Characterization of streamers in liquid nitrogen under AC voltages using UHF techniques

Pre-breakdown phenomena in liquid nitrogen (LN2), often referred as streamers, are the precursor of total breakdown due to the more conductive nature of density-change channels compared to the liquid. The paper characterizes streamer behavior in LN2 under AC voltages using ultra high frequency (UHF) and radio frequency current transformer (RFCT) measurements. The aim is to further understand the role of LN2 as insulation as well as the application of non-conventional partial discharge (PD) detection techniques in condition monitoring of future HTS devices. An experiment has been developed to measure PD events in LN2 using narrowband and wideband methods simultaneously. PD detection following IEC 60270 standards allows measurement of the amount of charge transferred at the terminals while the use of a UHF senor with a bandwidth up to 3 GHz can provide better resolution in the time domain. Results from the UHF sensor show that PD in LN2 appears in the form of multiple pulses which are suggested to be due to the expansion and collapse of the gaseous channels. The pulse burst phenomenon was not observed by the narrowband detection. Instead, a slowly damped signal was measured, which may result from the integration of multiple individual events. The experiment also allows correlation of PD measurements with images of pre-breakdown streamers using shadow photography. The characteristics of streamers in LN2 are discussed based on the observation of the density-change channels together with the corresponding discharge signature.