Research On Aging Status Of On-board Arrester Based On Frequency Domain Dielectric Response Method

Metal oxide surge arresters(MOSA)which have high nonlinear feature as well as quicker conduction response throughout the overvoltage protects systems from having disturbances as a result of switchgear operations and lightning strokes across the network,ensuring that railway system and power system can work securely.It is known that on-board zinc oxide surge arresters become degrade and damage its nonlinear features due to heavy weather,frequent vibration,lightning impulse,and lowering or raising pantograph and so on.Therefore,evaluating the aging status of arresters in time plays an important role in railway systems safety.Firstly,traditional characteristic parameters were measured,such as reference voltage measurement(1mA),total and leakage current.The limitation of traditional methods for aging status assessment was stated briefly by analyzing.Then frequency spectra of on-board arresters with different operation time were measured.In the lower frequencies,the real part and imaginary part of permittivity increase with impulse aging,while there is no evident change in higher frequencies.And it is noted that the order of magnitudes of permittivity of on-board arrester is large.Secondly,to further understand the internal relaxation processes of on-board arresters,the structure of the on-board arrester was analyzed,which were composed by several components,such as Zn O varistors,epoxy resin sleeve and rubber insulator.By analyzing the frequency spectra of these three materials,it is believed that the phenomenon of giant permittivity is mainly caused by ZnO varistors.Therefore,the influence of different impulse for ZnO varistor was researched,and measured frequency spectra were analyzed by multi-relaxation Harvriliak-Negami(H-N)model with dc conductivity,as a reference,at different impulse current degradation.The results reveal that the double relaxation H-N model was used to fit frequency domain spectra which were measured at room temperature.It is found that there are two relaxation processes with activation energy about 0.68eV and0.34eV,which should be related with intergranular Bi-rich microregions and the oxygen vacancy respectively.The intensity ofαrelaxation process increases with aging,whileβrelaxation process has no change as before.A new relaxation process with activation energy about 0.73eV is observed at a temperature about 393K.Andγrelaxation process is related to be the interface states due to ZnO-ZnO homojunction.With impulse times increased,its activation energy and relaxation time decrease,while its relaxation intensity and shape parameter increase.With the increasing of impulse times,DC conductivity increases obviously,and its activation energy drops off from 0.72eV to 0.64eV.At last,the double-relaxation Havriliak-Negami model with DC conductivity was introduced to analyze the frequency domain dielectric spectra of on-board ZnO surge arresters;the result shows that,there are two relaxation processes,which should be related with the interface states due to ZnO-Zn O homojunction and intergranular Bi-rich microregions,respectively,in on-board arrestor at measured frequencies.With running,the the intensity ofαrelaxation process andβrelaxation process increases,and the value of shape parameterα_βincreases,while the value of shape parameterβ_βdecreases;the activation energy of DC conductivity andβrelaxation process decease with the running time increases.The aging mechanism of on-board arrestor was researched by analyzing and comparing the frequency spectra and relaxation processes of ZnO varistor and on-board arrestor.And a novel thought based on frequency domain spectroscopy(FDS)was presented for the aging status assessment of on-board metal oxide surge arrester(MOSA).