As renewable energy proliferates across the US power system, the North American Electric Reliability Corporation (NERC) continues to actively address reliability risks resulting from the implementation of inverter-based resources (solar and wind generation technology) connected at both transmission and Distributed Energy Resources (DER) levels.
Based on the findings of multiple disturbance reports and the work of a dedicated industry task force, NERC has issued guidance related to performance and interconnection that aims to protect reliability and limit disruptions as the energy landscape evolves.
What Is an Inverter-Based Resource?
An inverter-based resource (IBR) is a piece of equipment that regulates the flow of power by converting direct current (DC) electricity to alternating current (AC) electricity. It can have different specific uses depending on the application. This category of devices includes renewable energy generators and backup power storage options. IBRs are essential pieces of equipment in energy generation technology, as they serve as the “brains” of the entire operation.
Types of Inverter-Based Resources
Examples of IBRs include:
- Grid-tied: These inverters convert DC power into AC power. A grid-tied converter connects to the electrical grid and processes the power between the energy system and the electrical grid.
- Standalone: A standalone inverter works independently of the electrical grid and uses batteries or rectifiers to convert DC to AC power.
- Battery-based: As the name implies, battery-based inverters require a battery and process DC to AC power as a backup option when necessary.
All inverters are set to deliver AC power in one of three phases — single-phase, split-phase or three-phase — depending on the specific type of IBR and the application.
Applications of Inverter-Based Resources
IBRs have numerous applications wherever solar engineering or a wind power system is used, including:
- Residential use: The most common application for IBRs in a residential sense is for residential solar panels, where IBRs are necessary to create usable power. An inverter works in a solar panel system to convert DC energy created by sunlight to AC power that supplies electricity for the home. If homeowners have battery systems for their solar panel systems, battery-based inverters can supply backup power in situations when the electrical grid is down or the sun isn’t out.
- Commercial use: In a commercial sense, IBRs are necessary for converting power in different vehicles, such as marine vehicles. Additionally, commercial properties may use renewable energy generation or battery banks for backup power, which also require the use of inverters.
- Industrial use: On an industrial scale, IBRs are used to protect critical power loads and prevent costly power interruptions.
Advantages and Challenges of Inverter-Based Resources
IBRs come with various advantages and challenges that companies should understand to make the most of them.
Some specific advantages of IBRs include:
- Cost savings: Using systems powered by inverters can potentially lead to cost savings. For example, solar panel systems require inverters and can help properties save immensely on electricity by harnessing solar power. When battery-based inverters are used, properties still have electricity without needing to rely on the electrical grid.
- Reliability and efficiency: Depending on the type of inverter used, inverters can supply high levels of reliability and efficiency, so users never have to worry about their electricity or power levels.
- Flexibility and scalability: IBRs are versatile, so commercial and industrial users can implement them for various applications. If greater power is required down the line, users can scale their IBR use to meet their needs.
- Environmental benefits: Because inverters are commonly used in renewable energy systems, IBRs can help contribute to sustainability. Further, IBRs help supply efficient power, allowing users to mitigate their energy use.
These are potential challenges related to IBRs, including:
- High initial costs: Some IBRs are less cost-effective, meaning users may be faced with steep upfront costs when implementing IBRs.
- System complexity and maintenance requirements: In many cases, such as industrial applications, IBR management can require immense technical knowledge. To properly maintain IBRs, users may require the assistance of knowledgeable professionals, which can increase costs.
- Safety concerns for grid-tied systems: There can be considerable safety issues related to inverters, particularly when “islanding” occurs in solar panel systems. This issue happens when properties continue generating electricity despite the electrical grid being down. If electricity continues to go back to the electric grid when it’s down, workers could be severely injured. Fortunately, precautions can be put in place to prevent islanding, such as the use of batteries.
- Limited power storage capacity for battery-based systems: While battery-based IBRs can provide electricity in a backup form, they can’t provide unlimited amounts of power. This potential challenge is something to note for battery-specific applications.
Disturbance Analysis: Key Findings
In recent years, several transmission system disturbances have provided valuable information regarding the ability of inverter-based resources to ride-through and recover.
In August 2016, the Blue Cut Fire in California caused a fault on the Southern California transmission system. It was cleared normally. However, it resulted in a significant number of solar PV resources erroneously tripping due to miscalculated frequency. An investigation found that many other resources were operating in a previously unknown mode called “momentary cessation.”
