Fault-Tolerant Control of MMC With Hot Reserved Submodules Based on Carrier Phase Shift Modulation

Reliability is one of the most important challenges for the modular multilevel converter (MMC), which is composed of a large number of power electronics submodules (SMs). In order to increase the system reliability, the reserved SMs are often utilized. A novel strategy is proposed in this paper to implement the fault-tolerant control of MMC with the hot reserved SMs based on the carrier phase shift modulation. All the SMs, including the hot reserved SMs, work in the operating mode and the standby mode in turns. A rotating sliding choice box is adopted to select the operating SMs and the corresponding phase-shift angles, which enables the equal burden of each SM. Once the faults are detected and localized, the reserved SMs can simultaneously replace the bypassed failed SMs, which guarantees the fault-tolerant operation with a nearly seamless transition and very short recovery time. Furthermore, the line cycle is set as the rotating period in the control strategy, without increasing much switching loss. The mismatch pulses are greatly reduced as well. This technique can also be easily implemented in a DSP or microcontroller. Simulations in MATLAB/Simulink and experiments based on a downscaled MMC prototype have verified the feasibility and effectiveness of the proposed control strategy.