Metal oxide surge arresters are crucial for overvoltage protection in power systems, determining the insulation level of electrical equipment, with their core component being the ZnO varistor. However, modern stable ZnO varistors exhibit an anomalous decrease in power loss during aging, contradicting the increase in power loss predicted by the classical ion migration model. This discrepancy poses challenges for the condition assessment and life prediction of ZnO varistors due to a lack of theoretical foundations, thereby presenting a potential threat to the power system. Consequently, the study of the anomalous aging mechanism of stable ZnO varistors has been identified as a major challenge for the varistor community by CIGRE in both 2013 and 2017.

In this paper, stable ZnO varistors are subjected to accelerated DC aging at elevated aging temperatures to investigate their long-term stability transition. With increase in aging temperature, power loss trend transitions from a continuous decrease at 120℃ to an initial decrease followed by an increase at 150℃, and a sustained rise at 180℃. The decreasing power loss trend can be fitted by a double exponential decay function, while the increasing power loss is proportional to the square root of the aging time t^0.5. After transitioning to a mixed stable type at 150℃, the aging of stable ZnO varistors becomes irreversible. In-situ high-temperature dielectric measurements reveal that the interface space charge polarization relaxation process shifts to higher frequencies with decreased relaxation time and activation energy decreasing from 0.583 eV to 0.560 eV, indicating the destruction of the grain boundary structure. Low-temperature dielectric tests show that intrinsic point defects of zinc interstitials undergo irreversible consumption after aging. Upon transitioning to an instable type at 180℃, the "crossover" phenomenon of the forward current-voltage (I-U) characteristics disappears at 180℃, and both forward and reverse I-V characteristics shift towards increased leakage current region as a whole. Severe deterioration in reverse electrical parameters was observed, as breakdown voltage U_1mA decreases from 200.5 V to 92.9 V, the nonlinear coefficient α decreases from 16.3 to 2.0, and the leakage current rises from 19.5 μA to 479.3 μA. More importantly, offline physical and chemical structural tests show a reduction in the diffraction angles of ZnO crystal planes and decreased peak intensities. Additionally, a significant decrease in the binding energy of the Zn_2p orbital is observed, with Zn_2p3/2 and Zn_2p1/2 orbitals decreasing from 1 022.5 eV and 1 045.9 eV to 1 022.1 eV and 1 045.2 eV, respectively. This indicates the reduction of zinc interstitials and confirming that the interface states cannot maintain stability at high temperatures, thus becoming neutralized and consumed with zinc interstitials.

These findings demonstrate that the essence of the decreasing power loss in stable ZnO varistors lies in the stable interface states at the grain boundary, which, however, cannot maintain stable at certain high temperatures. The interface states would then neutralize with the zinc interstitials due ion migration, subsequently leading to the reduction of zinc interstitials and the destruction of the ZnO lattice, resulting in significant deterioration of ZnO varistors. Therefore, optimizing the high-temperature stability of the interface states is crucial for enhancing the long-term stability of ZnO varistors.