Understanding corona discharge activity in liquid nitrogen under harmonic AC voltages adopting UHF technique

A methodical experimental study was carried out to understand the corona discharge activity in liquid nitrogen under harmonic AC voltages, with different THDs adopting the UHF technique. A marginal reduction in corona inception voltage is observed with increase in harmonic voltages with increase in THD. The magnitude of UHF signal formed at the point of corona inception, irrespective of harmonics AC voltages, is nearly the same. The rise time of the injected current pulse generated in the negative half cycle is lower than the signal generated in the positive half cycle. The characteristics of injected current pulse formed under harmonic voltages are the same as with signal generated under 50 Hz AC voltage. The UHF signal generated by corona discharges, at the point of inception, in the liquid nitrogen electrode gap lies in the frequency range of 0.7 -1.5 GHz. Ternary diagram clearly indicates that, for corona generated UHF signal generated under harmonic AC voltage with different THDs, the location in the ternary diagram is the same. Phase resolved partial discharge studies indicate that corona activity under harmonic AC voltage is predominant when the rate of rise of voltage is high.