Abstract：Cascaded H-bridge (CHB) converters are a type of common topology for high-voltage high-power power conversion and reliability, especially fault tolerant control, is a key issue because a CHB converter consists of many converter submodules and power devices. There are two levels of fault tolerant control for CHB converters, namely system level, which makes use of redundant or hot standby converter submodules, and converter level, that enhances the fault-tolerant performances for a converter submodule. The system level control usually bypasses faulty submodules completely and needs extra components which introduce increased costs and decreased efficiency, and thus the converter level control is also important to enhance the fault tolerant performance of one submodule before being bypassed. This paper focuses on the converter level and proposes a modulation reconstruction method to output reduced power for H-bridge submodules with open-circuited power switches, which requires no additional redundant switches or changes in topology. For open-circuit faults in a H-bridge, it is found and analyzed that one open-circuit switches only affects its corresponding output voltage in half of a line cycle, either positive or negative, depending on whether upper or lower switches are open-circuited and on the polarity of its current. In the affected half line cycle, the faulty H-bridge can only output half of its nominal voltage with a dc offset of half of its dc bus voltage, which will result in distortion in the output current. In the other half of a line cycle, the faulty H-bridge can operate like normal H-bridges. It is possible to modulate the faulty H-bridge to work in the unaffected half line cycle and to be bypassed in the other half, instead of bypassing the H-bridge at all times. Thus, a modulation method is proposed to enhance the power transfer capability of a CHB converter with faulty H-bridge submodules, based on which switch is open-circuited and the polarity of the current, and the modulation waveforms for the faulty and other healthy H-bridges are updated every half of the line cycle accordingly. In the unaffected half of the line cycle, every H-bridge submodule acts like normal conditions, and then in the other half of the line cycle, the faulty H-bridge submodule output zero voltage and the healthy H-bridge submodules output an increased voltage to compensate the voltage loss of the faulty H-bridge with adjusted phase shifts. As such, the faulty H-bridge submodule can still transfer power half of the time, and the power transfer capability for the CHB converter is increased from (n-1)P to 2(n-1)/(2n-1)P, if only one submodule is faulty, where n is the total number of submodules and P is the nominal power of one submodule. The proposed method was validated in both simulation and experiment, using a 5-level CHB converter. After making one of the power switches in one H-bridge submodule faulty on purpose, the output voltage and current of the CHB converter were distorted and the dc bus voltages of each H-bridge submodule showed abnormal fluctuations. When applying the proposed modulation method, the CHB converter could output sinusoidal current again, although less than normal conditions, and the dc bus voltages were regulated stably. As such, the faulty module can still transfer certain power and the stable operation of cascaded H-bridge converter can be effectively maintained.
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2023, Vol. 38 
