that the North American bulk power system continues to deliver on the reliability promises that consumers and other stakeholders have come to expect.

End-of-Chapter Summary

The incorporation of inverter-based resources into the bulk electric system (BES) is well underway. As noted in the NERC Reliability Guidelines for Inverter-Based Resources draft, NERC and industry stakeholders are focusing on system-level performance, modeling, real-time monitoring, and synchronization of visibility and enforcement activities, all in the context of providing reliable service to consumers consistent with traditional reliability standards addressing changing BES configurations and technologies. These traditional reliability standards are designed to be dynamic, evolving with each new reliability analysis and innovation as determined by industry stakeholders.

Glossary

Glossary

Bulk Electric System As defined in the NERC Glossary of Terms, the Bulk Electric System includes: Facilities - Control systems necessary for the operation of an interconnected electric energy transmission network - excludes local distribution facilities The definition is used to determine the applicability of a particular NERC Reliability Standard.

Balancing Authority Reliability system term that describes an entity that operates to schedule resources, to maintain real time load and generation balance within a Balancing Authority Area (BA) and to provide real time support to interconnection frequency as defined in the NERC Glossary of Terms.

Blackstart Capability of a generating unit or resource to be able to start up without external power and thus to make contribution to recovery of the affected portion of the BES. (NERC)

Bulk Power System The old term for the transmission network and generation. The old term has been replaced in the NERC documentation and standards with the term Bulk Electric System (BES) but still shows up from time to time in old documents and technical discussions.

Frequency Response The ability of the bulk power system or an individual resource to follow changes in system frequency through an automatic change in real power output or load, as discussed in the technical material and standards introductions.

Grid-Following Inverter Inverter-based resource control method operating an inverterbased resource control device that synchronizes an output voltage and frequency with a grid voltage and frequency reference of a grid including injecting, into the grid, a

current injected based on a state detected by the monitoring device, whereby the grid voltage and frequency reference are provided by the grid, and in which the gridfollowing inverter requires a strong grid.

Grid-Forming Inverter An inverter-based resource control strategy used to regulate and set point voltages and frequencies, as opposed to tracking a given grid reference. Term used by NERC and others to describe a class of control behaviors.

Inertia The stored kinetic energy in rotating mass that opposes changes in rotational speed and system frequency. Inertia is an inherent synchronous machine characteristic and does not physically exist in inverter-based resources.

Inverter-Based Resource Non-Synchronous Resources A term used to describe a generator or storage resource that connects to the BES through power electronic inverters instead of being directly synchronous-coupled. Examples of non-synchronous resources include wind, solar photovoltaic (PV), battery energy storage and other types of distributed energy resources.

Momentary Cessation The intentional reduction or removal of current by means of modulation of the output of inverter-based resources in response to a disturbance on the power system and restoration of normal current flow without manually turning on or off the source of the modulation. This term has been used in some NERC disturbances analyses and other reports.

NERC The North American Electric Reliability Corporation (NERC) is the Electric Reliability Organization (ERO) designated and certified by FERC to develop and enforce Reliability Standards for the bulk electric system in North America.

Phase-Locked Loop Control mechanism applied by a large number of inverter-based resources to match the inverter output to the grid voltage and frequency by following the phase angle and frequency of the grid.

Reactive Power The part of the AC electric current used to create the electric and magnetic fields. Also called voltage required to hold the voltage on the bulk electric system in place. Measured in volt amperes reactive (VAR).

Reliability Coordinator According to NERC, the RTO is the entity that holds the greatest level of authority and responsibility for ensuring reliable grid performance within a defined geographic area of the bulk electric system.

Short Circuit Strength The Short Circuit Capacity at any point in the system expresses the ability of the power system at this point to supply fault current. Important for power system design and also for power system stability issues such as voltage stability.

Synchronous Generator A conventional rotating electric machine (Alt Current (AC) Generator), which is not controlled by the frequency or voltage of the grid voltage and does not include communication with other machines or sensors to track grid performance, and is synchronized to the grid frequency through mechanical controls and is directly connected to the grid through a synchronous transformer or direct connection.

System Strength There are a number of terms that are used in NERC and industry technical documents to describe grid stability, among them “Grid Resiliency” and “Grid Reliability.” In this context, a related generic term is used to describe the capacity of the grid to provide stable voltage and frequency conditions following any occurrence (such as changes in load or faults) and which takes into account the effects of grid inertia, level of available fault current, and grid topology.

Transmission Operator Reliability Entity (RE) An entity responsible for reliability of its local transmission system and for operation of that system in accordance with the NERC Reliability Standards and applicable operating agreements.

Voltage Ride-Through The ability of a power generating unit to stay online and to operate successfully during and after a transient disturbance (voltage instability event) at the desired output level, with all voltages and time frames associated with the disturbance within normal limits.

The definitions provided in this glossary are for general information only and reflect terminology as used in NERC standards, technical reports and public reliability publications. They are intended to be functional and not binding. In the event of any conflict with NERC’s Glossary of Terms (as defined in the NERC Rules of Procedure), the

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
• Reliability governance and program assessments
• Registration and applicability analysis
• Operational and engineering reliability alignment
• Compliance program design and improvement
• Audit and enforcement support (non-advocacy)
• Mitigation planning and Self-Report development
• Training and executive briefings on reliability frameworks
• 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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