Cleaning effect for oil in oil impregnated all-film capacitors

Loss tangent (tan delta) is an important parameter for designing oil impregnated all-film shunt capacitor. To obtain low tan delta, tan delta of film and oil should be low. But in our study model capacitors of low tan delta are obtained using oil of high tan delta. Accelerated aging test and Partial Discharge (PD) measurements are carried out on these model capacitors, and it shows that oil in the elements of these capacitors is cleaned. Model capacitor impregnated with purified oil is aged for nearly 3400 hours and still of low loss, but the oil in it but outside the elements is of loss as high as 22.62%. With chromatography - mass spectrometry and gas chromatography analysis, large amount of an alcohol is found in this oil, which is thought to be the main reason for high tan delta. The elements in the capacitor seem to have a cleaning effect and be just like a wall that separates the low tan delta oil from the high one. The reason for this cleaning effect is discussed at the end of the paper.