High-voltage and high-power voltage source converters (VSCs) are usually operated at low switching frequencies to minimise the switching losses. This study presents a new control method suitable for low switching frequencies. It consists of two stages. In the first stage, a simple proportional controller is used to suppress any overvoltage or overcurrent during the transients. Then, after the control variables have settled to steady-state values, the iterative controller is activated to finely adjust the modulation index and firing angle of the converter to fulfil the control objectives. To justify the proposed control method, various control objectives are realised in a 10 MVA VSC system, and the results are validated by PSCAD/EMTDC and hardware-in-the-loop simulation. The results show that state variables converge within a few iterative steps, and the transient response is sufficiently fast. The proposed method is shown to perform better than the conventional PI control for eliminating the power imbalance and DC second harmonics due to the imbalance at the grid voltage.

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Iterative control method of voltage source converters for various high-power applications

References

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