An Iterative Control Method for Voltage Source Converters to Eliminate Uncharacteristic Harmonics Under Unbalanced Grid Voltages for High-Power Applications

When voltage source converters (VSCs) are subjected to disturbances or faults, they may not be able to perform as per the desired control objective, which presents a significant challenge to system operations. One well-known situation is when the converter is subjected to an unbalanced ac voltage with a negative-sequence fundamental voltage injection. Under this condition, the VSC may generate undesired double-frequency ripples at the dc voltage and uncharacteristic harmonics at the ac current. This paper presents a new two-stage control method suitable for VSCs used in high-power applications. In the first stage, a simple proportional controller is used to suppress overshoots during the transients. Then, the iterative controller is activated to finely adjust the modulation indices and firing angles of the converter to compensate the uncharacteristic harmonics. To verify the performance of the proposed control method, the control algorithms are implemented in a 10 MVA VSC system and validated by means of PSCAD/EMTDC and hardware-in-the-loop simulations. The results show that the state variables converge within a few iterative steps, and the transient response is sufficiently fast. The proposed method is shown to perform better than conventional methods for attenuating and eliminating the uncharacteristic dc and ac harmonics caused by imbalances in the grid voltage.