Research On The State Evaluation Technology Of Generator Insulation Based On Dielectric Response Method

Turbine generator is the core equipment of power system.Its main insulation is mainly composed of epoxy mica-composite dielectric insulation,whose aging state will seriously affect the life of turbine generator.In this paper,the aging condition assessment of generator insulation is studied based on frequency domain dielectric spectroscopy(FDS).Theoretical analysis,physical modeling and experimental verification are used to study the aging characteristics and dielectric response characteristics of main insulation.The research is beneficial to assess the aging state of generator insulation based on dielectric response method.In the laboratory,a combined aging test platform and FDS test platform was built for the stator bar,and IR test,dielectric loss test and FDS test are carried out on the samples which are in different aging state.In addition,the influence of temperature on the frequency domain dielectric spectrum is analyzed through experiments.The change mechanism of frequency domain dielectric spectrum of the samples is analyzed using the dielectric polarization theory.The FDS test values are fitted using double relaxation Cole-Cole model modified by dc conductivity to study the effect of aging and temperature on the relaxation model parameters.Experiment results show that the change rule of absorption ratio K and dielectric loss tan?reflects the gradual deterioration of stator insulation with accelerated aging state,which verified the effectiveness of the aging test platform.The test values can be effectively fitted using Cole-Cole model.With the increase of aging time,the values of real part and imaginary part of complex permittivity of samples gradually increase.The increase trend is more obvious at low frequencies(10-3?100Hz).As the aging,the dc conductivity and interfacial polarization increases quickly,the interfacial polarization relaxation time constant decreases,distributed parameter has no obvious change rule,whose value distributes at between 0.7?0.9.There is a big difference of the relaxation parameters between the first two periods and the last three periods.When bar insulation aging state becomes serious,relaxation mechanism of the insulation changes,which produces a new relaxation process.The FDS test results and relaxation model parameters have high sensitivity and present certain regularity as aging degree of samples changes.So the FDS method can be used in aging assessment of the main insulation.The main insulation characteristics of models at different temperature are studied.Experimental results show that the entire spectrums of real and imaginary parts moved to the high frequency as the temperature goes up.Temperature increase enhances the strength of interfacial polarization and dipole polarization.In the spectrum of ?",the frequency range of interfacial polarization plays a leading role broadens with the increase of temperature.The interfacial polarization relaxation time and the direct current conductivity satisfy the Arrhenius equation.According to the transition state theory,the value of activation enthalpy is 103.1kJ/mol.The activation entropy increases with the increase of temperature,which indicates the order of the whole system decreases.The decrease of activation energy indicates that all kinds of polarization will be established much easier than normal.So the temperature-dependence of frequency domain dielectric spectroscopy of generator insulation can be explained by the transition state theory.Finally,the structured main temperature curve can not only eliminate the influence of temperature on the dielectric spectrum,but also expand the test frequency range.At last,FDS field test was carried out on 02#generating unit of Zhuhai Power Plant,and the field test results are basically identical to laboratory research conclusions.So frequency domain dielectric spectroscopy can be preliminary applied to on-site state assessment of generator insulation.