Modeling And Analysis Of Electromechanical Transient Characteristics Of DFIG-Based Wind Turbines

With increasing wind power penetrations, wind turbine generators(WTGs) has become another kind of significant power supply in addition to the synchronous machine in the power system. Existing models only consider WTGs as constant power source or current source, ignoring the fact that WTGs are equally important with synchronous machine, and is not appropriate for the system transient stability analysis. Besides, traditional WTG control and modelling is generally designed assuming that the external grid is strong enough. However, in the case of weak grid, the terminal voltage is more vulnerable to power variations of grid equipment. Especially in contingency situation, some of the WTG control structures may cause interaction between WTGs and other system equipment, which may lead to loss of synchronization. So, in order to study the transient characteristic of WTGs in complex grid environment and its influence on dynamic stability of power system, this paper takes doubly-fed induction generator(DFIG) as the research object, and establishes its electromechanical transient model.This paper first introduces the basic control principle of DFIG and its mathematical model. Second, method for simplifying DFIG electromechanical transient model is put forward considering the typical control of the rotor side and grid side converter. The physical meaning of the internal voltage of DFIG is then explained and the internal voltage is regarded as the link of DFIG model and the power grid. Phase-locked loop(PLL) dynamic response characteristics are analyzed in the weak grid environment, as well as its effect on DFIG electromechanical transient behavior and transient synchronization characteristics. Thus the importance of considering PLL dynamic in the DFIG electromechanical transient model is proved. Finally, the simplified model of DFIG suitable for asymmetric fault is established. Thereby the influence of negative sequence control on DFIG electromechanical transient model is analyzed. The 1.5 MW DFIG electromechanical transient model is established in MATLAB/SIMULINK and simulated results verify all the obtained analysis.