Nonlinear Stability Study of DFIG Wind Energy Generation Systems

With the increasing concern drawn for the environment protection and nonrenewable energy conservation, more wind energy infrastructures have been put into wide deployment on the power grid. With the integration of renewable energy such as wind energy into the system, a more stringent requirement is laid for power system evolution where the single-direction power flow of the traditional power generation system cannot satisfy. In light of these ongoing situation, i.e., bi-directional power flow, more concerns should be considered for the mutual interaction between all these sources when power system integration tasks are dealt with. Typically, doubly-fed induction generation wins the most popularity for wind energy industry for its variable-speed operation, maximum power capture within a suitable region and low power losses. The deployment of a fraction power rating makes DFIG as a profitable solution for wind energy industry as well.;The objective of this thesis is to address the stability and performance issues of grid-connected DFIG wind turbine system. The stability issue for negative-sequence-injected standalone DFIG is studied symmetrically. Corresponding instability phenomenon will be gathered for further studies. Based on the implication from the negative sequence, the interaction between DFIG and loading with different electrical characteristics will be examined in the light of sustained oscillation at low frequency and the second harmonic of line frequency. Two categories of loadings are adopted to build the grid-connected DFIG wind turbine system. They include unbalanced active loads and passive loads. For each kind of loads, by varying different parameters, nonlinear analysis method will be conducted to analyze and predict the occurrence of the inherent bifurcation behaviors. Simulation results will be provided to discover the instability numerically. In addition, useful loading spaces information will be derived to assist the design-oriented bifurcation analysis in system integration tasks. After that, control schemes will be focused on broadening the system stable operation regions under unbalanced grid conditions. By addressing the critical problems existent in the ongoing control for unbalanced condition, one new sequence decomposition method will be pointed out to facilitate a windup control and power oscillatory control. With this new combined control method, two critical objectives such as the elimination of DC-link voltage fluctuation and torque pulsation can be achieved simultaneously with easy discrete-time domain implementation.;This thesis consists of five chapters. The first chapter provides an introduction of the wind energy system and an integral overview of the DFIG technologies. The second chapter presents elaborations on basic knowledge about DFIG, such as transformation basis and modeling as well as commonly-used conventional vector control in detail. The remaining parts will be devoted to report the contribution of the tasks in this project. Throughout the tasks, they are compliant with one common objective to facilitate the design-oriented bifurcation analysis for system integration tasks by means of various parameter spaces in light of ensuring stability issues and present a theoretical analysis procedure so that a better understanding of system behavior can be achieved with control scheme improvement. For each task, simulation verification and numerical analysis will also be conducted accordingly. It is expected that this thesis can offer useful reference of practice for wind energy system integration engineering.