Cryogenic Dielectric Behavior of Gases in Uniform and Weakly Non-Uniform Electric Fields

Environmental preservation has now become paramount as natural resources are rapidly being consumed. A large part of the emissions rejected into the atmosphere comes from the transportation vehicles due to their combustion-based propulsion systems. This has created a paradigm shift towards the electrification of transport systems. All-electric ships and electric aircraft require high powered devices of few tens of MW. They also need devices with high power densities. Using medium voltage helium gas-cooled superconducting power device technology in transport systems will provide the required power densities, both gravimetric and volumetric. One major challenge of operating cryogenic helium gas-cooled superconducting devices at medium voltage is the weak dielectric strength of helium gas. Over the past several years, the research team at Florida State University Center for Advanced Power Systems (FSU-CAPS) has been working on enhancing the dielectric strength of helium-based gas mixtures. It was discovered that the addition of a small mol% of stronger dielectric gas such as hydrogen or nitrogen to helium results in a significant improvement in the dielectric strength of the gas mixture. The research for this thesis continued the study of dielectric strength of gas mixtures with additional compositions using measurements of dielectric strength of gas mixtures at room temperatures and at cryogenic temperature of 77 K. The results show further enhancement of dielectric strength in some gas mixtures compared to the previous compositions both at 293 K and 77 K. The enhanced dielectric strength was expected in compositions with higher mole fractions of H2 and N2 compared to the compositions studied earlier. The study conducted for this thesis experimentally verified the predictions. The research team at FSU-CAPS has proposed, designed, and demonstrated a first of a kind gas insulated superconducting power cable (S-GIL) results. Breakdown measurements on the model cables of S-GIL cooled with helium gas mixtures revealed that the enhancements in the dielectric strength of gas mixtures did not fully translate to the enhanced performance of the S-GIL model cables. One of the suspected reasons for the observed discrepancy is the inherent non-uniform electric field in the SGIL cables. All the previous measurements of dielectric strength of gas mixtures were carried out in a uniform electric field. This research focused on the behavior of gases in uniform and weakly non-uniform electric fields at room temperatures and at 77 K to explain the discrepancy discussed earlier. A new experimental set up for measuring the breakdown strength of gas media in a weakly non-uniform electric field was established as part of the research for this thesis. The setup was used to measure the dielectric strength of pure helium and helium-based gas mixtures at room temperature and at 77 K at high pressures up to 2 MPa. Measurements of breakdown strength of helium-based gas mixtures at 293 K in a non-uniform electric field with a field efficiency factor 62.5% were conducted to match the field efficiency factors typically exist in the optimized S-GIL. The experimental results suggest that the breakdown strength in the non-uniform electric field relate to that of the uniform electric field by the field efficiency factor, η. This relationship was observed to hold good for both the 293 K and 77 K data. This observation expands our previously reported systematics of dielectric strength in uniform electric field to the non-uniform electric field conditions, which will expand our fundamental understanding of the dielectric behavior of gas media. Furthermore, the results give us a useful relationship between the dielectric strength data in uniform electric field and weakly non-uniform electric field, which will be useful in estimating the dielectric behavior of S-GIL power cables and other applications that use gases as dielectric media. The established relationship eliminates the need for costly experiments for measuring the dielectric strength at a specific operating temperature and pressure and in the required non-uniformity of the electric field.