| With the rapid development of wind power in China,the role of wind power in some regions’ power system changes from auxiliary energy to dominant energy.However,existing research on dynamic issue of power system with high penetration of wind power under grid fault still think of wind power as auxiliary energy and mostly studies the influence of wind power on dynamic behavior of traditional power system dominated by synchronous generator.Since wind power gradually takes the place of conventional power source and becomes the main power source in some regions’ power system,and the characteristics of wind turbine is very different with that of traditional synchronous generator,it is necessary to investigate the dynamic behavior of power system dominated by wind turbine under grid fault.In this thesis,multi-time scale control and transient switch of full-size power converter wind turbine and dynamic behavior of its grid connected system charactered by multi-time scale and discontinuity is investigated.And dynamic issue in DC-link voltage control time scale gets special attention.Due to the change of wind turbine’s control structure resulted by transient switch,amplitude-phase motion equation model of wind turbine in stage of grid fault duration and fault recovery are developed,respectively.Based on the model,wind turbine’s characteristics in DC-link voltage control time scale is investigated.Then system dynamic behavior of single wind turbine infinite bus system in DC-link voltage control time scale is explored.And the details are in the following:(1)The multi-time scale control and transient switch of full-size power converter wind turbine is investigated.Based on this,the characteristics of multi-time scale and discontinuity of dynamic behavior of wind turbine grid connected system under grid fault is revealed.And then the overall framework of the system dynamic behavior under grid fault is defined and the concerned subject is this thesis is made clear.The meanings and challenges of dynamic issue in concerned DC-link voltage control timescale with transient switch control considered is explored.(2)The amplitude-phase motion equation model of full-size power converter wind turbine in DC-link voltage control timescale is developed.Due to the transient switch control,the models in stages of grid fault duration and fault recovery are separately developed.In stage of fault duration,the influence of current control and phase locked loop on dynamic characteristics of wind turbine in DC-link voltage control timescale is specially focused.And the motion equation model that reflects internal voltage’s dynamics induced by current is developed.In stage of fault recovery,amplitude-phase motion equation model that reflects internal voltage’s dynamics induced by unbalanced active and reactive power is developed.And the controllers in DC-Link voltage control timescale and ramp rate limit of active current order get specially concerned.(3)Based on the motion equation model of full-size power converter wind turbine during grid fault,the small signal dynamic characteristics of wind turbine itself in DC-link voltage control timescale is analyzed.And then the dynamic behavior of single wind turbine infinite bus system in DC-link voltage control timescale is investigated.From the spectrum of two dimensional transfer function that reflects the response of internal voltage induced by current disturbance,the small signal dynamic characteristics of wind turbine itself is analyzed.It is found that the cross coupling relationship between the input and output of wind turbine is strengthend when the bandwidths of phase locked loop and current control loop tends to be close,which will deteriorate the small signal stability of grid conneted system.Furtherly,based on the dynamic characteristics of wind turbine,the stability mechanism in single wind turbine infinite bus system is revealed from the viewpoint of damping.The damping component of the system can be divided into two parts: inherent damping component contributed by phase locked loop and additional damping component contributed by current control loop.And inherent damping component is usually positive and additional damping component is usually negative.When the additional damping component is larger than the inherent damping component,oscillation instability will occur due to lack of damping.Then from the influence on damping,the factors including bandwidths of phase locked loop and current control loop,current orders during grid fault,structures of phase locked loop and current filter on the stability is explored.(4)Based on the motion equation model of full-size power converter wind turbine after active power climbing in stage of fault recovery,the transient characteristics of wind turbine itself in DC-link voltage control timescale is analyzed.And then the nonlinear oscillation behavior and stability of single wind turbine infinite bus system in DC-link voltage control timescale is investigated.First,from the viewpoint of piecewise linearization based on the geometric interpretation of Euler integral,the transient characteristics analysis of wind turbine itself is carried out.Based on the transient characteristics of wind turbine,general understanding and judgment about the characteristics of the nonlinear oscillation can be obtained.It is known that the oscillation characteristic quantities are not fixed.Then,the numerical analysis of the nonlinear oscillation behavior and stability in this stage is implemented using the frequency-time analysis based on Hilbert-Huang transform.It is found that the oscillation frequency and amplitude attenuation rate is time-varying.And on the whole,the amplitude attenation is deteriorated due to the influence of nonlinerity.In certain cases,oscillation with increased amplitude may occur,resulting in instability.Last,the mechanism explanation of the nonlinear oscillation’s characteristics and stability is carried out from the piecewise linearized system based on the specific model in this stage.(5)Based on the motion equation model of full-size power converter wind turbine during active power climbing in stage of fault recovery,the transient characteristics of wind turbine and the transient behavior of single wind turbine grid connected system in DC-link voltage control timescale is analyzed.And then the influence of ramp rate limit and current orders during grid fault on the nonlinear oscillation and stability after active power climbing is investigated.In stage of active power climbing,transient characteristics of internal voltage in phase branch induced by active current order climbing gets special attention and transient behavior of the grid connected system is explored.It is found that the transient response of phase branch is identified as approximate monotonicity at excitation of active current order climbing.The transient behavior in stage of active power climbing is related with ramp rate limit and current orders during grid fault,and the initial states after active power climbing is mostly influence by the transient behavior in stage of active power climbing.Due to the nonlinearity,the nonlinear oscillation and stability after active power climbing is closely related with the initial states.As a result,based on the transient behavior analysis in stage of active power climbing,the influence of ramp rate limit and current orders during grid fault on the nonlinear oscillation and stability after active power climbing is investigated.It is found that oscillation instability is easier to occur when ramp rate limit or active current order during grid fault tends to be large. |
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