A power control apparatus arranged to supply power to sub-modules of a Modular Multilevel Converter, MMC the power control apparatus comprising: a half-bridge circuit unit, comprising multiple switches (113, 114) and capacitors (111, 112) connected to form the half-bridge circuit unit, the half-bridge circuit unit being connected to the P and N buses of the MMC and arranged to convert a voltage across these P and N buses into a relatively lower voltage and to output the lower voltage; a transformer arranged to transfer the lower voltage, which is output from the half bridge circuit unit, from a primary side to a secondary side of the transformer, and to output an output voltage; a DC/DC converter arranged to receive a voltage corresponding to the output voltage which is output from the secondary side of the transformer, to convert the received voltage into a voltage required for the operation of the sub-module, and to output a converted voltage to the sub-module; a photocoupler arranged to output a reference signal corresponding to a magnitude of the secondary side output voltage of the transformer; a Pulse Width Modulation control unit arranged to control switching of the switches (113, 114) of the half-bridge circuit unit in response to the reference signal output from the photocoupler; and a starting circuit unit arranged to supply an initial starting voltage to the PWM control unit, wherein the PWM control unit is arranged to start in response to the starting voltage initially supplied from the starting circuit unit, and is configured to receive the voltage corresponding to the output voltage which is output from the secondary side of the transformer as an operating voltage. a half-bridge circuit unit, comprising multiple switches (113, 114) and capacitors (111, 112) connected to form the half-bridge circuit unit, the half-bridge circuit unit being connected to the P and N buses of the MMC and arranged to convert a voltage across these P and N buses into a relatively lower voltage and to output the lower voltage; a transformer arranged to transfer the lower voltage, which is output from the half bridge circuit unit, from a primary side to a secondary side of the transformer, and to output an output voltage; a DC/DC converter arranged to receive a voltage corresponding to the output voltage which is output from the secondary side of the transformer, to convert the received voltage into a voltage required for the operation of the sub-module, and to output a converted voltage to the sub-module; a photocoupler arranged to output a reference signal corresponding to a magnitude of the secondary side output voltage of the transformer; a Pulse Width Modulation control unit arranged to control switching of the switches (113, 114) of the half-bridge circuit unit in response to the reference signal output from the photocoupler; and a starting circuit unit arranged to supply an initial starting voltage to the PWM control unit, wherein the PWM control unit is arranged to start in response to the starting voltage initially supplied from the starting circuit unit, and is configured to receive the voltage corresponding to the output voltage which is output from the secondary side of the transformer as an operating voltage.