Improvement of Surface Flashover Performance in Vacuum of A-B-A Insulator by Adopting ZnO Varistor Ceramics as Layer A

In this paper, a new insulation system of ZnO varistor ceramic-Al₂O₃ ceramic-ZnO varistor ceramic insulators (defined as A-B-A insulators) is proposed to improve surface flashover performance in vacuum so that it is possible to reduce the electric field at the vacuum-insulator-cathode triple junction (CTJ) by changing the conductivity and permittivity of the ZnO varistor ceramic layer of the A-B-A insulators. Then, the influences of the conductivity and permittivity of ZnO varistor ceramics on the dc and impulse surface flashover voltage in vacuum were experimentally investigated, respectively. It was found that the dc surface flashover voltage of the samples increases greatly with a decrease of the dc conductivity of the ZnO varistor ceramic layer and is improved by 81% compared with the Al₂O₃ ceramic sample. It is attributed to the decrease of the electric field at the vacuum-layer A-CTJ caused by the conductivity of layer A larger than that of layer B and, furthermore, by the nonlinear current-voltage characteristics of layer A, i.e., ZnO varistor ceramics. Additionally, the impulse surface flashover voltage first increases and then decreases with a decrease of the permittivity of the ZnO varistor ceramic layer and is increased by 136%. It is explained by the decrease of the electric field at the triple junction by the permittivity of layer A larger than that of layer B and, furthermore, by the nonlinear current-voltage characteristics of layer A. These results demonstrate the insulation system of the ZnO varistor ceramic-Al₂O₃ ceramic-ZnO varistor ceramic insulators as an effective way to improve surface flashover performance in vacuum.