The present invention relates to an electric power supply system for an aircraft having a transformer-less HVDC architecture, the system comprising: at least one multiphase engine generator in a star configuration which provides a multiphase alternating (AC) voltage; a converting circuit for converting the multiphase AC voltage into a corresponding direct (DC) voltage for supplying at least one aircraft load; an electric power supply interface (27) for connection with a standardized GPU plug (50) of an external GPU (28), the electric power supply interface (27) comprising a supply voltage output terminal coupled to a self-contained power source (55) for providing a low voltage control signal for driving a GPU internal switching circuit (54) wherein the power source is decoupled from the AC voltage and DC voltage. The present invention further relates to an aircraft and an airport power supply system.