The Research On Control Strategy Of Rotor-side Converter For Doubly-fed Induction Generator Under Unbalance Grid Conditions

With the depletion of fossil energy,the proportion of renewable energy generation in the power system is gradually increasing,and wind power generation is gradually becoming the main force of new energy generation because of its good performance,and double-fed induction generator(DFIG)has become one of the mainstream wind generator models at present.However,its stator structure,which is directly connected to the grid,is often affected by grid unbalance faults,resulting in a degradation of the quality of the power delivered to the grid.Therefore,this paper investigates the control strategy of the rotor-side converter of DFIG with the aim of reducing rotor overcurrent,electromagnetic torque fluctuation and DC bus overvoltage,taking the grid voltage unbalance dip as a prerequisite.Firstly,this paper constructs a mathematical model of DFIG in a three-phase stationary coordinate system and two simultaneous rotating coordinate system under ideal grid voltage conditions,and analyzes the classical vector control to achieve decoupled control of stator active and reactive power.And the conclusion that the negative sequence component in the unbalanced grid voltage is the main reason for the steady operation of the DFIG is obtained by analyzing each electromagnetic quantity of the DFIG under the condition of unbalanced grid voltage dip using the symmetrical component method.Then,for the problems of rotor overcurrent,electromagnetic torque oscillation and DC bus overvoltage in the DFIG with slight grid voltage unbalanced dips,dual proportional-integral control is proposed on the basis of classical vector control.At the same time,deadbeat current predictive control strategy is applied to the control of rotor-side converter of the DFIG,and the traditional deadbeat current predictive control strategy is improved by extending the current prediction by one beat in order to eliminate the digital system delay.Finally,parallel connection of a crowbar protection circuit to the rotor-side of the DFIG is proposed to improve the low-voltage ride-through capability(LVRT)of the wind turbine under the grid voltage depth symmetry fault condition.In the simulation software experimental platform MATLAB/Simulink,firstly,the effectiveness of the dual proportional-integral control strategy and deadbeat current predictive control strategy are demonstrated under the dip condition of grid voltage unbalance of 20%.Then,the performance of each control strategy was compared under different grid voltage unbalance dips,using the rotor current overrun,the electromagnetic torque fluctuation range and the DC bus voltage overrun as evaluation indexes.The improved deadbeat current predictive control was found to be effective in suppressing rotor overcurrent and DC bus overvoltage,reducing diametrical oscillation of electromagnetic torque,and maintaining excellent control performance compared to other control strategies under 30%unbalanced dips.Finally,it is demonstrated that the rotor-side paralleling crowbar protection circuit can enhance the LVRT capability of the DFIG under deeply voltage dip fault conditions on the grid.In this paper,the research work provides a viable control and protection scheme for the stable operation of the DFIG under grid fault conditions,which has some practical engineering value.