混合型MMC-MTDC直流输电系统故障新型控制策略研究

Modular multilevel converter is a research frontier of HVDC transmission technology. Half-bridge Converter(HBMMC) and full-bridge Converter(FBMMC) constitute a hybrid combination of multi-terminal HVDC system which combines the main advantages of the two converters. In this project, hybrid MMC-MTDC HVDC system is the research object, building control strategy for AC side fault and DC side fault is the major issue. Based on the mathematical model, firstly, the operation mechanism of MMC-MTDC system under the normal condition and fault condition is researched, and coordinated control strategy which combines the improved voltage error method with voltage margin control is put forward. Secondly, according to the features of AC side fault, the improved control strategy which can estimate the positive and negative sequence virtual flux based on DSOGI-QSG and the model predictive control strategy are proposed. When the AC side fault occurs, the above strategies can reduce fault current and active power of the second harmonic fluctuations. Thirdly, taking advantage of the DC fault ride-through capability of full-bridge modular multilevel converter, the control algorithm of DC fault ride-through capability for sub-module in FMBBC is presented in order to improve the stability of system. Finally, in the foundation of studying on simple faults, control method and strategy of feature component extraction under compound faults are researched. The research of this project will provide an important theoretical basis for the development of flexible DC transmission technology in China, which has important scientific significance.

模块化多电平换流器是柔性直流输电技术的研究前沿，半桥型换流器(HBMMC)和全桥型换流器(FBMMC)相结合构成混合型多端直流输电系统可以使二者优势互补。本课题以混合型MMC-MTDC直流输电系统为研究对象，以构建交、直流侧故障的控制策略为主要问题，在数学模型的基础上，首先研究MMC-MTDC正常运行和故障状态时的运行机理，提出改进电压偏差控制与电压裕度控制相结合的协调控制策略；其次根据交流侧故障特征，提出改进的基于DSOGI-QSG的正负序虚拟磁链控制策略和模型预测控制方法，以减小故障电流及有功功率的二倍频波动；再次利用全桥模块化多电平换流器(FBMMC)的直流故障穿越能力，提出FBMMC子模块穿越直流故障能力的控制算法，以提高系统的稳定性。最后在简单故障的基础上，研究复合故障特征分量提取方法及其控制策略。本课题的研究将为我国柔性直流输电技术的发展提供重要的理论依据，具有重要的科学意义。

基于模块化多电平换流器（MMC）的多端直流（MTDC）输电技术，是柔性直流输电技术的研究前沿，它兼具MMC技术和多端柔性直流输电技术的优势，具有广阔的应用前景。本课题研究了混合型MMC-MTDC直流输电系统正常运行和交、直流侧故障的控制策略。根据MMC-MTDC的运行机理，建立了数学模型，提出了MMC-MTDC交直流混合系统潮流计算的交替迭代求解法，并设计了MMC-IDCPFC直流潮流线间控制器，能够使得MMC换流器更加平稳和快速的控制系统潮流。根据交流侧故障特征，提出了改进的基于DSOGI-QSG的正负序虚拟磁链控制策略和模型预测控制的方法，以减小故障电流及有功功率的二倍频波动；对直流侧故障提出了改进电压裕度控制策略，均达到了良好的控制效果。同时采用基于HHT的EMD方法提取故障特征信号，利用固有频率获得故障距离，通过最小二乘支持向量回归（LSSVR）算法设计回归模型并对故障距离预测值进行优化，实现故障测距和故障定位；进而设计智能故障诊断器（IFDD）对系统发生的故障进行诊断。针对MMC-MTDC系统内部环流抑制与子模块均压控制的问题，提出了基于二倍频注入的环流抑制控制策略（DFI-CCSC）和基于鸡尾酒排序的子模块电压均衡控制策略，这两种控制策略均能能更有效地抑制环流，较大程度地改善子模块电容电压的波动率。最后研究了复合故障特征分量提取方法及其控制策略，考虑到柔性直流输电系统的强耦合、非线性和不确定性，提出了一种基于新型变指数趋近律方法的滑模控制策略，通过对滑模面的优化设计和等效控制律的求取，来提高系统在正常或故障状态下的稳定运行能力；针对传统控制方法忽略了MMC内部的耦合因素，会导致有功与无功之间出现耦合的问题，提出了一种改进解耦控制，以实现有功与无功的完全解耦。课题的研究为我国柔性直流输电技术的发展提供了理论依据，具有重要的科学意义。