Periodical Searching Based Nonlinear Droop Control for Multi-terminal Flexible DC Transmission System

When the voltage source converter based multi-terminal high voltage direct current (VSC-MTDC) system adopts the linear droop control strategy of active power and DC voltage, the accuracy of the power-sharing among converter stations will be affected by the DC network characteristics. In the scenario that the DC network topology is unknown, a generalized mathematical model between the active power and the DC voltage of the converter station is established. Under the influence of DC network characteristics, the nonlinear feature of active power and DC voltage droop characteristic of the converter station is analyzed. On this basis, a periodical searching based nonlinear droop control strategy for the VSC-MTDC system is proposed, in which the accurate droop characteristic curve of the converter station operating within the possible ranges can be obtained through the searching technique without the prior knowledge of DC network topology and its parameters. Then the accurate active power control of the converter station can be ultimately implemented after the obtained accurate droop characteristic curve is updated to the bottom controller of the converter station. By establishing a four-terminal VSC-MTDC system model with a ring network in MATLAB/Simulink, not only the analysis conclusion of the nonlinear feature for the active power and DC voltage droop characteristics of the converter station is verified, but also the accuracy of the proposed control strategy in restoring nonlinear droop characteristics of converter station as well as power control compared with the traditional linear droop control strategy is demonstrated. © 2023 Automation of Electric Power Systems Press. All rights reserved.