FERC Rule to Improve Transmission Line Ratings
Transmission line ratings represent the maximum transfer capability of each transmission line and can change based on weather conditions. Line ratings are typically based on conservative, worse-case scenario assumptions about long-term air temperature and other weather conditions that can lead to underutilization of the transmission grid. Conversely, dynamic line ratings adjust at least hourly based on ambient temperatures and align more closely with actual operating conditions. To support the shift to dynamic line ratings, and to enhance the efficient utilization of the U.S. transmission grid, the Federal Energy Regulatory Agency (FERC) approved Order 881 on December 16, 2021.
Order 881 (which was clarified through Order 881-A in May 2022) requires U.S. transmission providers, including Independent System Operators and/or Regional Transmission Operators, to develop and implement Ambient Adjusted Ratings (AAR) for transmission lines over which they provide service by July 12, 2025.
Specifically, industry will need to:
- Use at least four seasonal line ratings when evaluating longer-term point-to-point transmission service ending more than 10 days in the future.
- Determine AARs for at least every hour for near-term (10 days into the future based on predicted ambient temperatures) requests for point-to-point and network service.
- Calculate those AARs for both day and night with the knowledge that there is no solar heating during the nighttime calculations.
While FERC Orders 881 and 881-A aim to enhance the accuracy and transparency of transmission line ratings and improve asset utilization, the use of AARs requires industry to account for the impact ambient conditions have on transmission facilities. These changes to the transmission line ratings will have some impacts on loadability (the ability of a transmission line to carry power). These impacts may require utilities to reevaluate or adjust their protection system settings to ensure that they do not limit transmission loadability under the higher AAR values, while still providing adequate protection for all fault conditions. Changes of this magnitude may lead to an extended implementation period.
To provide subject matter expertise and technical support for this effort, the NERC System Protection and Control Working Group (SPCWG) wrote a position paper aimed at helping utilities understand how the use of AAR impacts the protection system loadability requirements of NERC Reliability Standard PRC-023-4. The paper addresses how protection system settings can comply with both orders and with PRC-023-4. It also provides some examples and recommendations for different scenarios.
The position paper is available on NERC’s website here.
– Lynn Schroeder, PE, Sr. Manager Engineering at Sunflower Electric Power Corporation and MRO Protective Relay Subgroup member, and John Grimm, MRO Principal Systems Protection Engineer