基于调制理论的高压直流输电系统混合谐振型谐波不稳定判据

Modulation Theory Based Criterion on Composite Resonant Harmonic Instability in HVDC Transmission System

高压直流输电系统中交流侧二次谐波分量和直流侧基波分量经换流器调制后形成混合谐振,会诱发谐波不稳定。为快速判断混合谐振型谐波不稳定是否发生,在等间隔脉冲触发方式下,将换流器视为二端口网络,并通过开关函数的调制原理推导出了描述换流器的H矩阵。H矩阵可将换流器连接的交直流系统等效为1个RLC电路,从而计算出系统的谐振衰减因子。通过计算得到了谐振衰减因子与换流器的触发角和换相角、换流变的变比和漏抗、交流侧二次谐波阻抗、直流侧基波阻抗之间的关系,给出了判断混合谐振型谐波不稳定的可靠判据。最后在PSCAD/EMTDC中建立仿真模型验证了该谐波不稳定判据的有效性,并验证了可以通过修改相关系统元件参数来避免谐波不稳定现象的发生。

In an HVDC transmission system, the intermodulation between the secondary harmonic component on the AC side and the fundamental frequency component on the DC side via converters will generate composite resonance, which will further lead to resonant harmonic instability of the system. In order to quickly judge whether a resonant harmonic instability occurs or not, by treating converter as a two-port network and keeping the firing angle constant, we deduced the H matrix of converter based on the modulation theory of switching function. Through the H matrix, a DC-AC system connected by converter could be equalized as a RLC network, and hence a harmonic damping factor could be calculated. We calculated and analyzed the relationship among the harmonic damping factor, the firing angle and commutation duration of converter, the converting ratio and leakage reactance of converter transformer, the second harmonic impedance at AC system side, and the basic harmonic impedance at DC system side. Analyzing the results, we obtained reliable criteria for determining resonant harmonic instability. The criteria are verified by simulation in PSCAD/EMTDC, and it is proved that harmonic instability can be avoided through changing the correlative parameters of some elements in an HVDC system.