Drivetrain Efficiency Investigation of Fuel-Cell-Powered Vehicles Using a Triple Active Bridge, FOC and Six-Step Operation

This paper presents a case study on the use of a variable dc-link in fuel-cell-powered vehicles. Motor and inverter losses are analyzed under three scenarios: field-oriented control (FOC) with a fixed 400 V dc-link, FOC with the minimum possible dc-link voltage required for proper controller operation at each operating point, and six-step operation enabled across all investigated operating points by the variable dc-link. Measurements show a slight efficiency gain at the motor and potential inverter efficiency improvement of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{1 1}$</tex> percentage points when the dc-link voltage is minimized. To evaluate the system efficiency implications of operating at reduced dc-link voltages, loss simulations of a three-port dc-dc converter, specifically a current-fed triple active bridge (CF-TAB3), are conducted. The simulation results show a notable drop in the converter efficiency at minimum voltage, making six-step operation unsuitable for low speeds when the CF-TAB3 is controlled using the single phase shift strategy. However, moderate dc-link voltage reduction enhances inverter efficiency without significantly increasing converter losses, supporting the potential of multiport converters in fuel-cell-powered drivetrains. Further optimization of systemlevel efficiency is recommended to identify the optimal tradeoff between inverter and converter losses that maximizes overall performance.