By Scott DiSavino
(Reuters) - Power companies in the U.S. Northeast and Mid-Atlantic kept electricity flowing to most consumers throughout a three-day heat wave that had air conditioners humming despite the shutdown of three nuclear plants.
New York City electric company Consolidated Edison, which powers the biggest and most congested city in the United States, reduced voltage in neighborhoods in Manhattan and Brooklyn on Wednesday, allowing workers to fix some failed equipment.
Customers do not lose power in a voltage reduction, but incandescent lights, for example, glow dimmer, hot water heaters take longer and some motors run slower.
Con Edison reported a new 2012 peak for electrical demand at 1 p.m. of 12,836 megawatts, eclipsing the previous high for the year set on Tuesday but below the record peak of 13,189 MW set on July 22, 2011. One megawatt can power about 1,000 homes.
Temperatures in New York City hit 91 degrees Fahrenheit (33 Celsius) on Monday and 96 on Tuesday, and were expected to reach 100 on Wednesday. Thunderstorms that hit the New York area Wednesday afternoon were expected to break the heat wave, bringing temperatures closer to normal levels in the 80s by Thursday, according to AccuWeather.com.
The nuclear units shut by early Wednesday included Constellation Nuclear Energy Group's Nine Mile Point 1 in New York and Calvert Cliffs 1 in Maryland, and Exelon Corp's Limerick 1 in Pennsylvania.
Calvert Cliffs 1 however was back up and running earlier Wednesday morning.
Constellation Nuclear Energy is owned by units of Exelon and French power company Electricite de France SA (EDF).
Two of those nuclear units are located in the PJM power grid, which told generators and transmission owners to prepare for heavy air-conditioning demand but did not take any steps that customers would notice to reduce usage on Wednesday.
PJM is the biggest power grid in the U.S., serving more than 60 million people in 13 Midwest and Mid-Atlantic states and the District of Columbia.
On Tuesday, PJM activated its demand-response programs in parts of Virginia, West Virginia and Ohio to help maintain voltage levels as a lot of power flowed west across those states into the Midwest region.
Some of that power was moving into the grid operated by MISO, the Midwest transmission system operator that oversees 11 U.S. Midwest states and Manitoba in Canada. Demand in the MISO regions on Tuesday fell short of the record high set in July 2011, and lower demand was forecast for Wednesday.
No other power grids forecast that peak demand this week would reach record levels.
In New York, demand-response programs to relieve some of the stress on power lines were also activated.
Demand response programs pay consumers to cut back on electric use during peak times or when power prices are high by shutting off unnecessary lights, elevators and other equipment, reducing air conditioning and even turning on backup generators to reduce the amount of power taken from the grid.
The biggest power companies in regions that baked during the latest heat wave include units of Duke Energy, Exelon, FirstEnergy Corp, American Electric Power Co Inc, Xcel Energy Inc, Con Edison, National Grid PLC and Northeast Utilities.
(Additional reporting by Jeanine Prezioso; Editing by David Gregorio and Ciro Scotti)