Abstract
HVDC systems are used for bulk power transfer where reliable operation of the system is very important. A novel fault tolerant method under sub module (SM) failure is proposed in which the faulty SMs are bypassed and system is operated with reduced number of SMs using simple phase disposition pulse width modulation (PD-PWM) until the faulty SMs are replaced. The number of levels in the modulation signals for the faulty phase are adjusted depending on the number of faulty SMs in each arm. Once the fault is cleared, the number of levels in modulation signals will revert to normal mode, bringing the MMC to normal operation. This approach provides fault tolerant operation with full power handling capacity without significant performance degradation of the system during fault. Compared to conventional fault tolerant (FT) methods, proposed approach results in fast and simple changes in PWM signals, better efficiency, higher reliability with least number of spare or redundant SM units. The proposed FT strategy is verified through simulation. The simulation results are presented.
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