PD pulse waveform and charge distribution in oil-pressboard insulation with needle-plate model at positive DC voltage: Their characteristics and relationship

Abstract

A better understanding of the complicated characteristics and interrelationships between the charge distributions and partial discharge (PD) pulse waveforms occurring in oil-pressboard insulation can be used to characterise the PD mechanism as a basis for the optimisation of the insulation structure to minimise PD activity. These characteristics were studied by using a non-inductive resistor to measure the PD pulse waveforms produced in oil-pressboard insulation by a needle-plate at positive DC voltage and through additional simulations based on bipolar charge transport and hydrodynamic drift-diffusion models. It was found that the PD pulse waveforms are unimodal, and with increased applied voltage, their rising and falling edges and equivalent times decrease, and their apparent charge amplitudes and equivalent frequencies increase. These parameters have a saturation state at high voltages (>45 kV) and the oil streamers have voltage-dependent shapes and evolution velocities (vs); the higher the voltages, the higher the values of vs, and the wider are the streamer bodies. Accumulated charge can form a shield layer that hinders streamer evolution. Analysis of the PD mechanism in terms of the relationships between charge distribution and PD pulse waveform reveal that the rising and falling edges of a PD pulse correspond to streamer propagation in the oil gap and charge accumulation at the pressboard surface (i.e., the charging process in the pressboard), respectively. The peak value of the PD pulse waveform is shown to be related to the conduction current in the coupling loop as reflected by the current density at the streamer head.