New control technique for sensorless grid synchronization of modular multilevel converters for HVDC systems

High- and medium-voltage direct current (HVdc and MVdc) electric power transmission have become more common for transmitting power across long distances. Advantages such as lower conduction losses, lower capital cost, and flexibility make HVdc more attractive than conventional ac transmission systems, especially when integrating remote renewable energy resources with the power grid. Within voltage source converters (VSCs), the modular multilevel converter (MMC) using a half-bridge in each module has recently become the preferable solution for VSC-HVdc due to its multiple advantages when compared to the other VSC topologies. A control technique not requiring grid sensors at the MMC-based HVDC terminals is proposed simplifying the controller physical implementation. The proposed technique controls separately the exchanged active and reactive powers. The robustness of the proposed controller is demonstrated through simulations for different grid disturbances.