| With the sustained and rapid growth of wind power penetration in power system,the dynamic characteristics of wind power,which are distinctly different from those of traditional synchronous generators(SGs),are profoundly influencing the dynamic behaviors of voltage and frequency,seriously threatening the secure operation of power system.Among these threats,currently wind power under typical vector control(VC)based on phase-locked loop(PLL)synchronizing technique does not possess inertial response capability.High penetration of wind power will undoubtedly weaken the system inertia and seriously deteriorate the system frequency dynamics.However,the present research about improving frequency response characteristics of wind power basically focuses on the implementations of diverse control strategies,which are still lacking deep understandings and recognitions on the basic physical characteristcis and the inner mechanisms of wind power,and systematic thinking of the basic approaches to achieve the optimization of frequency response characteristics of wind power.Based on the deep understandings of wind power’s dynamic charateristics,this thesis will systematically give the basic idea to optimize the frequency response characteristics of wind power.And then,the amplitude-frequency control method of inner potential,viz.,virtual synchronous control(VSynC),will be firstly proposed for doubly-fed induction generator(DFIG)-based wind turbines(WTs)to improve the frequency response characteristics.Furthermore,the related issues about inertial response,primary frequency regulation,and low voltage ride through(LVRT)capability of VSynC-based DFIG WT are respectively studied.Details are as follows:(1)The essential problem of grid-connected control for WTs is to solve the power flow problem or achieve the balance between the input and output power,and maintaining the stability of operating states of the energy storage elements in WT is the necessary and sufficient condition to achieve the power balance.Based on this,the basic guiding thought for WTs to realize grid-connected operation is proposed.That is,each operating state of the storage elements in WT,such as the mechanical rotation speed,DC-link voltage,and the AC current,can be maintained stability by regulating the inner potential through either the instantaneous value control method or the amplitude-frequency control method.For DFIG-based and full-capacity WTs under the typical VC based on PLL,i.e.,the instantaneous value control method of inner potential,the phase motion equations of inner potential in electro-mechanical time scale driven by the active power unbalance are respectively developed to deeply understand the basic physical characteristics and the inner mechanisms of WTs.Based on the deep understandings and recoginitions of the dynamic characteristics of WTs,the basic idea to optimize the WTs’ frequency response characteristics is systematically proposed.More specially,the frequency response characteristics of WTs under the instantaneous value control method can be improved by regulating the power control angle or optimizing the PLL dynamics,besides,changing the synchronization mechanism,viz.,directly adopting the amplitude-frequency control method,can be another effective way to improve the WTs’ frequency response.(2)The amplitude-frequency control method of inner potential is first proposed for DFIG-based WT,which is called VSynC here,to provide the dynamic support capability for system frequency.The corresponding phase motion equation in electro-mechanical time scale driven by the active power unbalance is then developed to investigate the inertial characteristic of DFIG-based WT with VSynC,which is compared with that of typical VC-based DFIG WT including the df/dt inertial control.Theoretically,WTs with diverse inertial control strategies can always be transformed into the form of phase motion equations driven by active power unbalance,which makes them behave just like "virtual synchronous generators(VSGs)" in electro-mechanical time scale and essentially the same,the only difference is that the values and characteristics of the manifested inertia and damping of WTs with differenct inertial control strategy may be different.The small-signal model of VSynC-based WT is also built based on the single-machine infinite bus system(SMIB),and the influences of key controllers parameters and grid strength on system stability are studied.Comparative analysis of the operating stability between VSynC and VC based DFIG when assessing the weak AC grid is also perfomed,which demonstrates the superior stability of VSynC-based DFIG WT and seriously deteriorative stability of DFIG-based WT when adopting PLL-based VC.(3)The operating mechanisms of several typical frequency regulation strategies are detailed elaborated,the corresponding advantanges and disadvantanges are also summarized from the perspective of deloading methods and the stall risk resulted by governor control methods.Based on this,the primary frequency regulation strategy is further proposed for VSynC-based DFIG WT.Then,a detailed comparative analysis is conducted on the response characteristics of WTs under several typical frequency regulation strategies.Based on this,the constructive thinking of frequency response model(FRM)is proposed for WTs with frequency regulation strategies.More specially,for WTs with fast primary frequency regulation response,which may have large time overlap with the fast inertial response process,a united phase motion equation driven by active power unbalance can be built to reflect the overall FRM;For WTs with relatively slow primary frequency regulation response,which may be relatively independent from the fast inertial response process,the form of WTs’ FRM in this case is more closer to that of traditional SGs,that is,the phase motion equation driven by active power unbalance is used to reflect the fast inertial response process,and the primary frequency response process is then reflected by the input mechanical power variations resulted by system frequency changes.(4)The major chanllenge faced by VSynC-based DFIG WT when riding through grid faults is elaborated,that is,the strong transient electromagnetic force and the difficulty to provide fast dynamic support of reactive power and coordinate control of active power caused by the relatively slow dynamic response characteristics of VSynC.Quickening the dynamic response speed of the rotor excitation voltage when grid faults occur is the core thought to successfully ride through grid faults for VSynC-based DFIG WT.Based on the developed equivalent model of DFIG during grid voltage sags,the influnces of virtual resistace in VSynC on the transient response characteristics of DFIG are studied.Besides,the effects of adding the feed-forward compensation of stator natural flux dynamics and the feed-forward compensation of rotor transient back electromotive force(EMF)on the transient response characteristics of DFIG are also investigated,respectively.Based on the above comparative analysis,a strategy based on the dynamic feed-forwad compensation of rotor transient back EMF is proposed for VSynC-based DFIG to enhance LVRT capability during symmetrical faults.(5)Based on the imitative experimental platform of DFIG-based WT,the experiments of flexible grid-connection,step response,inertial response,and LVRT are performed,respectively,which demonstrate the validity and effectiveness of the proposed VSynC and the corresponding LVRT strategy. |
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