Virtual model predictive control of the inverter to achieve unbalance compensation for islanded and grid-tied operations

Voltage unbalance is a frequent problem in the power system, and using renewable energy power generation devices to achieve unbalance compensation is a promising solution compared to adding additional equipment. Most of the existing unbalance compensation schemes for inverters are implemented based on cascaded linear controllers, which have obvious disadvantages of complex structure and cumbersome parameter adjustment process. In order to develop a simpler unbalance compensation scheme, this paper first derives the equivalent sequence network (ESN) model of the unbalanced three-phase power system, and it reveals the important role of transmission line impedance in the unbalance problem. Inspired by the ESN model, an unbalance compensation scheme based on negative sequence virtual impedance (NSVI) is determined. Then the NSVI is embedded in the inverter mathematical model, and the virtual model predictive control (VMPC) of the inverter to achieve unbalance compensation is proposed. Finally, the proposed VMPC method is validated by experiments, and a comparative analysis is carried out with linear control methods. The development process and validation results show that, compared with the unbalance compensation scheme based on cascaded linear controllers, the VMPC method proposed in this paper has a simpler structure and stable performance.