A Novel Fault Current Limiting Method for MMC-HVDC System Based on VSG Control in LVRT Process

With the wide application of flexible DC transmission technology in the field of power transmission and distribution research, the power electronic characteristics of the power system become more and more obvious, the overall strength is reduced, and the anti-interference ability is weakened. In order to solve this problem, the grid-forming control strategy of converter is proposed. This paper is based on the virtual synchronous generator control technology, so that the grid-connected inverter has the inherent inertia of synchronous generator as well as damping characteristics to improve the overall stability of the system. However, the traditional VSG control will have the problem of fault current overrun when facing the AC grid fault, which may damage the power electronic devices. For this reason, this paper investigates the fault current development mechanism and the fault current limiting method when an ac-side symmetrical fault occurs in a grid-configured MMC under VSG control. The methods of adjusting the reactive power sag coefficient during the fault period and adjusting the virtual internal potential at the fault instant are proposed to realize the fault current limiting control. Finally, simulation verification is carried out based on the MATLAB/Simulink simulation platform, and the results better show the effectiveness of the proposed control strategy.