Research On Control Strategy Of VSC-MTDC

The multi-terminal DC (MTDC) is anticipated as an attractive solution to the large scale wind power transmission bottleneck. The topologies selection and design of control strategies is directly related to the operation reliability of VSC-MTDC, and they have a guiding significance for engineering application. This paper will focus on the mathematical modeling and control strategies of VSC-HVD、the topologies and coordinated control of VSC-MTDC, and application of the VSC-MTDC technology in large-scale wind power transmission.1. Based on the mathematic model of VSC-HVDC in abc three phases static coordinate system, the per-unit mathematical model and the equivalent circuit diagram of VSC-HVDC in dq synchronous rotation coordinate system are established. The double-loops and direct-current control strategies of VSC-HVDC is studied, the current inner loop controller based on the current state feed-forward and decoupling theory is designed, and then the outer loop control strategy (e.g. constant DC voltage control, active and reactive power control and frequency control). The control strategy for grid integration of large DFIG-based wind farms using VSC-HVDC is detailed studied. In order to control wind farm side AC voltage amplitude and frequency constant value, and then automatic collect wind power to transmission to grid side converter station.2. The topologies of VSC-MTDC are studied. The advantages and disadvantages of different topologies are compared in the following areas:the amount of DC circuit breakers and isolating switches, control of the realization of the degree of difficulty, DC line length and capacity, failure to the impact on the system, etc. By comprehensive consideration of technical and economic factors, the star topology suitable for the VSC-MTDC applied to large-scale wind power outer delivery is proposed. 3. Based on the analysis of the VSC model and control, the coordinated control strategy for the proposed VSC-MTDC system is discussed. According to DC power and voltage fluctuation in DC network, the coordinated control strategy automatically switches the converter station control mode to redistribution the power flow for keeping the system stable operation under different large disturbance conditions.4. In this paper, a star topology of VSC-MTDC system in Matlab/Simulink for large-scale wind power delivery to load center distance is established. The simulation results show that the VSC-MTDC system can maintain the stable operation in the following three large disturbances, such as wind speed changes abruptly, trip wind farm and three-phase short circuit transmission line failure according to coordinated control strategy.