Research On Surge Degradation Of Metal Oxide Surge Arrester Based On Leakage Current Harmonic Analysis

Zinc Oxide arresters are widely used in overvoltage protection of power systems due to their excellent nonlinear properties and surge energy absorption capabilities.Zinc Oxide Varistor is the main component of the surge arrester,and it is the fundamental reason that the surge arrester has voltage sensitive protection characteristics.While the surge arrester protects the electrical equipment,it also deteriorates due to continuous impact of the current gradually.Eventually the performance of the arrester is reduced,the capability of protecting electrical equipment is weakened,and the risk of power system operation is greatly increased.Therefore,it is very important to study the surge degradation of the zinc oxide varistor.In this paper,starting from the microstructure of zinc oxide varistors,the equations describing the volt-ampere characteristics of the grain boundary barriers are established based on the conduction mechanism of double Schottky barriers.The microscopic circuit model of the varistor plate is constructed based on the Voronoi network.The circuit model combines the macroscopic electrical properties with the microscopic physical parameters and plays an important role in the study of the internal parameters of the zinc oxide varistor during the surge aging process.The simulation results for a single grain boundary are in good meet with the volt-ampere characteristic curve of the grain boundary: as the applied voltage to the grain boundary increases,the charge on the surface state gradually increases and the barrier height gradually decreases;the barrier height decreases sharply and the current increases rapidly after the surface state is filled.The surge aging platform is established for zinc oxide varistor accelerated aging test,the power frequency leakage current test platform is established for the measurement power frequency leakage current waveform and conventional performance parameters of zinc oxide varistor during the aging process.Through the fast Fourier transform of the leakage current,each harmonic content is obtained.The amplitude of each harmonic shows an upward trend during the aging process,and the harmonic more than three order changes are more sensitive than the fundamental wave.The change trend of proportions of the harmonics of the fundamental wave are the same,and both show a tendency of increasing first and then decreasing.The causes of the harmonic changes in the aging process of the zinc oxide varistor are: in the initial stage of aging,the ion migration and the Schottky barrier distortion due to the local overheat caused by the current surge lead to the increase of the harmonic ratio;the zinc oxide varistor is aged seriously,and the grains and grain boundaries are seriously damaged,and the grain boundary barrier is reduced,and the nonlinear volt-ampere characteristic is greatly weakened.According to the variation law of the harmonic component of the leakage current,a method for judging the aging state of the zinc oxide varistor based on odd harmonic current distortion ratio is proposed.The method combines the stability of traditional performance parameters assessment and the sensitivity of single leakage current harmonics.It is very suitable for on-line monitoring of zinc oxide arresters.At the same time,the aging process of the zinc oxide varistor was simulated.Through the simulation of the zinc oxide varistor,it was found that with the increase of the density of the donor,the leakage current of the varistor rose significantly,and the reference voltage and nonlinear coefficient decreased rapidly.The grain boundary state density and the donor density have the opposite effect.By changing the microscopic physical parameters of the varistor to simulate different aging stages,the simulation results of the aging process show that as the aging degree deepens,the density of the grain donor gradually increases,the interface state density at the grain boundary gradually decreases.The grain resistance decreases not obvious,only after the aging of the grain has been slightly reduced after the molten plate.The change of interface state density and donor density led to the decrease of grain boundary Schottky barrier,while the decrease of the performance of zinc oxide varistor was mainly due to the decrease of grain boundary potential barrier,and the grain boundary potential barrier almost disappeared in the late aging period.Finally,the varistor is equivalent to a resistor with a lower resistivity.