Control Strategy Implemented in Modular Multilevel Converter to Improve the Operation During Variable DC Bus Voltage

Abstract

The modular multilevel converter (MMC) in high voltage direct current (HVDC) is equipped with hundreds of sub-modules (SMs), so the detailed model consist of hundreds of switching devices and consequently large computational effort but mathematical converter model with Arm Level Averaged (ALA) allows to know the dynamic and converter control decreasing simulation time. This paper presents a control strategy decou- pling the modulation index implemented in MMC-ALA model, it allows MMC operation with variable DC bus voltage but keeping constant the capacitor voltage in upper and lower arms, the evaluations consider the next operations: boost mode reducing DC bus voltage to 0.5.p.u., DC fault and SM voltage increased. The upper and lower capacitor voltages in all cases follow the reference independently to DC bus voltage value, the strategy is simulated in 150 MW converter using Matlab/Simulink.