Fractional order sliding mode control for circulating current suppressing of MMC

The bridge current distortion and system power loss of modular multilevel converter (MMC) are increased by the circulating current. In this paper, combined with fractional order theory, a circulating current controller based on fractional order differential sign function sliding mode control (FO-SMC) is proposed. By adding fractional order calculus into the sign function, the response speed is improved on the premise of reducing the chattering of the system. Under the same conditions, the system with PI, SMC and FO-SMC circulating current controller is simulated under AC power disturbance, DC side voltage step and received power step conditions, respectively. The simulation results show that the FO-SMC circulating current controller is better than the other in both steady and dynamic conditions and is more suitable for nonlinear systems as MMC.