DC Voltage Control and OLTC Control Strategy for MMC-Based HVDC System Under SM Failure

The Modular multilevel converters (MMCs) have excellent performance when operated with high modulation index such as the case of gird-connected converters in high voltage direct current (HVDC) system. Reliable and fault tolerant operation (FTO) of the system is very important in HVDC. A novel FTO algorithm under sub module failure without using any additional redundant SMs is proposed. In the proposed method, the direct current bus voltage control and on-load tap changer (OLTC) control strategy are combined for interruption free operation of the system without any additional voltage stress on switching devices and performance degradation of the system during fault. The analytical details of the normal as well as FTO operation are given. The proposed FTO strategy is verified through simulation and also experimentally using a prototype. The simulation and experimental results are presented.