The Modular Multi-level Converters (MMCs) have emerged as the most suitable converter technology for HVDC application. Besides the recognized advantages over conventional VSCs, one of the remarkable features of the MMC is its ability to store energy in the distributed sub-modules. In DC system, the DC voltage is the indicator of power balance as with the frequency in AC systems. In the similar sense as AC systems, the dynamics of the DC voltage is characterized by the energy stored in the form of electrostatic potential energy in the capacitors. However, compared to the conventional AC system, the system energy level of the DC system is anticipated to be far fewer than that of AC system. This implies that, without explicit control, the DC system voltage can be extremely volatile against power disturbances.
Patent FR3039940 (B1) WO2017021642 (A1)