Dual Adaptive Nonlinear Droop Control of VSC-MTDC System for Improved Transient Stability and Provision of Primary Frequency Support

This paper presents an adaptive nonlinear droop control (ANLDC) strategy for the voltage source converter (VSC) in multi-terminal high voltage direct current (MTDC) system, which enables the converters to provide primary frequency control for connected AC grids. The presented strategy inherits the concept of an emergency control approach. In normal condition when there is no significant deviation in RoCoF and DC voltage, it will be in inactive mode. The combination of two control techniques, nonlinear frequency-droop control, and nonlinear voltage-droop control results in a dual nonlinear controller that includes the advantages of both of them at the same time. The ANLDC strategy bears the advantage of the traditional droop control that is based on local measurements and there is no need for a communication system. The simulation results indicate that the AC/DC station with the proposed dual control strategy can improve simultaneously the primary frequency response and transient stability of the connected AC grid.