Impact Of Gird Integration Of Doubly Fed Induction Generator On Power System Small-Signal Stability

With the depletion of coal energy and deterioration of natural environment, different types of renewable energies are attracting more and more attention due to their clean and environmental friendly properties. The wind power generation is the fastest-growing and most technologically sophisticated one among them, and it has been utilized all over the world. However, with the continuous expansion of the scale of wind power integration, the impacts on small-signal stability of the grid brought by wind power are becoming increasingly significant. Therefore, it is worthwhile to put research efforts on this topic.In this thesis, the small-signal stability of a single-machine infinite-bus power system integrated with a doubly-fed wind generator (DFIG) which has been widely used in power systems is studied. The main contents of this paper are listed as follows.Firstly, the mathematical model of each part of a DFIG is presented and by using which the full linear model of a single-machine infinite-bus power system integrated with a DFIG is derived. According to the linear model, the block diagram of the Phillips-Heffron model can be drawn. The damping torque contributed from the excition system of the synchronous generator and the DFIG to the electromechanical oscillation loop can be derived individually by using the Damping Torque Analysis (DTA) method. The impacts of the generator power output and the line reactance on damping torque have been investigated in case studies. This thesis also presents a method to estimate the electromechanical oscillation mode when the DFIG is integrated by using damping torque and sensitivity analysis methods.Secondly, virtual inertia control strategy is applied to DFIG which has no interia contribution to power systems to simulate the inertia response of synchronous machines. It can also increase the speed of frequency recovery of the power system. Two methods of virtual inertia control along with their mathmatical models are introduced in this thesis. Moreover, the damping torque contributed from the virtual inertia control is derived. The effects of the control parameters, output power of generators and line reactances on the damping torque have been studied in examples.Lastly, the DFIG is displaced by a synchronous generator and the damping torque contributed from the displacing synchronous generator is also calculated. In accordance with the research method when the DFIG is presented, the effects of output power of generators and the line reactances on damping torque are invertigated in case studies. By comparing the two cases, the similarities and differences between DFIG and synchronous machine on the aspect of impacts on the small-signal stability of power systems can be concluded.The contents of this thesis have a certain practical significance to the long-distance power transmission of bundled wind andthermal power generation system.In addition, guidances can be supplied tothe future potential application of virtual inertia control of DFIG. Furthermore, the study in this thesis have reference value to the comparative study of impacts of DFIGs and synchronous machines on power system small-signal stability.