Virtual Torque and Power Control of a Three-Phase Converter Connected to an Unbalanced Grid With Consideration of Converter Current Constraint and Operation Mode

The paper presents a three-phase power converter control under grid voltage imbalance. The proposed method adapts the current imbalance in relation to the inverter or rectifier operation. Under the inverter operation, the control target is to have the nonoscillatory p component of instantaneous power and the x(v) component of instantaneous virtual torque calculated as the dot product of the grid virtual flux vector and the converter current vector. Then, the phase with lower voltage is supported by higher current. In the rectifier mode, the control target is to have the nonoscillatory q component of power and the T-v component of virtual torque calculated as the cross product of the virtual flux and the converter current. This way, the phase with the highest voltage is loaded by higher current. Unbalanced current obtained in both targets is limited in the way that allows us to limit true phase current and retain sinusoidal current waveforms. However, it does not allow maximum power transfer at the given grid voltage asymmetry, so in order to increase the power value, current progressive balancing is proposed in both operation modes. Finally, converter current balancing is applied also in the case in which a significant amount of reactive power is referenced.