The PJM Interconnection is the first grid operator to fully adopt “ambient air” adjusted transmission line ratings, a measurement that can allow for fuller use of a transmission line’s capacity, PJM said Monday.
PJM started using hourly ambient-adjusted ratings, or AARs, across its system on March 4.
“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,” the grid operator said. “PJM system changes also synchronized markets functions to adapt to the new advanced data structures and dispatch methods.”
AARs can increase capacity on transmission lines by 15% to 40% compared with static ratings, according to Ampacimon, a grid technology company.
PJM’s move was in response to Federal Energy Regulatory Commission Order 881, issued in 2021, which directed grid operators and transmission owners to use AARs for near-term transmission service.
Grid operators rate their transmission equipment by assessing how much capacity it can safely carry. Those ratings have typically been static ratings based on conservative assumptions about worst-case, long-term air temperature and other weather conditions. Those assumptions can lead to inefficiencies on the transmission system, FERC said when it issued its decision.
FERC set a July 17, 2025, compliance deadline for adopting AARs, but it has approved numerous extension requests.
The Midcontinent Independent System Operator, for example, doesn’t expect to be in full compliance with Order 881 until the end of 2028, according to a mid-January presentation.
FERC also extended the New York Independent System Operator’s compliance deadline to Dec. 31, 2028.
The Southwest Power Pool expects to go live with AARs on Sept. 1, according to an update on the grid operator’s compliance efforts. ISO New England expects to start using the ratings on Dec. 15, and the California Independent System Operator plans to start two days later.
In an offshoot from its Order 881 rulemaking, FERC has proposed requiring dynamic line ratings, which go beyond AARs by considering wind and solar conditions to set transmission line ratings. FERC issued a notice of proposed rulemaking in June 2024 and has accepted public comments on the issue.
