Research On Droop Control Of DFIG Wind Farm Under Grid-connected And Off-grid Conditions

In recent years,renewable energy generation,represented by wind power,has developed rapidly,and its grid-connected capacity has increased year by year.However,large-scale wind power integration also has a certain impact on the stability of the power system due to its volatility and intermittent nature.The research object of this paper is mainly DFIG wind farm.Doubly-fed induction generators(DFIG)have become the most widely used wind power generators due to their wide operating speed range,small generator size and low cost.The rotor of DFIG is connected to power system through the power electronic device.And the output power of DFIG is decoupled from the system frequency,which cannot respond to the change of the system frequency like the traditional synchronous machine.Consequently,the frequency stability of the power system is being challenged in the high proportion wind power generation area.More and more research concentrate on the feasibility of wind power actively participating in frequency modulation.The main research content of this paper is the frequency droop control of the doubly-fed wind farm under off-grid conditions.This paper firstly outlines the basic operation control principle and mathematical model of DFIG,including the maximum power control of the wind turbine,PWM converter control,and pitch angle control.Secondly,the deloaded control method of DFIG is introduced,including overspeed control and pitch control.This paper deeply analyzes the operating range,response speed and energy changes in frequency regulation of the two deloaded control methods.The paper writes its control and rotor motion equations of DFIG using overspeed control.And a system frequency response model with DFIG frequency regulation is established.This model can analyze the influence of various parameter changes on the system frequency.Under grid-connected conditions,the frequency droop control of the DFIG wind farm is mainly used to participate in the primary frequency regulation.The active power control strategy of wind farm mainly includes active power dispatch strategy and frequency control strategy.This paper proposes a novel power allocation strategy of DFIG wind farm that uses overspeed control preferentially to retain more kinetic energy in wind turbine generators(WTGs).The wind farm frequency regulation control strategy adopts the combination of deloaded control and droop control.The droop coefficient of each WTG is deduced by formula derivation.The variable droop coefficient control strategy enables the wind farm to effectively release the maximum reserve active power.The simulation results show that the dynamic response characteristics and steady-state values of the system frequency are significantly improved.A DFIG islanding power system model is established by introducing the concept of internal voltage in this paper.The mechanism of the stable operation of the islanding power system with droop control was explained.The influence of each parameter on the system frequency dynamics and stability is analyzed,and the simulation results verify the correctness of the proposed model.