The feasibility of using UHF sensors to measure partial discharges in liquid nitrogen

Liquid nitrogen (LN2) has been proposed as a coolant for high temperature superconducting (HTS) applications also as electrical insulation. Partial discharge (PD) activity indicates overstressing of the liquid dielectric or contamination which may provide an indication of the health of the dielectric and provide a useful precursor to the electric failure of LN2. The conventional method for PD measurements is restricted to a narrow bandwidth (less than 600 kHz). It has been shown that streamer discharges in liquid dielectrics consist of many small pulses, which due to the limited bandwidth of the PD detector would be integrated overtime by conventional equipment and recorded as single pulse. The application of ultra high frequency (UHF) sensors can provide greater precision in the measurement of the discharge pulses during the pre-breakdown streamer phenomena in LN2. In addition, the UHF technique can be installed as on-line monitoring system with the capability of locating the discharge sources using the time-of-flight method, as has been proposed for oil filled transformer designs. This paper explores the use of UHF sensors to measure PD in LN2. The experiments utilise point to plane electrode configuration located inside a cryostat with high voltage bushing rated to 50kV PD free. The sensor is mounted on the outside wall of the cryostat with uses of dielectric windows and signals are captured by a digital oscilloscope with a bandwidth of 2.5GHz. The conventional PD technique (IEC 60270) is also applied simultaneously and the results are presented and compared.