Investigation of Partial Discharge Activity by a Conducting Particle in Transformer Oil under Harmonic AC Voltages Adopting UHF Technique

A methodical experimental study was carried out to understand the partial discharges initiated due to particle movement in transformer oil under high frequency ac voltage and under harmonic ac voltages (with different THDs), by adopting UHF technique. It is also observed that irrespective of order of harmonics, increase in THD of the supply voltage shows reduction in levitation voltage. The rise time of the partial discharge current pulse formed due to particle movement is few ns. It is observed that the frequency content of the signal radiated due to partial discharges generated due to particle movement in the electrode gap lies in the UHF signal range, with its dominant frequency near 1 GHz. The supply voltage frequency and the time required for 100 UHF signals to form shows inverse relationship. When the applied voltage is increased above levitation voltage, the UHF signal magnitude formed due to particle movement increases, irrespective of supply voltage frequency. Ternary diagram clearly indicates that irrespective of frequency of the supply voltage, the UHF signal location in the ternary diagram is the same.