Insulation diagnosis for 220kV oil-immersed current transformer by frequency dielectric spectroscopy

Current transformer (CT) is used for protection of major components and to provide continuity of operation in power system. Failure of CT may lead to an energy explosion and result in phase to ground fault that will hinder the operation of a substation. Therefore insulation diagnosis is of great importance for evaluation of CT. As a non-destructive dielectric testing tool, Frequency Dielectric Spectroscopy (FDS) is widely used by power utility for assessments of the condition of electrical equipments such as transformer and cable, but seldom used on CT. Due to the similarity of transformer´s insulation, the insulation condition of oil-paper insulation CT may be evaluated by FDS. In this paper, with the frequency range from 10^-2~10³Hz, FDS has been investigated on insulation condition assessment for several 220kV oil-immersed CTs. Complex permittivity and dissipation factor data are obtained as a function of frequency for each CT. To verify the validity of FDS, high voltage dissipation factors from 10kV to 145kV have been measured by Schering Bridge and compared with the FDS result at power frequency. Results indicate that FDS evaluation is a reliable tool for determination of moisture content in papers and of oil conductivities.