More than 60% of oil-immersed current transformers are the vertical type in the Inner Mongolia Power Grid, which affects the operation stability of power system directly. This paper presents a diagnosing case of partial discharge defects of a 110kV vertical oil-immersed current transformer (VOCT). It is found that the Dissolved Gas Analysis (DGA) and dielectric loss of the VOCT exceeded warming thresholds. To explain the abnormal experimental results, byproducts in oil were examined and dielectric loss were re-measured under different voltage levels. In addition, the results of dielectric loss and grounding capacitance measurements indicated that the insulation of cable-paper tapes was good and the discharge was not attributed to moisture. Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS) showed that the copper belts and cable tapes are neither oxidized nor aged. Based on the diagnosis and analysis, the causes of defects were cleared, and technical suggestions for instrument transformer were proposed.
 

Capacitance,Dielectric loss factor,DGA,Main capacitance screen,Primary winding,VOCT