Study On Partial Discharge Properties And Multi-Variable Failure Models Of Oil-Paper Insulation At Pulsationg DC Voltages

The valve-side windings of converter transformers are subjected to pulsating DC voltage consisting of AC, DC, repetitive pulse and strong harmonic voltage components. The electric field distribution of vale side insulation is very different from that of traditional power transformers. The valve side winding insulation accumulates space charge at the pulsating DC voltage field, which will influence the local electric field distribution of winding insulation. Partial discharge properties as well as electrical life characteristics have close relationship with the type of applied voltage on the insulation system. Thus, there are great difference in statistical characteristics of partial discharge and electrical aging characteristics of oil-paper insulation between converter transformers and traditional power transformers. According to the operating condition characteristics of converter transformers, the partial discharge(PD) behavior, electrical breakdown properties and multi-variable failure model of oil-paper insulation under pulsating DC voltage are systematically studied in this thesis. The main contents are as follows:(1) Cavity partial discharge characteristics in oil-paper insulation under pulsating DC voltage were experimentally studied and theoretically analyzed. The PD inception voltage, statistical distribution of PD pulses and the PD process were analyzed through PD experiments. The influence of temperature on PD properties was studied. The simulation model cavity partial discharge in oil-paper insulation was established, and the influence of temperature on PD inception voltage, PD magnitude and statistical characteristics were simulated and analyzed. The results showed that the PD phase-resolved distribution and characteristics of cavity discharge under pulsating DC voltage are very different from that under AC voltage. Furthermore, the temperature has greater influence on the PD properties of oil-paper insulation under pulsating DC voltage.(2) The electrical breakdown experiments of oil-impregnated paper insulation and oil gap-paper insulation structure under pulsating DC voltage were implemented. The influence of ripple factor and temperature on breakdown voltage of oil paper insulation were investigated. In addition, the mechanism of oil paper insulation breakdown voltage under pulsating DC influenced by temperatures was studied combined with the oil paper dielectric properties and space charge distribution characteristics under different temperatures. The experiment results showed that ripple factor has great influence on the electrical breakdown voltage of oil-paper insulation under pulsating DC voltage and the increase of temperature decrease the breakdown voltage of oil paper insulation under pulsating DC voltage.(3) The accelerated electrical aging test of oil-immersed paper insulating and oil gap-paper insulation under pulsating DC voltage was carried out. A large amount of failure data were obtained and the Weibull distribution function was used to analyze the failure data. The failure properties of oil-paper insulation under pulsating DC, DC and AC voltage were compared. The influence of temperature on failure properties of oil paper insulation under pulsating DC voltage was analyzed. Experimental and analytical results showed that with the increase of temperature the voltage endurance ability of oil paper insulation decrease significantly. And the more DC components contained in pulsating DC voltage, the more pronounced the influence of temperature will be.(4) The multi-Weibull failure model considering electric field strength and temperature under pulsating DC voltagee was studied. The failure data of oil-immersed insulating paper and oil gap-paper insulation under different electric field stresses and temperature were analyzed. The functional relationships between electric field stress, temperature and characteristic life were established. Combining the two-parameter Weibull distribution function, the dual-variable Weibull failure model considering the electric filed and the multi-variable Weibull failure model considering the electric field stress and temperature were studied and established. The hypothesis testing results showed that the proposed Weibull failure models have good adequacy and adaptability of the proposed models were test in the thesis.The above work is an active exploration in PD properties and electrical ageing assessment of oil paper insulation under pulsating DC voltage. The results offer the theoretic support for fault diagnosis and life prediction of converter transformers and provide the theoretic and experimental basis for inner insulation design of converter transformers.