Optimal Synchronization Interval in the PS-PWM based MMCs with Sub-module Asynchronism

Modular Multilevel Converters for HVDC applications normally adopt a distributed control system to manage considerable submodules (SMs) in the system, where a large number of local controllers are employed. The manufacturing tolerance of digital controller crystal oscillators introduces controller clock inconsistency and finally leads to the asynchronism of triangular carriers implemented in individual SM for phase-shifted PWM scheme. Considering the tolerance of controllers having a normal distribution, the asynchronism of the triangular carriers is modelled based on the probability theory and the resulting switching harmonics are evaluated in this paper. The switching function model taking the asynchronism into account is proposed, in order to quantitively analyze the contents of switching harmonics around the low frequency and the equivalent switching frequency. The theoretical calculation of switching harmonics can be used to determine the optimal interval of the SM synchronization scheme according to the system THD requirement. The communication stress is largely eased because of the optimal synchronization scheme. The validity of the switching function model and the proposed optimal synchronization scheme are verified experimentally on a prototype of MMC.