Polarization and Depolarization Current (PDC) tests on biodegradable and mineral transformer oils at different moisture levels

Polarization and Depolarization Current (PDC) testing is a non-destructive dielectric testing method to determine the conductivity and moisture content of transformer insulations. This research was carried out to find the dielectric responsive function and maximum conductivities of biodegradable and mineral transformer oils for comparative analysis. Each insulation oil with different moisture levels (dried, normal, or wet) was tested. The dried condition was achieved by processing the oil in a dry-out plant to evacuate the moisture. Normal condition refers to the oil as taken directly from the drum while wet condition is the oil after 200 ppms of moisture have been added. The polarization and depolarization current at test DC voltage of 1000 V was recorded for 10,000 seconds. Each test was repeated five to ten times to collect the most consistent PDC data and to check the repeatability of each PDC pattern. In the analysis, the PDC curve was plotted and the DC conductivity of the oil was calculated. It was found that the trends of the PDC response for the two transformer oils are different.