Flashover Improvement of AC-DC Mixed Voltages on HVDC-GIS Spacer with Functionally Graded Materials

In HVDC systems with bipolar high voltage, the gas-insulated switchgear (GIS) may experience mixed voltages of AC and DC. To enhance the insulation capability of the basin spacer, this study presents a collaborative optimization approach for AC and DC electric fields utilizing the multi-dimensional functionally-graded material (MFGM). A novel spacer was developed by integrating materials with volume graded permittivity (ε-FGM) and surface nonlinear conductivity (SNCM), followed by conducting electric field simulation and flashover voltage testing under various AC, DC, and AC-DC mixed voltages. When exposed to DC voltage, the central region of the convex surface of the MFGM spacer tends to accumulate homopolar surface charges. In contrast, heteropolar surface charges tend to accumulate in the surrounding area. As a result, this accumulation leads to a 25% reduction in electric field strength at the HV triple junction point and an increase in flashover voltage by 14.5% compared to traditional spacers. Under the AC voltage, the electric field distortion of the MFGM spacer is decreased by 21% and the flashover voltage is raised by about 18%. Under the AC-DC mixed voltage, the flashover voltage of the MFGM spacer is raised about 12.7%. The MFGM spacer demonstrates satisfactory performance in electric field relaxation for both steady-state and transient-state, indicating its potential application in HVDC-GIS.