Stage pre-warning based on leakage current characteristics before contamination flashover of porcelain and glass insulators

Contamination flashover is the ultimate result of the creeping discharge of polluted insulators. Since there are close ties between the contamination flashover and the leakage currents, it is critical to research the leakage current characteristics during the entire contamination discharge process. The leakage currents of porcelain and glass insulators are monitored and analysed through a number of laboratory tests in various polluted cases. The goal is to find the characteristics that are useful for the pre-warning of the contamination flashover. The emphasis of this paper is on the leakage current root-mean-square values and waveforms and power spectrum estimation of the leakage currents. Results of the experiments (repeated several times) show that the progress of the contamination discharge process can be classified into three stages, that is, security stage, forecast stage and danger stage. In addition, the boundaries of the three stages in both the time domain and the power spectrum domain are given based on the analysis of the test results of various insulators. That is very helpful for the stage pre-warning of the contamination flashover. Also, a new characteristic index, K, is introduced. It is the rate of rise of the maximum peak value of the power spectrum. It can provide a more comprehensive theoretical guidance for the contamination flashover stage pre-warning together with the leakage current and waveforms. The three-stage classification for the entire flashover process is also meaningful in order to perfect the prediction of the contamination flashover. Finally, other characteristics of the power spectrum estimation are also discussed based on the three-stage classification.