Optical measurement of partial discharges in silicone gel under repetitive pulse voltage

Silicone gels are used in power semi-conductor assemblies to seal the system against moisture and atmospheric contaminants, and also to eliminate partial discharges that may occur in air at high voltage (up to 6.5 kV in recent insulated gate bipolar transistor -IGBT- modules). The electrode geometry considered here is constituted by a ceramic substrate used in modules. PD´s can be initiated at the sharp edges of copper tracks. Taking advantage of the transparency of gels, optical measurements of PD´s are carried out in this work both under AC and repetitive pulses. It is shown that optical measurements lead to a very high PD detection sensitivity (<1pC). Results obtained allow to distinguish voltage thresholds at which electroluminescence, partial discharges, and irreversible gel degradation successively occur when the voltage is increased. The influence of the shape of the applied voltage and frequency are investigated. These measurements clearly show the influence of space charges on partial discharge inception, that depends on the peak to peak voltage rather than on the instantaneous applied voltage. It is concluded that classical PD measurements under AC voltage do not adequately represent the actual performance of gels subjected to repetitive pulses.