Investigation of Alternative Cryogenic Dielectric Materials and Designs for High Temperature Superconducting Devices

The consumption of electricity is seen by society as a certainty and not an uncertainty; however, there are several uncertainties about how the topology of the electrical grid will look in the future. For instance, it is expected that the demand for electricity is set to considerably increase, there will be a greater incorporation of renewable generation sources, and society will call for a decrease in the spatial footprint of the electrical power grid. To address these uncertainties, new technology has been proposed to replace the conventional copper devices currently utilized. One of the new technologies that has shown great promise over the last decade are superconducting power devices. The appeal of superconducting technology lies in its ability to operate at significantly higher current densities than equivalently sized copper or aluminum technologies. This increase in current density will potentially allow for the electrical power grid to operate at higher capacity and greater efficiency. In order to develop superconducting devices for high power applications, knowledge of the critical boundaries with regards to temperature, current and magnetic field need to be studied. High-voltage engineering principles also need to be studied in order to ensure that an optimal design is produced for the superconducting power device. These theoretical and practical challenges of designing superconducting power devices are discussed in Chapter 1. Chapter 2 focuses on the high-voltage engineering and dielectric design aspects of a specific superconducting power device: HTS power cables. In particular, this chapter discusses the different dielectric design topologies, cable layouts, and reviews successfully demonstrated HTS power cables. One of the current limitations of designing superconducting power devices is the lack of dielectric materials compatible with cryogenic temperatures, and this area has been the focus of my research. The main focus of my Ph.D. is the investigation of new cryogenic dielectric materials and designs, which can be separated into two main areas. The cryogenic studies on increasing the dielectric strength of gaseous helium (GHe) focused on the addition of a small mol% of various gases such as nitrogen (N2), hydrogen (H2) and neon (Ne) to GHe (Chapter 4). The studies to increase partial discharge inception voltage of GHe cooled high temperature superconducting (HTS) power cables focused on using a Polyethylene Terephthalate heat shrink to individually insulate HTS tapes (Chapter 6), as well as the development of a novel HTS cable design referred to as the Superconducting Gas-Insulated Transmission Line (S-GIL) (Chapter 7). While the research conducted can be split into different categories, the experimental techniques in preparing samples and performing measurements are consistent and are discussed in Chapter 3. From completing this research, several key findings were discovered that will help advance the development of GHe cooled superconducting devices. Here is a summary of these discoveries: • The addition of 4 mol% of hydrogen gas to GHe increases the dielectric strength by 80% of pure GHe for all pressures. This trend was seen with both AC and DC voltages and DC breakdown strengths were approximately 1.4 times higher than the AC, as expected. • By measuring the breakdown strength of 1, 2, and 4 mol% hydrogen gas mixed with GHe, a linear relationship exists between hydrogen mol% and breakdown strength. The saturation limit does not appear to have been reached, so there is potential for higher breakdown strengths with higher hydrogen mol%. However, there are potential safety concerns with regards to flammability that need to be considered for higher mol% hydrogen mixtures. • Tertiary mixtures containing 8 mol% nitrogen gas, and 4 mol% hydrogen gas mixed with GHe yielded approximately a 400% increase in the dielectric strength when compared to GHe. With the introduction of the nitrogen gas to the mixture the maximum operating pressure was limited to approximately 0.85 MPa before condensation occurred. • The partial discharge inception voltage (PDIV) measurements for a cable measured in the 4 mol% hydrogen mixture and then in GHe showed a 25% higher value when the cable was measured in the 4 mol% hydrogen mixture than in GHe. This improvement in PDIV is not as great as the 80% improvement seen in the breakdown measurements. • The Polyethylene Terephthalate heat shrink selected to provide individual insulation to HTS tapes did not allow for a high operational voltage when used as the insulation method for a HTS cable as breakdown occurred between 1-2 kV. • The development of the S-GIL allows for the full benefits of increasing the dielectric strength of GHe to be exploited. • The S-GIL will allow for higher operating voltages and better thermal characteristics than currently available for GHe superconducting power cables.