Model Predictive Control Of Dfig Under Unbalanced Grid Voltage Conditions

The doubly-fed induction generator(DFIG)has become the mainstream model of wind farms because of its small converter capacity and flexible power adjustment capabilities.Since the grid-connected node of DFIG is at the end of the grid,and its stator is directly connected to the grid,the grid voltage becomes an important factor that affects the running state of DFIG.When the grid voltage is unbalanced,the generator will generate serious heat and increase the loss.Negative sequence voltage will cause abnormal vibration of the generator,reduce the reliability of the gearbox and bearings,and in severe cases will cause the generator to stop and affect the safe operation of the power grid.Therefore,the research on the control technology of DFIG under unbalanced grid voltage has important theoretical and practical significance.This paper conducts in-depth research on the control strategy of DFIG under unbalanced grid voltage from the aspects of mathematical prediction model,objective function design,simulation and experiment verification.Aiming at the problem of complex control structures using existing control strategies such as PIR,PR or multiple PI current regulators,this paper uses finite control set-MPC(FCS-MPC)to design control strategies in a two-phase static coordinate system,which simplifies the control structure.The mathematical model of DFIG is established in the ideal grid voltage state and the unbalanced grid voltage state respectively,and the rotor current prediction equation is derived.In order to reduce the negative influence of unbalanced grid voltage on the running state of DFIG,four rotor current control strategies are proposed according to four control objectives.In order to further enhance the ride-through capability of DFIG asymmetric faults and expand the advantages of model predictive control in DFIG multi-objective optimization control,research on multi-objective optimization control strategies is carried out.The reference equations of DFIG stator current,rotor current,stator active power,stator reactive power and electromagnetic torque are derived.Predicting equations and using the idea of hierarchical control provide two design methods of objective functions.On the one hand,it avoids the design process of traditional multi-objective control strategy weight coefficients,and on the other hand,it enhances the coordination of multiobjective control.Finally,the number of control targets was increased to three,and the effectiveness of the control strategy was proved by assigning reasonable weight coefficients.The model of the doubly-fed wind power generation system was built in MATLAB/Simulink environment,and the FCS-MPC program was written to verify the proposed control strategy.With TMS320F28335 as the core chip,a physical experiment platform was built.The comprehensive results of simulation and experiment showed that compared with the traditional multi-objective optimization control strategy,the proposed control strategy helped to improve the dynamic and steady-state performance of the doubly-fed wind power generation system.The ability of doubly-fed wind turbines to ride through asymmetric faults was improved.