Smart All-SiC Solid-State Transformers

Solid-state transformers (SSTs) are power electronic interfaces between a medium-voltage (MV) system (AC or DC) and low-voltage system (AC or DC), which provide galvanic separation by means of transformers that are operated with a frequency that is significantly higher than the grid frequency of 50Hz or 60Hz, i.e., in the range of several tens of Kilohertz. Especially in environments where the permissible weight and/or volume are restricted, such as future high-speed trains, future civilian or naval vessels with on-board MV AC or MV DC distribution systems, and even future all-electric aircraft concepts, SSTs can outperform solutions based on conventional low-frequency transformers in terms of efficiency and power density. Also in DC-DC applications such as MV DC collecting grids of wind parks or large photovoltaic plants, SSTs are required in case a galvanic separation and/or high voltage conversion ratios are needed.

Since several years, the Power Electronic Systems Laboratory of the D-ITET is dedicating various research projects to this emerging concept, whereby a holistic approach based on multi-objective optimization methods is employed, considering a wide range of aspects including but not limited to topologies (e.g., single-cell vs. multi-cell approaches), reliability, costs, efficiency, power density, isolation, protection, control, and also hardware prototype systems. As an example, a current project investigates low-complexity single-cell SST systems, which are enabled by the use of novel high-voltage silicon carbide (SiC) power semiconductors. These devices feature a yet unseen combination of very high blocking voltage capabilities (10kV and beyond) and switching characteristics that allow for very high switching frequencies. Therefore, a further reduction of the volume and weight of the isolation transformers as well as of the passive elements employed in EMI filters may be possible – be it in low-complexity single-cell SSTs for lower MV levels or in multi-cell SSTs whose applicability could be extended to higher MV levels.