Understanding the breakdown characteristics of liquid nitrogen under non-standard transient voltages

Liquid Nitrogen (LN₂) is used as an insulant as well as coolant in high temperature superconducting power equipments. Recently it is identified in HTSC transformers; the voltage distribution in the winding is oscillatory in nature. An attempt has been made to generate such unidirectional oscillatory impulse voltage (UOIV) of various frequencies to understand the breakdown characteristics of liquid nitrogen with non-uniform electric field configuration (Needle-Plane configuration). For the purpose of comparison breakdown characteristics under standard lightning impulse voltage were obtained. It is observed that the breakdown strength of UOIV and the lightning impulse voltage (LIV) are nearly same. The discharges initiated by UOIV in liquid nitrogen measured using Broadband sensor. It is observed that discharges occur at every peak of UOIV. It is high when the effective frequency of UOIV is high. It is also observed that number of discharges of various magnitudes occurs in the negative OIV. The magnitude of discharges is almost the same under positive polarity of UOIV. This high and low magnitude discharges can cause early failure of insulation. The signal measured using broadband sensor indicates that the signal lies in the UHF range. The result of the study indicates that PD occurs at the every peak of the applied OIV indicating severity of OIV.