Research On Control Strategy Of Brushless Doubly-fed Wind Power System In Grid-connected Stable Operation

The Brushless Doubly-fed Induction Generator(BDFIG) is reliable and durable in structure due to its absence of brush and slip rings and it can realize Variable Speed Constant Frequency(VSCF) with low capacity frequency converter.Therefore,it has a strong competitive advantage in wind power generation greatly affected by the replacement of brush and slip rings,especially compared to the Doubly-Fed Induction Generator(DFIG).However,a complete and detailed theoretical system about control of Brushless Doubly-Fed Wind Power Generation System(BDFWPGS) connected to the grid has not formed.There are few theoretical studies on the Maximum Power Point Tracking(MPPT) control of the whole wind power system from wind turbine,drive shaft to the generator and there is also a lack of research on the Low Voltage Ride Through(LVRT) of BDFWPGS.On this basis,this paper further studies the control strategy of the BDFWPGS for grid-connected steady state operation.Firstly,the power model and the first-order model of the transmission shaft system are established successively in combination with the power characteristic expression of the wind turbine,and the maximum power tracking principle of the wind turbine is explained.Based on the speed expression of brushless doubly-fed motor and the flux linkage voltage equation in three-phase abc coordinate system,the brushless doubly-fed motor model under arbitrary rotation dq coordinate system and rotor speed dq coordinate system is derived.The power conversion relationship of BDFIG is analyzed.Secondly,the design methods of speed controller and pitch angle controller in the wind turbine control system are presented.Based on flux orientation of Power Winding(PW),a power decoupling vector control algorithm for brushless doubly-fed machine with double closed loops of power and current is derived.According to the power relationship between two sets of stator windings of BDFIG obtained above,the given value of PW active power with maximum wind energy utilization coefficient is calculated,thus achieving maximum power of BDFWPGS.In addition,a software Phase-Locked Loop(PLL) method is given to realize the flux orientation of the PW.Then,based on the voltage flux equation in the static coordinate system of the PW,the transient change of the Control Winding(CW) induced electromotive force of the BDFIG is analyzed during the grid voltage sag.In order to limit the transient impact of the CW voltage and CW current,the machine-side converter is equivalent to a virtual impedance.Based on the flux estimation of the PW,the given value of CW current is recalculated when the power grid fails.On the basis of the above vector control method,a new LVRT method of BDFIG is proposed,so as to realize the stable operation control of brushless doubly-fed wind power generation system when the grid voltage drops.Finally,the above two control methods are verified by simulation and experiment.The simulation model of the wind power system is established in Matlab/Simulink.The motor speed and power tracking under different wind speeds and the LVRT performance of the system are simulated and analyzed respectively,proving the feasibility of the above two control methods.Then,on the basis of the existing 30 kW BDFIG hardware platform,the controller software is designed to complete the experimental verification of the BDFWPGS grid-connected stable operation control.