双向交直流变换器桥臂故障容错有限控制集模型预测控制

Bidirectional AC/DC converter is the hub device in hybrid microgrids. The transient surges generated in high frequency switching process may easily cause converter leg failure which will result in hybrid microgrids splitting. In order to maintain continuous operation under fault conditions, the study of fault-tolerant topologies and control become significant and urgent issues. A non-redundant two stages bidirectional AC/DC converter with leg fault-tolerant capability is presented in this project. The converter is reconfigured by utilizing high frequency isolated DC transformer and three phase four switch (FSTP) converter. The frequent faults in this converter include open circuit, short circuit and disconnection of converter legs. The main research topics of this project are as follows. (1) To analyze the influence of faults on bidirectional AC/DC converter, and reveal the optimal rules of switching voltage vector selection under the condition of fault operation by replacing the fault leg with virtual capacitance leg. (2) To establish the power prediction models in different fault operation modes, and propose the finite control sets model predictive control (FCS-MPC) method for multi-modes flexible switching in fault-tolerant operation. (3) To discover the influence of unbalanced voltage and the cost function optimal matching rules by combining the theoretical analysis with experimental research, and to clarify the power quality regulation mechanism of the model predictive compound control combining power decoupling compensation with time-delay compensation. This project will provide theoretical basis for improving the operation reliability and fault-tolerant ability of bidirectional AC/DC converter.

双向交直流变换器是连接混合微电网的枢纽装置，其故障停运会导致混合微电网解列。为了维持多种故障模式连续运行，双向交直流变换器故障容错结构与控制方法研究成为重要而紧迫的课题。本项目分析开路、短路、断线等桥臂故障对双向交直流变换器的影响，采用高频隔离直流变压器和三相四开关并网变流器构建一种具有桥臂故障容错能力的非冗余两级式双向变换器结构，揭示电容虚拟桥臂替代故障桥臂容错运行时最优开关电压矢量的选择规律；建立容错双向变换器多种故障模式功率预测模型，提出一种多模式柔性切换容错运行的有限控制集模型预测控制方法；在此基础上，探索电网不平衡对故障容错运行的影响和代价函数优化匹配规律，阐明双向功率解耦补偿与时滞补偿相结合的模型预测复合控制对并网电能质量的调控机制。本项目有望为提高双向电能转换可靠性提供理论基础，也为研制容错双向交直流变换器提供技术支撑。

在分布式发电和微电网中，双向交直流变换器控制着交流侧和直流侧之间的双向功率流动，对改变和优化能源结构、促进清洁能源友好接入、改善电能质量等方面起到重要作用。当双向交直流变换器的开关器件发生故障后，为实现其故障容错运行，重构了一种三相四开关变换器。模型预测控制具有良好的自适应性和鲁棒性，设计了三相四开关容错结构的模型预测控制策略。首先阐明故障后的变换器结构变化及容错运行机理；建立了容错运行模式的功率预测模型，分析了故障情况下电压矢量变化关系。然后，设计了价值函数及延迟补偿策略。传统的模型预测控制开关频率不确定，导致其输出电流频谱分散，谐波含量大。提出了一种多矢量恒频模型预测容错控制方法，通过控制三个矢量的作用顺序，固定功率器件开关频率，使电流谐波频率集中在开关频率的整数倍，解决受滤波电感限制较大的问题。分析电网不平衡对变换器故障容错运行输出功率波动、电流畸变的影响关系。在研究变换器工作机制及故障后电压矢量变换关系的基础上，通过对复功率与电压、电流正负序分量的关系进行了分析和推导，设计了抑制功率波动的模型预测直接功率控制策略，提高容错运行电能质量。

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