In the high-voltage and high-power applications of modular multilevel converters(MMCs)

large amounts of sub-module(SM)capacitor voltages need to be sampled and maintained in an acceptable range

but the increase in the number of voltage sensors compromises both economy and reliability of the overall system. To solve this problem

a new method of measuring SM capacitor voltages based on voltage estimation and group voltage measurement is proposed. The proposed method divides SMs in the same converter arm into groups

with each group equipped with only one voltage sensor to reduce the number of voltage sensors. On the basis of the new measuring method

a SM open-circuit diagnosis strategy is introduced

which can detect and locate faulty SM rapidly by comparing voltage measurement and estimation

and voltage sensor redundant technique is given to guarantee system normal operation in case voltage sensor fault should occur. A 21-level MMC based high voltage direct current simulation system is set up in PSCAD/EMTDC software environment. It is shown by simulation results that the new method has an approximate measuring accuracy as the traditional one. Meanwhile

SM open-circuit fault and voltage sensor fault can be diagnosed within a relatively short time.