Improved Voltage Disturbance Rejection of Grid-connected VSC under Unbalanced Voltage Sag Based on Grid Dynamic Model

This paper presents a new approach in the mitigation of unbalanced voltage sag by grid-connected voltage source converter (VSC). The feed forward (FF) controller architecture is adopted with the objective to minimize the impact of unbalanced voltage in the disruption of the dc-bus voltage. To deal with the existing independent control of active and reactive power, two FF controllers are needed. The first FF controller trims the dc-voltage controller output signal and provides correction for the d-axis current (active power) reference. The other feed forward controller trims the reactive/voltage support controller output signal to provide correction on the q-axis current (reactive power) reference. The computation elements of the FF controller are based on the linearized grid model in synchronous frame. The FF controller is tuned at -2 omega(1) which is the frequency in synchronous frame where the unbalanced voltage takes place. For verification of the proposed controller feasibility, simulation is used. The results show that the proposed FF controllers can seek to minimize the impact of unbalanced voltage sag particularly in maintaining the oscillating voltage closer to the dc-voltage reference point.