The PJM Interconnection is the first grid operator to implement the Federal Energy Regulatory Commission’s requirement to maximize the usable capacity of existing grid infrastructure by employing more granular and precise thermal ratings, going live with hourly “ambient air” adjusted transmission line ratings on March 4.
PJM dispatches electricity across its transmission system to flow within safe transfer limits, or ratings, designated for all energized equipment, such as power lines, circuit breakers and transformers.
"PJM has long been at the forefront of more sophisticated ratings systems, having used temperature-adjusted ratings, day/night switches and seasonal study ratings for years before FERC Order 881," it said.
Now additionally, conforming with Order 881, PJM strictly uses hourly ratings from real-time to 10 days out and monthly seasonal ratings for longer-term studies 12 months out.
The flow of current across high-voltage transmission lines generates heat. Accordingly, weather conditions at any given time affect transmission lines’ physical ability to safely convey that electricity without overheating.
Overheated lines can expand in length and droop down until close enough to ground objects to catastrophically short out, or the lines can “anneal,” permanently losing tensile strength to stay taut while suspended. Before these potentially catastrophic events occur, PJM manages the transmission congestion or overloads by re-dispatching generation to reduce flow on the lines.
Cold weather or wind cools transmission lines so they may carry more power without overheating. In contrast, transmission line capacity contracts during hot days with little wind. In this way, wind and cold weather directly contribute to a line’s power-carrying capacity. Sometimes, cold or windy weather can dramatically increase a line’s capacity.
Cold and windy weather can increase a line’s usable transmission capacity by 10% to 40%, researchers for the U.S. Department of Energy found.
In 2021, FERC Order 881 ordered all transmission owners within grid operator regions like PJM, or standalone entities, to change how they rate transmission line capacity to incorporate weather impacts to make the grid more efficient.
FERC said more power could be safely conveyed over power lines using more granular and precise thermal limits, also known as Ambient-Adjusted Ratings (AARs). The results, FERC reasoned, would save on costs to ratepayers by conveying more power safely where it is needed, easing transmission constraints and expanding the efficient use of the existing transmission system, PJM said.
A few minutes past midnight on March 4, PJM’s systems switched over to monitoring and dispatching the grid based on the new AARs. Now, rating sets are adjusted hourly according to ambient temperatures forecast 10 days ahead for 47 separate regions with a shared weather forecast within the PJM footprint.
The switchover was the result of a multiyear effort to adapt multiple PJM systems to AARs required by FERC Order 881. Affected systems included many PJM Operations functions governing real-time monitoring and reliability studies.
PJM systems now leverage new, complex and dynamic data sets of line ratings that update every hour according to a stream of weather data of ambient air temperatures with forecasts up to 10 days ahead. PJM system changes also synchronized markets functions to adapt to the new advanced data structures and dispatch methods.
“This was an enterprise-wide effort, involving a comprehensive stakeholder process and Operations, Markets and IT working with three vendors on the design, deployment and go-live event on March 4,” said Darlene Phillips, Executive Director – Operations Engineering Support. “PJM pushed the boundaries of technology and support that other RTOs will be able to follow, and our real-time monitoring and studies of the grid are now enhanced by ratings adjusted for ambient temperatures and the best available forecasts.”
