Dielectric Designs of Power Cables for Enhanced Resiliency in High Temperature Superconducting Electrical Transport Systems

The electrical power industry is currently going through one of the most dramatic changes in its history. With an ever-increasing demand for power, environmental cleanliness, and reduction in electronic device sizes, the use of electrical power is continuously expanding to new types of electrical devices and systems. Electric transport systems are one of the newest large-scale power systems being developed. Electric ships and aircraft require innovative ideas and novel designs of power system components to meet their anticipated power demands. These immense power demands are coupled with ambitious goals of energy efficiency, power density, and resiliency. These demands add strict constraints of size and weight, while at the same time call for substantially high-power capabilities in the megawatt (MW)-range. High temperature superconducting (HTS) power cables have been proposed as a solution to meeting these goals of power-dense, resilient, and efficient electrical transmission. HTS power cables possess a current density of 50-100 A/mm2, compared to conventional conductors such as copper, which generally have a current density of 1-30 A/mm2. Hence, HTS power cables achieve high capacity power transmission without the need to operate at a high voltage, which can reduce the size and weight of electrical components within a power system. There have been only a few studies in the past on understanding electrical faults in power systems consisting of HTS cables; however, there were no comprehensive studies on the response of HTS cables during various types of electrical fault conditions. Understanding the response of the system during various electrical fault scenarios is crucial for satisfying the requirement of resiliency for electric transport systems. These studies provide the fundamental electrical characteristics HTS cables must be designed to be implemented within the system The researched performed for this thesis focused on three primary studies: a study was performed on understanding various types of electrical faults that can occur in a medium voltage direct current (MVDC) power system and the additional design constraints that come to play when using HTS power cables as a means of power transmission in these systems. Another study was conducted on characterizing the intrinsic dielectric strength of liquid nitrogen (LN2) and analyzing the discrepancies in measured data that occur from gas bubble formation in the LN2 medium. This study also analyzed if the substance and pressure used to pressurize and subcool LN2 had a significant impact on its measured dielectric strength. The final study performed in this thesis was on investigating the feasibility of a novel HTS cable design referred to as superconducting power cable with hybrid cryogens (SPCHC). The SPCHC design utilizes LN2 as the dielectric medium and GHe as the primary cooling medium by utilizing commercially available HTS conductors and cryostats, along with the previously published work on LN2-cooled HTS cables. Several key findings were obtained from completing this research. These key findings are essential for enhancing the resiliency of HTS power cables, and for characterizing the intrinsic dielectric strength of LN2. The following is a summary of the conclusions that were made in this research: • The average measured dielectric breakdown voltage of LN2 in a stagnant bath was approximately 6% lower for vertically positioned electrodes compared to horizontally positioned electrodes. Also, vertically positioned electrodes had approximately 5% lower maximum breakdown voltage compared to the horizontally positioned electrodes. The lower measured values of dielectric breakdown for the vertically positioned electrodes indicate that a larger volume of gas bubbles was likely trapped between the electrodes when vertically assembled. • The measured intrinsic dielectric strength of LN2 increased by a maximum of 87% when pressurizing and subcooling the LN2 compared to the dielectric strength of a stagnant unpressurized bath of LN2. This study showed that even slight gas bubble formation in the LN2 medium has a significant impact on the measured dielectric strength of LN2. • The substance and pressure utilized to pressurize LN2 did not have a significant impact on its intrinsic dielectric strength. In the experiments performed in this study, LN2 was pressurized at pressures of 35-65 PSIA and with cases utilizing nitrogen gas (GN2), gaseous helium (GHe), and a binary gas mixture consisting of 6 mol% hydrogen gas (H2) and 94 mol% GHe. The lower condensation temperature of pure GHe and the binary gas mixture allowed for varying pressure levels to be achieved easier compared to nitrogen gas, which condenses at 77 K. • The SPCHC power cable design can potentially reduce the LN2 volume required for the cable by up to 95% and reduce the maximum gas volume expansion in the event of a cryogen leak by up to 89% compared to conventional LN2-cooled HTS power cable technology. The increased dielectric strength of LN2 compared to GHe may allow for the SPCHC design to have more HTS cables installed within a single cryostat compared to utilizing just GHe. Installing more HTS cables into a single cryostat allows for a reduction in the required cryogenic components of a system.