NERC Recommends Approaches for Underfrequency Load Shedding Programs

In a recently released reliability guideline, NERC recommends additional approaches for Underfrequency Load Shedding (UFLS) program design to help utilities effectively consider the effects of Distributed Energy Resources (DERs). The guidance was developed to address the accelerated transition of the power system to locally installed, decarbonized resources that depend on inverters. These new technologies introduce operational controls issues into the electric grid. UFLS data gathering and analysis methodologies may require modification to address reliability risks.

Underfrequency Load Shedding

UFLS programs are the last line of defense for the reliability of the power system. They are designed to disconnect predetermined amounts of end-use load automatically if system frequency falls below pre-specified thresholds. Some UFLS programs include multiple levels of load disconnection to combat falling frequency. All UFLS frequency thresholds are set below the frequency drop expected from the largest contingency events in each Interconnection to avoid spurious load disconnection. UFLS programs are also designed to coordinate with generator underfrequency protection to avoid underfrequency operation induced generator damage. Typically, the first stage of UFLS operation occurs in the frequency range of 59.5 Hz to 59.3 Hz; however, various regions of the Bulk Power System may have different thresholds for UFLS operation based on regional preferences for local reliability needs.

UFLS Compliance Guidance

NERC’s new guidance calls for UFLS program designs to be reviewed because the programs can be significantly impacted by DER penetration levels on distribution feeders. DER penetration should be considered as part of Planning Coordinator studies and in the management of UFLS relay settings and locations as part of program implementation. While the arming of UFLS feeders plays an important role in the implementation of the program, the major decision points on quantity of load armed for UFLS, intentional time delays, and study case setup call for revisiting best practices in the study process to mitigate any potential reliability risks that the high penetrations of DER may have on UFLS programs.

NERC recommends that utilities and other organizations performing UFLS studies should:

• Include dynamic models of both utility-scale DER (U-DER) and retail-scale DER (R-DER) for DER modeled in their simulation.
  • At a minimum, U-DER voltage and frequency trip models should be included.
• Ensure accurate modeling of BPS-connected generators, including the following:
  • On-line operating reserves
  • Governor response
  • Voltage and frequency trip protection settings
  • Over excitation limitations and under excitation limitations if present
  • Power system stabilizers if present
• Include additional study cases reflecting load conditions other than peak load when developing the UFLS program.

The guideline discusses the impacts of  DERs on electrical island frequency, which UFLS programs are designed to support. The guideline provides detailed recommended practices to overcome the identified issues and preserve the UFLS program’s ability to act as the last line of defense.

Next Steps

TRC clients with UFLS programs are advised to carefully review the guideline’s findings and recommendations. TRC clients should be prepared to modify their UFLS data gathering and analysis programs as necessary to address the risks to UFLS programs brought about by the high penetration of DER some regions are experiencing.

With expertise in all power system planning and power delivery protection system engineering, TRC can provide independent project management services to review your company’s UFLS program. TRC can also offer an independent review of your company’s UFLS program under the mandatory PRC-006-5 standard requirements and any applicable regional variations of the standard.

TRC is well positioned to assess your UFLS program’s ability to perform as expected and in compliance with NERC mandatory standard including this new guidance in the face of a changing industry.

Resources

• NERC: Recommended Approaches for UFLS Program Design with Increasing Penetration of DERs
• TRC Protection and Controls Solutions
• TRC System Planning Solutions

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