| The converter transformer is an important equipment to ensure the safe,reliable and stable operation of the UHVDC transmission system,due to the complicated operating conditions of the transformer in the actual environment,the oil-paper insulation inside the converter transformer can withstand the effects of aging factors such as thermal stress,AC and DC composite electrical stress,and mechanical vibration stress for a long time.In recent years,the research on the aging test of transformer oil-paper insulation is mostly carried out under a single thermal aging state.However,under the coupling action of "electricity-heat-vibration" multi-stress field,the multi-factor combined aging characteristics and failure mechanism of converter oil-paper insulation the research is relatively scarce.In order to study the aging characteristics of converter transformer oil-paper insulation under multi-factor combined aging,and at the same time provide effective support for the evaluation of converter transformer insulation status.This paper conducts in-depth basic research on the state change rule and failure mechanism of oil-paper insulation under the "electricity-heat-vibration" multistress field coupling environment in a laboratory environment,and characterizes the change rule between the insulation state and characteristic parameters.In order to simulate the aging environment of oil-paper insulation of converter transformers in actual operation,this paper designs and builds a mult ifactor joint accelerated aging test platform in a laboratory environment,and prepares multiple sets of oil-paper insulation XY models.Subsequently,accelerated aging tests under multiple stresses such as thermal aging,combined electrothermal aging,combined aging of electrothermal vibration and combined aging of AC and DC superimposed electrothermal vibration were carried out,and samples were taken regularly for time domain dielectric response test and physical and chemical performance test.In order to avoid the inrush current caused by instantaneous pressure and ensure the synchronization of signal sampling,this article uses the electrometer as the basis under laboratory conditions,and uses the time relay and the high voltage relay control module t o design the current time domain spectrum test system with bypass switch protection.Through the analysis of the polarization and depolarization current test curves of the oil-paper insulation sample,it can be seen that with the increase of the aging time,the polarization and depolarization current curves under the same aging factor show an obvious overall upward shift,that is,the more the current value is larger,the more serious the overall aging of oil-paper insulation.With the increase of the aging factor,the overall PDC curve shows a trend of upward and leftward shift.The longer the aging time,the more significant the promotion of electrical stress and vibration stress on insulation aging.According to the test results of the degree of polymerization,it can be known that the more the aging factor,the greater the rate of decline of the degree of polymerization,and the faster the aging rate of oil-paper insulation under multi-factor combined aging.Comparing the change trend of time-domain dielectric characteristic quantities under different aging conditions,it can be seen that the impact of vibration stress and direct current stress on the aging of oil-paper insulation is greater than that of alternating current stress.Compared with insulating paperboard,multi-factor combined aging has a more obvious promotion effect on transformer oil aging.In order to deeply study the correlation between the multi-factor combined aging characteristic parameters of the converter oil paper insulation and th e aging state,a calculation method for predicting the steady-state value of the relaxed polarization intensity through the change trend of the relaxed polarization intensity over time is proposed.Based on this,a characteristic quantity "relaxed polarization coefficient" that is sensitive to the aging state of oil-paper insulation is proposed,and the quantitative relationship between polarization coefficient and aging days is established by using a nonlinear regression method.Analyze the variation of the degree of polymerization with the number of aging days under different aging factors,and perform fitting with the cumulative loss model of the degree of polymerization to obtain the fitting values of the model parameters under different aging factors.According to kinetic analysis,the acceleration factor relationship between different aging factors under the reference aging rate is defined.This parameter can also be used as an auxiliary criterion to further explore the aging characteristics and failure mechanism of oilpaper insulation under multi-factor combined aging. |
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