The Interpretation of Corona and Dielectric Measurements on the Mica-Asphalt Insulation of a 30-Year-Old WaterWheel Generator at Grand Coulee Dam

It is important to have reliable field techniques for assessing the insulation quaility of large rotating machine stators in order to reveal any defects which could lead to a winding failure in service. Toward this objective, a variety of nondestructive measurements of internal discharges (corona) have been applied and compared on an old mica-asphalt insulated generator. Extensive field measurements of RIV corona and integrated corona charge per cycle (related to power factor tip-up) are reported and interpreted, and verified by tests on coils returned to the laboratory.