Stator Winding Insulation Diagnosis by Very Low Frequency Techniques

Laboratory studies were undertaken to understand the low frequency response of thermally degraded stator winding insulation. The dielectric diagnostic methods namely capacitance and tan δ measurements at different frequencies (Dielectric spectroscopy a frequency domain method) and Recovery voltage measurement (time domain method) has been used to study the dielectric response of epoxy mica insulation. Two different types of insulation systems namely Epoxy-Mica resin poor and epoxy-Mica resin rich system are evaluated. Low voltage measurements at 140 volts peak were made in the frequency range of 1 mHz to 1 kHz using Dielectric spectroscopy. The Recovery voltage measurement technique was adopted to investigate the slow polarization process in the dielectric and to assess the insulation ageing based on the analysis of polarization spectrum. This paper presents the dielectric response of epoxy mica insulation system subjected to thermal ageing.