| Converter transformer is the key equipment and directly affects the reliability inHVDC transmission system. Operating conditions of converter transformers are moresevere than the power transformer which causing the coil insulation failure probabilityof converter transformer is higher than the power transformer. With the problems ofreliability evaluation methods of oil-paper insulation of converter transformer, thecharacteristics of withstand voltage, space charge and the failure model of oil-paperinsulation at the pulsating DC voltage are presented in the thesis. The main contents areshown as follows:①The withstand voltage characteristics of oil-paper insulation at pulsating DCvoltage were researched. The influence of the temperature and ripple factor on thewithstand voltage and failure time of oil-paper insulation at pulsating DC voltage wereresearched. The withstand voltage under pulsating DC, AC, and DC voltages wereobtained using short-term pressure method. The variation rules of relative dielectricconstant and volume resistivity of the oil-paper insulation were analyzed which changedwith temperatures. The failure time of oil-paper insulation were obtained using constantmethod; The variation rules of failure time of oil-paper insulation were analyzed whichchanged with voltage and temperatures. The results showed that, compared with the ACand DC voltage, the electrical aging properties of oil-paper insulation at pulsating DCvoltage have significant difference.②The space charge characteristics of oil-paper insulation were researched atpulsating DC voltage during electrical aging process. The space charge injection,accumulation and dissipation processes were analyzed using space charge density ofspecimens which tested by electro-acoustic pulse method; and the effect of electricalaging time and temperature on the space charge accumulation and dissipationcharacteristics of oil-paper insulation were researched. The results showed that thespace charge density of specimens increased with the electrical aging time extended andincrease of aging temperature. With temperature increasing, the field distortion distortedwas more serious.③The variation rules of space charge characteristics of electrical aging oil-paperinsulation at pulsating DC voltage were researched for electrical aging characteristics ofinsulation. The variation rules of failure time and temperatures which changed with space charge behavior were analyzed by space charge density of specimens. Theanalysis method of combined space charge characteristics and electrical aging ofoil-paper insulation was proposed. The influence of temperature and aging time on theelectrical aging of oil-paper insulation was researched. And the influence mechanism ofaging time and temperature on space charge characteristics of electrical aging insulationwas analyzed. The results showed that the extended aging time accelerate oil-paperinsulation aging; the temperature further strengthens its aging effect on oil-paper theinsulation.④The variation rules of weibull distribution of the failure time and electricstrength were analyzed. The multi-risk failure model and modeling method of oil-paperinsulation at pulsating DC voltages was proposed. And the dual-variable Weibull failuremodel was established. It shows the accuracy of failure probability of electrical aginginsulation at pulsating DC voltages. It solved the problem that the traditional weibullfailure model can’t reflect the relationship between failure time, electric strength andfailure probability of insulation. The evaluation method was proposed which the failureprobability distribution of comprehensive test index was used to estimate the accuracyof failure model.and it verified the validity of dual-variable weibull failure model andthe effectiveness of modeling method. The results show the dual-variable weibull failuremodel could effectively show the relationship between electric strength and failureprobability, and improve the accuracy of the calculation results.The above work is active exploration for electrical aging properties of valve-sidewindings insulation in converter transformer.It reveals the influence mechanism ofaging time and temperature on the electrical aging of oil-paper insulation at pulsating dcvoltage. The results offer the theory and experiment basis for the failure diagnosis andassessment of oil-paper insulation in converter transformer. |
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