Research On Control Strategy Of Brushless Doubly-fed Wind Generator Under Grid Disturbance

As a new type of AC induction motor,brushless doubly-fed induction machine gradually applied to wind power generation.Because brushless doubly-fed induction machine eliminates brush and slip ring,it improves the stability of operation and reduces the maintenance cost of operation,and has a broad application prospect.With the rapid development of wind power generation system,the power system puts forward higher requirements for wind power generation technology.During the operation of the power grid,the power quality of the power system is seriously affected by the power grid voltage imbalance and symmetrical drop caused by problems such as three-phase load asymmetry and three-phase short circuit fault.Therefore,this paper studies the control of brushless doubly-fed induction machine under the disturbance of power grid.Firstly,based on the basic principle of brushless doubly-fed motor,the mathematical models of brushless doubly-fed induction machine in the three-phase abc coordinate system and the two-phase dq rotating coordinate system are established respectively,and the corresponding simulation models are built in Matlab/Simulink.The state equation of direct power control of brushless doubly-fed induction machine is derived based on the instantaneous power theory,which provides a theoretical basis for the subsequent research of direct power control strategy.Secondly,the mathematical model of brushless doubly-fed induction machine in unbalanced power grid is derived,and the positive and negative sequence voltage decomposition of power grid voltage is achieved by using the fast positive and negative sequence separation method without delay.Compared with the traditional T/4 cycle delay method,the fast positive and negative sequence separation method can accurately and quickly decompose the unbalanced power grid voltage.It provides accurate positive and negative sequence voltage for direct power control of brushless doubly-fed induction machine in unbalanced power network and reduces control error.Based on the mathematical model of brushless doubly-fed induction machine in unbalanced network,a new direct power control strategy of brushless doubly-fed induction machine is proposed in this paper,which combines the convergent rate integral sliding mode variable structure with proportional integral controller,and can solve the stator current distortion and power fluctuation problems of brushless doubly-fed induction machine in unbalanced network.In the environment of Matlab/Simulink and hardware in the loop experiment platform,the mathematical model of brushless doubly-fed induction machine under unbalanced power network is simulated and tested.The results verify the correctness of the mathematical model of brushless doubly-fed induction machine under unbalanced power network,the accuracy of the delay free rapid positive and negative sequence separation method and the effectiveness of the proposed new direct power control method.Finally,based on the mathematical model of the brushless doubly-fed motor,the operation state of the brushless doubly-fed induction machine during the voltage drop of the grid is deduced.When the grid voltage drops,the flux of the stator control winding of the brushless doubly-fed induction machine changes,leading to the increase of the voltage amplitude of the control winding and waveform distortion,which affects the stable operation of the brushless doubly-fed wind power generation system.Therefore,a brushless doubly-fed induction machine low voltage traverse control strategy is proposed in this paper.The reference current tracking is used to replace the feedback of the control winding voltage in the power inner loop,improve the reactive power and maintain the stability of the grid voltage,reduce the influence of grid voltage sag on the brushless doubly-fed motor,and complete the brushless doubly-fed induction machine low voltage traverse under the voltage sag of the grid.Through Matlab/Simulink simulation platform and hardware simulation and experiments on the loop simulation experiment platform,the results verify the correctness of the paper’s derivation of the brushless doubly-fed motor’s operating state during the voltage sag period and the effectiveness of the low voltage crossing control strategy of brushless doubly-fed induction machine proposed in this paper.This paper analyzes the operation state of brushless doubly-fed induction machine under grid disturbance,deduces the direct power state equation of brushless doubly-fed induction machine during grid voltage unbalance and sag,analyzes the influence of grid disturbance on brushless doubly-fed induction machine and proposes corresponding control strategies to improve the stable operation ability of brushless doubly-fed induction machine under grid disturbance.