Elucidation of Impedance Frequency Characteristics of Cast Resin Transformer and Application to Partial Discharge Diagnostic Method

Abstract

It is desired to establish an insulation diagnosis method for cast resin transformers (CRTr), which is often used in distribution systems. Partial discharge (PD) detection is considered to be an effective method, but its phenomenon has not yet been clarified. This paper presents investigation on the frequency characteristics of PD generated inside CRTr to improve PD detection technology. It is clarified that dominant frequency of PD signal detected by a high frequency CT depends on the frequency characteristics of the impedance between the primary and secondary windings. It is suggested that the dominant frequency could be predicted in advance by analysis using a high-frequency equivalent circuit model of CRTr. PD diagnostic technology that utilizes the frequency characteristics of the CRTr impedance has also been proposed. It is found that a PD generated external to CRTr does not have resonance at the dominant frequency of the CRTr itself, even though it passes through the CRTr. In conclusion, it is suggested that the frequency analysis of CRTr impedance could identify degraded CRTr and external noise.