Research On Transient Characteristics And Fault Current Of Doubly Fed Induction Generator Based Wind Turbines During Short Circuits Of Power Grid

With the emerging challenges in energy safety and environmental protection,the energy mix of China is now evolving from a fossil fuel dominated structure to a more renewable energy structure.As a leading solution for the wind power generation,doubly fed induction generator(DFIG)based wind turbine(WT)has already become an important power source other than the synchronous machine(SM)in modern power systems.Different from SM,DFIG-based WT generally rides through the grid fault with the switching of additional protection circuits and control strategies,because the electromagnetic energy storage elements(ESEs)and IGBT-based converters,inside the DFIG-based WT,are vulnerable from the overload.Consequently,the responses of DFIG-based WT to power grid faults are quite different from that of SM,and the dynamic behaviors of modern power systems during short circuits evolve significantly.Applications that operate on the principles of SM's fault behaviors,such as the conventional methods for rating circuit breakers and designing protective relays,are more likely to fail.Moreover,the regularities and mechanisms underlying the transient phenomena are complicated and unexplored.Thus,the transient behaviors of DFIG-based WT and its underlying characteristics are worth studying to enhance the reliability of future power systems.The existing studies on modeling of DFIG-based WT for short-circuit faults mainly aim at numerical simulations.The existing studies on DFIG-based WT's transient behaviors in faulted power systems mainly focus on data mining and analytical calculation,without fully considering the switching of these additional protection circuits and control strategies.The main viewpoint of this dissertation is that DFIG-based WT's transient behaviors are dominated by its transient characteristics.From this point of view,the general control and protection structures of DFIG-based WT are summarized.Then,the transient problems are described with mathematical equations.A novel analytical model is proposed to depict the relationship involved in the transient characteristics.Finally,with these cognitions,the regularities of DFIG-based WT's behaviors can be succinctly depicted for some practical applications.The main topics of this dissertation are as follows,(1)The control and protection structures of DFIG-based WT,both in normal and short-circuit scenarios,are summarized in Chapter 2.The multiple ESEs and the normal control strategies are re-recognized from the perspective of energy storage and energy conversion.It shows that the involved relationship follows a multi-scale control structure.Chapter 2 also depicts the typical sequence of charging and accelerating of different ESEs,after the short circuit occurs.The performances of the additional protection circuits and control strategies,for riding through the faults,are summarized as the switching structure in multi-scales.The sequential characteristics of these structures during severe faults are the main focuses of this dissertation.(2)An analytical model is proposed in Chapter 3 to depict the transient characteristics of DFIG-based WT.The original mathematical relationships are organized as the state space form,and the elements show that the transient analysis corresponds to a complicated mathematical problem with multiple challenges.With the proposed concept of DFIG-based WT's internal voltage vector,an internal-voltage-behind-inductance model is proposed to interface with the power system through a controlled voltage source back of a constant-parameter inductance.The internal voltage vector depicts the relationship involved in the transient characteristics,and helps to synthetically represent the transient behaviors of DFIG-based WT in faulted power systems.(3)The differences between DFIG-based WT and SM,both in transient characteristics and transient behaviors,are summarized in Chapter 4.The relationship and branch that decide the internal voltage vector of DFIG-based WT are different from that of SM as the sequential switching,nonlinear,high-order coupling features.By comparing their internal voltage vectors in single-machine infinite-bus power systems and symmetrical fault scenarios,the time-varying phenomena of DFIG-based WT's internal voltage vector,both in magnitude and frequency,are discovered.The complicated internal voltage vector is decomposed into three simple separated vectors to identify the underlying regularities.(4)The regularities of DFIG-based WT's fault current are analyzed in Chapter 5 with assumptions for a kind of fault scenarios that three-phase short circuits locate closely to wind power plants.The concept of operational inductance and a corresponding analytical method are proposed.As a result,an analytical expression of DFIG-based WT's fault current is derived to show the regularities in magnitude,frequency and decay with a unified and simple form.The analytical results are verified with experiments on a 10 kW DFIG platform as well as detailed electromagnetic transient simulations in Matlab/Simulink,in both single and multi WTs scenarios.(5)The physical mechanism of fluxes inside the DFIG-based WT during the fault is analyzed in Chapter 5.The crowding-out phenomenon is discovered as the determinant of the general relationship between flux linkage and current.The overall relationship is further depicted with the proposed equivalent inductances.As a result,the relationship between components of flux linkage and fault current can be represented with algebraic operation and figures.It introduces a new useful perspective to grasp the regularities of DFIG-based WT's fault current,and to generalize the flux figures of DFIG-based WT and SM.(6)The primary applications of these analytical results are explored in Chapter 7.The differences between DFIG-based WT and SM,on aspects of regularities and the physical mechanism of fault current,are summarized.Based on these differences,two simple applications,in rating circuit breakers and evaluating distance protection's performances,are discussed.In all,with the proposed internal voltage vector and internal-voltage-behind-inductance model,this dissertation is expected to help readers capture the basic cognition of DFIG-based WT in aspects of both transient characteristics and transient behaviors.The proposed analytical results,including analytical expression of DFIG-based WT's fault current and the flux linkage-current model,can be directly applied to rate circuit breakers and to study issues relating to protective relays.