Experiment analysis of influence of external coupling network on partial discharge monitoring of transformer

This paper discusses experiments on the propagation of a partial discharge (PD) pulse between a transformer and other apparatus inside a 500 kV substation. The results could be used to construct a practical PD calibration model and be helpful to design an on-line PD monitoring system for transformers. Some of the frequency characteristics of the external coupling network (the connection lines and neighboring equipment-often referred to as coupling impedance) of the transformer are presented. It is found that with the variation of the monitoring point, the influence of external coupling network on PD monitoring of the transformer varies greatly. When the simulated PD signal is injected through bushing tapping and the monitoring point is at tank grounding, it is sufficient to consider only lines and equipment near the transformer. The external coupling network of the transformer could be represented by a lumped capacitance approximately if the monitoring frequency is below 400 Hz. When the simulated PD signal is injected through the 500 kV terminal of the transformer, the distant lines and equipment of the transformer could not be ignored