Comparison and Evaluation of Three Main Types of Wind Turbines 1
Abstract— Many different types of wind turbines have been connected to the power systems of the world in recent years, and their primary focus was to design them to be more efficient and reliable. In this paper, three main  types of wind turbines are modeled and compared. The performances of these wind turbines during a severe three-phase fault at the end of a transmission line are simulated and their characteristics are analyzed. Finally the operation and maintenance costs of these wind turbines are discussed and proper applications of these wind turbines are pointed out.
Index Terms—wind turbine, modeling, fixed speed wind turbine, doubly fed induction generator, multipole permanent magnet synchronous generator。
I. INTRODUCTION   
As the demand for energy is steadily increasing, experts have been looking for new forms of resources to reduce dependence on fossil energy. Compared with solar and other renewable
energy, wind energy is cheaper and more reliable. Over the years wind power became the most rapidly growing renewable energy in electricity generation . The installed capacity and net generation of wind power in US from 1992 to 2005 are shown in Fig. 1.   
The total amount of electricity that could potentially be generated from wind in the United States has been estimated at 10.8 trillion kWh annually , but installed wind energy generating capacity now totals only 9,149 MW, and is expected to generate about 24.8 billion kWh of electricity in 2006 — that is still less than 1% of U.S. electricity generation (4.0 trillion kWh in 2004). According to Fig. 1, the average capacity factor of wind turbines is 21.7%, which is acceptable for a variable energy source with no fuel cost.
As a practical green source, wind power can provide electric power  with minimal  environmental pollution. Compared to coal and natural gas, the wind source is renewable without any worry about the future depletion of wind reserve. But wind energy can provide only a part of the electrical demand and consumer cannot rely completely on wind powered electric energy because wind is not available all the time. Experts have looked into the pos
sibility of storing electrical energy. But as large scale electric energy storage is not cost effective, direct grid connected wind power is the most viable option.
                Fig. 1 Installed capacity and net generation of wind power in US
Similar to other generation processes, a wind turbine converts mechanical energy into generator torque and the generator converts this torque into electricity and feeds it into the grid. The difference is that the mechanical energy is from wind. A wind turbine is a complex system because the knowledge from the areas of aerodynamics and mechanical, electrical,
and control engineering is applied. To make the wind turbines more efficient and match the grid requirements better, more and more technologies are being applied to them. There are currently three main types of wind turbines available as shown in Fig. 2 :
(a)  Fixed speed wind turbine with an asynchronous squirrel cage induction generator (IG) directly connected to the grid via a transformer.
(b)  Variable speed wind turbine with a doubly fed induction generator (DFIG) and blade pitch control.
(c)  Variable speed wind turbine using a permanent magnet synchronous generator that is connected to the grid through a full-scale frequency converter. This is called direct drive (DD) wind turbine.
With the application of new technologies, the cost of electricity from utility-scale wind systems has dropped by more than 80% over the last 20 years. Now, state-of-the-art wind power plants at excellent sites are generating electricity at less than 5 cents/kWh, which is
actual cost without other economic benefits such as production tax credit [2]. Compared to the costs of conventional generation technologies, it is very attractive and no other resources are needed to deal with expensive pollution emission control equipments.
                   
                       
(a)
(b)
(c)
(a) Fixed speed wind turbine with an induction generator
(b) Variable-speed wind turbine with a doubly-fed induction generator
(c) Variable-speed wind turbine with a permanent magnet synchronoureactive翻译s generator
Fig. 2 General structures of three different types of wind turbines
Wind power will be looked at as same as a common electrical energy resource eventually, so the associated requirements of conventional generation technologies, such as power factor correction, supplying reactive power to the transmission system, stability improveme
nt and so on, are being increasingly applied to wind turbines. Which type of wind turbine should be selected for a particular wind farm project is becoming one of the key issues in the decision making process. This paper introduces the characteristics of the above mentioned three types of wind turbines and presents their mathematical models. Computer simulations of their responses to a severe three-phase to ground fault at the end of a transmission line are presented. Finally they are compared and discussed from both technical and economic points of view.

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