Research On Frequency Modulation Control Strategy Of Doubly-fed Wind Power Generation System

As one of the mainstream wind turbines,the doubly-fed wind turbine has the characteristics of zero inertia due to the decoupling of rotor speed and grid frequency.With the large-scale integration of wind power,the equivalent inertia of the system will be reduced.Moreover,the doubly-fed wind turbine usually operates in the maximum power tracking mode,and it cannot provide additional standby power to participate in system frequency regulation.In addition,wind power generation has the characteristics of randomness,which increases the burden of system frequency regulation.Therefore,this paper studies the frequency modulation control strategy of doubly fed wind turbine.Firstly,according to the working principle of doubly-fed wind generator and energy storage battery,the mathematical model is established in the two-phase rotating coordinate system,and the control system is built through vector control.Secondly,according to the power characteristics of the doubly-fed wind generator,the calculation process of the corresponding speed and pitch angle of the doubly-fed wind generator under different wind speeds and load shedding degrees is deduced.According to different wind speeds and load shedding conditions,the rotor over-speed is used for load shedding in the low wind speed region,rotor over-speed and pitch angle are combined to reduce load in middle wind speed region,and variable pitch angle is used to reduce load in high wind speed region.Then,based on the load shedding control of DFIG,the frequency modulation control coefficient is adjusted to improve the real-time frequency modulation ability.The maximum negative frequency deviation maintaining link is introduced to make the doubly-fed wind turbine provide constant additional power at the lowest point of system frequency.The speed recovery control link is introduced to prevent the system frequency from falling out of the frequency modulation dead zone in the speed recovery process.On this basis,the frequency modulation control strategy of DFIG is designed at low,medium and high wind speeds and its effectiveness is verified by simulation.Finally,aiming at the shortcomings of frequency regulation of DFIG in low,medium and high wind speed regions,the corresponding frequency regulation control strategy is introduced on the basis of considering the charging state of energy storage battery.In the low wind speed region,the energy storage battery compensates the difference between the frequency modulation active power and the real active power of the doubly-fed wind turbine.In the middle wind speed region,the energy storage battery compensates the active power difference and responds to the frequency change of the system actively.In the high wind speed region,energy storage battery participates in frequency modulation alone.The simulation results show that the frequency modulation effect of energy storage battery assisted frequency modulation is improved compared with that of doubly fed wind generator alone.