Insulating Capacity of a Sphere-Plane Electrode Arrangement with Thick Dielectric Coating under Negative Lightning Impulse Voltage Stress in Synthetic Air at High Pressures

Synthetic air is an environmentally friendly alternative to sulfur hexafluoride in gas-insulated switchgear. Hybrid insulation systems are a possible solution to compensate for the inferior dielectric properties. Thick coatings of several millimeters affect the macroscopic electric field distribution. Thus, the maximum electric field strength in the gas volume can be reduced, resulting in a higher streamer inception voltage. The improvement was investigated theoretically and experimentally. A sphere-plane arrangement with different gap distances (20 mm to 60 mm), coating thicknesses (2 mm to 10 mm) and relative permittivities through different groups of materials (EP, RTV 2 SiR, PU) was investigated. Exclusively negative lightning impulse voltage as the critical polarity was applied. The gas pressure of the synthetic air was varied between 0.1 MPa and 1.0 MPa. FEM simulations of the electric field distribution combined with the streamer criterion applied to the gas gap were used to predict the breakdown voltage of the hybrid insulation system. Comparisons between experimental results and simulations of the streamer inception voltage indicate good agreement. The results of this contribution provide a broad basis for the design of hybrid insulation systems in highly pressurized synthetic air.