基于M3C的低频输电系统不对称故障穿越控制策略

低频输电技术兼具高压交流及高压直流输电技术的优势，具有良好的发展前景。但当系统发生不对称故障时，子换流器间电容电压均衡被破坏，影响低频输电系统的安全运行。鉴于此，提出了一种(modular multilevel matrix converter, M3C)不对称故障穿越控制策略。该方案既能在一定程度上限制短路电流，又能平衡子换流器间电容电压，有效提高M3C不对称故障穿越能力。首先介绍M3C的拓扑和工作原理，分析M3C不对称故障期间的运行特性。进而在dq坐标系下推导故障侧电压电流，计算M3C故障侧有功功率表达式并对其中的直流分量部分进行提取。通过将直流分量不均衡抑制为零的方式确定负序电流参考值，用以实现故障侧的负序控制。最后，搭建基于M3C的低频输电系统模型，通过仿真验证了所提方案的有效性和可行性。

Asymmetric fault ride-through control strategy for a low frequency AC transmission system based on a modular multilevel matrix converter

Low frequency AC transmission technology has the advantages of both HVAC and HVDC transmission technology and is a good development prospect. However, when an asymmetric fault occurs in a low frequency AC transmission system, the balance of capacitor voltage between sub-converters will be destroyed, threatening the safe operation. In view of this, an asymmetric fault ride-through control strategy for an M3C is proposed. This method can not only limit the short circuit current to some extent, but also can balance the capacitor voltage between sub-converters, effectively improving the asymmetric fault ride-through capability of the M3C. First, the basic topology and working principle of an M3C are introduced, and the operational characteristics of an M3C during asymmetric faults are analyzed. Then the fault side voltage and current are expressed in the dq coordinate system, the expression of active power on the fault side of the M3C is deduced and calculated, and the DC component is extracted. The negative sequence current reference value is determined by suppressing the DC component imbalance to zero to realize the negative sequence control of the fault side. Finally, a low frequency AC transmission system model is established. Simulation results verify the effectiveness and feasibility of the method.