The US energy industry is undergoing a significant transformation as the country pursues its target to achieve a carbon-free electricity sector by 2035. This has resulted in an increasing shift towards the integration of inverter-based resources (IBRs) such as solar photovoltaic (PV), wind, battery energy storage systems (BESS), and hybrid plants into the bulk power system (BPS). While IBRs offer several benefits such as increasing energy efficiency, they also pose several challenges related to their variability, the reliability and adequacy of the BPS, and the lack of comprehensive standards and processes for IBRs. The rapid integration of IBRs into the BPS is driving significant changes in the power grid throughout North America. The increasing variety of resources is making it difficult for grid planners, protection engineers, operators, and others in the electricity sector to cope up with the challenges they pose to grid reliability and security.
In the past, IBRs had a small role to play in the power generation mix, and their only function was to generate energy during normal operation and step aside during grid disruptions. However, with the increasing prevalence of IBRs on the BPS, this approach is no longer viable and they must now provide essential reliability services, support the BPS during both normal and disruptive conditions, and providing sufficient data and information to enable transmission entities to effectively operate the grid. Previously, the North American Electric Reliability Corporation (NERC) has used disturbance reports and reliability guidelines to identify and address reliability issues associated with the increasing use of IBR technology in the BPS and has developed measures to mitigate these issues. However, as the use of IBRs continues to grow, mere reliance on recommendations and guidance is not adequate to manage the associated risks effectively and it is crucial to rely on technical solutions. To this end, NERC held a technical session at its Member Representatives Committee/Board of Trustees meetings in February 2023 to discuss the challenges posed by BPS-connected IBRs, and offer potential solutions.
The following are some of the highlights from the session…
Interconnection requirements and process for IBRs: The changing resource mix has made it difficult for transmission owners to keep up with developing stricter interconnection requirements. This has resulted in ambiguous requirements that are challenging to enforce, leading to inconsistent performance and misinterpretations by the different stakeholders. Consequently, BPS-connected IBRs have had unreliable operation and abnormal performance during grid disturbances, which have become more frequent and severe over the past seven years.
Additionally, the Federal Energy Regulatory Commission (FERC) pro forma interconnection agreements, which are relied on by many entities in the US, do not provide enough detail on the performance of IBRs to ensure BPS reliability. The current focus of the interconnection process is on the transmission owner specifying interconnection requirements and the transmission planner and planning coordinator conducting reliability studies, but all entities need to establish high-quality requirements, models, and studies to ensure IBRs comply with BPS reliability standards. NERC’s jurisdiction for assuring BPS reliability is limited, but it can be expanded by bifurcating performance requirements from facilitation of the interconnection queue.
The requirements should be updated to ensure that interconnection customers comply with established performance and modelling requirements throughout the process, or face corrective actions. Ensuring BPS reliability is crucial in the interconnection process, but it may face conflicts with legal, financial, and political pressures. An effective and efficient process that prioritises the mitigation of possible BPS reliability issues is crucial for a sustainable electricity delivery system. IBRs pose increasing reliability risks to the BPS, and NERC plans to address this by advocating for stronger requirements and submitting a work plan to FERC for more comprehensive registration. It aims to collaborate with the industry to identify owners and requirements.
Some of the solutions on the purely technical front include equipment standardisation, changing needs for modelling and reliability studies, enhanced commissioning practices, agile modernisation of NERC reliability standards, risk-based compliance enhancements and addressing cyber security concerns.
Equipment stadardisation: The lack of equipment standards for IBRs has created challenges in their interconnection. However, the recently developed IEEE 2800-2022 provides minimum performance specifications for IBRs based on their state-of-the-art capabilities. This standard establishes uniform technical minimum requirements for IBRs that interconnect with transmission and sub-transmission systems. Its purpose is to provide a framework for reliable integration of IBR into the BPS. The standard outlines performance requirements that IBRs must meet, such as voltage and frequency ride-through, active power control, reactive power control, dynamic active power support under abnormal frequency conditions, dynamic voltage support under abnormal voltage conditions, power quality, negative sequence current injection, and system protection. The standard is also applicable to isolated IBRs that are connected to an AC transmission system via dedicated voltage source converter high-voltage direct current (VSC-HVDC) transmission facilities.
Notably, the IEEE 2800-2022 alone is not adequate to address the ongoing reliability risks associated with IBRs as the procedural challenges encountered during the interconnection and commissioning processes of IBRs cannot be fully resolved through this standard. Moreover, the standard only applies to new IBRs and does not address the existing ones. That said, the new IEEE standard is a step in the right direction.
Challenges in interconnection studies for IBRs: The reliability studies carried out by the transmission planners during interconnection studies and annual planning assessments require more detailed and accurate modelling, which can be challenging. In particular, the need for detailed electromagnetic transient (EMT) studies in many areas, to get an insight into expected power system over-voltages and over-currents resulting from switching operations, fault conditions etc., poses a problem for many entities. This is so because the study requires significant computational resources and expertise. As the penetration of IBR increases, it is important to quickly up-skill existing resources or hire additional expertise to ensure the reliable operation of the BPS.
