Research On Active Support And Fault Ride-Through Of Wind Farm Via Flexible Direct Current Transmission System

Wind power output has the characteristics of fluctuation,randomness,intermittence,etc.,so it needs a flexible control system to match it.Large scale wind power directly connected to the traditional AC power grid has many problems,such as frequency modulation,voltage regulation and low-voltage ride through,which makes the current AC power grid have problems in the absorption of wind power.VSC-HVDC has full control characteristics.It does not need to use external power to implement commutation,so it is very suitable for wind farms,which are relatively weak power sources,to be connected to the grid.However,there are still some technical challenges for the wind farm to be sent out through the VSC-HVDC System.For the wind power generation system,it needs to have strong low-voltage ride through capability;for the whole system,it needs to solve the problem of fault ride through when the AC power grid fails.Therefore,the research on active support ability and fault traversal ability of wind power grid connected system is of great significance to the safety and stability of the whole system.Firstly,the DFIG is taken as the research object,and the mathematical model of rotating coordinate system and grid side converter is established.The mathematical model of VSC-HVDC System is established,and the outer and inner control strategies are designed.The simulation analysis shows that: when the voltage drops at the grid point,the wind farm should have the ability of low-voltage ride through and provide voltage support for the grid;when the AC side of the system grid fails,the control scheme must be adopted to realize the fault ride through of the whole system.In order to realize the low voltage ride through capability of wind turbine,an improved crowbar and chopper control scheme is proposed.At the same time,STATCOM is used as additional reactive power support equipment to comprehensively improve the control of grid side converter to provide voltage support for the grid.The effectiveness of the scheme is verified by simulation.Finally,in order to realize the fault crossing of the whole system,a coordinated control strategy is proposed,which combines the self regulating energy dissipation resistance circuit with the maximum power tracing load shedding method.In the early stage of fault,the self-regulation energy consumption sub module is put into use to restrain the voltage rise caused by communication delay of load reduction method;after the maximum power tracking load reduction method is put into operation,the number ofenergy consumption sub modules can be reduced.The simulation results show that under the coordinated control strategy,the voltage of VSC-HVDC DC bus does not exceed the threshold value during the whole fault period,and it has a very stable fault ride through effect during the whole fault period,and the voltage fluctuation is smaller than the traditional method,and the system cost is lower.The effectiveness of the coordinated control strategy is proved.