A Modular Multilevel Converter comprising an apparatus for supplying power to a sub-module of the Modular Multilevel Converter, the apparatus comprising: a capacitor configured to be charged up to an input voltage between P and N buses of the Modular Multilevel Converter; a relay unit; a resistor connected in series with the relay unit; a TVS diode connected in series with the resistor; a Zener diode connected in series with the TVS diode; a transformer including a primary winding, a first secondary winding, and a second secondary winding to deliver the input voltage received in the primary winding to the first secondary winding and the second secondary winding to output an output voltage of the first secondary winding to a relay switching unit and to output an output voltage of the second secondary winding to a control unit; a switch connected to the primary winding of the transformer in series for switching a flow of current supplied to the transformer; the control unit for enabling current to flow to the transformer by turning on the switch upon receiving a clamping voltage of the Zener diode; and the relay switching unit for operating the relay unit upon receiving the output voltage from the first secondary winding of the transformer, wherein the series connection of the relay unit, the resistor, the TVS diode and the Zener diode is connected in parallel to the capacitor, wherein, when the input voltage reaches the rated voltage, the control unit turns on the switch such that the switch and the transformer constitute a closed circuit to make the current flow through the switch and the primary winding, then the transformer outputs the low voltage as the output voltage from the second secondary winding of the transformer, which is input to the relay switching unit, wherein the relay switching unit operates to open the relay unit by using the output voltage from the first secondary winding of the transformer so that no current is applied to the control unit through the relay unit, and wherein after the relay unit is opened, the control unit receives the output voltage from the second secondary winding of the transformer. a capacitor configured to be charged up to an input voltage between P and N buses of the Modular Multilevel Converter; a relay unit; a resistor connected in series with the relay unit; a TVS diode connected in series with the resistor; a Zener diode connected in series with the TVS diode; a transformer including a primary winding, a first secondary winding, and a second secondary winding to deliver the input voltage received in the primary winding to the first secondary winding and the second secondary winding to output an output voltage of the first secondary winding to a relay switching unit and to output an output voltage of the second secondary winding to a control unit; a switch connected to the primary winding of the transformer in series for switching a flow of current supplied to the transformer; the control unit for enabling current to flow to the transformer by turning on the switch upon receiving a clamping voltage of the Zener diode; and the relay switching unit for operating the relay unit upon receiving the output voltage from the first secondary winding of the transformer, wherein the series connection of the relay unit, the resistor, the TVS diode and the Zener diode is connected in parallel to the capacitor, wherein, when the input voltage reaches the rated voltage, the control unit turns on the switch such that the switch and the transformer constitute a closed circuit to make the current flow through the switch and the primary winding, then the transformer outputs the low voltage as the output voltage from the second secondary winding of the transformer, which is input to the relay switching unit, wherein the relay switching unit operates to open the relay unit by using the output voltage from the first secondary winding of the transformer so that no current is applied to the control unit through the relay unit, and wherein after the relay unit is opened, the control unit receives the output voltage from the second secondary winding of the transformer.