Effect Of Moisture Content Under Ac And Or Dc Electric Field On Space Charge Characteristics In Oil-paper Insulation

Converter transformer is one of the most important components in HVDC transmission system,thus its insulation system is related to the stability and operation safety of the transmission system.And the insulation performance for oil-paper insulation in converter transformer is greatly influenced by its moisture content.In this paper,oil-paper insulation samples with 0.31%,1.12%,2.04%and 3.12%moisture contents were investigated,and the effects of moisture content on physical,chemical and electrical properties of oil-paper were analyzed by means of infrared spectroscopy,dc conductance and DC breakdown.Based on Auto Equal Phase Shift(AEPS)principle,the space charge distribution under DC electric field,power frequency AC electric field and DC superimposed AC electric field is detected,and the relationship between moisture content and space charge characteristics of oil-paper is explored to provide a theoretical basis for the condition evaluation of oil-paper for converter transformer.The infrared spectra show that the change of moisture content will not lead to significant changes for the main components of oil-paper.The moisture is partially present in oil-paper in the form of ions,and continuously reacts with various functional groups of cellulose as concentration changes.The dc conductance characteristics show that the rise of moisture content not only raises the electrical conductivity of oil-paper insulation under DC electric field,but also increases the rising rate of oil-paper conductivity when field strength increased.The breakdown strength shows that the increase in moisture content causes the breakdown field strength of oil-paper to increase first and decrease later,and enhances the pressure resistance diversity at different positions of the insulation.It can be inferred that most of the water exists in oil-paper in the form of ions,and continuously reacts with the functional groups of oil-paper,thereby affecting the electrical conductivity characteristics,breakdown characteristics and charge injection process of oil-paper insulation.Space charge studies have shown that the effect of moisture content on the charge characteristics of oil-paper can be divided into two stages:(1)During the process of increasing the moisture from 0.31%to 1.12%,the functional groups such as methyl,methylene within oil-paper are increased and the hydrogen bonds are decreased,thereby lowering the charge injection threshold and increasing the total amount of traps.Under DC electric field,the injected charge combined with traps increases,leading to the increase of average bulk charge density of oil-paper.Under AC electric field,the charge injected from upper electrode increases rapidly,and continuously neutralizes with injected charges of lower electrode,resulting in the decrease of average bulk charge density at the phase of 90~o and 270~o.Under DC superimposed AC electric field,the negative charge injected by lower electrode decreases and the positive charge injected by upper electrode increases;(2)During the process of increasing the moisture from 1.12%to3.12%,the functional groups such as methyl,methylene are decreased and the hydrogen bonds are increased.Therefore,under DC electric field,the injected charge combined with the trap is reduced,and the average bulk charge density is decreased.Under AC electric field,the charge injected from upper electrode is reduced,so that the charge of the lower electrode combined with the trap increases,resulting in an increase in the average bulk charge density at the phase of 90~o and 270~o.Under DC superimposed AC electric field,the negative charge injected by lower electrode increases and the positive charge injected by upper electrode decreases.