A Space Vector Modulation Approach for a Back-to-Back Connected Four-Level Converter

The phenomenon of DC-capacitor voltages drift is the main technical challenge of a back-to-back connected diode-clamped multi-level converter (DCMC). This paper (i) develops a mathematical model for a back-to-back connected four-level DCMC and formulates the capacitor voltages drift phenomenon, and (ii) based on the developed model proposes a space vector modulation (SVM) approach to control the capacitor voltages drift. The online voltage balancing task is achieved based on a coordination between the two SVM modulators that minimize a cost function related to voltage unbalance of DC-capacitors. The main feature of the proposed SVM-based balancing strategy is that it enables online DC-capacitor voltages balancing with no requirement for lookup tables, additional controls or auxiliary devices. Performance of the overall back-to-back DCMC system and effectiveness of the proposed SVM-based balancing strategy are evaluated based on time-domain simulations in the PSCAD/EMTDC software environment.