A coordinated event analysis and disturbance report published in June 2017 concluded that this is a systemic issue requiring action by industry. NERC issued a Level 2 Alert to collect data on the extent of the reliability risk and to provide recommended performance improvements for existing and new solar PV resources connecting to the Bulk Power System (BPS).
In October 2017, the Canyon 2 Fire in California resulted in a transmission disturbance that again triggered the loss of solar PV resources as a result of a fault that was cleared normally. In this case, the predominant cause of tripping was attributed to sub cycle transient over-voltages resulting in inverter tripping at solar PV facilities.
Momentary cessation was also a contributing factor. NERC ERO, working with California ISO and Southern California Edison, published a disturbance report in February 2018 to outline key findings and recommendations. A Level 2 NERC Alert was issued to provide recommended performance improvements and to initiate modifications to eliminate momentary cessation for existing resources to the greatest possible extent.
As the NERC Alert for the Canyon 2 Fire disturbance was being rolled out, two additional disturbances occurred, related to the April 2018 Angeles Forest Fire and May 2018 Palmdale Roost Fire. NERC issued a disturbance report to document the primary contributors to solar PV output reduction in these events. The causes were similar in nature to the Canyon 2 Fire disturbance. However, a unique finding in these two events was that some amount of DER also tripped in the local area, causing a net load increase after the disturbance.
Inverter-Based Resource Performance Task Force
Each disturbance analysis has led to new findings and recommendations regarding inverter-based resource performance. The causes of tripping and reduction of solar PV output have led to industry efforts to mitigate these issues. Collaboration between multiple industry groups working together to understand the system needs and the capabilities of inverter-based resource technology was essential.
NERC formed the Inverter-based Resource Planning Task Force (IBRPTF), which reports to both the NERC Planning and Operating Committees. While the IBRPTF started as a small group of industry experts that helped in the analysis and understanding of grid disturbance, it has grown to well over 200 members and has wide-reaching industry support and participation.
The group includes the major inverter manufacturers in North America; generation entities; transmission planning and operations entities; national labs and research institutes; modeling and simulations experts; renewable energy integration experts; and other subject matter experts in the area of inverter-based resource performance, control and protection. Through its ongoing work, the IBRPTF has informed the development of important NERC reliability guidance.
NERC Reliability Guideline: BPS-Connected Inverter-Based Resource Performance
The response of inverter-based resources to grid disturbances, unlike synchronous generation, is predominantly driven by the controls and logic programmed into the inverters that interface these resources with the grid. The current FERC Generator Interconnection Agreements and NERC Reliability Standards do not prescribe the exact behavior of generating resources.
However, industry leaders (including inverter manufacturers) acknowledged that the implementation of standardized performance specifications would help support consistent response from generating resources connected to the transmission and distribution system. For that reason, the IRPTF developed a reliability guideline recommending performance specifications.
The guideline has been adopted in many ways by the inverter manufacturers that work with generator owners and project developers as a cornerstone document regarding the expected behavior of inverter-based resources connected to the BPS. While NERC has jurisdiction over Bulk Electric System (BES) resources, the guideline explicitly states that the recommendations provided are applicable to all resources connected to the BPS (not just BES resources). Refer to this guideline for detailed information regarding the expected and recommended performance of inverter-based resources connected to the BPS.
NERC Reliability Guideline: Improvements to Interconnection Requirements for BPS Connected Inverter-Based Resources
This guideline builds on previously published guidance which recommended performance specifications and a repository of technical reference material regarding inverter-based resource behavior during normal grid operation and disturbances. It aims to provide more clear and consistent interconnection requirements for newly interconnecting inverter-based resources.
While these recommendations are applicable to all inverter-based resources connected to the BPS (including BES and non-BES resources), many of the recommended specifications are not mandatory nor enforceable. Further, most resources connected to the BPS are not subject to NERC reliability standards since they have a capacity of less than 75 MVA. It is particularly important that the local transmission owner interconnection requirements, per NERC FAC-001-3, and interconnection studies per NERC FAC-002-2, evolve with new technologies and the capabilities of inverter-based resources.
Achieve NERC Compliance With TRC
Transmission owners are advised to carefully review the NERC Inverter-Based Resources Guideline and Interconnection Reliability Guideline for Inverter-Based Resources. Generation owners should review these same documents to understand why the guidelines are being implemented.
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