As the key protection device in the power system, the ageing characteristics of ZnO varistor can directly affect the safe operation of power system. In order to study the influence of Co2O3 content on the electrical properties of ZnO varistors, thermally stimulated current (TSC) characteristics test, capacitance-voltage (C-V) characteristics test, scanning electron microscope (SEM) test, voltage-current (V-I) test were carried out on ZnO varistors before and after AC ageing tests. The results show that the mean grain size decreases while the varistor voltage increases as the Co2O3 content increases, the barrier height and the defects of ZnO varistors increase with the increase of Co2O3 content, and the barrier height decreases at a certain degree after AC ageing test. As the Co2+ ion radius is close to that of Zn2+ ion, it is easy for Co2+ ions to enter into ZnO lattice during sintering process, which is account for the increase of zinc interstitial of ZnO varistors. When the Co2O3 content exceeds a certain range, the AC degradation degree of ZnO varistors will become much more serious. From the experimental results, ZnO varistors doped with 1.0 mol% Co2 O3 exhibit a better performance.
