Operation And Analysis Of Doubly Fed Induction Generator In Weak Grid With High Penetration Of Wind Energy

China has been the largest market of wind turbines in the world in recent years,and it is raising concerns about the stability of the electrical grid due to a large number of wind turbines on-line.Grid connected wind turbine must have the capability of fault ride-through(FRT)which consist of low voltage ride through(LVRT)and high voltage ride-through(HVRT)for the stable and secure operation of power system,however,due to high penetration of wind into weak grids the power system presents operational characteristics of weak inertia,low short-circuit ratio(SCR),high line impedance,weak electrical damping,and weak voltage support,so the safe and stable operation of power system faces major challenges.In this dissertation,the impact of the grid impedance,adding a novel cascade converter,and the resonance mitigation techniques are studied in aims to improve the dynamic performance of the DFIG system.First,the dynamic model of the DFIG system with both mechanical and electrical components is discussed in detail.It elucidates the energy conversion process in the wind turbine,the dynamic electrical behavior of the machine,and a brief description of the mechanical dynamics.Additionally,the classical vector control of the DFIG system,the voltage control of the dc-link,and the maximum power point tracking(MPPT)algorithms are also established.Moreover,a comprehensive overview of the grid code requirements and stipulations for the grid-connected operation of wind turbines are studied.A detailed investigation of the FRT grid code requirement,including the LVRT and HVRT characteristics and major technologies for LVRT and HVRT,are presented for the DFIG system.Second,a detailed theoretical and mathematical analysis is established to elaborate on the dynamic behavior of the DFIG system during grid voltage dips and swells.Detailed investigation of the LVRT and HVRT of DFIG,including the voltage dip and swell profiles,transient characteristics,and the behavior of the DFIG at the moments of voltage dip and swell is presented.A DFIG mathematical model is used to analyze the response of wind generation systems under the influence of grid faults.Third,the classical rotor crowbar technologies are discussed for the LVRT of the DFIG system.The limitations of traditional crowbar resistance selection methods are outlined considering the impact of grid impedance.Furthermore,the modified resistance selection method is put forward to improve the LVRT performance of DFIG taking the grid impedance into considerations.A complete set of computer simulations are also provided in order to validate all theoretical results.Fourth,the novel cascade converter(CC)at Y-point of DFIG is proposed for the first time to improve the dynamic performance of the DFIG system.Additionally,the connection scheme,the rating,and the control of the CC are developed.The overall control algorithm is developed to show how and when the CC will improve the LVRT or the stability of the DFIG system.The simulations in MATLAB/SIMULINK are provided to validate the theoretical and mathematical analysis.Fifth,the dynamic performance of the DFIG system under a weak grid with high penetration of wind energy is carried out.Different methods of resonance,such as subsynchronous resonance(SSR)and high frequency resonance(HFR)identification are discussed.The impedance modeling approach for a DFIG system is developed.The virtual impedance-based phase angle compensation method is introduced into the back to back power converter control of DFIG system.The limitations of the virtual impedance-based resonance mitigation technique are outlined.Additionally,the CC at Y-point of the DFIG system for resonance mitigation under a weak grid is proposed for the first time.Theoretical and mathematical analysis is validated through extensive simulations.Additionally,the impact of the phase locked loop(PLL)on the resonance in the DFIG system is presented.Three different types of PLL,such as synchronous reference frame(SRF),Lead/Lag and an improved PLL are presented in this thesis.Additionally,the impact of converter dead-time on impedance modeling is studied.The impact of PLL dynamics with different types on resonance in the DFIG system is validated through simulations and experiments.Sixth,the operation under certain disturbances and stability analysis of the CFDRG are carried out to find the safe operating limit of the machine.Theoretical analysis and Matlab simulations have validated the effectiveness of the proposed wind power generation system.