Due to the large number of submodules (SMs)

and modular multilevel converters (MMCs) in high-voltage applications

they are usually regulated by the nearest level modulation (NLM). Moreover

the large number of SMs causes a challenge for the fault diagnosis strategy (FDS). This paper proposes a currentless FDS for MMC with NLM. In FDS

the voltage sensor is relocated to measure the output voltage of the SM. To acquire the capacitor voltage and avoid increasing extra sensors

a capacitor voltage calculation method is proposed. Based on the measurement of output voltages

the faults can be detected and the number of different-type switch open-circuit faults can be confirmed from the numerous SMs in an arm

which narrows the scope of fault localization. Then

the faulty SMs and faulty switches in these SMs are further located without arm current according to the sorting of capacitor voltages in the voltage balancing algorithm. The FDS is independent of the arm current

which can reduce the communication cost in the hierarchical control system of MMC. Furthermore

the proposed FDS not only simplifies the identification of switch open-circuit faults by confirming the scope of faults

but also detects and locates multiple different-type faults in an arm. The effectiveness of the proposed strategy is verified by the simulation results.