A droop line tracking control for multi-terminal VSC-HVDC transmission system

•A novel droop control implementation for multi-terminal VSC-HVDC system is proposed.•The converter is ensured to operate along a droop line following a disturbance.•Instead of droop constant, displacement distance is used to generate the new reference.•The proposed method becomes an additional layer of the existing converter control.•The concept simplifies realization of the advanced converter control (piecewise linear droop line). In theory, the control mode of a voltage-sourced converter (VSC) within a multi-terminal HVDC (MTDC) transmission system can be represented by using a droop line characteristic in the active power and DC voltage relationship (Pac–Udc) curve. However, operating the droop control as an extension of the Udc control mode with a steep slope was found to be difficult since it introduces instabilities. This also happens in the extended Pac control with a shallow slope. This paper proposes a droop line tracking (DLT) method for mitigating these issues, i.e. by calculating the new operating point of the converter based on the displacement distance from the droop line. The simulation results show that the proposed method enables the converter to operate in different control modes by simply changing the droop line characteristic. Furthermore, the same concept can be extended to implement the advanced droop control, i.e. represented by a multi-slope droop line characteristic such that the converter can be operated in different control modes depending on the DC system disturbance level. Simulation using a three-terminal HVDC system with an offshore wind farm (OWF) station has been performed to demonstrate the functionality of the proposed method in realizing the advanced droop control mode.