Distortion of partial-discharge images caused by high-voltage harmonics

Nowadays, one of the main methods in diagnostics of high-voltage insulating systems is registration of partial-discharge (PD) pulses and their visualisation in the form of phase-resolved images. The paper describes the influence of high-voltage harmonics on partial-discharge phase-resolved images, which is crucial for proper identification and classification of PD patterns. The evolution of these images, depending on the total-harmonic-distortion (THD) parameter being an assessment factor, calculated by means of fast Fourier transform (FFT), is shown. Numerical simulation which illustrates the harmonic influence on PD is reported. Different harmonic-distortion instances of test voltage have been analysed along with the corresponding PD phase distributions. The measurement results were obtained on thermosetting insulation of a stator bar with the controlled harmonic content of the test voltage. The basic principles of harmonic analysis, along with the measurement set-up, are presented. It is shown that THD may only be a rough and global assessment factor. For the practical analysis of PD images, as a tool for insulation diagnostics and monitoring, the test-voltage frequency spectrum will be required.