Virtual-Torque-Based Control of Three-Phase Rectifier Under Grid Imbalance and Harmonics

Abstract

The paper deals with a new control method of three-phase rectifier operating under simultaneous grid voltage imbalance and harmonics. The proposed method uses virtual electromagnetic torque, which is calculated on the basis of converter current and virtual flux of the power grid. Direct calculation of reference alpha/beta components of current vector does not require any synchronization procedure or grid voltage/virtual flux orientation of the reference frame. On the other hand, the proposed control is still as simple in implementation as other vector control methods such as voltage-oriented control or virtual-flux-oriented control. The introduced variable (virtual torque) allows us to calculate reference current vector components in alpha/beta frame without symmetrical components decomposition. The current vector obtained by using the proposed control method has the same asymmetry factor as unbalanced grid voltage. It means that the rectifier is seen by the grid as a resistive load with the same resistance in each phase. For the grid it is preferable target because the phase with the highest voltage is loaded most, while the other phases are less loaded. Converter power can be increased by progressive balancing of three-phase converter current keeping the phase current limit.