Research On Thermal Effect Of ZnO Varistor Under Multi-pulse Lightning

ZnO varistor often causes thermal collapse and fire due to heat generated by lightning strikes in practical applications.Aiming at the problem that the single-pulse lightning impulse experiments of ZnO varistors in the current laboratory cannot realistically and effectively simulate natural lightning strikes,the current and thermal effects of ZnO varistors under multi-pulse lightning strikes are studied.Safety performance has important theoretical significance and application value.In this paper,a multi-pulse lightning impulse test platform is designed and built.The thermal damage characteristics and damage mechanism of ZnO varistor under multi-pulse are studied from two aspects: macro damage and micro damage characteristics.Based on the changes in electrical performance parameters and temperature,the effects of different waveform parameters on the aging characteristics of ZnO varistors under multiple pulses were studied.The energy absorption characteristics of ZnO varistor under multi-pulse are simulated and simulated.Based on the impact experiments and simulation results,an improvement method for lightning protection performance is proposed.The main research contents and results of this article are as follows:(1)A 20-pulse lightning impulse test experiment system with a discharge time of 1(?)s to999(?)s(short interval)and 1ms to 999ms(long interval),accuracy within ± 1(?)s,adjustable continuous flashing time interval,and different waveforms was designed and carried out.Test of ZnO varistor withstand shock under multi-pulse.The results show that the average impact life of different types of ZnO varistor is significantly different,and the impact resistance is closely related to its own material and structure.(2)The thermal damage mechanism of ZnO varistor under multi-pulse is studied,and the thermal damage characteristics of ZnO varistor under the condition of power frequency operation load,salt spray and humid environment are analyzed.The results show that the thermo-physical properties and non-uniformity of the structure lead to uneven temperature rise on the surface,and the thermal gradient of the temperature gradient causes the ZnO varistor to be damaged.Under multiple pulses,the greater the non-uniformity of the micro-grain boundaries of the ZnO varistor,the higher the concentration of energy absorption,and the area with the largest non-uniformity first reaches the fracture damage energy.When the non-uniformity coefficient is 1.8,the energy absorption capacity is only 400 J / cm3.Under the action load,the ZnO varistor will be short-circuited due to the thermal equilibrium imbalance.After being treated in a humid and salt mist environment,water molecules enter the grain boundary layer to chemically react to generate alkaline OH-which destroys the structure of the grain boundary layer,greatly reducing the ZnO varistor.Impact resistance.(3)The structure of the grain boundary layer and the properties of the crystalline phase material of the ZnO varistor under multi-pulse current were studied at the micro-level using a scanning electron microscope.It was found that the types of trace elements and the size of the grain boundaries in the grain boundary layer will affect the ability of the ZnO varistor to withstand multi-pulse shocks.As the number of shocks increases,the grain structure of the ZnO varistor continues to degenerate.The heterogeneity of internal ion migration and the heterogeneity of the micro-grain boundary layer cause the local energy density to be too large and the local temperature to be too high.The cell-variable grain boundary resistance melts through,and local high temperature may cause the Bi element in the ZnO varistor to change in different crystal phases.(4)The influence of pulse current with different waveform parameters on the aging characteristics of ZnO varistor was studied.The relationship between the pulse parameters,the number of pulses,the number of impacts,the aging rate and the average surface temperature rise was established.The aging characteristics of ZnO varistors under 3 pulses,4pulses,and 5 pulses were analyzed based on the equivalent energy method.The results show that the waveform parameters such as the number of pulses and the pulse interval determine the aging rate of the ZnO varistor,and the increase of the pulse energy will accelerate the aging of the ZnO varistor.When the same energy is injected,the multi-pulse destructive force is greater than a single pulse.A theory of energy superposition cumulative effect is proposed to explain the multi-pulse aging mechanism of ZnO varistor.(5)The Voronoi network geometric model was used to simulate the microstructure of the ZnO varistor.The double Schottky barrier conduction mechanism model was used to simulate the equivalent circuit of the micrograin boundary.Impact of energy absorption capacity.Based on the ZnO varistor impact experiments and simulation results under multiple pulses,an improved method for the ZnO varistor edge defect is proposed,and a SPD dedicated short-circuit current decoupler is proposed to deal with the problem of fire and combustion based on thermal effects.