Diagnosing high-potential test failures in large water-cooled hydrogenerators

The Bureau of Reclamation owns and operates three 700-MW, 15-kV hydrogenerators that have water-cooled stator windings at the Grand Coulee power complex on the Columbia River in the State of Washington. Recently, routine maintenance AC high-potential tests of 21-kV RMS resulted in a steadily increasing number of stator winding groundwall insulation failures. The authors describe the effort of the Bureau of Reclamation to determine the reason for this increasing failure rate when performing routine stator winding maintenance AC high-potential test. They also describe the Bureau´s efforts to develop a measurement system to determine which stator bars are in imminent danger of failure and to develop a repair program to alleviate the failure problem