Development of On-Line Monitoring for Power Transformer Bushing

Abstract

This paper presents the development of on-line monitoring for condenser type bushing of power transformer in order to detect a degradation of bushing and to provide the alarm before bushing failure. For the bushing degradation analysis, the capacitance and power factor of bushing internal insulation are on-line monitored and measured by installing the sensing device at the test tap of bushing. The changing of internal insulation capacitance leads to the change of voltage across tap capacitor and the change of leakage current through bushing insulation at the test tab. Such changes result in the change of power factor or phase angle of leakage current. Thus, the leakage current is additionally detected by installing the Hall Effect sensing device to observe the trend of bushing failure. A total of eight cases of both normal and degraded insulation are investigated. The internal insulation capacitance as well as the magnitude and phase angle of three phase leakage current were measured and compared with the commissioning value or observed the increasing trend to set the alarm level. In case of degraded bushing, the leakage currents changed and caused the summation of leakage current value to be greater than the standard value. Then the alarm is activated. This knowledge was used to develop the detection and decision making algorithms in LabVIEW program to determine the condition of transformer bushing.