Transmission line ratings to Consider ambient weather conditions

​On December 16, 2021, in Docket RM20-16, FERC issued a final rule on Managing Transmission Line Ratings (Order 881).  Through this rule, FERC is requiring:

1. Public utility transmission providers to implement ambient-adjusted ratings for transmission lines over which they provide transmission service and to use uniquely determined emergency ratings;
2. RTOs and ISOs to establish and implement systems and procedures to allow transmission owners to update line ratings;
3. Transmission providers to maintain a database of TOs’ transmission line ratings and methodology on its OASIS site.

 
FERC defines a transmission line rating as the maximum transfer capability, computed in accordance with a written methodology and good utility practices, considering the technical limitations on conductors and other equipment (thermal flow limits), as well as technical limitations of the transmission system (voltage and stability limitations).  The transfer capability of a transmission line can change with ambient weather conditions.  Increases in temperature lower the transfer capability while decreases in temperature increase transfer capability.  The continued use of seasonal or static transmission line ratings based upon conservative, worst-case assumptions, results in suboptimization of the transmission line. 
 
FERC requires the following:

1. Transmission providers use an ambient adjusted rating (“AAR”) when a service request ends within 10 days.  AAR applies to a time of not greater than one hour and reflects the absence of solar heating during nighttime periods.  Additionally, ARRs are to be used for determining curtailment, interruption or redispatch of transmission service anticipated to occur during the 10 day or less period.   
2. For longer-term service, FERC requires the use of seasonal line ratings, including for determining curtailment, interruption or redispatch of transmission service.
3. There are exceptions to the use of ARR to ensure the safety and reliability of the transmission system.

 
Though not ordered in this proceeding, FERC initiated a subsequent proceeding, Docket No. AD22-5, to consider dynamic line ratings (“DLR”), which presents opportunities for transmission line ratings that are more accurate than those established with AARs. Unlike AARs, DLRs are based not only on forecasted ambient air temperatures and the presence or absence of solar heating, but also on other weather conditions such as (but not limited to) wind, cloud cover, solar heating intensity (instead of mere daytime/nighttime distinctions used in AARs), and precipitation, and/or on transmission line conditions such as tension or sag.   FERC adopted the definition of DLR as a transmission line rating that: (1) applies to a period of not greater than one hour; and (2) reflects up-to-date forecasts of inputs such as (but not limited to) ambient air temperature, wind, solar heating intensity, transmission line tension, or transmission line sag.           
 
In this rule, FERC also requires transmission providers to use uniquely determined emergency ratings for contingency analysis in the operations horizon and in post-contingency simulations of constraints.  Such uniquely determined emergency ratings must also incorporate an adjustment for ambient air temperature and daytime/nighttime solar heating, consistent with our AAR requirements for normal ratings. Most transmission equipment can withstand high currents for short periods of time without sustaining damage. Emergency ratings reflect this technical capability, defining the specific additional current that a transmission line can withstand and for what duration the transmission line can withstand that additional current without sustaining damage. Because emergency ratings reflect this capability, uniquely determined emergency ratings will ensure more accurate transmission line ratings.
 
FERC requires each transmission provider to submit a compliance filing within 120 days of the effective date of this final rule, revising their OATT to incorporate pro forma OATT Attachment M.   FERC further require that all requirements adopted in the rule be fully implemented no later than three years from the compliance filing due date.