Research On The Withstand Performance And Failure Mechanism Of SPD Varistor Under Multi-waveform Multi-pulse Currents

The Class I Surge Protective Devices(SPDs),which are used for lightning protection in low-voltage power distribution systems,are employed for discharging most of the lightning surge currents.The main voltage-limiting element of limiting-type SPD is the metal-oxide varistor(MOV).The single impulse currents adopted in the standards and the multi-pulse with single waveform could not simulate the pratical lightning stroke currents that usually contain multi-components and multi-pulse.It is worth to investigate whether nowadays test methods are sufficient to test such kind of SPDs.In order to reduce the failure probability of Class I SPDs,and improve their lightning withstand capabilities,it is necessary to investigate the withstand performance and failure mechanism of Class I SPDs under the impulse currents that are more close to pratical lightning stroke currents.The Multi-Waveform Multi-Pulse Currents(MWMPC)are adopted in this paper to simulate the pratical lightning stroke currents.The withstand capability tests and operating duty tests are carried out on Class I SPD varistors,and the withstand performance of MOVs are analyzed.The failure modes,microstructural characteristics and failure mechanisms of the damaged SPD varistors are analyzed.The main research contents and conclusions are presented as follows.(1)Considering the characteristics of common lightning surge currents in distribution systems,the application problems of SPDs in low-voltage distribution systems are investigated,including the SPD configuration and its cascaded coordination.The accuracy of various MOV simulation modes on evaluating the residual voltages and energy absorption under different impulse currents are investigated,considering the dynamic characteristics of MOVs under different impulse currents.The recommendations for SPD model selection are also proposed.Considering different influence factors,the configuration and cascaded coordination of SPDs in distribution systems are analyzed,which are verified by corresponding experiments.The results clarify that the nonlinear resistance model has higher accuracy for estimating the energy absorption of MOVs,while the IEEE dynamic model is more accurate to evaluate the residual voltages of MOVs.As for the cascaded SPD configuration in distribution systems,the SPD combinations with clamping voltage of upstream SPD a little higher than the downstream SPD are recommended for the optimized SPD configuration in low-voltage distribution systems.(2)The MWMPC,which contain“10/350?s impulse current+2ms rectangular current+1?s steep impulse current+1?s steep impulse current”,are adopted to simulate the effects of the full-scale lightning stroke currents on SPD varistors.According to the standard test methods,the experimental procedures for the MWMPC withstand tests and the Multi-waveform multi-pulse Operating Duty Test(MODT)are proposed.In addition,the pass criteria are proposed.(3)The MWMPC withstand tests are carried out on Class I SPD varistors with different manufacturers,types,and coatings,considering the influences of different current amplitudes,multi-pulse sequences,and time intervals.The influence factors on Class I SPD varistors under MWMPC are analyzed.Moreover,the different effects of MWMPC and the single pulse currents on SPD varistors are analyzed and compared.The results show that under the MWMPC,the SPD varistors with higher maximum continuous operating voltage(U_c)have better withstand capabilities than the MOV with low U_c.Comparing with the continuous current occur between the first lightning return stroke current and the first subsequent return stroke current,when the continuous current occurr between the subsequent return strokes,the SPD varistors are more easily damaged.Under the MWMPC,the bare Zn O has better performance than the Zn O with coating.(4)The withstand performance of SPD varistors,when the MOV suffer MWMPC during operation,are investigated through MODT.The experimental results are compared with those under same tests without power frequency voltage application,as well as those under standard single impulse Operating Duty Test(ODT).The results show that when the continuous current occurr between the subsequent return strokes,the damage on MOVs are also more serious.After the MODT,the decrease of the MOV DC reference voltages is more than the experimental results without power frequency voltage application,and the MOVs are more easily damaged under MODT.The MODT have much more degradation effects on the electrical and physical properties of SPD varistors,comparing with the single-pulse ODT.(5)The failure modes and failure mechanisms of SPD varistors under MWMPC are analyzed through the visual inspection,microstructural observation,elemental examination,and phase analysis.The forming reasons for different microstructural characteristics are explained.The results show that there is a new MOV failure mode,besides the common coating crack and puncture failures,which is the MOV cracked and punctured holes appearing on the fractured surface.For the punctured MOVs,the decrease of the Zn O grains is observed on the punctured inner surface.The bismuth-rich intergranular layers become thicker and surround the grain surfaces.For the cracked MOVs,the decrease of the grain size is also observed on the fractured surfaces.The coating crack is caused by the flashover between the Zn O sintered body and the electrode sheet,resulting in the crack on the MOV coating.The puncture failure is caused by the nonuniform microstructures of MOVs,which lead to the localization of the impulse currents,resulting in the puncture of the localized materials.The bismuth(Bi),which is solid solution in the Zn O matrix,would precipitate from the Zn O matrix and form more Bi-rich phase in the intergranular layers during the puncturing process,and the Bi-rich phase wetting the surface of the Zn O grains.The cracked MOVs with punctured holes appearing on the fractured surface are caused by the synthetical effects of both Joule heating and thermal stress under MWMPC that include currents with both large energy and steep wavefront.The large degree of undercooling,which is generated during the puncturing and cracking moment,leads to the Zn O grain refinement at the fractured surfaces and the punctured inner surfaces.The investigation and conclusions of this paper could provide optimization suggestions for the cascaded SPD configuration in low-voltage distribution systems.The proposed MWMPC test methods could provide reference for expanding the test technology of Class I SPDs that are installed at high exposures and may be subject to direct lightning strokes.The obtained MWMPC experimental results,the analysis of MOV microstructural characteristics and failure mechanisms could provide references for improving the MOV withstand capability and manufacturing process,as well as the optimization of material formulation.