Study On Low-voltge Ride-through Strategy Based On Rotor-terminal Voltage Control For Doubly Fed Wind Power Generation System

As a dominant type in wind power field,doubly fed wind power turbines are occupying a great proportion in today's fast development wind market throughout the world.The direct connection between the stator circuit of DFIG and the power grid results in that the transient terms of the stator flux under grid fault can introduce high current or voltage into the rotor circuit,which brings negative influence to the low-voltage ride-through(LVRT)of this type of turbines.To meet the requirement of grid-tied operation during grid fault,crowbar circuit is provided at the rotor circuit in present commercial wind turbines ready to discharge the transient energy.However,once the crowbar is activated,the control to DFIG will be lost.As a result,not only the active and reactive power cannot be controlled effectively,but also a heavy mechanical impact will be seen.Therefore,how to reduce the crowbar activation,namely the way how to use the drive converter itself realizing the LVRT operation to a great extent,has been a hot topic in today's academic community.With the help of the knowledge about DFIG electromagnetic characteristics and with respect to its operating specialty,this thesis study and propose a LVRT scheme based on rotor terminal voltage control.By this scheme the fault range being rode through is enlarged mostly.The researches presented in this thesis can be summarized as follows.Firstly,with the mathematic model,the basic vector control of DFIG and the electromagnetic response under grid fault are analyzed and studied deeply.Secondly,based on analyzing the demagnetization theory of DFIG,the virtual inductance self-demagnetization scheme,proposed by our research team,is further studied and experimented comprehensively.Compared with those conventional demagnetization method,the virtual inductance self-demagnetization scheme not only can decrease the rotor terminal voltage further,enlarge the fault range being rode through,but also the stator flux is not needed in the implementation.Moreover,this scheme directly controlling the rotor-terminal voltage,producing demagnetization effect passively,therefore,the effects do not rely on the dynamic performance of the rotor current closed loop.Thirdly,although the LVRT performance can be improved by the virtual inductance self-demagnetization scheme,involving differential operation is its shortcoming in implementation.For all this,a LVRT scheme of tracking control to transient inductance voltage(TCTIV)is proposed in this thesis to set the rotor terminal voltage during grid fault.In this scheme,the transient inductance voltage in the rotor circuit is observed with a sliding mode observer(SMO),and the rotor-terminal voltage is controlled to track the transient and negative-sequence components involved in the inductance voltage.By this,not only the aim of demagnetization control is achieved,but also the differential operation is avoided.Fourthly,a set of emulation system testing doubly fed wind power generation systems is built in laboratory,based on which associate experiments are made to validate the design and analysis.