By Rich Heidorn Jr.
The power outage that darkened the White House and much of D.C. on April 7 began with the failure of a 230-kV lightning arrester in the Pepco portion of the Ryceville, Md., substation 40 miles south of the district, according to a briefing by the North American Electric Reliability Corp. last week.
The outage, which caused a “severe, prolonged voltage sag” in the D.C. area, began about 12:39 p.m. when Pepco’s protection systems failed to isolate a fault on the 230-kV line.
Two separate and redundant protection systems failed, the first as a result of a loose connection to an auxiliary trip relay circuit, the second due to “intermittent discontinuity” in an auxiliary trip relay circuit, according to a presentation to NERC’s Member Representatives Committee.
Pepco and Southern Maryland Electric Cooperative lost 532 MW of load for as long as two hours. Some customers automatically switched to back-up power sources, while customer protection systems separated others from the grid due to low voltage. The outage affected the Maryland peninsula bounded by the Potomac River on the west and the Chesapeake Bay on the east.
Panda Power’s Brandywine 202-MW combined-cycle plant and the Calvert Cliffs nuclear units 1 and 2 (1,779 MW) tripped offline. Brandywine returned to service after about an hour; Calvert Cliffs returned two days later.
Investigators found damage to an A-frame structure at the Ryceville substation, including pitting near burned arresters and a downed static wire. An A-phase conductor was found detached outside the fence line.
There was no evidence of burning to the A-phase arrester, suggesting that mechanical failure resulted from the arc burning off the insulator and the weight of the line breaking the arrester free from the structure.
