Control strategy for the voltage source converter based HVDC transmission system under unbalanced AC grid conditions

An electromagnetic transient model for the voltage source converter (VSC) based HVDC transmission system under unbalanced AC grid conditions was proposed. Based on this model, a control strategy was proposed to suppress the double line-frequency ripples in the DC link voltage. According to the relationship between the αβ reference frame and the dq⁺ and dq^- reference frames, it can be obtained that the negative-sequence appears as 100 Hz component in the positive-sequence synchronous reference frame; while, the positive-sequence component appears as 100 Hz component in the negative-sequence synchronous reference frame. Based on a simplified equivalent circuit of the VSC transmission system, a voltage source converter based high voltage direct current transmission (VSC-HVDC) electromagnetic transient model considering the loss of the phase reactors was constructed. To mitigate the additional stresses on the VSC valves and DC capacitor caused by the DC side ripples, the inner loop dual current controller which based on the positive-sequence synchronous reference frame and negative-sequence synchronous reference frame were designed. The outer loop DC voltage controller was designed. Simulation results demonstrate the validity of the mathematical model and the effectiveness of the proposed control strategy.