Method to Balance Grid Voltage Condition Injecting Three-Phase Currents by Controlling Three-Phase CSC

A topology for an interface of DC grid with an AC grid using single stage bidirectional Current-Source Converter (CSC) has been reported in the literature. Under a balanced-grid voltage condition, the DC-link inductor current can be regulated over a wide range (zero to rated value), while the AC-side current has low harmonic distortion. However, unbalanced grid voltages result in second-harmonic pulsation in the current and power on the DC side of the converter. In addition, the AC-side currents will be unbalanced due to the presence of a negative sequence component. This would result in undesired tripping of the converter if one of the phase currents exceeded its rated value. Various control loop structures for the operation of voltage-source converter under unbalanced grid voltage conditions has been reported in the literature. However, use of similar control loop structures for CSC may lead to unstable operation. In this paper, a control scheme to inject balanced three-phase currents into the AC grid under an unbalanced grid voltage condition. The stability of the proposed control scheme is studied using a small signal model of the converter. The performance of the proposed control scheme is studied MATLAB/Simulink and validation by experiments.