Estimation of the void content in HV stator coil insulation using AC bridge measurements

It is shown that tan δ tip-up alone is not indicative of the void content of HV (high-voltage) stator coil insulation. A more reliable indication of the void content is obtained by calculating the dissipated energy from bridge measurements, assuming that all of it is directly due to partial discharges (PDs), and using the equivalent void model circuit of Gemant. The significance of other dissipative mechanisms of interfacial and space-charge polarization, particularly during the occurrence of PDs, has been recognized, as have the shortcomings of the void model of Gemant, which does not account for these phenomena. A modification to the circuit of Gemant is suggested by considering the effect of space charges. A method for calculating the effective discharge inception voltage at a given test voltage is proposed