Requirement of enhanced commissioning practices: The NERC reliability standards currently lack commissioning requirements for newly interconnected IBRs. The Electricity Reliability Organisation (ERO) Enterprise has also noticed a pattern of facilities being commissioned in ways that do not match the model used for reliability studies during the interconnection process. This mismatch creates a situation where the system is operated in an unknown state, making it susceptible to unexpected or abnormal performance issues. Such issues have been recorded multiple times in the NERC disturbance reports. To prevent this, it is necessary to introduce requirements that ensure the commissioned facility matches the model used during the interconnection process. Any discrepancies between the two should be resolved before commercial operation begins. The gaps in interconnection studies can result in last-minute changes in settings of IBRs during commissioning. These changes may not have been properly studied and there may be a lack of feedback loops to assess their impact on prior study results. Industry practices fail to identify possible configuration, settings, or other design issues, leaving the system open to unexpected behaviour from poorly commissioned IBRs. Non-compliance with interconnection requirements or NERC standards is typically only identified after a significant event has already occurred, rather than through proactive auditing. To address this, it is necessary to establish commissioning requirements and to proactively identify potential compliance issues through ongoing auditing and study practices.
Proposed reliability standards for IBRs NERC is working on several projects aimed at updating their reliability standards to make sure they are clear, consistent, and applicable to inverter-based technologies. At the same time, the FERC has issued a Notice of Proposed Rulemaking (NOPR) that seeks to enhance NERC standards and address the risks posed by IBRs as highlighted by the ERO. Under this, the FERC has proposed to direct NERC to develop new or modified reliability standards pertaining to IBRs. IBRs can produce real and reactive power like synchronous generators, but they do not react to disturbances on the BPS in the same way. This is because synchronous generators are physically linked to the power system and are synchronised to support system voltage or frequency during voltage or frequency fluctuations by continuing to produce real and reactive power. IBRs, on the other hand, must be programmed to support the power system and to ride through a disturbance because they are not directly synchronised to the system. The operational characteristics of IBRs and their equipment settings may cause them to reduce power output or cease operation without tripping offline, individually or in the aggregate in response to a single fault on a transmission or sub-transmission system, which may exacerbate system disturbances and have a material impact on the reliable operation of the BPS. Therefore, FERC is proposing that NERC develops new or modified reliability standards that address specific matters for IBRs, such as data sharing, model validation, planning and operational studies, and performance requirements. International experience has shown that process-oriented requirements are not enough, and that functional specifications and performance requirements are necessary to ensure the reliability of systems with increasing levels of IBRs. NERC must expedite the process of enhancing their standards to address these risks, recognising the necessity and urgency of the situation.
The new reliability standards as proposed by FERC should ensure that NERC registered entities have access to required data to predict the behaviour of all IBRs and their impact on the system. To achieve this, the standards should require generator owners, transmission owners, and distribution providers to share validated modelling, planning, operations, and disturbance monitoring data for IBRs with planning coordinators, transmission planners, reliability coordinators, transmission operators, and balancing authorities. Validated IBR models should be included in planning and operational studies to assess the reliability impacts. Planning and operational studies should assess the impacts of all IBRs within and across planning and operational boundaries for normal operations and contingency event conditions. Finally, the reliability standards should also ensure that registered IBRs provide frequency and voltage support during frequency and voltage excursions in a manner necessary to contribute toward the overall system needs for essential reliability services.
Risk-based compliance enhancements: To achieve this, the ERO Enterprise Compliance Monitoring and Enforcement Program (CMEP) has been underway to address the increased risk that IBRs pose to BPS reliability. NERC has issued and will continue to update CMEP Practice Guides to assist regional auditing teams in this area. CMEP Practice Guides are developed independently by the ERO Enterprise and reflect the professional judgment of its staff. In some cases, the development of the guides may be initiated after policy discussions with industry stakeholders. Once the guidelines are developed, they are posted on the NERC website for transparency and accessibility to the industry.
Enhanced cyber security: Finally, there is a need to target cyber security, which is a top concern for the industry. The increasing amount of generation that is not subject to the NERC Critical Infrastructure Protection (CIP) standards is a top concern for the electricity industry with regards to cyber security. The rise of distributed energy resources (DERs) and the use of DER aggregators, which control many DERs across a large geographic area due to FERC Order 2222 increases the vulnerability of the electricity sector to cyber-attacks by changing the attack surface of the electricity sector. These entities and the resources they control often lack regulatory cyber security standards, such as the NERC CIP standards.
Additionally, there is a concern about the prevalence of vendor/manufacturer remote access and potential cross-border control centre operations. To address these issues, NERC is supporting the industry with advances in DER cyber security and recommended security practices for unregistered IBRs on the BPS as part of the Security Integration Strategy. This is important for securing the overall electricity ecosystem as the resource mix continues to change. NERC is identifying areas that require additional support for entities to develop, procure, and operate a secure high renewables grid. This includes guidance on equipment standards and certification, integrated security practices, and risk assessments. Regulatory oversight and understanding possible security vulnerabilities in critical infrastructure areas is crucial for identifying potential BPS reliability issues and ensuring adequate levels of security across the BPS.
The industry should not let current challenges, such as the reliable provision of essential reliability services from IBRs, distract itself from the need to focus on future issues related to system integration and interoperability. The variability of these resources, which are mostly renewable energy resources, can also affect resource and energy adequacy, posing reliability risks to the BPS that must be addressed moving forward. It is important to acknowledge that the challenges associated with BPS-connected IBRs are not due to any deficiencies in the technology, but rather due to shortcomings in processes and the absence of comprehensive standards for IBRs starting from the interconnection process through their entire lifespan. NERC is committed to executing its IBR strategy to effectively and efficiently mitigate the risks to the BPS in this area.