PJM Implements Ambient-Air Adjusted Transmission Line Ratings Under FERC Order 881

PJM has implemented the Federal Energy Regulatory Commission’s requirement to maximize the usable capacity of existing grid infrastructure by employing thermal ratings, which are live with hourly ambient air adjusted transmission line ratings from 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 deployed ratings systems, having used temperature-adjusted ratings, day/night switches and seasonal study ratings for years before FERC Order 881.

Additionally, conforming with Order 881, PJM used hourly ratings from real-time to 10 days out and monthly seasonal ratings for longer-term studies 12 months out. PJM manages the transmission congestion or overloads by re-dispatching generation to reduce flow on the lines.

Researchers for the U.S. Department of Energy found that cold and windy weather increases a line’s usable transmission capacity by 10% to 40%.

On March 4, PJM’s systems switched over to monitoring and dispatching the grid based on the new AARs. The 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 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 coordinated 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.”