Influence of moisture and ageing on dielectric response of ester and mineral oil impregnated pressboard insulation

Frequency Domain Spectroscopy (FDS) technique has been widely used as a reliable tool to assess the condition of transformer insulation. It has been identified that FDS is largely influenced by ageing condition of insulation system including moisture in solid insulation. Therefore, adequate understanding on the influence of these factors on FDS is necessary to accurately interpret the condition of insulation system. This issue well understood for mineral oil-paper insulation system. However, there are limited number of studies performed to investigate the impact of ageing and moisture on FDS of natural ester (NE)-paper insulation system. To fill the gap, this study measures the FDS of transformer grade pressboard insulation samples aged in a mineral oil and two different commercially available NE based insulating liquids at different ageing and moisture conditions. Results of this comparative study reflect that influence of moisture and ageing on FDS of NE impregnated papers insulation is completely different in quantitatively to that of mineral oil impregnated paper insulation.