Tuned Medium Band and Wideband UHF PD Measurement System for GIS

Abstract:
In order to replace the lightning impulse test during commissioning procedure or for a regular check, a very sensitive and resilient PD-measurement is required for onsite tests of GIS. The most sensitive UHF-PD-measurement method consists of low-noise broadband amplifiers applied close to the PD-sensors and the manual selection of the measurement frequency to cover possible resonant frequencies. Further a correlation of detected pulses with the test voltage will lead to phase resolved signal display. The disadvantage of narrowband measurement systems are the time consuming sequential check of the centre frequency compared to broadband systems. The broadband system design with a detection band of often more than 1 GHz, has the disadvantage of significantly reduced measurement sensitivity as soon as interfering frequencies are located within the measurement bandwidth. The here shown tuned medium band UHF PD measuring system design consists of a manually tuned band-pass filters with a bandwidth of 80 MHz applied in a frequency range of approx. 0.1 to 2 GHz. The 80 MHz bandwidth is significant wider that typically tunable narrowband measurement systems. The selection of the center frequencies should be primarily based on the individual resonant frequencies of the PD-sensors determined by the CIGRE sensitivity check (CIGRE Technical Brochure 654) on site. The medium bandwidth allows to integrate the individually shifted resonant frequencies of a PD-signal at a PD-sensor within the measurement band, caused by different PD locations. Due to the wider bandwidth the probability of missing resonant frequency at a specific centre frequency is much lower than with the narrow band technique which typically have just a bandwidth of around 3 MHz or less. An earlier evaluation of measurement at different environment and different types of GIS showed a high possibility that a medium bandwidth is still narrow enough to avoid fix frequency disturbances by the use of a sufficient centre frequency. This result in an optimized system design for PD-measurements at on site tests of GIS and monitoring purposes and therefore in a high sensitivity of the measurement even in difficult situations due to interfering frequencies by making use of the combination of different methods and settings.