Hybrid control strategy of double closed-loop and improved repetitive control based on three-level three-phase four-leg inverter

Three-level three-phase four-leg (3L3P4L) inverter is widely used in uninterruptible power supply because it can provide a path for zero-sequence current components and realize operation under asymmetric load conditions. In order to solve the power quality problem of 3L3P4L inverter under nonlinear load, this paper proposes a hybrid control strategy based on double closed-Ioop+improved repetitive control. First, aiming at the optimization of voltage balance under asymmetric load, the three-phase four-leg inverter control is simplified to three single-phase inverters for control. Then, the overall control block diagram of the system is given, and the parameter design of the double closed-loop+improved repetitive control controller is deduced. In order to achieve the best performance between system stability and tracking error, the influence of each link of the improved repetitive controller on the system performance is analyzed in detail and the corresponding parameter design criteria are given. Finally, the effectiveness of the proposed control strategy under nonlinear load and asymmetric load is verified on the 30 kW 3L3P4L inverter experimental platform.