在模块化多电平变换器(MMC)中，IGBT和电容是最关键也是故障率最高的元件，这些元件的故障会降低输出电能质量，并引起局部高电压大电流，波及其他元件，如不及时诊断，并把故障子模块旁路，会导致MMC系统瘫痪，造成较大的经济损失。为应对IGBT的开路、短路故障及电容老化，本项目：1）提出基于滑模观测器的方法诊断IGBT开路故障和监测电容老化。利用滑模观测器实时估计子模块电容电压和电容量，通过比较实际与估计的电容电压诊断IGBT开路故障；比较估计电容值与其历史记录，判断电容是否老化。2）提出基于电容电压变化率的IGBT短路故障诊断方法。IGBT发生短路故障后，对应子模块电容电压急剧下降，本项目提出一种电路检测电压变化率，并结合门极信号判断故障。这些方法可诊断多个并发的IGBT开路或短路故障。本项目旨在为基于MMC的柔性输电系统提供一套可靠实用的IGBT和电容故障诊断方案，为其不间断运行保驾护航。

In a Modular Multilevel Converter (MMC), IGBTs and capacitors are the most crucial but most failure-prone components. Failures of an IGBT or a capacitor will degrade the output power quality and will cause high voltage and high current near the faulty components, damaging other components, and finally, if the faulty operation is allowed, the shut-down of the MMC plant will follow, leading to considerable economic losses. In order to guarantee the uninterruptible operation of a MMC, this project proposes a group of methods to diagnose open- and short- circuit failures of IGBTs as well as the aging of capacitors: 1)open-circuit faults and capacitor aging are diagnosed using a Sliding Mode Observer (SMO). With the SMO, the capacitor voltage as well as the capacitance of each submodule are estimated. The estimated and measured capacitor voltages are compared to detect the open-circuit fault of an IGBT; the estimated capacitance is compared to its historical record to monitor the capacitor health condition. 2）a short-circuit fault diagnosis method for an IGBT based on changing rate of capacitor voltages (dVc/dt) is proposed. In the presence of the short-circuit fault of an IGBT, the corresponding capacitor voltage will drop dramatically and the fault can be diagnosed by detecting dVc/dt using a simple circuit. All the proposed methods are working on each submodule, thus multiple open- or short- circuit failures of IGBT occurring at the same time can be detected. The project will provide robust and useful methods to guarantee the uninterruptible operation of a MMC based High Voltage DC Transmission system.