U.S. Northeast power grids survive heat wave

* Three nuclear power plants shut in region
* New York, PJM activate demand response programs
* Con Edison reduces voltage in New York City

By Scott DiSavino
July 18 (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 a couple neighborhoods in Manhattan
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 to heat water and some motors run slower.
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 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 that 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 the 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 United States 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 the
voltage 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 the
Midwest Transmission System Operator (MISO), which oversees 11
U.S. Midwest states and Manitoba in Canada. Demand in the MISO
on Tuesday was expected to top the region's all-time usage
record of 98,526 megawatts (MW) set in July 2011.
One megawatt can power about 1,000 homes.
Officials in the MISO were not immediately available for
comment on Tuesday's actual usage.
No other power grids forecast peak demand this week would
reach all-time levels.
For a factbox on record peaks, see

DEMAND RESPONSE
In New York, the New York ISO, like PJM activated its demand
response programs to reduce relieve some of the stress on the
power lines.
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 they take from the grid.
The biggest power companies in the 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.