Study On Dielectric Response Characteristics And Electrical Aging State Evaluation Method Of The VPI Main Insulation Of Large Generator

The main insulation of stator bar is an important part of the insulation structure of a large generator.Its electrical aging state will directly affect the safe and stable operation of the generator.Electrical aging,under high electric field,is an important aging factor which affects the life of the insulation.Therefore,the study of electrical aging state of generator insulation can effectively evaluate its life and avoid the occurrence of sudden accidents during the operation process.At present,dielectric loss factor,partial discharge and breakdown field strength at power frequency are used as the measuring and testing methods.However,the state of insulation can not be fully and effectively diagnosed by the methods above.Therefore,in order to provide theoretical basis for exploring the non-destructive evaluation method of insulation aging state,the vacuum pressure impregnation(VPI)epoxy-mica insulation specimens subjected different time of electrical aging were tested by the frequency domain spectroscopy,polarization and depolarization current and thermally stimulated depolarization current.The aging state of the insulation are described by measuring the dielectric response characteristic parameters change with the aging time.The main contents include:The temperature characteristics of the dielectric relaxation process of epoxy-mica insulation were analyzed by using the frequent domain dielectric spectrum method combined with the temperature normalization principle.Meanwhile,the Havriliak-Negami model was used to fit the dielectric constant dielectric spectrum curve in the frequent-domain of epoxy-mica insulation,which was divided into two parts: the conductivity process and the relaxation process,and the temperature and electrical aging characteristics of the insulation were analyzed.The main results showed that the spectrum curves of the real and imaginary parts of the dielectric constant of epoxy-mica insulation increased and moved to the high frequency region with the increasing temperature.The real part of the spectrum curve and the imaginary part of the spectrum characteristic peak of the polarization of epoxy-mica insulation moved to the high frequency region and the amplitude increased.The effects of external electric field,polarization time and test timing on the dielectric relaxation process of epoxy-mica insulation with different electrical aging time were analyzed by using the polarization and depolarization current method combined with Debye charge decay model.The main results showed that due to the non-Debye relaxation processes in the epoxy-mica insulation,the polarized and depolarized charges of the insulation follow different charge decay laws.The polarization charge of the insulation decreased with the increasing number of tests,but the charge difference between the two tests increased with the increasing electrical aging time.The depolarization charge of the insulation increased with the increasing polarization time,and the total charge released during the depolarization process also increased with the increasing electrical aging time.The thermal stimulation depolarization current method was used to analyze the change rule of the current peaks of the epoxy-mica insulation with aging time.Meanwhile,the biased voltage thermal stimulation depolarization current measurement was used to analyze the generation mechanism of the current peaks of epoxy-mica insulation,and quantified and characterized the influence of electrical aging on the charge each current peak.The main results showed that the thermal stimulation current of epoxy-mica insulation had two current peaks,low and high temperature peak,low temperature peak was caused by shallow trap charge,high temperature peak was caused by deep trap charge and dipole polarization in epoxy resin,the total charge of low temperature peak decreased at first and then increased,and the total charge of high temperature peak increased monotonically.In order to evaluate the electrical aging state of epoxy-mica insulation comprehensively,the dielectric constant and polarization/depolarization charge of real machine bars with different electrical aging time were fitted.Conductivity,relaxation time,dielectric loss,maximum absorption charge difference,depolarization charge and corresponding slope parameters were extracted as the characteristic parameters,which was used to calculate the comprehensive evaluation parameter of the electrical aging state of the insulation.This method eliminates the influence of different dimensions of each characteristic parameter on the evaluation results,objectively highlight the characteristic parameters with large fluctuations,and has low requirement on the test accuracy.It is suitable for the field nondestructive testing of the main insulation of stator bar in the electrical aging state.In addition,we also calculated the comprehensive evaluation parameter of the electrical aging state of testing bars with the same method,and verified the effectiveness of the evaluation method.