Synchronous Flux Weakening Control With Flux Linkage Prediction For Doubly-Fed Wind Power Generation Systems

In recent years,doubly-fed wind generators have been widely used in power generation systems because of their unique structures,which can achieve variable speed constant frequency and have relatively small excitation capacity.The control strategy of the doubly-fed wind generators under the normal operation has become mature.At present,the trend of the main research is switched to the grid dips from the normal operation of the grid,the doubly-fed wind generators have achieved the fault ride-through capability to avoid grid paralysis in a large area.The main research is in a single grid drop and the mixed grid drops in this paper,a synchronous flux weakening control strategy with flux linkage prediction is proposed for realizing the fault ride-through capability of the doubly-fed wind generators.Firstly,this paper analyzes a fast and effective fault detection method of the grid voltage.The doubly-fed wind generator is very sensitive to the grid disturbance,especially to grid dips.This is due to the fact that the stator windings are directly connected to the grid.In order to change the two kinds of control strategies quickly at the moment of the grid dips,this paper puts forward the methods of amplitude detection and phase detection of the grid voltage to achieve the purpose of detecting voltage drops from two aspects.The vector model for the doubly-fed wind generator in the two-phase stationary reference frame can be described.This paper introduces the proportional resonant control of the doubly-fed wind generator under the grid voltage dips.The control algorithm is simple and has no redundant compensation terms.It can realize the zero steady state error control and improve the robustness of the control system.There exists DC component in the symmetrical drop,and there exist DC and negative sequence components in the stator flux under the asymmetrical dip.This causes a large slip rate.As a result,higher transient overcurrent appears in the rotor.Based on its mathematical model,the transient electromagnetic relationship under the normal the grid voltage operation and the grid voltage dips.At the same time,it analyzes the magnetic circuits of the stator and rotor magnetic field for the doubly-fed wind generator under the normal operation and the grid faults,respectively.The following control strategy is based on a theoretical basis.To enhance the fault ride-through ability of the doubly-fed wind generator,this paper presents a synchronous flux weakening control strategy with flux linkage prediction.The variation of the stator and rotor flux is analyzed during grid faults.During grid faults,the stator flux is tracked by the rotor flux to realize synchronous variation between the stator and rotor flux quickly,which weakens the large slip rate and suppresses the stator and rotor overcurrents.The relationship between the stator and rotor flux is deduced in two-phase stationary reference frame and the optimal rotor flux reference is obtained by the relationship between the stator and rotor flux,which is deduced in two-phase stationary reference frame.The deadbeat predictive control is applied to the rotor flux for rapid synchronization between the stator and rotor flux,which weakens their interaction.It improves the low-voltage ride-through capability under the grid dips.The results indicate that the proposed control strategy is of simple structure,and has faster dynamic response.Besides,it also effectively suppresses the stator and rotor fault overcurrents,and reduces oscillations in torque.The performance of the doubly-fed wind generator can be largely improved under grid faults.At the end of the paper,in order to further verify the applicability of the control strategy,the different types of the voltage dips are set,the dips types are from the three-phase symmetrical faults to the more common asymmetry faults.The analysis of the results of the operation can be obtained that the rotor current is controlled to be within the maximum current.