Control device and control method for modular multilevel converter

A control method for a Modular Multilevel Converter (MMC) composed of a plurality of sub-modules connected in series and including a converter arm connected to a DC link, the control method comprising:

• determining, by an arm controller, whether a DC failure occurs by detecting an arm current of the converter arm;
• transmitting by the arm controller, a bypass control signal to a sub-module controller and notifying a main controller of an occurrence of the DC failure when determining that the DC failure occurs;
• controlling, by the sub-module controller, the sub-module to bypass a DC failure current in response to the bypass control signal;
• detecting, by the main controller, in real time an arm current of the converter arm and a voltage of the DC link and determining whether the DC failure is a temporary DC failure or a permanent DC failure on the basis of the detected arm current and DC link voltage when the main controller is notified of the occurrence of the DC failure;
• and
• transmitting, by the main controller, a normal operation control signal for normal operation of the sub-module or a bypass control signal for bypassing of the DC failure current to the arm controller, depending on whether the DC failure is the temporary DC failure or the permanent DC failure,
• wherein the arm controller transmits the bypass control signal for turning off power switches and turning on a bypass switch to the sub-module controller without receiving a control signal from the main controller when determining the occurrence of the DC failure on the basis of the detected arm current, and then notifies the main controller of the occurrence of the DC failure,
• wherein the sub-module controller turns off the power switches in the sub-module and turns on the bypass switch in response to the bypass control signal received from the arm controller,
• wherein, after turning off the power switches and turning on the bypass switch, the main controller transmits the normal operation control signal for normally operating the sub-module to the arm controller when the DC failure is the temporary DC failure, and the sub-module controller turns on the power switches in the sub-module and turns off the bypass switch in response to the normal control signal received from the arm controller, and
• wherein the main controller transmits the bypass control signal for bypassing the DC failure current to the arm controller when the DC failure is the permanent DC failure, and the sub-module controller keeps the power switches, which have been turned off, turned off and keeps the bypass switch, which has been turned on, turned on in response to the bypass control signal.