The report says transmission lines have been functioning normally and available for use an average of 99.6 percent of the time, not including for planned outages, since tracking began three years ago.
Most outages residents experience stem not from the bulk system, but from smaller failures in distribution systems managed by local utilities and regulated by states. Not including storm-related outages, the average U.S. customer goes without power 1.2 times annually for a total of 112 minutes, according to PA Consulting Group.
The bulk power system is changing, a result of the declining use of coal and nuclear power and the rising use of natural gas and renewable power.
One-sixth of the existing coal capacity is projected to close by 2020, much of it at small, inefficient units in the Ohio River Valley, the Mid-Atlantic and the Southeast, according to the Energy Information Administration. The permanent closure of four nuclear reactors in California, Florida and Wisconsin was announced this year, and reactors in New York, Vermont and elsewhere may also close.
Plant shutdowns mean there’s less of a cushion in electrical capacity when power demand is high or problems arise. Shutdowns also create pockets of transmission congestion or regions where power is scarce. Both situations drive up power prices for customers, make the grid less stable and present planning challenges.
“That is a new stress that we hadn’t thought about” a decade ago, said Scott Moore, vice president of transmission engineering and project services at American Electric Power, one of the nation’s biggest utilities.
The reliability report raises concerns about the Texas grid, one of the three major U.S. grids, where the amount of wiggle room in capacity is expected to dip below targeted minimum levels.
Growing reliance on natural gas-fired generation also creates weak spots.