Research On Voltage Support Control Strategy Of PMSG Wind Turbines During Unbalanced Fault

Wind energy,as a form of renewable energy development and utilization,has developed rapidly in recent years.As the proportion of wind power generation’s power supply continues to increase,in order to maintain the safe and stable operation of the power system,wind turbines are required to have the capability of asymmetric fault ride through.Relevant departments in China have put forward requirements for low-voltage ride-through,that is,to ensure that the wind turbine does not go off-grid during the voltage sag,and can provide certain reactive power support to achieve friendly grid-connected access to wind turbines.Permanent magnet synchronous generator(PMSG),as the mainstream model of wind power generation,has the advantages of no gearbox,and low failure probability.And it is simple to maintain and reliable.The control strategy of the PMSG wind power system will have a direct impact on the grid connection of the system.In view of this problem,this paper proposes a voltage support control strategy for the PMSG wind power system during grid asymmetrical faults to achieve the purpose of supporting the point of common coupling(PCC)voltage and improve the wind turbine’s symmetric fault ride-through capability.Firstly,this paper analyzes the structure and principle of the PMSG wind power system,and lists two structures of dual PWM converter and uncontrollable rectifier converter.The mathematical models of the PMSG and grid side converters are established.The vector control principles of the network side converter and the machine side converter are analyzed,respectively.And the control strategy block diagrams are drawn through their control mode.The simulations experiment verify the effectiveness of the control method in the ideal situation of the power grid.The influence of grid fault on the PMSG wind turbines is analyzed,and the mathematical model of grid side converter under grid unbalance fault is established by using symmetrical component method.It lays a theoretical foundation for the voltage support control strategy.Grid guidelines require that wind turbines have the ability to regulate the voltage at the PCC.In view of this problem,this paper analyzes the principle of positive and negative sequence double dq phase locked loops(PLL),proposes a positive and negative sequence current control strategy for the PMSG wind power systems,and establishes corresponding positive and negative sequence equivalent sequence network models.The simulation model of the PMSG wind turbine is established by DIgSILENT/PowerFactory simulation software,and the asymmetric fault situation of grid is simulated and studied.And it is verified by MATLAB that the proposed equivalent sequence network model can be used to calculate the fault current.Finally,according to the relationship between the grid side converter injecting positive and negative sequence reactive currents into the power grid and the phase voltages of the PCC,a voltage support control strategy is proposed to reduce the degree of PCC three phase voltage asymmetry.Double dq,PI vector control method is used for grid side converter,and the system control block diagram is given.A simulation model of the PMSG wind power system is built on the DIgSILENT/PowerFactory simulation platform.The simulation results make clear that the proposed voltage support control strategy from this paper can support the voltage at the PCC in the event of an asymmetric fault in the grid.The proposed control strategy is compared with the positive and negative sequence current control strategies.And the simulation results show that the new control strategy in this paper is effective.