Study Of Moisture Content Influence On The Dielectric Properties Of Oil-pressboard Insulation Under Compound Voltage

Converter transformer is an important device in UHV DC transmission system.The valve side winding of the operating converter transformer not only withstandsAC voltage, operative-over voltage and lightning impulse, but also subjects to DCvoltage and polarity reversal voltage. So the performance of the insulation structureis very different from the general power transformer. The insulation characteristiccan be seriously affected by the moisture content. In order to study the influence ofmoisture content on the dielectric properties of oil-pressboard insulation undercompound voltage, following questions are carried out in this paper:The breakdown test platform of oil-pressboard insulation was build. Then thetreatment process of transformer oil and pressboard and the measurement methods ofmoisture content and dielectric parameters of oil-pressboard insulation were present.According to the structural characteristics of the converter transformer, typicalphysical model and circuit model were established.The influence of moisture and temperature on the resistivity and relativepermittivity of transformer oil and oil-immersed pressboard were studied. Based onthe measured results, the electric field distribution of oil-pressboard insulation wascalculated when the water reached equilibrium state. Meanwhile the breakdownexperiment of oil-immersed pressboard with different moister content under DCvoltage, AC voltage, AC superimposed on DC voltage was carried out, then theeffects that the moisture and AC content had on the breakdown voltage of oil-immersed pressboard were studied.The results show that, with the increasing of moisture content, the resistivity ofoil-immersed pressboard and transformer oil both decline exponentially, the relativedielectric constant of oil-immersed pressboard increases exponentially and therelative permittivity of transformer oil keeps unchanged basically. Electric fieldstrength in oil-immersed pressboard will be reduced, meanwhile the electric fieldstrength in oil will be enhanced with the increasing of moisture content when moisture reaches equilibrium state in oil and pressboard. The effect of moisturecontent on breakdown voltage of oil-immersed pressboard will be influenced by ACcontent. The breakdown voltage will not change when AC voltage is applied on oil-immersed pressboard. When the rate of AC voltage is low, the breakdown voltage ofoil-immersed pressboard will increase until a peak voltage, then decrease to a certainvoltage. The research results can provide a reference for the design and operation ofconverter transformer.