Dielectric Response Analysis of Mineral Oil Immersed Transformer Insulation during Manufacturing Process

Polarization and Depolarization current (PDC) method is one of non-destructive insulation testing methods that can be used for diagnosis the insulation conditions of high voltage equipment. This paper presents the PDC analysis of an oil immersed distribution transformer measured during the manufacturing process. A 22 kV, 30 kVA single phase transformer was designed and constructed. During manufacturing process, the polarization current and depolarization current of the transformer insulation were measured for 6 case studies as follows: 1) paper insulation between iron core and low voltage (LV) windings before dry and vacuum in the oven, 2) paper insulation between LV and high voltage (HV) windings before dry and vacuum in the oven, 3) paper insulation between iron core and LV windings after dry and vacuum in the oven, 4) paper insulation between LV and HV windings after dry and vacuum in the oven, 5) oil-paper insulation between iron core and LV windings after dry and impregnation process and 6) oil-paper insulation between LV and HV windings after dry and impregnation process. Then, PDC test results were analyzed. It was found that the moisture content in the paper insulation clearly affected the capacitance at power frequency, and PDC shapes. Besides, the impregnation process had a good effect on the capacitance ratio and dielectric dissipation factor. Moreover, the current difference of dry paper insulation between HV and LV windings presented the nonlinear characteristic both before and after impregnation process.