Effect of Electrical–Thermal–Mechanical Ageing on the Partial Discharge Characteristics of Oil–Pressboard Insulation Under AC–DC Voltages

Abstract

Converter transformers are the core components of ultra-high voltage (UHV) transmission systems. The main cause of faults in converter transformers is irreversible deterioration of oil–pressboard insulation under combined electrical–thermal–mechanical stress over long operating times. In this paper, the chemical characteristics of oil–pressboard insulation samples subjected to electrical–thermal–mechanical ageing for different times are studied. An image processing algorithm is used to analyse the discharge propagation characteristics of the samples under combined alternating current (AC)–direct current (DC) voltage, and the current pulse curves and phase resolved partial discharge spectrogram corresponding to the discharge images are analysed. An improved wavelet packet algorithm is used to denoise the discharge current pulse. Finally, the influence of electrical–thermal–mechanical ageing on discharge characteristics is analysed using radar charts. The condition of oil–pressboard insulation is one of the main factors determining the life expectancy of converter transformers. The results obtained here therefore have practical significance for understanding the process of insulation failure caused by accelerated ageing of oil–pressboard insulation.