Operation Limits of the Hybrid DC/DC Modular Multilevel Converter for HVdc Grids Connections

The dc/dc modular multilevel converter (MMC) is one of the promising solutions for the interconnection of two different HVdc systems. The operation of the conventional topology of this converter is limited due to structural restriction. Recently, a hybrid topology is proposed to improve the power transfer capability and reduce ac circulating current. This topology can neutralize dc power flow at any phase difference and transmitted power by adding full-bridge-based submodules (FBSMs) and half-bridge-based submodules (HBSMs). However, using FBSMs beside HBSMs to generate negative voltage can unbalance the capacitor's voltages. This article identifies the operation limits of the hybrid dc/dc MMC, which is caused by an unbalance in SMs capacitor's voltages. Using the proposed analysis method, minimum achievable ac circulating current and maximum transmitted power are obtained. The minimum achievable ac circulating current is important for the calculation of conduction loss and calculating the current rating of components. This study enables designers to find the optimal operating point and avoid the capacitor's voltages unbalance. The impact of transmitted power, conversion ratio, and arm inductance on operation limits of the hybrid topology is investigated through different case studies. The obtained results revealed that the operation range of hybrid converters with larger transmitted power, higher conversion ratios, and larger arm inductance is more limited.