Research On Control Strategy Of Multi-terminal VSC-HVDC For Wind Power

The technology of VSC-HVDC(Voltage-Sourced Converter High Voltage Direct Current)is anticipated as an attractive solution to the large scale onshore and offshore wind power transmission bottleneck. This paper focuses on the control strategy of VSC-HVDC for wind farm grid interconnection, as well as DC(direct current)voltage optimal control strategy which used for several wind farms connected with several AC grid.(1)Based on analysis of steady state characteristics and the mathematic model of the VSC-HVDC system, this article designed a direct current control strategy of the VSC-HVDC system. Frist, based on three phase static coordinates, the per-unit mathematical model and the equivalent circuit diagram of VSC-HVDC in dq synchronous rotation coordinate system are established. Secondly, By introducing the discrete time mathematical model and deadbeat control, the paper analyzes the current inner loop controller based on the current state feed-forward and decoupling theory, and then the outer loop control strategy(e.g. constant DC voltage control, active and reactive power control and frequency control). Finally, this paper designed the direct current control strategy for the VSC-HVDC system.(2)The control strategy of VSC-HVDC system for doubly fed wind farm network is studied. The paper give the mathematical model of doubly-fed wind power generation, and design a vector control strategy of DFIG. The AC voltage source control strategy for grid integration of large DFIG-based wind farms is studied. The control strategy can control wind farm side AC voltage amplitude and frequency constant value, and then automatic collect wind power to transmission to grid side converter station.(3)DC voltage optimization control strategy of multi terminal VSC-HVDC system for grid connected wind farm is studied. This strategy combines the advantages of master-slave control, dc voltage margin control strategy and dc voltage droop control strategy, presents a dc voltage optimal control strategy that is suitable for DC system. When the system power occurs a small fluctuation,part of converter station that are connected to the active system adjusts the power to maintain system stability;When the system power occurs a large fluctuation,all converter stations that are connected to the active system adjusts the power to maintain the stability of dc voltage,and to avoid large fluctuations of dc voltage. Finally,We analyze the characteristics of dc voltage optimal control strategy in Matlab/Simulink simulation platform. The simulation results verify the correctness and feasibility of the control strategy.(4)The Physics experimental system and simultaneous operation characteristics of multi-terminal VSC-HVDC system is built. When the VSC-HVDC system access passive or weak AC network, can provide active support stability, and improve the frequency stability. Experimental results show that, this article studies the VSC converter station control strategy can realize the steady operation.