Research On The Transient Stability Of Power System Connected With Large Scale Wind Power

In recent years,the demand for the energy is continuously increasing.With the issue of the depletion of fossil energy,environment pollution and climate issue,the wind power generation has developed rapidly.Among them,the doubly fed induction generators(DFIG)have been extensively interconnected to the electric power system due to its stable characteristics.However,the interconnection of the large-scaled wind power not only offers us green energy,but also brings new technical problems,among which,the transient stability issue of the power system with DFIGs has attracted great attentions.This thesis focuses on the developing the simplified DFIG models for electromechanical transient analysis and analyze the approaches for evaluating transient stability of power system interconnected with large-scaled DFIGs.The main works of the thesis are as follows:(1)The time scale of flux and current in DFIG transient process is studied.Firstly,a second-order differential equation of the DFIG in complex field is derived.Then,the approximated analytical solutions to the free responses of the flux and current and the formula for the corresponding time constants are obtained.Furthermore,the impacts of faults position and resistance on the time scales of the free responses are analyzed by simulations.Finally,the adaptability of the electromechanical transient model with or without the stator or rotor transient in the electromechanical transient analysis is discussed.(2)Simplified models of DFIG for the electromechanical transient analysis are established.Firstly,under different working conditions,both the transient and steady-state models of DFIG during & after the fault are analyzed.Then,with the ignorance of the short time-scaled response,the models are simplified by maintaining the energy accumulation during the fault.Furthermore,the error analysis and simulations of the established models are carried out.Finally,simplified DFIG models for electromechanical transient analysis are proposed.(3)The impact of interconnection of large-scale DFIGs to the power-angle-curve and transient stability of a single-machine infinite-bus(SMIB)system is studied.Firstly,the change of the power-angle-curve of the synchronous machine due to the connection of large scale of DFIGs is analyzed.Then,with the equal-area criterion and voltage phasor diagram,the influence of the characteristics of DFIG during/after fault to the transient stability of a SMIB system is analyzed under different fault conditions.(4)The transient stability assessment method for the power system interconnected with large-scaled DFIGs based on the transient energy function is proposed.Firstly,the concepts of the classical energy function and the branch energy function are introduced,and the relationship between the above two energy functions is discussed.Then,the generators are clustered according to their speed during the fault.Furthermore,the effects of the interconnection of large-scaled DFIGs on transient stability of the multi-machine systems are quantitatively assessed with the change of the energy of the interconnected generators,with respective to the different replacing conditions,i.e.,DFIGs replacing the same capacity of synchronized machines/clusters whose speed are lead,lag or middle.