Hybrid Unidirectional MMC-Based Rectifier

More-electric ships (MESs) and subsea oil processing electric power systems are typically fed by synchronous generators. These are primarily driven by gas turbines in the case of O&G or diesel engines in MESs. In neither of these applications the generators should experience regenerative operation, i.e., they do not operate under reversed power flow. Medium voltage dc unidirectional rectifiers are well suited and provide natural protection against power flow reversals. Thus, a hybrid rectifier concept that is based on line commutated diodes to transfer a large portion of electric power allied with the redundancy and higher operating frequency of modular multilevel converter (MMC) submodules (SMs) is proposed. The hybrid rectifier operates the MMC SMs to synthesize waveforms that are ideally composed of triplen harmonics. Therefore, the frequency of the power oscillations at the capacitors are shifted to frequencies higher than those of a conventional MMC. The overall required capacitance is consequently reduced. Furthermore, a much lower number of SMs is required when compared to a conventional three-phase MMC. These characteristics are welcome in the cited applications. This work discusses how to operate and control the proposed system.