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FERC looks to improve accuracy and transparency of transmission line ratings

November 23, 2020

by Paul Ciampoli
APPA News Director
November 23, 2020

The Federal Energy Regulatory Commission recently issued a notice of proposed rulemaking (NOPR) aimed at improving the accuracy and transparency of transmission line ratings, which represent the maximum transfer capability of each transmission line. 

FERC took the action at its monthly open meeting on Nov. 19.

At the meeting, FERC staff noted that under current typical practices, transmission line ratings are seasonal or static ratings. 

These ratings are based on conservative assumptions about the worst-case, long-term ambient conditions that equipment might face.  They are typically updated only when equipment is changed or ambient condition assumptions are revised, and therefore may not accurately reflect the near-term transfer capability of the system. 

FERC staff said that more accurate ratings include ambient-adjusted ratings (AARs) and dynamic line ratings (DLRs), both of which are the subject of the NOPR. 

Unlike seasonal or static-based ratings, ambient-adjusted ratings incorporate near-term forecasted ambient air temperatures.  Dynamic line ratings are based not only on forecasted ambient air temperature, but also on other weather conditions such as wind, cloud cover, solar irradiance intensity, precipitation, and/or on transmission line conditions such as tension or sag. 

There can be consequences to using an inaccurate representation of system transfer capability, FERC staff said. 

For example, FERC staff said that because ambient air temperatures are typically less extreme than worst case assumptions, seasonal and static transmission line ratings typically indicate that there is less transmission system transfer capability available than the transmission system can actually provide. This increases congestion costs.  At other times, however, seasonal or static transmission line ratings may overstate the near-term transfer capability of the system, creating potential reliability problems and inaccurately low congestion pricing, which may prevent occurrences of rates for scarcity pricing.  In either case, the use of seasonal and static assumptions results in transmission line ratings that do not accurately represent the transfer capability of the transmission system.

To address this concern, the NOPR proposes to require transmission providers to implement ambient-adjusted ratings and seasonal line ratings on the transmission lines over which they provide transmission service.  Transmission providers would use AARs for evaluating requests for near-term transmission service, and would use seasonal ratings for evaluating other, longer-term transmission service requests. The NOPR also proposes to revise the rules for setting seasonal ratings to make them more accurate.

In addition, the NOPR proposes to require RTOs and ISOs to establish and implement the systems and procedures necessary to allow transmission owners to electronically update transmission line ratings at least hourly. 

The NOPR recognizes that there may be instances in which transmission owners may wish to implement transmission line ratings that may be more accurate than AARs, such as dynamic-line ratings, but are unable to have such ratings reflected in RTO/ISO markets under those markets’ current capabilities.  This proposed requirement seeks to remove this barrier to adoption of these more accurate line ratings. 

The NOPR also proposes to require transmission owners to share transmission line ratings and transmission line rating methodologies with their respective transmission provider(s) and, in RTOs/ISOs, with their respective market monitor(s).  Such information sharing would increase situational awareness and improve the ability to verify the accuracy of transmission line ratings.

Finally, the NOPR seeks comment on whether to require transmission providers to use unique emergency ratings.

Comments are due 60 days after publication in the Federal Register.

The NOPR is available here.

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