Experience with UHF partial discharge measurements

Some tests on UHF measuring techniques on transformers and oil-paper insulation models have been reported in literature. The results indicate the feasibility of this method for oil-paper insulation, but are not conclusive. To have a good perspective of this method, UHF partial discharge (PD) measurements were conducted on small oil-paper insulation models, large transformer windings and PD suspect jobs. Classical phase-resolved PD patterns were recorded alongside using an in-house developed system, which permit cycle-by cycle replay and analysis of the PD patterns. Acoustic PD measurements were simultaneously recorded for comparison of relative sensitivities. Experimental investigations reveal that UHF PD measurements can detect, with good sensitivity, small levels of discharges and those buried within insulating layers. In contrast, acoustic measurements can detect small levels of discharges only when a direct line of sight is available between the discharge source and sensor. For discharges buried within insulating layers, acoustic detection is possible only for some cases when discharge level is high. This paper describes the results of the experiments.