This presentation will discuss the Federal Energy Regulatory Commission’s (FERC) December 2021 Order 881, which requires FERC-jurisdictional electric transmission providers to (among other things) implement ambient-adjusted transmission line ratings (AARs) on certain transmission lines by July 2025. Transmission line ratings are power flow limits that prevent transmission lines from experiencing various technical problems, including overheating. Overheating a line can cause reliability, safety, equipment damage, and/or legal compliance issues. Assumptions about weather conditions can affect how such line ratings are calculated. For example, on a cold winter day, more power can be flowed over a line before overheating than can be flowed on a hot summer day. Order 881 requires the adoption of AARs that reflect both up-to-date forecasts of ambient air temperature and the different amounts of solar heating that lines experience during daytime and nighttime hours. AARs are significantly more accurate than the traditional static or seasonal line ratings, which assume worst-case weather conditions across long time periods. This improved accuracy both increases economic efficiency (by increasing transmission capacity in most hours) and protects reliability (by reducing transmission capacity to reflect system limitations in other hours). The presentation will review transmission line ratings generally before discussing Order 881's requirements for AAR implementation and related information exchange.
