Transient Behavior Of Dfig And Its Influence On Power System Stability

In order to penetrate to the nature of transient behaviour of doubly-fed induction generator under grid disturbance, the conditions of electromechanical decoupling is concluded and the results of electromechanical decoupling is investigated. Power angle of DFIG is defined and its dynamic behavior is analyzed based on a single machine (DFIG) vs infinite system. It is demonstrated that the transient behavior of the power angle, not depending on rotor position and featuring complete electromagnetism transition, often shows fast-change character. Therefore, while disturbances happen in power grid, the active power of DFIG can be restored quikly to original value. Both digital simulation cases and physical experiments verify the proposed theory viewpoints.In order to investigate the influence of DFIG-based wind farms on small-signal stability of power system, the conception of frequency-power dynamic characteristic for DFIG was presented in this paper. Based on the proposed conception, two propositions are proved to be available for examining the damping of generic component. The phase of frequency-power dynamic characteristic of generic component can tell whether the damping performance is positive or not, and its magnitude can tell how strong the damping is. The research reveals that the regulator parameters of excitation system, such as integrator and proportion coefficient, have strong influence on the frequency dynamic characteristic of DFIG. Hence, the daming contribution of DFIG may extensively vary if the parameters of excitation regulator change. Using theoretic viewpoints above, an additional control strategy is designed for DFIG convertor to improve power system damping. Based on Power System Analysis Software Package (PSASP), 2-area and multi-area power systems cases show that the proposed control strategy can increase power system damping. The proposed strategy has some advantages of simple construction, fast tuning parameters, convenient debugging, and easy realization.For studying the transient stability of power systems with DFIG-based wind farms, the influence of DFIG on the first swing stability of power systems is investigated through illustrating the fast-change characteristic of DFIG power angle. It is revealed that two sorts of power angle curve cross will be produced when DFIG and parallel synchoronous generators are integrated into power grid. In this paper, these sorts of angle curves cross are defined as positive cross and passive cross, which have a significant effect on the stability of synchoronous generators. When a large disturbance happen in power grid, the positive cross will appear in the period of first swing and transient stability of power system will be weakened. But, the passive cross will occur in the succeeding seconds and DFIG will accordingly enhance damping of synchoronous generators. According analysis results above, an excitation control scheme is designed for DFIG convertor to improve transient performance of power angle of DFIG, and the proposed control strategy can also restrict the first swing of the synchoronous generators nearby the wind farm during grid disturbance.To reveal the mechanism of torsional oscillation of DFIG shaft, based on electromagnetic torque conception, it is demonstrated that the parameters of PI regulator and forward feed compensation are key factors that have great impacts on the electromagnetic torque. Because the controller design value of mutual inductance between stator and rotor windings is hardly equal to its real value, the error between design and real value can be positive or negative, which is the other crucial factor relating to the sign and magnitude of damping torque. Four kinds of cases are studied to find out the feature of damping torque, and two of them are confirmed to be the mechanisms of torsional oscillation of negative damping torque. To monitor shaft oscillations, an on-line identifying scheme for mechanical modes and electrical modes is developed, which employs subspace identification algorithm to identify system model and extract the dominating modes under the natural operation conditions.