Modular Multilevel Converters (MMC) are becoming increasingly popular with the development of HVDC connection and, in the future, Multi Terminal DC grid. A lot of publications have been published about this topology these last years since it was first proposed. Few of them are addressing explicitly the two different roles that are held by this converter in a HVDC link: controlling the power or controlling the DC voltage level. Most of the time, the DC-bus voltage is supposed to be constant. In an HVDC link, this corresponds to the substation which controls the power. This paper addresses the cases when the voltage is regulated by the converter and presents the different ways of voltage control.