在混合双馈入直流输电系统中，由于电网换相换流器型高压直流输电(line commutated converter based high voltage direct current，LCC-HVDC)的存在，系统在不同的联络线长度下呈现出不同的高频振荡特征。为研究混合双馈入直流输电系统的高频振荡特征及机理，首先建立系统的状态空间模型，基于此研究系统的高频振荡特征，结果表明，由于LCC子系统的存在，在不同的联络线长度下混合双馈入直流输电系统呈现出不同的高频振荡特征。然后，建立混合双馈入直流系统的高频阻抗模型，研究LCC子系统、联络线长度、模块化多电平换流器(modular multilevel converter，MMC)控制链路延时、交流线路长度等对系统高频阻抗的影响。结果表明，对于无电气距离的混合双馈入系统，LCC滤波器的存在使其无高频振荡风险;对于有电气距离的混合双馈入系统，当LCC和MMC子系统之间的电气距离较近时，LCC 交流滤波器对系统高频振荡的抑制效果随着联络线长度的增大而减弱。最后，提出混合双馈入系统无高频振荡风险的联络线临界长度确定方法，并通过理论分析和电磁暂态仿真进行有效性验证。
Research on High-frequency Oscillation Characteristics and Mechanism of the Hybrid Dual-infeed HVDC System
The hybrid dual-infeed HVDC system presents different high-frequency oscillation characteristics with different tie-line lengths,due to the existence of line commutated converter based high voltage direct current(LCC-HVDC)system.To study the high-frequency oscillation characteristics and mechanism of the hybrid dual-infeed HVDC system,the state-space model of the system is established,and the high-frequency oscillation characteristics of the system are studied.The results show that the hybrid dual-infeed HVDC system presents different high-frequency oscillation characteristics under different tie-line lengths due to the existence of LCC subsystem.Then,a high-frequency impedance model of the hybrid dual-infeed system is established,and the effects of the LCC subsystem,tie-line length,MMC time-delay and AC line length on the system high-frequency impedances are studied.The results show that for the hybrid dual-infeed HVDC system without electrical proximity,the existence of the LCC AC filter eliminates the risk of high-frequency oscillation,and for the hybrid system with electrical proximity,the suppression effect of LCC AC filter on high-frequency oscillation decreases with the increase of tie-line length.Finally,a method to determine the critical length of tie-line without the risk of high-frequency oscillation for the hybrid dual-infeed HVDC system is proposed,and the effectiveness is verified by the theoretical analysis and electromagnetic transient simulation.
hybrid dual-infeed high voltage direct current(HVDC)systemhigh-frequency oscillationAC filtertie-line length
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