Effect of Moistures on Space Charge and Trap Level Characteristics of Oil-Impregnated Pressboards

Yi Guan,Minghe Chi,Qingguo Chen,Qian Wang,Xinlao Wei,Jinfeng Zhang

doi:10.1109/access.2020.3024768

Abstract

Oil-paper insulation as an irreplaceable insulation system in the electrical power transformer is inevitably suffering the water intrusion on working condition, which will make dielectric performance degraded and insulation aging accelerated. In high interest of explaining the deterioration effect of absorbing moisture on the dielectric properties of oil-paper insulation system and corresponding mechanism, the space charge and trap level characteristics are investigated for the specially prepared oil-impregnated pressboards with different moisture contents. The Pulsed Electro Acoustic and Isothermal Surface Potential Decay experiments are performed to test the space charge distributions and evaluate the trap level depths, respectively. It is indicated that the polarity of injected space charges is alternated with the decaying rate being raised when the absorbing moisture is aggravated in oil-impregnated pressboards. The charge carriers are more prone to be captured by the shallow-level traps that have been introduced by water permeation near pressboard surface than the intrinsic deep-level traps inside pressboard. The captured charge carries accounts for the significantly exacerbated space charge accumulation as manifested by the surface charges of oil-impregnated pressboards.

Highlights

Converter transformer, as a key equipment of high voltage direct current power transmission, works under the combined voltages of alternating and direct current (AC and DC) electrical fields, where space charges are apt to accumulate inside insulator so as to distort internal electric field and accelerate dielectric aging [1]
The mean volume densities of space charges accumulating inside the oil-impregnated pressboards with different moisture contents being polarized under a relatively lower DC electric field of 10kV/mm and dissipating with time under short-circuit are shown in the upper and bottom panels of Fig.3, respectively
Homocharge accumulate inside the oil-impregnated pressboards with the moisture content of 0.34% and 3.74%, while heterocharge accumulate inside the oil-impregnated

Summary

Introduction

As a key equipment of high voltage direct current power transmission, works under the combined voltages of alternating and direct current (AC and DC) electrical fields, where space charges are apt to accumulate inside insulator so as to distort internal electric field and accelerate dielectric aging [1]. The space charge characteristics is always a hot spot pertaining to the electrical insulation performances of converter transformers [2]. Space charge accumulations are essentially determined by charge mobility and trap level distribution [3], [4]. Pressboards and bearing the deterioration of space charge characteristics caused by the environmental moisture, dielectric aging, and operating temperature elevation [5]–[7]. It has been reported that the temperature elevation will significantly increase charge mobility and cause considerable homocharge and heterocharge accumulations inside the oil-impregnated pressboards when DC voltage is applied or polarity reversal of electric field occurs [8].

Methods

Results

Discussion

Conclusion