Tuned medium-band UHF PD measurement method for GIS

In order to replace the lightning impulse test, a very sensitive PD-measurement is required for onsite tests of GIS (CIGRE Joint Working Group 33/23.12 in Electra No 176, 1998). The most sensitive UHF-PD-measurement technique consists of low-noise high-gain broadband amplifiers applied directly to the PD-sensors combined with the manual selection of possible resonant frequencies in the frequency spectrum for narrowband signal extraction with a bandwidth of some MHz. Further, a correlation with the test voltage will lead to phase resolved signal display. The only disadvantage is the time consuming visual selection of the optimal centre frequency of the narrowband measurement system done sensor by sensor compared to broadband or fixed band measurement techniques. The broadband system design has the disadvantage of significantly reduced measurement sensitivity as soon as interfering frequencies are located within the measurement bandwidth. Beside this, in principle, the signal to noise ratio is lower for a broadband measurement system compared to a narrowband system. The narrowband system with fixed frequencies show poor or no sensitivity when the GIS resonant frequency caused by a PD source do not correspond with the narrow band measurement frequency or when interfering frequencies appear at the fixed measurement frequencies.

The tuned medium band UHF PD measuring system design consists of one or more manually pre-tuned band-pass filters with a bandwidth of 50–150 MHz applied in a frequency range of approx. 100–2000 MHz. The selection of the centre frequencies should be based on the individual resonant frequencies of the PD-sensors determined by the CIGRE sensitivity check (CIGRE task force 15/33.03.05 in Electra No 183, 1999) 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 PD-signals at a specific centre frequency is much lower than with the narrow band technique (Judd et al. in IEEE Trans. Dielectric Electric. Insulat. 3(2), 1996). The evaluation of measurements carried out in 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 suitable centre frequency. Using several tuned frequency bands the probability of missing the resonance frequencies of a PD-signal is even lower and additionally allows to conduct a first coarse localization of the PD-source based on the frequency dependent damping of the PD-signal. The main advantage of the proposed design is the combination of simultaneous measurement with optimal sensitivity based on selective avoiding of fixed band interfering frequencies together with tuning into the most sensitive frequency band of each individual PD sensor of a GIS.

This results 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.