PJM Interconnection has become the first grid operator to implement ambient-air adjusted (AAR) transmission ratings, meeting the Federal Energy Regulatory Commission’s (FERC) Order 881 requirement to maximise the usable capacity of existing lines. The new system went live on 4 March.
Transmission line capacity depends on temperature and wind conditions, as high-voltage lines heat up when current flows. Hot, still weather reduces capacity, while cold or windy conditions allow lines to carry more electricity safely. AARs use hourly forecasts up to 10 days ahead across 47 regions in the PJM footprint, enabling more precise and dynamic management of the grid.
Previously, PJM employed temperature-adjusted, day/night and seasonal ratings, but Order 881 requires granular hourly ratings from real-time to 10 days and monthly seasonal ratings for longer-term studies. By adjusting line ratings to actual ambient conditions, PJM can dispatch electricity more efficiently, reduce congestion, and expand safe utilisation of the transmission network.
The upgrade involved a multiyear effort across Operations, Markets and IT, integrating complex real-time data streams, synchronising market functions, and updating reliability studies.
“This enterprise-wide initiative enhances PJM’s monitoring and dispatch capabilities,” said Darlene Phillips. “Our grid now operates with ratings tied to ambient temperatures and the best available forecasts, setting a benchmark for other RTOs.”
Source: PJM
