Hybrid one-cycle control scheme for fault-tolerant modular multilevel rectifiers

This paper customises the classic one-cycle control (OCC) scheme for modular multilevel rectifiers (MMRs) and overcomes the inherent defect of the OCC. To be specific, a hybrid one-cycle control scheme is proposed combining the OCC and the virtual loop mapping (VLM). First, on the basis of the classic OCC and the volt-second equivalence principle, the relationships between the MMR arm equivalent duty cycle and the sub-module (SM) duty cycle are derived. Then, by making use of VLM methods, the switching frequency of the SM is reduced and the dynamic capacitor voltage balance is obtained as well. Further, to achieve the single-line-to-ground fault-tolerance capability and eliminate the second harmonic ripple in the DC voltage, the constant power control is presented using the negative-sequence voltage compensation. Specifically, the whole control scheme only needs one proportional integration controller, which greatly reduces the complexity of system control and the system cost. The validity of the proposed scheme is verified through simulation and experimental studies.