Low frequency harmonic components of leakage current as a diagnostic tool to study aging of silicone rubber insulators

Abstract

The paper presents the results of an investigation in which the low frequency harmonic components of leakage current (i.e. the third and fifth harmonic components) have been used to study the aging of silicone rubber (SIR) insulators in salt-fog. The test was conducted for 120 hours; 100 hours with fog and 20 hours without fog. The leakage current was measured for different SIR insulators during salt-fog and analyzed using fast Fourier transform (FFT). Two different conductivity levels were used during the test under 22.8 V/mm electric stress for all insulators. It was noticed that the leakage current waveform changed from a small capacitive sinusoidal current to highly distorted resistive current during salt-fog. Also, a noticeable difference in the behavior of the fundamental component of the leakage current and its corresponding third and fifth harmonic components was seen. At some instances when the fundamental component of the leakage current was relatively high and the low harmonic components are low, no arcing was seen. Arcing was seen only when the value of the harmonic components increased. Also, it was found that the increase in the low frequency harmonic components is more than the increase in the fundamental as the conductivity level of the salt-fog is increased. This suggests that low frequency harmonic components of leakage current could be a better indication of arcing and hence aging of SIR insulators than the peak value of the leakage current alone.