Experimental Study Of Moisture Effect On Dielectric Properties Of Oil-paper Insulation During The Cooling Process

Oil-paper insulation is the main insulation type for transformers. Theoperation reliability of transformers is largely determined by dielectric propertiesof oil-paper insulation. For a long time, research on aging of the oil-paperinsulation system has always been carried out by a lot of scholars, especially theeffect of thermal aging. However, in recent years, extremely cold weather occursfrequently in the northeast, malfunction of gas protection in the main transformeroften happened due to the cold weather. Therefore, it is necessary to study ondielectric properties of oil-paper insulation at low temperature. The research canprovide theoretical guidance and reference data for the low-temperature operationof the transformer, and has a certain theoretical and engineering significance.The low temperature oil-paper insulation test platform is built firstly to meetwith needs of further work. Then dielectric properties of oil/oli-paper insulationare tested by the platform at different initial moisture, different cooling rate anddifferent oil-paper proportion conditions. The results show that during the coolingprocess, the conductivity-temperature curves of transformer oil are single peakcurves. When initial moisture increases, the conductivity increase and theextreme point of conductivity moves to high temperature. The conductivitycurves of oil-paper insulation decreases with the decrease of temperature. Thedecline rate of conductivity decreases with the increase of initial moisture and thedecrease of paper proportion. The trends of loss factor curves of transformer oilare the same as that of conductivity. The loss factor of oil-paper presents thetrend of first decrease then increase with the decrease of temperature, and theextreme point of loss factor moves to high temperature with the decrease of initial moisture content and increase with the proportion of paper. The relative dielectricconstant of oil and oil-paper composites increases monotonously with thedecrease of temperature, and the value increase with initial moisture content. Thebreakdown voltage of transformer oil presents the trend of first decrease thenincrease with the decrease of temperature.It can be seen that moisture has a great effect on dielectric properties ofoil-paper insulation during the cooling process through the results. Therefore, it isimportant to control the moisture content in the operation of transformer at lowtemperature and it is the guarantee of transformer’s stable operation at lowtemperature.