Failure of the electrical insulation in hydrogenerator windings is the main cause of hydrogenerator outages and rewinds. The condition of the insulation can be evaluated by either on-line tests or off-line electrical tests. For stator winding testing, the only on-line test is the partial discharge test. For off-line stator tests, the insulation resistance, polarization index, power factor, power factor tip-up, off-line partial discharge and the AC or DC hipot test can be used. For on-line rotor winding testing, only the air gap flux test can determine if rotor winding shorted turns on present. Rotor off-line tests include the insulation resistance, surge test, pole drop and AC or DC hipot tests. These various tests will be briefly described, as well as what problems they could detect.
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