CONTROL OF PARALLEL -CONNECTED MODULAR MULTILEVEL CONVERTERS

Abstract

This paper presents an integrated control and switching scheme for a grid-connected modular multilevel con- verter (MMC). The control consists of the conventional vector control (outer PQ control, inner line current control) used in two-level voltage source converters (VSCs) and additional circulating current mitigation control. Circulating currents in a modular multilevel converter (MMC) will be eliminated using a proportional resonant controller. The generated upper and lower arm reference voltages will be sent to the switching sequence generating blocks to generate gate signals to submodules. Phase- shifted PWM scheme will be used to determine how many submodules to be turned on in an arm while the voltage balancing block will determine which submodules to be turned on. Matlab/SimPowerSystems simulation environment is used to model a grid-connected MMC with the integrated PQ control. Simulation case studies demonstrate the effectiveness of the proposed control strategy. The contribution of this paper lies in the modeling and presentation of the entire control and switching procedure for a grid-connected MMC, with reference to the control of two-level VSCs. This paper will provide readers with a clear understanding of the difference between MMC control and two-level VSC control