Applying UHF PD On-site Monitoring to Large-Size Power Transformer and Pulses Analysis

The use of partial discharge (PD) on-site monitoring as an indicator of HV equipments insulation condition is well known. However, the existence of external interferences has greatly reduced the credibility of PD on-site measurement technology of HV equipments. This paper describes practical application of the UHF PD detection method on a new oil-immersed power transformer and to suppress interference, an internal UHF sensor (antenna) was designed and successfully installed to power transformers for its insulation condition assessment which can effectively avoid the frequency band of the external interferences. As the frequency band of this UHF sensor was 300~1500MHz, envelope detection technique was used to extract UHF signals excited from PD sources inside the transformer. The results of MATLAB simulations have shown that UHF PD signals can be effectively extracted by this method. Using the monitoring system established in this paper, plenty of external interferences pulses and UHF PD pulses were recorded and investigated, PD pulses with small amplitude can be effectively detected, which reveal that UHF PD measurements can effectively measure small level of discharges with good sensitivity.