The process of creeping discharge-caused damage on oil/pressboard insulation

Authors: RUIJIN LIAO, ENDE HU, LIJUN YANG, LIAN DUAN

Abstract: This paper presents experimental research on creeping discharge by using cylinder-plate electrode configurations under AC voltages. The process of creeping discharge-caused damage on the oil/pressboard insulation was studied. First, the electric field distribution was achieved by Multiphysics software simulation. Afterwards, the phenomena that occurred in the entire damage process, such as ``white smoke'', ``white mark'', and ``black mark'', were recorded and analyzed. Furthermore, the micromorphology of the oil-impregnated pressboard was observed via scanning electron microscope (SEM). Finally, the inner mechanism of the damage to the oil/pressboard insulation was explored according to the phenomena and the SEM morphologies. Results showed that during damage processes, the high electrical field strength (nearly 25.853 MV/m) at the weak-link point between the cylinder electrode and the pressboard directly caused the incipient discharge. The cavity, moisture, impurity, solid particle, and formation of a gaseous channel all contributed to the development of the damage. The ``white smoke'' consisted of gases that stemmed from the ionization of oil and evaporation of moisture. The ``white mark'' was the gas channel and pressboard carbonization was caused by discharge and high temperature, both of which were also the main causes of the emergence of ``black mark''. SEM images revealed that the pressboard successively experienced ``white solid'', ``crack'', and ``pitting'', which changed its surface roughness. The distorted electric field caused by gases, solid particles, and pitting further damaged the oil/pressboard. The pitting evolved into the starting point of the electrical trees and gradually led to the final breakdown.

Keywords: Transformer, oil/pressboard insulation, creeping discharge, damage process, COMSOL simulation, surface morphology

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