混合直流系统中直流侧故障暂态电流的解析表达与故障特性的研究

由基于线性换流器高压直流输电系统（LCC-HVDC）和基于电压源换流器高压直流输电系统（VSC-HVDC）共同构成的混合直流输电系统，其故障特性与传统直流输电系统不同。针对此问题，对混合直流输电系统中直流侧故障暂态电流特性进行了研究。首先建立了送端电网采用LCC型换流站、受端电网采用VSC型换流站的两端混合直流输电系统，利用拉普拉斯变换定理推导了直流侧故障时的等效电路，解析了LCC侧和VSC侧直流故障电流简易表达式。其次，在简易表达式的基础上，充分考虑送端LCC侧换流站的触发角动态变化过程和受端VSC侧换流站交流电流的馈入，进一步解析了两侧精确的故障电流表达式。然后，从故障电流幅值、谐波等方面对比分析了三种高压直流系统中直流侧故障电流的变化特征。最后，通过MATLAB/Simulink仿真验证了所提故障电流解析表达式的正确性。

Research on Analytical Expression of DC Side Fault Transient Current and Fault Characteristics in Hybrid HVDC System

The hybrid high voltage direct current (HVDC) transmission system consists of the line-commutated converter based HVDC (LCC-HVDC) transmission system and the voltage source converter based HVDC (VSC-HVDC) transmission system, which have different fault characteristics than conventional HVDC transmission system. To solve this problem, the DC side fault transient current characteristics in hybrid DC transmission systems are investigated. Firstly, a two terminal hybrid HVDC transmission system with LCC converter station in the sending-end grid and VSC converter station in the receiving-end grid is established. The equivalent circuit in case of DC side fault is derived by using laplace transform theorem, and the simple expressions of DC fault current on LCC side and VSC side are analyzed. Secondly, on the basis of the simple expressions, the accurate fault current expressions on both sides are further analyzed by fully considering the dynamic change process of the trigger angle of the LCC converter station at the sender side and the feed-in of the AC current from the VSC converter station at the receiver side. Then, the variation characteristics of DC side fault current in three HVDC systems are comparatively analyzed in terms of fault current amplitude and harmonic. Finally, the correctness of the proposed analytical expression of DC side fault current is verified by MATLAB/Simulink simulation.