The quality of the stator winding insulation system should be verified according to the following brief description of the test procedure.
The test requires an AC power source capable of supplying PD-free 50 or 60Hz variable voltage up to at least the rated line-to-ground voltage of the insulation system under test. Partial discharge measurements are taken in voltage steps up to the rated line-to-ground voltage of the insulation system to measure the discharge inception voltage (DIV) and the discharge extinction voltage (DEV). The maximum PD magnitude at line-to-ground voltage should be less than 50pC.
"IEEE Recommend Practice for Measurement of Power Factor Tip-Up of Electric Machinery Stator Coil Insulation"
"Test of insulation of bars and coils of high-voltage machines"
"Guide for a test procedure for the measurement of loss tangent of coils and bars for machine windings"