Sensorless Virtual-Flux Direct Power Control of Grid Connected Converters under Unbalanced Weak Grid Conditions

Power grids are weakened by increasing penetration of renewable energy sources. Nevertheless, the conventional control methods of voltage source converters (VSCs), cannot work well under weak grid conditions. Direct power control (DPC) due to the simplicity of its system structure, the elimination of current controllers, and modulation blocks has attracted widespread attention. Some researchers have enhanced the control method by reducing the effects of unbalance under stiff grid conditions. Nevertheless, the weak grid conditions lead to voltage fluctuations at the point of common coupling (PCC) and consequently cause converter instability. This research proposes a sensorless virtual flux DPC of VSCs enjoys a stable performance under the unbalanced weak grid conditions using the proposed active and reactive power references. The proposed control system provides features such as high-power quality and better current THD in comparison with the conventional direct power control method. Also, the voltage sensors have been eliminated. The stability of proposed control system is provided using a small-signal model, where the model is directly extracted from a conventional switching look-up table. The stability analysis shows that it is possible to simultaneously change system parameters. These findings are verified through extensive simulation.