Research On The Adaptive Control Strategy Of MMC-HVDC Systems For Wind Farm-grid Connection

In recent years,the technology of wind power generation is developing rapidly in China,and the total installed capacity of it ranks the first in the world.The characteristics of fluctuation and intermittence of wind power have exerted adverse impacts on the grid and restricted the capacity of wind power integrated into the grid through AC system.Flexible DC transmission technology is one of the best ways of wind power delivery because of its high controllability,reliability and flexible operation mode.However,in the traditional flexible DC transmission system,the control mode of constant active power is mostly adopted in the wind farm side converter station.When the wind power fluctuates,the outlet frequency of the wind farm will beyond the limit,thus endangering the safe operation of the whole system.To solve this problem,the control strategy of VSC system for wind power integration is studied in this paper.First of all,this paper introduces the research status of MMC-HVDC and the wind power grid,summarizes the advantages and disadvantages of the existing control strategy,and introduces the topology and working principle of the MMC and its sub modules.In the PSCAD/EMTDC simulation software,a simulation model of MMC-HVDC is built,which provides the basis for the ensuing research.Then,according to the mathematical model of squirrel-cage induction wind generator,the relation between the output power of wind farm and the frequency of the system is obtained.The active power reference control strategy based on frequency assistance is proposed.The control strategy changes the active power reference value of the converter station with the fluctuation of the output power of the wind farm to ensure the stability of the system,and the simulation software is used to verify the proposed control strategy.Finally,considering the unified grid of large scale wind farm output power,a MMC-HVDC topology with double converters is proposed.On the basis of this topology,the improved droop control and adaptive droop control based on the active power frequency assistant controller are proposed which changes the P-f droop characteristics and the P-f droop coefficient,allocate the wind power reasonably to the two converter stations.The control strategies further improves the transmission efficiency on the basis of frequency stability.Finally,a simulation model based on the proposed control strategies is built in the simulation platform,and simulation verification.