Whose Line Rating Is It Anyway? FERC Enters the Line Rating Discussion

On November 19th, the Federal Energy Regulatory Commission (FERC) issued a Notice of Proposed Rulemaking (NOPR) in Docket No. RM20-16-000 to reform the pro forma Open Access Transmission Tariff and its own regulations under the Federal Power Act to improve the accuracy and transparency of transmission line ratings.1 For the first time, FERC is proposing that transmission providers adopt ambient adjusted ratings for shorter-term transmission service requests, day ahead and real time market operations. The NOPR was issued following the Commission’s August 2019 staff whitepaper and September 2019 technical conference in Docket No. AD19-15-000.

These are the key elements of FERC’s NOPR:

All transmission providers must use Ambient Adjusted Ratings (AARs) (1) as the basis for evaluation of transmission service requests that will end within 10 days (start to finish) of the request and (2) as the basis for determining the necessity of certain curtailment, interruption, or redispatch of transmission service that is anticipated to occur within those 10 days, including posting of available transfer capability (“ATC”) or other information on OASIS.

All transmission providers must use Seasonal Line Ratings (1) as the basis for evaluating “longer-term” transmission service requests that will last longer than 10 days, including Network Transmission Service and (2) as the basis for determination of necessity of curtailment, interruption, or redispatch that is anticipated to occur more than ten days in the future, including posting of ATC or other information on OASIS.

FERC is not proposing the use of Dynamic Line Ratings (DLRs) at this time. However, FERC is seeking comment: (1) on whether to require Regional Transmission Organizations (RTOs) and Independent System Operators (ISOs) to conduct a one-time study of the cost effectiveness of DLR implementation, and if so, what details and format that study should include and (2) on the additional costs, if any, needed to comply with the proposed requirement that RTOs/ISOs also be able to accommodate frequently updated transmission line ratings from transmission owners and (3) on whether there is any need to extend the requirement to accommodate frequently updated ratings to transmission providers that operate outside of an RTO/ISO.

FERC is also seeking comment on whether to require transmission providers to implement unique Emergency Ratings that would be used during post-contingency operations.


RTOs and ISOs will be required to establish and implement the systems and procedures necessary to allow transmission owners to electronically update transmission line ratings at least hourly. RTOs/ISOs must revise their dispatch and unit commitment models to implement AARs for both day ahead and real time markets and any intra-day reliability unit commitment or reliability assessment commitments. For day ahead markets, systems must have at least hourly capability for utilizing AARs.

Transmission owners must 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). FERC is also seeking comment (1) on whether to require transmission owners to share their ratings and ratings methodologies with other transmission providers upon request and (2) on whether to require transmission owners to make their transmission line ratings and rating methodologies available to other interested stakeholders, including posting information on their OASIS pages or other password protected online forum.


FERC is requesting comment on the proposed rule within 60 days after publication in the Federal Register (yet to occur as of this writing) and is proposing a staggered implementation for the ratings components of the rule. For historically congested transmission lines, AARs must be implemented within one year from the time that a compliance filing is made by the transmission provider with all other lines requiring AARs or seasonal line ratings.

Based on recent rulemakings at FERC, it is possible a final rule could take effect around Q1 2022. That would require that AARs be implemented for historically congested transmission lines in Q1 2023 with all other lines in Q1 2024.


More data and equipment. In order to comply with requirements for AAR or DLR calculations, additional data on ambient temperature, humidity, wind speed, wind direction and other factors will need to be captured in real-time. This will include field equipment, more granular weather/wind forecasting and modifications to SCADA to provide real time ratings for state estimation and ATC calculations. Additional training will also be needed for system operators and operations support staff to ensure AARs are properly monitored in real-time and accounted for in short-term planning studies.

Revised methodologies, processes and documentation for NERC compliance. New rating methodologies need to be documented to ensure compliance with regional and NERC facility rating standards and FERC guidance from the NOPR. New processes will need to be developed for AAR and DLR calculations. Other affected documents will include the Capacity Benefit Margin Implementation Document (CBMID) and Transmission Reliability Margin Implementation Document (TRMID). On the operations front, calculation of IROLs/SOLs will need to be reviewed and adjusted as necessary.

Open Access Transmission Tariff revisions. Changes to line ratings inevitably impact the calculation of Available Transmission Capacity/Available Flowgate Capacity (ATC/AFC). The methodology for ATC/CBM/TRM calculation needs to be revised to reflect changes made to the ATCID, CBMID and TRMID. These changes may also lead to changes to short-term and secondary non-firm transmission service.

OASIS Changes. Depending on the method of automation used in the calculation and posting of ATC, software enhancements for real-time calculation tools, algebraic decrementing of ATC based on confirmed service, incorporating real-time transmission modeling for ATC values all need to be implemented.


GDS recommends the following steps to be ready for the line rating NOPR and its eventual implementation:

Be Engaged. Change is coming. Stay abreast of the FERC NOPR and subsequent technical conferences. Get with your consultant and FERC counsel to let your voice be heard. Participate in various associations such as APPA and NRECA.

Be Prepared. Start doing your research now to look at tools and equipment you might need to implement the NOPR. Begin assessing your documentation and processes such as your Facility Rating Methodology, ATCID, CBMID, TRMID, TPL practices and IROL/SOL methodology. Leverage the expertise of your consultants to assist in the transition.

Be Communicating. Talk to your SCADA and OASIS vendors to help guide them by communicating what you need. Initiate the conversation with your utility neighbors to discuss how to best coordinate ratings on shared facilities.

Leverage GDS. Let us help you evaluate your organization’s readiness and develop a workplan for closing any gaps.

For more information or to comment on this article, please contact:

Paul Kelly, Managing Director  | CONTACT
GDS Associates, Inc. – Marietta, GA
770-799-2359 or paul.kelly@gdsassociates.com





John Chiles, Principal  | CONTACT
GDS Associates, Inc. – Marietta, GA
770-799-2423 or john.chiles@gdsassociates.com