Conclusion
The MMC converter would be the natural choice to design medium voltage power converters for step-up-transformer-less grid integration of renewable generation units. A common high-frequency magnetic link can be considered to generate multiple isolated and balanced dc supplies for all the H-bridge inverter cells of MMC converter, which automatically minimizes the voltage imbalance problem. The leakage currents and safety problems will also be solved inherently due to electrical isolation provided by the high-frequency link. Although the additional windings and rectifiers may increase the loss of the high-frequency link MMC converter, it is still similar to a 2-level converter-based traditional system, because the use of an MMC converter can eliminate three main parts, i.e., 2-level converter, line filter, and power frequency step-up transformer, from the classical system. Moreover, the elimination of heavy and large step-up transformer and line filter will enable large amount of cost savings in installation, running, and maintenance of renewable power generation systems. Regardless of the type of power transmission system, the medium voltage converter is equally applicable in high voltage ac (HVAC) or HVDC system.