HVDC shunt tap based on three single-phase half-bridge series-connected MMCs operated under 2L modulation

In this study, a cost-effective HVDC shunt tap with low-number of relatively low-voltage semiconductor devices and small-sized passive components is proposed. The proposed architecture is based on employing three single-phase half-bridge Modular Multilevel Converters (MMCs), where the DC sides of MMCs are connected in series across the total DC-link voltage. The MMCs are adopted instead of conventional Two-Level Voltage Source Converters (2L-VSCs) to avoid the complications of series connection of Insulated Gate Bipolar Transistors (IGBTs). The involved MMCs in the suggested architecture are operated with the conventional 2L modulation, which results in insignificant arm inductors and Sub-Modules (SMs) capacitances (in range of µH and µF, respectively). This, in turn, affects positively the converter cost and footprint. Each arm of the involved MMCs can be considered as a high-voltage valve of a 2L-VSC. To maintain the balance of SMs capacitors, each arm of the involved (N + 1)-level MMC has an extra SM (a balancing SM) to select N out of N + 1 SMs during the turn-off condition. The operational concept, design, and assessment of the proposed architecture are presented in this study. Simulation results are provided for substantiation of the proposed concept. Finally, a scaled down single-phase prototype is used for experimental validation.