Interlinking a Tram Traction Network and Bipolar DC Microgrid via an Isolated Bidirectional DC-DC Converter with Input Stage

Abstract

Existing DC tram traction networks (DCTTN) provide intermittent regenerative energy and quasi-persistent surplus capacity as they are dimensioned for peak power demand. The deployment of DC microgrids is increasingly on the rise largely due to the compatibility with DC-native loads and avoiding AC grid congestion issues. In particular, the bipolar DC microgrid (BDCMG) is considered, as it enables different voltage levels, requires less copper for cabling and enhances reliability compared to unipolar architectures. Interlinking these two energy networks through isolated bidirectional DC-DC converters (IBDCs) offers a favorable opportunity to enhance the operation of the DC microgrid, particularly during periods of local demand fluctuations or utility disturbances.
This thesis considers three IBDC topologies for the interface: the CLLC resonant converter, single- and three-phase dual active bridges (DABs). Simulation-based comparisons of the 1-phase and 3-phase DAB demonstrate that the 3-phase DAB achieves higher efficiencies across operating points considered based on industry standards, requires smaller output capacitance, which enables a higher power density. This is advantageous for facilitating a modular architecture with high-frequency SiC MOSFETs, which not only allows passive components to be smaller in size and thus lower the overall system cost, but also ensures redundancy and scalability towards different energy demands of systems (DCTTN and BDCMG) at different locations or cities. To ensure robustness against the electrically demanding tram traction environment, it is necessary to consider a dedicated input circuit to protect the downstream components against damage from surge overvoltages as well as ensuring that the input circuit does not compromise the stability of the IBDCS. Simulation-based comparisons of two different passive damping methods (RC and RL) used on multi-stage LC filter topologies (part of the input circuit) were investigated. Results indicate that the RL damped filters require smaller footprint size, achieve lower output impedance and improved transient voltage attenuation. Based on Middlebrook’s impedance-based stability criterion, the inclusion of the input circuit and the DCTTN’s overhead line impedance did not compromise the stability of the system (input circuit + IBDC). Adopting a modular approach of stacking IBDCs, two distinct configurations of connecting the input circuit with the IBDCs were taken into consideration. A single input circuit for a 150 kW load showed a smaller size and weight and lower cost per kW compared to using three smaller input circuits for 50 kW loads each.