A Novel Fault-Tolerant Control Method for Modular Multilevel Converters

Recently, with the rapid development and the application of modular multilevel converter (MMC) in high voltage direct current (HVDC) transmission system, motor drive and other fields, more and more attention has been paid to it's safe and reliable operation. This paper proposed a semi-hot reserved redundancy method under MMC sub-modules (SMs) failure. In this method, redundant SMs participate in uncontrolled precharge as ordinary SMs. After uncontrolled precharge, the redundant SMs are controlled in voltage balance state under MMC normal operation. If an ordinary SM failed, the faulty SM will be bypassed immediately and the redundant SM will be recharged at the same time. When the redundant SM is recharged to the rated voltage, it will be inserted quickly to replace the faulty SM to maintain the stable and continuous operation of the MMC system. The proposed method can quickly reduce the circulating current oscillation and guarantee the MMC system normal operation under SMs failure. The method is simple to achieve and there is no need to change the original modulation strategy and capacitor voltage balance control of the MMC system. Finally, a 21-level two-terminal MMC system is built in PSCAD/EMTDC to validate the effectiveness of the proposed methods.