Stability And Torsional Characteristics Analysis Of Grid-Connected Doubly-Fed Generators

With the accelerating development of clean renewable energy sources, expanding wind power, it makes Wind Turbine Generators'(WTGs) operations in the stability of the grid increasingly prominent. Due to the limitation of WTG' key components connected by shafts stiffness, when the WTG connection operate, the complex conditions may provoke the torsional oscillations between the different sections of the WTG Therefore, depth study of the grid-connected stability of WTGs and the unit potential oscillation modes have a very important practical significance.In this paper, doubly-fed induction wind turbines are used as the research objects. By setting up a grid-connected power system simulation model, in-depth analysis of the WTGs at different disturbance transient stability, as well as disturbance on the drive train may impact. When the drive train-related parameters are changing, the effects of transient stability of the WTGs are researched. Lastly the natural frequencies and mode shapes of the torsional oscillations that may appear between different sections of the WTGs'drive train are discussed. Specific research and analysis topics are as follows.(1) The mathematical models of basic wind, gust wind, ramp wind and random wind are investigated, and the simulation results of these models in Matlab show that all of models can well imitate the basic characteristic of wind speed. The basic principle of wind energy conversion is analyzed; by pitch angle adjusting and rotate speed controlling, the wind turbine can capture wind energy at maximum. According to the actual physical structure of WTG'drive train, three different equivalent models of drive train are established.(2) The equations of voltage and flux linkage between stator and rotor winding of doubly-fed induction generator (DFIG) are derived in detail, and then the fifth-order transient model and the third-order simplified model of induction generator are established. By adopting stator flux linkage-oriented vector control, the d-axis and q-axis current between stator and rotor winding of generator can be decoupled, the active and reactive power in DFIG can be regulated independently. The mathematical models of dual PWM converter are derived, and the simulation of this model in Matlab show that the active and reactive power in it can be regulated independently, the power can bidirectional blow.(3) A grid-connected power system simulation model with DFIG has established in Matlab, and compared the simplified model with the transient model of induction generator. Thanks to the transient model fully considers the transient process of stator and rotor flux linkage, taking into account brake torque of generator at the initial moment of fault. Therefore, after the failure, the transient model can more accurately calculate the transient current; describe the generator speed transient characteristics; predict the response to other changes in various parameters; and also for the associated power system stability analysis. By contrast with the different equivalent models of drive train, it is proved that the lumped-mass model after fault provides the optimistic result; the three-mass equivalent model can more accurately describe the transient response process of the other parameters of WTG, which is more appropriate to be used in the power system stability analysis.(4) On account of the randomness for the change of wind speed and the gravity of system fault, for the grid-connected operation of WTGs on condition of wind disturbance and short-circuit fault are simulated respectively. The results show that in these two types of disturbances, the system established can maintain transient stability very well. When the drive train-related parameters are changing, the impacts of transient stability of the WTGs are researched. The results show that either by increasing the inertia constant of equivalent masses or the stiffness coefficient of shafts can improve transient stability of WTGs after system fault.The simulation results found that by adopting the three-mass equivalent model for the WTG'drive train can accurately describe the transient response process of WTG and calculate the torque which appears between different sections of the WTGs'drive train. The results have important directive significance for the design and increase service life of drive train, and the security and reliability of power network operation.(5) The torsional oscillation characteristics of WTG'drive train is analyzed; it is found that there are decaying and oscillation modes between different sections of the WTGs'drive train. For the mode of decaying, the blades of wind turbine and the rotor of generator have the central influence and the individual masses could be able to maintain the synchronous rotation, no oscillation. For the mode of oscillation, the hub of wind turbine and the rotor of generator play a major influence and there are two modes of oscillation. The torsional frequencies are 1.05Hz and 2.45Hz. The achievements have great practical value for the influence of natural torsional oscillations of drive train to the security and reliability of power network operation.