Investigation of capacitor voltage transformers

An investigation has been carried out on 330-kV and 132-kV CVTs (capacitor voltage transformers) with seven to ten years of service life. The population total was 148 with an extensive DGA (dissolved gas analysis) testing history. The failures and high DGA results could not be correlated with location operating conditions or associated equipment. Units removed from service for high gas production but standing deenergized for more than a few months showed low PD (partial discharge) activity. Laboratory measurements showed that all units increased in capacitance with voltage, time, and temperature. For normal units tan delta remained constant or slightly decreased for these same variables. Abnormal units showed increasing tan delta with temperature. Thermal runaway could be induced in these abnormal units at normal operating voltage and about 25 degrees C above ambient temperature. Examination of failed capacitors showed puncture of the paper between the foils in the center of the pack. This same mode of failure could be reproduced in bench tests on individual capacitor packs. It was concluded that DGA testing is important and should be continued, that failed or suspect units should be tested within two weeks of removal from service, and that the failure mechanism in the CVTs investigated is thermal runaway due to increasing dielectric loss.<>