The Research On Low Voltage Ride Through Based On Direct-driven Permanent Magnet Synchronous Wind Generator System

As the development of wind technology, the problem of low voltage ride through on grid-connected wind power generation system is concerned gradually.When grid penetration is higher, if wind generators quit operation during the grid fault period, it will cause serious effects on power grid.The advanced countries have successively released quantitative standard of fault ride through, the requirement is that the wind generator operate without grid-off during grid fault, and provide reactive power to the grid, until the fault cleared.Compared with constant speed wind power generation system, variable speed wind power generation system has better ability on wind power capture, and also the conversion efficiency from wind energy to electrical energy is much higher.Direct-driven permanent magnet synchronous generator has some advantages such as low speed, high efficiency, high power factor.It is very suitable for variable speed wind power generation system.This paper focus on direct-driven permanent magnet synchronous wind power systems and research on low voltage ride through, then prove the correctness of the main theory results by simulation.Firstly the basic theory of wind power is introduced,mathematical model of the wind turbine and PWM converter is established, and through the analysis of converter energy transmission,do research on the mathematical model of double PWM converter.The rotor field-oriented vector control strategy id=0is used in generator side converter,grid voltage oriented vector control strategy is used in grid side converter.Secondly simulation model is established respectively, then prove the correctness of simulation model which is established and control strategy by simulation.In this way,a good simulation platform for the technology of low voltage ride through of direct-driven permanent magnet synchronous wind power systems is establishedAnd then analyze the converter dynamic operation of direct-driven wind generator grid side of in detail during the grid voltage three-phase symmetric falling period, and prove the correctness of the above by simulation of direct-driven wind generator system operation when grid voltage falled, and the damages that maybe happened if any measures won’t be taken to wind generation system are discribed.Regarding dc bus over-voltage, the Crowbar protection circuit is used.According to the reactive power demand during the grid voltage falling period, a new reactive power control strategy is used during grid side converter fault.Shortly introduced two representative auxiliary control strategy including pitch angle control,and tip speed ratio control.At last, analyzed power transmission characteristics of the direct-driven permanent magnet synchronous wind turbine grid side converter during grid unbalanced fault period, put forward a voltage vector control strategy based on the positive and negative sequence, effectively control the oscillation of dc bus voltage.At same time increased energy discharging circuit on dc side,energy will be discharged by hysteresis control when dc bus voltage is high,reduced dc bus over-voltage.Therefore we realized direct-driven permanent magnet synchronous wind power generator operation without grid-off when the grid asymmetric fault happened.