Understanding the incipient discharge activity in liquid nitrogen under AC voltage by adopting UHF technique

Incipient discharges formed due to particle movement, corona and surface discharges in liquid nitrogen under ac voltages radiates UHF signals. The frequency content of the UHF signal generated due to particle movement at the point of inception lies in the range 0.5 - 2 GHz. Corona and surface discharge radiated UHF signal which vary with applied voltage. At higher applied voltage to the electrode gap, the UHF signal generated due to surface discharges and corona, have increased energy content of the signal in the frequency range 2 - 3 GHz. It is observed that irrespective of type of incipient discharge formation, multiple discharge action occurs, which could be observed through zero span analysis. Ternary diagram can be used as a condition monitoring tool and could help one to classify the type of discharges (partial discharge/corona/surface discharge) that occurs, for any remedial action, during operation.