Space charge dynamics in polyethylene under periodical high voltage electric fields

Abstract

During the last two decades, space charge has been recognised to be a major factor influencing the electrical performance of cable insulation. A significant amount of work has been carried out to investigate space charge dynamics within polymeric insulation under high voltage direct current (HVDC) fields. Modern charge mapping techniques are adopted to obtain the necessary information about space charge within the insulation. However, the underlying physics of charge transport and the charge trapping characteristics of space charge are not well understood. Employing theoretical modelling based on numerical simulation to analyse the space charge features can provide an insight view into the charge distribution in dielectrics under realistic conditions. This thesis focuses on the analysis of space charge phenomenon within polyethylene insulation under common operating electric fields through both experimental and numerical investigations.

An improved pulsed electro‐acoustic system along with a data processing procedure has been developed to investigate space charge in polyethylene under AC and superposed AC and DC voltages. Raman spectra and Attenuated Total Reflectance Fourier‐transform Infrared (ATR‐FTIR) spectra are collected to confirm the influences of the magnitudes and frequency of AC fields on the physical characteristics of polyethylene. Evaluation of pure AC and DC voltage tests as specified in the international standard, BS EN 61378‐2:2001, has also been done by comparing the space charge profiles under the real superposed AC and DC voltage and the deduced testing voltages.

A numerical simulation model based on bipolar charge transport theory has been developed to analyse space charge phenomenon in polyethylene under periodical complex electric fields. The build‐up of space charge in polyethylene under DC electric fields has been modelled, and the simulation setting has been optimised based on the measured results. The model is also introduced to simulate the charge dynamics under AC and superposed AC and DC fields. The simulation results exhibited good agreement with the measured profiles.

Besides, the effects of applied field characteristics (frequency, field magnitude, and field composition) on the charge formation and transportation have also been investigated and using both the experimental and numerical approaches. Furthermore, the numerical model has been further applied to analyse the relationship between space charge phenomenon and electrical breakdown in insulation. It has been found that the different region where breakdown happens caused by different charge dynamics is a significant reason leading the various breakdown strengths of the same material different, under AC and DC voltages.

The outcome of this dissertation can aid the fundamental understanding of charge dynamics in the insulating materials under general operating high voltage electric fields.

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