| The surge protection device (SPD) is an important device to protect the normal operation of the system from over-voltaging damage in low-voltage distribution systems, and it has no significant effect on the operating characteristics of the applied system especially when there is no surge voltage in the distribution systems. But when the surge appears in the power distribution system:SPD will play a role to discharge current and clamp voltage in its effective protection. As the SPD’s core components, MOV (metal oxide varistor) and GDT (gas discharge tube), and its related properties are critical to the SPD.In the long-running process for the SPD in low-voltage distribution systems, it may face aging and deterioration problems due to the external environment such as lightning current, over-voltage operation, high temperature, high humidity etc. We analysis of the reasons for the formation, and combine with the related theory and further draw different pulse currents and different milliamperes DC voltage characteristics under dynamic aging MOV (dynamic voltammetry curves and dynamic resistance) variation by SJTU-ICG-100-20platform and SJTU-TST impact thermal stability testing. At the same time, experiments demonstrate that it is efficient to avoid the static parameter values belonging to the range already qualified aging serious situation when the dynamic resistance is brought into the rating system.For effective anti-aging, we study multi-chip parallel MOV performance issues and draw a conclusion that the difference between UlmA and Rd are the main cause of the current deviation IL· Based on electromagnetic transient analysis software ATP-EMTP, we also consider MOV voltage characteristics in the whole region and the current role of the large current response characteristics, and build a circuit model, which uses8/20μs pulse current loop and2ns rise rate of the square wave circuit conduct measurement. We simulate the residual pressure and flow characteristics under different number of pieces MOV, and draw MOV multi-chip parallel flow through the change trend of residual pressure, further give the relative number of the best pieces in parallel.Concerning problems of the matching performance between multi-electrodes gas discharge tube and metal oxide varistor, it can be inferred that when the multi-electrodes gas discharge tube is in parallel with MOV, according to the operational principle of them, residual voltage of the gas tube can be effectively reduced during the stage of glow discharge and gas tube will be shunted by MOV. Based on the Townsend theory, the equivalent discharge model of gas tube during the stage of glow and arc discharge is founded. Through impulse experiment by the open circuit voltage wave of (1.2/50μs), impulse current wave of8/20μs and combine wave of (1.2/50μs,8/20μs), the accuracy of the model was verified; during the experiment, it was also found that when MOV was in parallel with the negative electrode of gas tube during the impulse, the speed of discharge is significantly boosted and the response time of the multi-gap will be markedly shortened. According to the phenomenon and theoretical analysis, it is proposed that a new method of parallel connection between multi-electrodes gas discharge tube and metal oxide varistor, which has a certain reference value in the practical application. |
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