Elimination of zero sequence circulating currents in paralleled three-level T-type inverters with a model predictive control strategy

The configuration of modular paralleled three-level T-type inverters (3LT(2)Is) has been widely utilised to extend the system power rating. However, zero sequence circulating currents (ZSCCs) are generated when sharing the common DC-link and the AC side without isolated transformers. This study presents a novel model predictive control (MPC) based strategy to eliminate the ZSCC in parallel operating 3LT(2)Is. Both the differences of common-mode voltages (CMVs) and neutral point potentials (NPPs) between paralleled 3LT(2)Is are investigated to be the potential exciting sources of the ZSCC based on the developed model of the ZSCC. According to this conclusion, an MPC-based zero CMV method which adds an additional term to the cost function is used to reduce the differences of CMVs between paralleled 3LT(2)Is. Besides, a closed-loop ZSCC feedback control method is further introduced to compensate the ZSCC by modifying the cost function. With the proposed MPC-based ZSCC elimination strategy, the ZSCC between the paralleled 3LT(2)Is can be effectively eliminated in both steady and dynamic states. Besides, good performances can be achieved for both load current tracking and NPP balance control. Simulation and experimental results validate the effectiveness of the proposed ZSCC elimination strategy.