多端柔性直流输电系统动态附加频率控制策略

附加频率控制利用基于电压源型换流器的多端柔性直流输电(VSC-MTDC)系统直流电压变化传递故障交流系统频率变动,促使非故障交流系统所连电压源型换流器(VSC)消纳不平衡功率参与频率调节。然而固定下垂系数灵活性不足,在不平衡功率分配时忽略VSC实时运行状态与交流网络稳定性,无法保证系统参与频率调整的同时安全稳定运行。通过研究频率变动造成的功率不平衡量分配和直流电压下垂系数的定量关系,提出一种计及系统运行状态的VSC-MTDC动态附加频率控制策略,将VSC功率裕度和交流网络频率变化量引入下垂系数,动态调整不平衡功率的分配比例。仿真结果证明,应用动态附加频率控制进行频率调节后,频率偏差较小的交流网络在所连VSC功率裕度较大时能承担更多不平衡功率,而频率偏差较大的交流网络所连换流站不平衡功率配比下降,VSC-MTDC系统安全稳定运行水平得到显著改善。

Dynamic additional frequency control strategy for VSC-MTDC transmission system

The additional frequency control utilizes the DC voltage variation of VSC-MTDC(Voltage Source Converter based Multi-Terminal Direct Current) system to represent the frequency variation of the AC system with faults. In this way, the VSC connected to non-fault AC system is assigned to accommodate the unbalanced power and participate in frequency regulation. However, the flexibility of the DC voltage droop control with constant coefficients is relatively insufficient. The real-time operation status of VSC and the stability of AC system are neglected when allocating the unbalanced power among all remaining non-fault AC systems, which cannot guarantee that the operation of VSC-MTDC is secure and stable. By analyzing the quantitative relationship between the unbalanced power distribution caused by the frequency variation and DC voltage droop coefficient, a dynamic additional frequency control strategy of VSC-MTDC is proposed. The frequency deviation of the AC systems and the power margin of the VSCs are introduced into the DC voltage droop coefficient to dynamically adjust the unbalanced power distribution ratio. The simulative results show that, by adopting the proposed strategy, the AC system with smaller frequency deviation can accommodate higher level of unbalanced power when the connected VSC power margin is large, while the unbalanced power distribution ratio of VSCs connected to the AC systems with larger frequency deviation is reduced dramatically. Therefore, the proposed control strategy effectively improves the secure operating ability of VSC-MTDC system.