Single-Phase AC/AC Multilevel H-Bridge Transformerless Converter with SST Functionalities

Solid State Transformers (SSTs) have been proposed as an alternative to standard Low-Frequency power Transformers (LFTs), in order to meet the demands of an increasingly complex electrical grid. Bidirectional power flow, active and reactive power control, unitary power factor and harmonic filtering are only a few advantages provided. However, all classic SST configurations use an intermediate High-Frequency Transformer (HTF) to adapt voltage levels, which increases the device's volume, weight and cost. This work proposes an AC/AC multilevel H-bridge transformerless converter with SST functionalities. The converter is based on a five-level back-to-back cascaded H-bridge (CHB-B2B) structure, in which the DC-link capacitors make the energy balance from the rectifier stage to the inverter stage. To avoid the possible short-circuit switching states of the converter and ensure a good tracking of the controlled variables, namely the input current and the output voltage, the fundamentals of Model Predictive Control (MPC) are applied. The proposed topology is then implemented for several load types with MATLAB/Simulink software.