Humidity and contamination severity impact on the leakage currents of porcelain insulators

The leakage current is the most important information for monitoring the insulator contamination, and especially for forecasting the contamination flashovers. Since the contamination severity and the ambient humidity can both result in the increase of the leakage currents, it is crucial to distinguish the exact reason for the leakage current changes. Experiments, repeated many times, have shown that the normal range of the leakage currents is less than 50 mA at rated operating voltage. This normal range is called the security stage, and it occurs far ahead of the contamination flashover. The characteristics of the leakage currents at that stage have been investigated when changes in the ambient humidity or the contamination severity were introduced in an artificial fog chamber. A joint approach of fast Fourier transform amplitude and power spectrum amplitude estimation is used to analyse the leakage currents in combination with the leakage current waveform. The main purpose is to improve the pre-warning of contamination flashover at the security stage and to obtain more time for the cleaning or replacement of heavily polluted insulators. Fog chamber test results show that there are two frequency spectrum related characteristics. Both characteristics play an important role in the state evaluation of insulator surface, considering the ambient humidity and the contamination severity. The pre-warning thresholds of these two characteristics can be recognised in case of very severe contamination together with high relative humidity conditions. These thresholds are very helpful for pre-warning in advance before contamination flashovers.