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FERC Proposes Rule on Ambient-Adjusted and Seasonal Transmission Line Ratings to Improve Accuracy and Transparency

On November 19, 2020, the Federal Energy Regulatory Commission (FERC or Commission) proposed a rule that would revise both the Commission’s pro forma Open Access Transmission Tariff (OATT) and its regulations under the Federal Power Act to improve the accuracy and transparency of transmission line ratings. Transmission line ratings represent the maximum transfer capability of transmission lines and factor into and affect not only determinations of available transfer capability (ATC) but into market models and curtailment and redispatch decisions. The proposed rule, Managing Transmission Line Ratings, would establish a new Attachment M to the pro forma OATT titled “Transmission Line Ratings” and would require:

  • Transmission providers to implement ambient-adjusted ratings (transmission line ratings that factor in near-term forecasted ambient air temperatures) on the transmission lines they use to provide service;
  • Regional transmission organizations and independent system operators (RTOs/ISOs) to establish and implement systems and processes so that transmission owners can electronically update their transmission facilities’ line ratings at least hourly; and
  • Transmission owners to share transmission line ratings and methodologies for determining transmission line ratings with their transmission providers and, if applicable, with their RTO/ISO market monitors. 

In addition to proposing these requirements, the Commission seeks comment on whether to require transmission line ratings and line rating methodologies to be shared with other transmission providers upon request and on whether to require transmission providers to implement “unique emergency ratings.”

The proposed rule would apply to transmission providers and transmission owners operating both within and outside RTOs/ISOs and includes a compliance schedule that would focus first on historically congested lines. While the proposed rule discusses dynamic line ratings, which are adjusted more frequently than ambient-adjusted ratings and are based on actual field data rather than forecasts, and gives transmission owners the option of implementing them, it does not require dynamic line rating implementation.

As the Commission explains, transmission line ratings affect not only transmission service availability but, because transmission line ratings affect transmission provider decisions concerning curtailment, interruptions, and redispatch and are used in RTO/ISO market models to establish commitment and dispatch, they can affect transmission congestion and the prices of energy, operating reserves, and other ancillary services. At present, however, most transmission owners and transmission providers in the United States are using static or seasonal transmission line ratings that do not accurately reflect the transfer capability of the transmission grid; instead, static and seasonal line ratings may show more or less ATC than is actually available, causing potential reliability issues and distorting congestion pricing. 

The Commission would replace static and seasonal ratings with ambient-adjusted line ratings that typically are adjusted on a daily or hourly basis. Ambient-adjusted line ratings would be used for evaluation of transmission service requests that will end within 10 days of the request and to determine the necessity of curtailment, interruption, or redispatch of transmission service that is expected to occur within 10 days. To address those issues beyond 10 days, the Commission proposes to require the use of seasonal line ratings; however, the Commission also proposes changes to current methods for implementing seasonal line ratings, such as limiting the duration of a season to 3 months.

Comments on the proposed rule are due on March 22, 2021.