Investigation of Maintenance Tests for Generator Insulation

A large water-wheel generator winding, about to be replaced, was subjected to all available tests. When the winding had cooled in air of summer humidity, nondestructive indications of d-c strength were readily obtained and were proved, by subsequent deliberate breakdowns, to be near enough to actual breakdown strength for practical judgment of condition. This had not been found possible with dry windings. After repeated and protracted exposure to direct voltages near breakdown, no damaging effects could be discerned by d-c leakage, 60-cycle power factor (pf), or physical dissection. Under high direct voltage, artificial breakdowns were applied; resulting surge voltages were measured and found to be harmless. Power-factor-voltage tests, slot-discharge tests, and corona tests were correlated with physical conditions found on dissecting coils. Phase-to-ground impulse measurements showed a relationship between oscillation frequency and fault location. Turn-to-turn impulse tests showed definite fault detection, but could be made on only one coil at a time. One incipient turn-toturn fault was located in situ by an ionization probe, but the method used was shown to need improved discrimination. Through the collaboration of various authorities in this series of tests, greater appreciation of the potentialities and limitations of the methods was obtained.