Research On Coordination Control Strategy Of VSC-HVDC Transmission System With Wind Power Integration

With the severe energy situation and the continuous maturity of wind power generation technology,more and more wind farms are appearing in the power system to replace the traditional energy power plants to supply power to the load center.However,due to the energy characteristics: randomness,fluctuation and far from the load center distribution,the problems of grid connection and absorption of wind power are becoming more and more serious.HVDC transmission technology based on voltage source converter(VSC-HVDC)has become one of the effective forms of power trans-regional transmission with renewable energy integration,depending on the flexibility and controllability of power electronic devices,transmission stability,engineering application performanceprice ratio and feasibility of renewable energy trans-regional absorption.It is an important link in the construction of smart grid in China.In this paper,the control strategy of converter station and wind power generator for VSC-HVDC system with DFIG grid-connected is studied.Firstly,the basic composition and structure of wind power grid-connected VSC-HVDC system are introduced.The mathematical models of DFIG and converter station are established in two-phase rotating coordinate system,and the traditional active power control strategies of DFIG and converter station are explained.Then,aiming at the problem that conventional decoupling control of converter station effectively isolates AC/DC system,which leads to the reduction of inertia and frequency modulation capability of AC system.Based on the principle of DC voltage droop control,a virtual governor coupled with DC voltage and AC network frequency is proposed.The inertia effect of generator is simulated by the rotor equation of virtual synchronous generator.The frequency response and power support between the receiving converter stations of multi-terminal HVDC system are effectively realized.Then,the control strategy of DFIG participating in frequency modulation via VSC-HVDC system is studied.Load reduction operation of DFIG is realized by rotor overspeed control,and a comprehensive inertial control structure with inertial response and primary frequency modulation effect is added to the DFIG speed control structure to realize effective response of DFIG to frequency variation of wind farm and to change output wind power.At the same time,combined with the frequency response control of receiving converter station and the frequency conversion control of wind farm side converter station,coupled DC voltage with the frequency of AC network at both side and using DC voltage as the medium of frequency information transmission,the wind power grid-connected VSC-HVDC system can achieve frequency response coordination control based on local detection.Finally,the VSC-HVDC system model of DFIG wind farm connected to grid is built on Matlab/Simulink platform,and the effectiveness of the proposed control strategy is verified by simulation.