The medium-frequency (MF) transformer in a phase-shift modulated dc solid state transformer (SST) is prone to partial discharge (PD) that impacts the system reliability and lifetime. Tightly packed MF transformer windings carrying pulsewidth modulation (PWM) voltages create favorable conditions for PD inception due to proximity between adjacent winding conductors and electric field enhancement because of spiral geometry. A peak potential condition is created between the adjacent posterior conductors of the primary and secondary windings as a result of phase-shifted modulation. This along with fast alternating electric fields incepts PD at lower than expected operating voltages. Early inception of PD leads to premature transformer failure leading to unanticipated dc SST down-time. The effects of PWM voltages containing high dv/dt switching transients on PD and lifetime characteristics of MF transformer in a dc SST are not yet understood, which is investigated in this article. A nonintrusive PD detection strategy for MF PWM ac voltage is presented using high-speed optical sensors. PD characterization is performed on Kapton polyimide insulated copper foil windings for a foil-type MF transformer. Winding samples are stress tested with steady state PWM voltages up to 2 kV and frequencies up to 50 kHz with high dv/dt up to 60 V/ns generated by a half-bridge inverter using 3.3 kV SiC devices while maintaining the transformer's geometric profile. Accelerated dielectric lifetime characterization is also performed and novel lifetime prediction models are derived based on the acquired results containing both frequency and dv/dt dependent variables.

Dielectric breakdown strengths of ternary mixture of gaseous helium, hydrogen, and nitrogen at room temperature and cryogenic temperatures (77 K) have been investigated experimentally in an effort to identify a gaseous cryogen with higher dielectric strength than pure helium for high temperature superconducting (HTS) power applications. Building on our previous success in enhancing the dielectric strength of helium gas by 80% by using a binary mixture containing4 mol% hydrogen + 96 mol% helium, a ternary mixture with a composition of 4 mol% hydrogen + 88 mol% helium + 8 mol% nitrogen was investigated. The composition of the ternary mixture was the result of theoretical modelling studies which used the Boltzmann equation to calculate the dielectric strength of various ternary mixture ratios. The dielectric breakdown strength of the ternary mixture was measured at room temperature and at 77 K at several pressures up to 2 MPa. A 300 % enhancement in dielectric strength was observed for the ternary mixture compared to pure gaseous helium. The dielectric strength of the ternary mixture was measured to be 12 kV/mm at 1 MPa and 77 K, which is approaching the dielectric strength range of liquid nitrogen. The results on the new mixture are compared to the previous studies on helium-hydrogen binary mixtures and the implications of the observed enhancement for gas cooled HTS power devices is discussed. The challenges posed by the addition of nitrogen in terms of limited operating temperature and pressure due to the condensation of nitrogen at higher pressures and/or lower temperatures is also discussed.

We propose a thermally efficient and power-dense high-temperature superconducting (HTS) cable (CLEAN Cable) as well as a thermally efficient, mechanically rugged, and easily attachable/detachable cryogenic-to-ambient power link (CLEAN Connector). The bundled design enables high gravimetric power density and node-less tapping of power. The CLEAN Connector minimizes ambient heat leaking into the cryogenic environment by magnetically coupling the cryogenic section to the ambient counterpart via a high-frequency isolated DC/DC converter. The proposed high-frequency operation reduces the core size of the cryogenic/ambient transformer that provides electrical and thermal isolation while enabling quick connection. The proposed CLEAN technologies provide the following advantages - (1) Reduced heat load; (2) High gravimetric power density; (3) N –1 system redundancy; (4) Minimum ambient heat leak into the cryogenic system; (5) High vibration/mechanical stress tolerance; (6) Partial discharge (PD) free at high-altitudes; (7) Easy cryogenic and ambient system connection; (8) Inherent fault detection and isolation.

Superconducting Gas Insulated Line (S-GIL) concept for high temperature superconducting (HTS) power cables was developed with practical designs that use tubular insulator spacers. The implications of spacer materials and designs were studied in terms of electric field enhancement and its role in limiting the maximum withstand voltage. Three different tubular insulator materials and two different sizes were explored using finite element analysis of electric field enhancement. Model cables were fabricated and tested for their response to high voltage tests at various pressures up to 2 MPa, with two different gas media, and at two different temperatures. The experimental results were compared with the expected behavior based on the finite element analysis. Implications of the designs on cryogenic gas flow and pressure drop along the length are also discussed. Maximum withstand voltage above 40 kV was achieved for designs suitable for gas-cooled HTS cables operating at 77 K.

Potential failure modes of high temperature superconducting (HTS) devices and related cryogenic systems in electric ship applications are analyzed to understand and plan for the necessary mitigation systems. Risk-based FMEA methodology was adopted to aid in improving the resiliency of large superconducting power distribution systems. A concurrent FMEA study of cryogenic thermal and electrical systems in an HTS power distribution network is discussed. A one-line architecture for a power distribution system was developed to identify the key parameters needed for performing FMEA for HTS power distribution systems. The failure modes are listed to show the interdependence of the electrical and cryogenic systems. The severity and detectability of various failures were used to formulate critical redundancy and mitigation tools to design resilient power distribution systems for electric ships.

High Temperature Superconducting (HTS) power cables are being developed for a variety of applications including the electrical power grid, electrification of transportation, and high energy physics. The appeal of superconducting technology is its high current density — generally greater than 100 A/mm ² — compared with conventional copper and aluminum conductors, which generally do not support greater than 5 A/mm ² . Thus, HTS wires and cables support high power ratings even at low and medium voltages, which reduce the size and weight of the electrical power systems. The size and weight reductions are essential to realize large all-electric ships and electric aircraft. HTS devices must be operated within the boundary of their respective critical temperature, critical current, and critical magnetic field [1]. Second generation HTS materials possess critical temperatures of ∼90 K. However, operating temperatures in the range of 50–70 K are typically required for HTS cables to support the desired operating current of many power-dense systems and to have a sufficient temperature margin to tolerate unexpected heat loads and fault currents. When a superconductor exceeds any of its three critical parameters, a quench will occur. A quench causes the HTS cable to transition to its normal state where it starts to display its high electrical resistance. The interdependency of the electrical and thermal properties of HTS materials also depends on the cryogen used to achieve the desired operating temperature. There are only a few cryogens compatible with the desired operating temperatures of <77 K, with liquid nitrogen (LN 2 ) being the most commonly used for HTS cable applications for the electric power grid due to its abundance, low cost, and good electrical insulation and heat transfer properties. LN 2 -cooled HTS cables have been successfully demonstrated in the electrical power grid operating at 10–200 kV [2], [3]. The LN 2 -cooled 10 kV AC HTS cable installed as part of the Ampacity project in Essen, Germany demonstrated the potential of HTS technology to be economically feasible in urban areas. Installing the 10 kV HTS cable instead of a conventional 110 kV cable to supply the downtown area of Essen allowed for 4 out of 10, 110/10 kV transformer substations located in the center of Essen to be decommissioned and removed [2].