Failure analysis on abnormal leakage of plastic floating ball in transformer gas relay for nuclear power plant

The plastic components are always more prone to property degradation than the conventional metallic components, so their safety reliability is paid more attention for the routine maintenance. The gas relay containing two plastic balls is an important safety protection device of the high-voltage transformer. The two plastic hollow floating balls inside the relay buoy in the insulating oil level, and float up and down with exhausting and generation of the gas and oil. Once the oil level increases or decreases beyond the safety limits, the protection switches are immediately actuated by the floating ball. In this paper, one plastic floating ball inside the relay in high-voltage transformer was found leaked during one refueling cycle of a nuclear power plant because quite a bit insulating oil was penetrated inside the ball. Although no serious consequence had been aroused, it will bring the potential safety hazards for the normal operation of the transformer. For this reason, the failure analysis was systematically performed to analyze and identify the causes of abnormal leakage of the plastic ball. The comprehensive testing and analysis results indicated that the unqualified fusion process of two plastic hemispheres was the root cause, thus the pertinent countermeasures were proposed.