Nonlinear stability study of DFIG wind energy generation systems

Abstract

With the increasing concern drawn for the environment protection and the conservation of nonrenewable energy, wind energy generation infrastructures have been widely deployed into the power grid system. The integration of the renewable energy such as wind energy into the power grid system, necessitates a more stringent requirement to protect the traditional power grid with single-direction power flow. In light of these ongoing situations, the renewable energy system features the benefit of the bi-directional power flow. Therefore, more concerns should be considered for the mutual interaction between all these generation sources and various loads. Typically, the doubly-fed induction generation wins the most popularity for the wind energy industry due to its variable-speed operation, the maximum power capture within a suitable region, low power losses and the use of fractional power rating converters. The objective of this thesis is to address the stability and performance issues of the grid-connected DFIG wind energy generation system. The stability issue is addressed at first for a standalone negative-sequence-injected DFIG. The corresponding instability phenomenon will be gathered for further studies. Based on the implication from the negative sequence, the interaction between the DFIG and loading with different electrical characteristics will be examined in the light of sustained oscillations at the low-frequency component 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 load, 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, the loading spaces will be derived to assist the design-oriented bifurcation analysis in system integration tasks. Furthermore, control schemes will be focused on broadening the system stable operation regions under unbalanced grid conditions. By addressing the critical problems existed in the ongoing control for unbalanced conditions, one new sequence decomposition method will be pointed out to facilitate a windup control and a 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 the basic knowledge about DFIG, such as the transformation basis and the DFIG modeling as well as the 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 the power system by means of various parameter spaces in light of ensuring stability issues. Furthermore, a numerical analysis procedure is presented so that a better understanding of system behavior can be achieved with control scheme improvement. For each task, the simulation verification and theoretical analysis will also be conducted accordingly. It is expected that this thesis can offer a useful reference of practice for the wind energy system engineering.