| Summary: | Measuring the leakage current and voltage of an insulator is widely used to calculate indices for the pollution estimation of overhead line insulator strings. These indices are intended to be used for the online condition monitoring of insulators to prevent flashover failure. The variability of the data over time causes questionable results when these indices are used in real time. To improve the performance of the indices, this study focuses on analyzing the leakage current and its harmonics over a time dimension to demonstrate the amplitude changes and their causes. For this purpose, the concept of harmonic impedance for insulators was introduced. In an artificial fog chamber, a U120B insulator is subjected to several tests with different levels of contamination and varying levels of constant humidity. The results showed that the equivalent impedance of the insulator varied significantly over time (up to four times). This variation is due to the water absorption process captured by the insulator and depends on the level of contamination and humidity. The 5th and 7th-order harmonic impedances are stable over time, with exceptional variations of less than 9%; therefore, harmonic currents depend only on the voltage harmonics. In contrast, the 3rd and 9th-order harmonic impedances exhibited amplitude variations of up to 9.1 times. Finally, several indices are evaluated. The phase difference angle (PD) index performed best at a constant humidity and steady-state leakage current.
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