交流电网不平衡情况下电压源换相直流输电系统的控制策略

推导了交流电网不平衡情况下电压源换相高压直流输电系统(voltage source converter based high voltage direct current transmission，VSC-HVDC)电磁暂态模型，提出了适用于该场合的抑制直流电压二次波动的控制策略。通过分析αβ坐标与dq+和dq-坐标之间的变换关系，得出结论：在正序旋转坐标下正序分量为直流量，负序分量是频率为100 Hz的交流量；而在负序旋转坐标下负序分量为直流量，正序分量是频率为100 Hz的交流量。通过简化交、直流侧电路，建立考虑换相电抗器损耗的交流系统不平衡情况下VSC-HVDC系统电磁暂态数学模型。为了抑制发生不平衡故障时直流电压的二次波动给VSC阀和直流电容器产生额外应力等问题，设计基于正、负序旋转坐标系的双电流内环控制器和直流电压外环控制器。仿真结果证明所提出的数学模型正确、可靠，所提出的控制策略能够有效地抑制直流电压二次波动。

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) electroma- gnetic 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.