Abstract
Solid State transformer (SST) is arising as a key technology in the establishment of megawatt charging infrastructure, addressing the growing demand for more efficient, flexible and scalable solutions. Medium voltage SST based on modular multilevel topologies shows advantage for fast adaptation in charging infrastructure, allowing the use of available semiconductor device technologies, however as the number of components increase, challenges in reliability and power density needs to be addressed. DC-link capacitors are one of the main components affecting SST reliability. Film capacitors are known for provide superior reliability compared to aluminum electrolytic capacitors (Al-Caps) but lower energy density, limiting its use in SST-modules with high capacitance requirements. This work analyzes a partial oscillating power control (pOPC) strategy that allows explore the DC-link capacitance reduction trade-offs in SST-modules targeted for MW applications. Assessment of capacitance reduction for a 3.3 kV-class SST module is presented and analysis of main requirements for DC-link capacitor bank design using film or Al-Caps is reported. It has been found that a reduction of 73% in the required DC-link capacitance can be achieved with pOPC strategy compared to the standard control approach. When film capacitor technology is considered, the capacitor bank design can meet the DC-link requirements for all pOPC cases without capacitance over-sizing, while for Al-caps technology, actual capacitance reduction is limited by thermal constraints linked to DC-link current requirements.