Modeling and Analysis of Doubly Fed Induction Generator Wind Energy Systems

Lingling Fan,Zhixin Miao

• Publisher: Academic Press
• Year: 2015
• ISBN:
  9780128029862
• Topic: Technology & Engineering
• Subtopic: Renewable Power Resources
• Index: Technology & Engineering

Wind Energy Systems: Modeling, Analysis and Control with DFIG provides key information on machine/converter modelling strategies based on space vectors, complex vector, and further frequency-domain variables. It includes applications that focus on wind energy grid integration, with analysis and control explanations with examples.

For those working in the field of wind energy integration examining the potential risk of stability is key, this edition looks at how wind energy is modelled, what kind of control systems are adopted, how it interacts with the grid, as well as suitable study approaches.

Not only giving principles behind the dynamics of wind energy grid integration system, but also examining different strategies for analysis, such as frequency-domain-based and state-space-based approaches.

• Focuses on real and reactive power control
• Supported by PSCAD and Matlab/Simulink examples
• Considers the difference in control objectives between ac drive systems and grid integration systems

Chapter 1

Introduction

Abstract

In this chapter, wind energy integration issues are separated into two categories based on time scales: operation issues that are in the time scale of hours, and dynamic issues that are in the time scale of seconds. The focus of the book is to introduce modeling and analysis of wind farms to investigate the dynamic issues. The structure of the book and the contents of each chapter are presented.

Keyword

Wind energy grid integration

1.1 WIND ENERGY INTEGRATION ISSUES 1

1.2 OBJECTIVES OF THIS BOOK 3

1.3 STRUCTURE OF THE BOOK 4

REFERENCES 6

1.1 Wind Energy Integration Issues

Integration issues related to wind energy can be roughly categorized into two types based on time scales: operation issues and dynamic issues. Operation issues relate to a long-term time scale, e.g., hours, days, or weeks. Operating the system and dispatching synchronous generators while accommodating intermittent wind energy at hourly base or daily base is an example. For this type of research, wind farms’ generation profiles of hours and days are of study interest. This type of problems belongs to the category of operations research. Optimization problems are usually formulated and solved. Example research can be found in [1–3]. In [1], a 24-hour unit commitment problem based on hourly generation, load, and wind profiles is formulated to evaluate the impact of wind penetration on market price. An economic dispatch problem with wind energy is presented in [2], where the stochastic nature of wind power is modeled based on wind speed Weibull probability distribution function. In [3], a short-term forward electricity market clearing problem is formulated with uncertain wind.

Compared to operation issues, dynamic issues are of very short time scales. For example, the time-scale of inter-area oscillations and electromechanical resonances is tens of seconds. The time scale of electric resonances is be even shorter.

Based on the above classification, many issues related to wind energy grid integration are dynamic issues, e.g., low voltage ride through, subsynchronous resonances (SSR), frequency support from wind, and control coordination of wind and high voltage direct current (HVDC) delivery systems. Dynamic modeling of wind energy system is important to approach this type of research. For example, in [4, 5], dynamic models of an induction machine are used to investigate the effect of stator voltage drop on transients in rotor voltages and currents. In our research on SSR in Type-3 wind farms with series-compensated networks [6], dynamic models of a Type-3 wind energy system and the grid were built. Analysis and simulation were conducted based on this model. In [7], frequency support schemes from wind energy system are validated through dynamic model-based simulation in the time scale of tens of seconds. Dynamic model-based simulation is also employed when designing DFIG (doubly fed induction generator) and HVDC control coordination [8].

To be able to investigate the aforementioned issues, we need a good understanding of the dynamic models of wind energy systems as well as the grid. Compared to conventional power systems where synchronous generators are the main source, wind energy systems include both rotating machines and power converter interfaces. Thus, modeling has to capture power converter control dynamics as well as the electromagnetic and electromechanical dynamics of AC machines. In conventional power systems, electromechanical dynamics (e.g., low-frequency oscillations) and torsional interactions between the turbine mechanical systems and the electric systems are expected. In wind energy systems, inclusion of converters could introduce resonances due to converter-grid interactions and machine-converter interactions. Investigation of emerging dynamic phenomena requires adequate dynamic models of wind energy systems.

Wind turbines are classified into four types based on AC generator type and converter type [9]. Type-1 systems deploy induction machines and work at a fixed wind speed. Type-2 wind turbines use wound rotor induction generators with rotor resistance control. Type-3 and Type-4 wind turbines are called variable speed fixed frequency wind generators. These generators can work at variable wind speed and produce fixed frequency electricity. Type-3 wind generator uses doubly-fed induction generator (DFIG) with a partial converter interface, which connects the rotor side to the grid through a back-to-back converter (a rotor-side converter (RSC) and a grid-side converter (GSC) connected via a DC-link capacitor), as shown in Fig. 1.1. Type-4 wind generator uses permanent magnet synchronous generator with a full-converter interface.

Figure 1.1 Type-3 wind energy system.

The state-of-the-art wind energy systems adopt voltage source converter-enabled wind turbines: Type-3 and Type-4. Type-3 occupies more market than Type-4. GE Wind’s 1.5/1.6 MW wind turbine (Type-3) was the most popular wind turbine in the U.S. market in 2012 [10].

The focus of this book is Type-3 wind energy systems. Dynamic modeling of DFIG-based wind energy system and its applications in dynamic studies will be elaborated in this book.

1.2 Objectives of This Book

With the focus on Type-3 or DFIG wind energy systems, throughout this book, analytical DFIG models will be derived and applied in various studies. Currently, there is a lack of monographs on systematic treatment of DFIG wind energy system in terms of modeling, analysis, and control.

There are classic textbooks on modeling and analysis of AC machines and drives, e.g., Krause’s Analysis of Electric Machinery and Bose’s Power Electronics and AC Drives. These books focus on drive systems. There are significant differences in control objectives for an AC drive system and a grid integration system. In drive systems, machine control focuses on flux and torque control, while grid integration focuses on real and reactive power control. In addition, many grid integration dynamic phenomena are system-level phenomena, e.g., SSR. System models including wind generation are required. These phenomena emerged and attracted interest from the research community and industry. These topics were not covered in a drive book.

The purpose of writing this book is to integrate the state-of-the-art wind energy grid integration research into education. The book can serve as a textbook for a graduate-level course on AC machine modeling with applications in wind energy integration. The book can also be read by researchers and practitioners in the field of wind energy integration.

1.3 Structure of the Book

The book is organized as follows.

Chapter 2 discusses AC machine modeling. Class notes from the authors’ course on “AC Machines and Drives” at University of South Florida will be used to develop AC machine modeling using space vectors and complex vectors. Following induction machine modeling, DFIG machine modeling will be discussed. Simulation and analysis examples will be provided.

Chapter 3 presents modeling of DFIG converter control. In this chapter, converter control of DFIG will be thoroughly explained. Differences between drive control and grid integration control will be discussed. In this chapter, a simulation example in PSCAD with power electronic switching details will be given. The purpose of the demonstration is to show that converter voltage outputs after filters are sinusoidal. Readers can then build visual connections between control of abc sinusoidal waveforms versus vector control or control of dq-variables. Modeling of converter controls of DFIG will then be explained based on dq-reference frame. The mathematical models will be derived and a simulation example will be demonstrated.

Chapter 4 discusses unbalanced DFIG analysis and control. In this chapter, steady-state analysis and transient analysis of DFIG rotor voltage/current due to unbalanced stator voltage drop will be presented. Following the analysis, mitigation of unbalance effect using converter control is presented. Simulation case studies are presented to verify the mitigation strategies.

Chapter 5 pres...