Harmonic and imbalance compensation in electric railway systems using modular multilevel converters under varying load conditions

This paper presents a novel control strategy for the compensator based on modular multilevel converter (MMC) applicable to electric railway systems. The compensation algorithm is composed of two level: A model predictive control (MPC) is used to control the three-phase output currents of the MMC, a closed-loop control based on energy equations is utilized to upper and lower capacitors balancing control. Also, the reference current components are calculated using instantaneous power theorem. A three-phase four-leg MMC configuration is considered as the compensator. The main challenges include three-phase output currents control under distorted and unbalanced conditions and elimination the circulating current in each legs of MMC in order to upper and lower capacitors balancing control in medium-voltage electric railway systems. The capability of the proposed controller is verified under sudden load variation conditions and it is demonstrated that the two-level proposed control strategy has ability to compensate the harmonic and imbalance distortions in electric railway systems as the distorted loads under load variation conditions. The simulations are carried out in MATLAB/Simulink and the results confirm the effectiveness of the proposed controller.