Research On Coordination Control Strategy Of VSC-MTDC System Considering Frequency Stability Of Weak AC System

With the development of multi-terminal flexible HVDC technology in the fields of new energy grid-connected,large-capacity long-distance power transmission and new urban DC distribution network,power coordination control between converter stations and frequency stability of weak AC system connected by converter stations in VSC-MTDC system are of great significance to ensure the stable operation of VSC-MTDC system.Initially,a new adaptive droop control strategy is proposed to solve the problem of coordinated power control among multiple converter stations.The unbalanced power in the system is flexibly allocated according to the power margins of the converter stations.In the process of power regulation,droop coefficients changes in real time according to power margins,which reduces the fluctuation of DC voltage and the influence of DC voltage errors(line length and other factors)among converter stations on power regulation.Furthermore,considering the frequency stability of weak AC system,in order to realize the bidirectional support of active power of DC system and weak AC system,the frequency of DC voltage and weak AC system are divided into two parts: the open-time operation area and the time-limited operation area.The direction of active power support for DC system and weak AC system is determined according to the interval of DC voltage.When the DC voltage is in the open-time operation area,the DC system can provide power support to the weak AC system to maintain frequency stability.When the DC voltage is in the time-limited operation area,the weak AC system sacrifices part of the frequency quality to provide short-term power support for the DC system to maximize the stability of the multi-terminal DC system.Finally,the simulation models of four-terminal HVDC system and five-terminal HVDC system are built based on PSCAD/EMTDC software.In the four-terminal HVDC transmission system,traditional fixed slope,improved fixed slope and adaptive droop control are compared and simulated.The bidirectional active power support capability of adaptive droop control considering frequency stability is simulated in five-terminal HVDC transmission system.The simulation results verify the feasibility and effectiveness of the proposed control strategy.