A New Fault Tolerant Control Method for a Three Phase Modular Multilevel Converter Under an Arm Failure

Modular multilevel converter (MMC) is widely used in medium to high power applications. One of the significant features that an MMC can provide is the fault tolerant operation, which improves the reliability of the converter. Although there are extensive efforts towards addressing the fault tolerant operation of a modular multilevel converter, the control of a three phase MMC when one of its arms is out of service, therefore, must be bypassed, is not yet discussed. The objective of the paper is to overcome this problem, by proposing a control method, which allows to control separately, the voltages of the capacitors in the upper and lower arms, except for the voltages of the capacitors belonging to the bypassed arm. In order to meet these requirements, the injected circulating current in each phase must contain a DC component, but also an AC component, oscillating at the fundamental frequency. The feasibility of the proposed fault tolerant method has been confirmed by simulations conducted on a three phase MMC connected to a low voltage grid using MATLAB/SIMULINK.