Characteristics Of Partial Discharge And Space Charge Transport Investigation In Oil-paper Insulation Under Dc

Converter transformer insulated with oil-paper, as the transportation and ac-dc isolation equipment, is one of the most important equipment in DC power transmission system. Different types of high voltages including AC, DC and AC superposed DC may act on the oil-paper insulation structure in operation.Many kinds of defects may be generated inside the converter transformer in the process of production, transportation, installation and operation. The electrical and chemical effects of partial discharge(PD), occurring on the point of insulation defect, can make the condition of the defect even worse. Thus PD is an important factor of insulation deterioration and failure. In fact, charge migration and recombination would happen when PD occurs. It is of great engineering and academic values to investigate the characteristic of PD and space charge distribution in transformer oil-paper insulation aged for different times. Also it can be used in equipment condition monitoring and life management.In this paper printed enhanced bowtie antenna was designed to set up PD detection system based on Ultra High Frequency method(UHF) and space charge measurement system based on Pulse Electroacoustic method(PEA). Soft threshold selecting de-nosing method based on Multi-Resolution Analysis(MRA) and characteristics extraction in spectra of UHF signals generated by PD were proposed in this paper. Effects of applied voltage scales, polarities and transformer oil aging on characteristics of UHF signals generated by PD and space charge distribution in oil-paper were analyzed. The results showed that energy distributions of UHF signals generated by PD in different frequency bandwidth were affected by scales, polarities of applied voltage and aging time of transformer oil. Higher applied voltage resulted in more energy distribution in low-frequency stage and less in high-frequency stage. UHF signals under negative DC had less energy distribution in low-frequency stage and more in high-frequency stage than that of positive DC. UHF signals in aged oil had less energy distribution in low-frequency stage and more in high-frequency stage than that of unaged oil. Homo-polarity space charges would be injected into the oil impregnated paper when high DC voltage was applied. The longer aging time of transformer oil was, the more homo-polarity space charges were injected, and the faster the negative charges would be dissipated.