Research On Thermal Characteristics And Failure Mechanism Of Surge Protective Devices Valve

Surge protective device(SPD)with metal oxide varistors(MOV)as the core components can stabilize circuit voltage and protect electrical equipment,but it is prone to appear thermal damage and failure problem due to its thermal effect.Thus,thermal characteristics and failure mechanism of surge protective devices were studied.The electric-thermal coupling finite element model of SPD based on the theory of heat conduction was established according to the working conditions and thermal physical properties and electrical performance of ZnO varistors.The normal working temperature field distribution characteristics of SPD were analyzed.The research showed that the whole temperature field distribution of SPD was homogeneous when it worked normally.The temperature field distribution and temperature increase of ZnO varistor valve after lightning current pulse with 8/20?s waveform and thermal damage were discussed.The changes of temperature and leakage current of ZnO varistor valve after ageing were also explored.The following conclusions can be drawn from the analysis results: the temperature field homogeneity of MOV after lightning current pulse with 8/20?s waveform was lower than the normal working temperature field homogeneity,and MOV was in good condition without damage because the maximum temperature increase of MOV valve was less than limit working temperature increase;the overall structure collapsed when SPD occurred thermal damage,and the highest temperature was over 1000K;the leakage current and highest temperature approximately demonstrated linear increase trend when the conductivity increased from 10-6S/m to 10-5S/m linearly after MOV ageing.The thermal damage situation was more serious when the tripping pieces of multi-channel surge protective devices cannot tripping synchronously.The existing structure of tripping pieces was optimized to ensure all tripping pieces with the same prestress.The upward slope of middle channel tripping pieces was increased by 0.5 degrees than the edge of channels.Predeformation detection device of tripping pieces was designed to ensure simultaneous tripping of tripping pieces.The changes of microstructure and material composition of ZnO varistors valve before and after the different failure modes were analyzed and its failure mechanism was explored.The aging damage of ZnO varistors tended to cause the extrusion damage between the cells,and the adding elements migrated and formed fragmental materials scattering inside ZnO varistors.The impact damage of ZnO varistors tended to cause grain crushing and perforation damage accompanied by adding elements volatilization.