Influence of Harmonic AC Voltage on Surface Discharge Formation in Transformer Insulation

The Ultra High Frequency (UHF) signal radiated during the surface discharge process under harmonic AC voltages with different Total Harmonic Distortion (THD) shows a dominant frequency at 0.8 GHz. The surface discharge inception voltage decreases with an increase in harmonics and THD as observed in the present work. The peak factor of the harmonic voltage and the surface discharge inception factor show inverse relationship. The rise time of injected current pulse due to surface discharges measured using High Frequency Current Transformer (HFCT) is about 600 ps and 900 ps for the positive and negative current signal, respectively. Cumulative energy spectra and Ternary plot confirms that UHF signal generated due to surface discharge process does not significantly vary under harmonic AC voltage with different THD's. The magnitude of UHF signal formed due to surface discharge under 50 Hz AC voltage is higher than that formed under the harmonic AC voltage. Surface charge measurement after corona charging process with the oil impregnated pressboard material indicates that the amount of charge deposited is high with the specimen damaged under harmonic AC voltage. Degraded zone of oil impregnated pressboard insulation due to surface discharges was analyzed using Attenuated Total Reflectance Fourier Transform Infra-red (ATR-FTIR) spectroscopy. Phase resolved partial discharge (PRPD) studies indicate that surface discharge activity predominates when the rate of rise of voltage is high.