Study of propagation of transformer partial discharge pulses in the coupling network for a 500 kV substation

This paper discusses the development of a simulation model used to study the propagation of partial discharge (PD) pulses of transformers in a 500 kV substation. The results are used to design an on-line PD monitoring system for transformers. The impedance matrix of a 500 kV single-phase transformer is computed with a special program based on geometry data and its external coupling network is calculated with EMTP. The factors that affect the coupling network (often referred to as coupling impedance) are also analyzed. It was found that the frequency characteristic of the coupling network is complex and an equivalent lumped coupling capacitance representation is only feasible below 60 kHz. The impedance varies greatly as frequency increases. The simulated PD current responses are obtained in tank grounding and 500 kV bushing tapping grounding and the optimal monitoring point is discussed. Applying resonant frequency of PD responses as monitoring frequency is suggested; this can increase the monitoring sensitivity effectively. A practical off-line PD calibration model for transformers in operation is designed according to the simulation results.