Frequency characteristics of leakage current waveforms of a string of suspension insulators

In order to establish a method for monitoring whether flashover occurs or not in a string of insulators based on leakage current waveforms and their frequency characteristics, the leakage current waveforms and the frequency characteristics of a string of 120-kN suspension insulators were investigated with artificial contamination tests and field tests. As a result, it was found that leakage current waveforms become nearly the symmetrical wave when strong local arcs occur; hence, the intensity of the odd order of harmonic components, e.g., 50, 150, and 250 Hz, is high. Furthermore, it was clarified that the transition of the leakage current waveforms until flashover occurs is classified into six stages and that a threshold exists in the magnitude of peak leakage current and prominent oddorder harmonic components by which the occurrence of flashover can be predicted.