Research On Modeling Of Wind Farms Applicable To Frequency Control And Frequency Modulation Strategy

Large-scale grid-connected wind turbines will reduce the overall inertia of the power system and affect the frequency stability of the system.Therefore,it is necessary to study the participation of wind farms in the frequency modulation process of the system.The single-machine equivalent model is usually used in wind farms involved in frequency modulation process.Because of the difference of frequency modulation capability of wind turbines under different wind speeds,the single-machine equivalent model is difficult to meet the accuracy requirements,and the traditional AC/DC grid-connected frequency modulation control strategy also has some limitations.Based on direct-drive wind turbines,this paper studies the unit modeling,wind farm equivalent modeling and AC/DC grid-connected frequency modulation control strategy.The ontology model and steady-state control model of direct-drive wind turbines are studied.Aiming at the participation of wind turbines in the system frequency modulation process,the inertial frequency modulation control strategy of rotor is added to the active power control link,including virtual inertial control,droop control and integrated control.The wind farm model and synchronous generator model are built on the PSCAD/EMTDC simulation platform,and the frequency of the system decreases or rises when load is connected or removed to verify the effectiveness of frequency modulation control strategy.The results show that the virtual inertial control can reduce the frequency change rate,the droop control can reduce the frequency change range,the comprehensive control combines the advantages of the two,and the frequency modulation effect is the best.In order to compensate for the low precision of single machine equivalent model in frequency modulation control of wind farm,the frequency curves of wind farm with different wind speeds are analyzed,and an equivalent method of wind farm suitable for frequency control is proposed.According to the clustering characteristics of the frequency curve,the clustering index is obtained.When the system frequency decreases and the frequency rises,the clustering index is different.According to the clustering index,the wind turbines in each group are divided into one and the whole wind farm is equal to multiple wind turbines.The simulation results show that the proposed equivalent method has better equivalent effect than the single machine equivalent model.The frequency characteristics of power system and the influence of inertia on frequency characteristics are analyzed.Aiming at the wind farm AC grid-connected participating in frequency modulation process,the influence of wind power permeability and frequency modulation control coefficient on frequency characteristics is studied.Considering the effective kinetic energy stored in direct-drive wind turbines,the virtual inertia time constant is calculated.Based on the virtual inertia time constant,a self-adaptive coefficient frequency modulation control method is proposed to provide effective power support for the system while ensuring its safe and stable operation.The integrated system model of direct-drive wind farm integrated into power grid via Modular Multilevel Converter High Voltage Direct Current(MMC-HVDC)is established.Aiming at the problem of delay time when wind farm is connected to grid via DC to participate in frequency modulation additional communication link,a coordinated frequency control strategy of wind farm and MMC is proposed,which mainly includes DC voltage control of GSMMC,frequency control of WFMMC and active power control of wind farm.The influence of DC voltage control coefficient and active power control coefficient on the coordinated control effect is studied.Finally,the frequency modulation effect of the proposed coordinated control strategy is verified by simulation when the system frequency decreases and the system frequency rises.