By Tim McLaughlin

PJM, the largest U.S. power grid operator, said it ordered generators to run at maximum output and bring idle power plants online immediately on Thursday evening, as it faced escalating stress from a heat dome.

PJM's orders, detailed on its emergency procedures website, were aimed at preserving reliability as it sought to maintain power on a grid serving 67 million people across the Mid-Atlantic, South and Washington, D.C., regions and the world's largest concentration of data centers.

Even before this week's heat wave that sent temperatures soaring toward 100 degrees Fahrenheit (38 degrees Celsius), PJM had been straining to overhaul a system pushed to the brink by surging energy consumption by data centers and electric vehicles.

PJM's "transmission security" action included preparing demand-response resources to reduce electricity consumption when called upon. The grid operator also instructed dispatchers to notify management and government agencies and consider public conservation appeals.

PJM data showed its operating reserves fell nearly 80% from 22 gigawatts at the start of the day on Thursday to about 5 GW by the evening, close to the minimum requirement of 3.2 GW.

The grid operator drew on standby reserve power to meet demand that approached but fell short of an all-time record.

Tight supplies and dwindling reserves on Thursday pushed market clearing prices briefly to nearly $28,000 per megawatt hour for idling power plants to come online, according to PJM operations data, up from just $100 per MWh before the start of the heat wave.

PJM stopped short of ordering voltage reductions or telling electric utilities to cut power to their customers.

PJM did not respond immediately to a request for additional comment.

The deployment of all available resources is designed to prevent more severe emergency actions and reduce the risk of customer outages if supplies tighten further or transmission conditions deteriorate.

In practical terms, the move appeared intended to avoid rolling blackouts or other last-resort measures in a region that includes the densely populated Baltimore-Washington metropolitan area.

The heat dome is forecast to last through the July 4 holiday weekend.