A Series-Connected Multilevel Converter: Topology, Modeling, and Control

Cascaded H-bridge (CHB) is a promising topology for medium-voltage high-power applications. CHB consists of low-power H-bridge modules in each phase, and these modules are connected in a cascade manner to achieve multilevel operation. However, CHB requires a phase-shifting transformer with multiple secondary windings to generate an isolated dc source for each module, and this requirement increases the overall system cost. In this paper, a series connection of low-power modules is employed to realize a new power converter called a series-connected multilevel converter. Each phase of the proposed topology requires a single isolated dc source. Hence, the required number of secondary windings is considerably reduced. This approach also minimizes the complexity and cost of the overall system. The operation of the proposed topology is presented by using three-level neutral point clamped modules in each phase. Model predictive control (MPC) is employed to control the dc-bus capacitor voltage and output currents of the proposed topology. A discrete-time mathematical model of the proposed topology is also developed to predict the future behavior of control variables. The performance of the proposed topology with MPC is verified through MATLAB simulations and a scaled-down laboratory prototype.