Fault-Tolerant Strategy for Modular Multilevel Converters With Combined Zero-Sequence Voltage Injection

This paper proposes a novel fault-tolerant strategy for the operation of modular multilevel converters after submodule (SM) faults, which combines dc injection, neutral shift (NS), and balanced zero-sequence voltage injection (BZVI). A new method to calculate the injected zero-sequence voltage is proposed, which greatly simplifies the calculation of the dc injection and enables the combination of BZVI with NS. The resulted strategy has stronger fault-tolerant capability than previous ones: First, it increases the maximum redundancy limit of SM faults under which it is unnecessary to decrease the line voltages. Secondly, it can achieve higher, balanced line voltages when faulty SMs exceed the above maximum redundancy limit. The proposed strategy also has the advantages of online implementation, optimal three-phase balance ability, and self-limiting of the phase with maximum phase output capacity.