Partial discharge measurements of streamers in liquid nitrogen under AC voltages using UHF sensor

The pre-breakdown phenomena, have been studied with the aim to further understand the use of liquid nitrogen as a part of insulation system in High Temperature Superconducting (HTS) devices. Previous studies have shown that streamer discharges in liquid dielectrics can appear in the form of pulse trains which are thought to be the results of density-change channels expanding and collapsing. This behavior cannot be observed by the conventional partial discharge (PD) measurement method due to its narrow bandwidth (less than 600 kHz). Instead, a slowly damped pulse is recorded, which shows the integration of many faster pulses. The application of ultra high frequency (UHF) sensor with the bandwidth up to 3 GHz can provide further details of discharge events during streamer initiation and propagation in LN2. An experiment has been designed to measure discharges using narrow and wideband sensors simultaneously. The conventional method following IEC 60270 standards allow measuring the apparent charge at the terminals while UHF sensor details individual events. In addition, discharge signals can be time correlated to images of density-change streamer captured by high speed digital imaging with stroboscopic backlighting. Analysis of obtained results reinforces the proposed mechanism for the characteristics of streamer production in LN2.