| Because of its long-distance, high-voltage, green economy, environmental protection and small floor area, high-voltage direct current(HVDC) becomes the best means in high-efficiency green power transmission in twenty-first century. The DC-arrester is an important guarantee to achieve the safe operation of HVDC. The key of DC-arrester is the high- performance DC-ZnO varistor, however, most of published literatures focused only on the potential gradient, there is a dearth of investigation on the long-term stability and high pulse-current impact. This thesis mainly aims to produce high surge current withstanding capability varistor with excellent surge absorption capability maintaining other electrical characteristics.In this work, the ZnO varistors were prepared by the two-step sintering which can reduce the grain size, increase the potential gradient and improve low current performance. We designed different two-step sintering processes, and the effect of different two-step sintering parameters, such as sintering temperature and socking time, for microstructure and electrical properties of ZnO varistors were studied. The optimized sintering procedure is following: T1=1180 ℃, t1=10 min, T2=850 ℃, t2=5 h. In the optimal condition, the potential gradient E1mA=4221 V/cm,the non-linear coefficient α=93.3, leakage current IL=0.24 μA, the clamping voltage ratio K5KA=2.34, the variation of breakdown field △E1mA(%)=-9.2% were achieved.Based on the contribution of improving conductivity of grain to reduce the residual voltage ratio, the effect of different quantities of Zn powder doping for high surge current withstanding capability was studied. The results showed that the grain sizes of ZnO varistors decreased and the intermediate phase increased after Zn power doping. Because Zn powder enter grains and provide free electrons, the clamp voltage ratio and the variations of potential gradient after applying a high pulse-current(4000-6000 A) have been significantly improved when the content of Zn powder was 3 mol%. In this case, when the pulse-current is 6000 A, the clamping voltage ratio K=2.40, the variations of potential gradient ΔE1mA(%)=-6.81%. In addition, the fails of samples by several large current impacting indicated that the cave structure is the dominated reason causing the sample failure inflection point.Finally, reducing the interstitial Zn ion concentration of ZnO varistor can improve stability against DC accelerated aging stress, we used amphoteric dopants Na. The microstructure of the sample was most uniform by SEM picture, the average grain size was 6.9 μm and the relative density reached 94.1% when the addition of Na2CO3 was 0.06 mol%. Na atoms can occupy the lattice sites to block the formation of zinc interstitials in the available sites and occupy the interstitial sites to restricted the migration of zinc interstitials which can improve stability against DC accelerated aging stress. The sample with 0.06 mol% Na2CO3 has the smallest coefficient KT(4), the smallest variations of potential gradient ΔE1mA(%)(-0.54) and the smallest variations of leakage current in following ageing procedure: 0.9V1mA/125 ℃/8 h. In addition, based on the influence on diffusion and migration of oxygen ions, hydrogen ions by thermal treatment, the samples with 0.06 mol% Na2CO3 were annealed. The result showed that the thermal treatment was 900 ℃, the coefficient KT=3.3, the variations of potential gradient ΔE1mA(%)=-0.5 in 0.9V1mA/125℃/8h. |
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