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... 10 600 MW, about half that of Germany. The Chinese market was increased by the country’s new Renewable Energy Law. China has more than doubled its total installed capacity by installing 1347 MW of wind energy in 2006, a 70% increase over 2005. This brings China up to 2604 MW of capacity, making it the sixth largest market worldwide. It is expected that more than 40 GW will be installed by 2020; this may become China the third major power supply by that year. Growth in African and Middle Eastern market also picked up in 2006, with 172MW of new installed capacity, mainly in Egypt, Morocco, and Iran, bringing the total up to 441 MW, a 63% growth (Blaabjerg & Chen, 2006). The European Wind Energy Association (EWEA) has set a target to satisfy 23% European electricity needs with wind by 2030. The exponential growth of the wind industry reflects the increasing demand for clean, safe and domestic energy and can be attributed to government policies associated with the environmental concerns, and research and development of innovative cost-reducing technologies. The large scale development of wind power results in the wind turbines/farms becoming a significant part of the generation capacity in some area, which requires that the power system treats the wind turbines/farms like a power source, not only an energy source. The wind power penetration would result in variations of load flows in the interconnected systems, as well as re-dispatch of conventional power plants, which may cause a reduction of reserve power capacity (Slootweg & Kling, 2003). Some actions become necessary to accommodate large scale wind power penetration. For example, the electric grid may need an expansion for bulk electricity transmission from offshore wind farms to load centers, and it may require reinforcement of existing power lines or construction of new power lines, installation of Flexible AC Transmission system (FACTs) devices, etc. The discovery of electricity generated using wind power dates back to the end of last century and has encountered many ups and downs in its more than 100 year history. In the beginning, the primary motivation for essentially all the researches on wind power generation was to reinforce the mechanization of agriculture through locally-made electricity generation. Nevertheless, with the electrification of industrialized countries, the role of wind power drastically reduced, as it could not compete with the fossil fuel-fired power stations. This conventional generation showed to be by far more competitive in providing electric power on a large scale than any other renewable one. Lack of fossil fuels during World War I and soon afterward during World War II created a consciousness of the great dependence on fossil fuels and gave a renewed attention to renewable energies and particularly to wind power. Although this concern did not extend long. The prices for electricity generated via wind power were still not competitive and politically nuclear power gained more attention and hence more research and development funds. It took two oil crises in the 1970s with supply problems and price fluctuations on fossil fuels before wind power once again was placed on the agenda. And they were these issues confronting many countries in the seventies which started a new stage for wind power and motivated the development of a global industry which today is characterized by relatively few but very large wind turbine manufacturers. The beginning of modern wind turbine development was in 1957, marked by the Danish engineer Johannes Juul and his pioneer work at a power utility (SEAS at Gedser coast in the Southern part of Denmark). His R&D effort formed the basis for the design of a modern AC wind turbine – the well-known Gedser machine which was successfully installed in 1959. With its 200kW capacity, the Gedser wind turbine was the largest of its kind in the world at that time and it was in operation for 11 years without maintenance. The robust Gedser wind turbine was a technological innovation as it became the hall mark of modern design of wind turbines with three wings, tip brakes, self-regulating and an asynchronous motor as generator. Foreign engineers named the Gedser wind turbine as ‘The Danish Concept’. Since then, the main aerodynamic concept has been this horizontal axis, three-bladed, upwind wind turbine connected to a three-phase electric grid, although many other different concepts have been developed and tested over the world with dissimilar results. An example of other concepts is the vertical axis wind turbine design by Darrieus, which provides a different mix of design tradeoffs from the conventional horizontal-axis wind turbine. The vertical orientation accepts wind from any direction with no need for adjustments, and the heavy generator and gearbox equipment can rest on the ground instead of on top of the tower. The aim of wind turbine systems development is to continuously increase output power, as depicted in Fig. 1. Since the rated output power of production-type units reached 200 kW various decades ago, by 1999 the average output power of new installations climbed to 600 kW. Today, the manufactured turbines for onshore applications are specified to deliver 2- 3 MW output power. In this sense, the world’s first wind park with novel "multi-mega power class” 7 MW wind turbines was manufactured by the German wind turbine producer Enercon (11 E-126 units) and put into partial operation in Estinnes, Belgium, in 2010 (to be completed by July 2012). The key objective of this 77 MW pilot project is to introduce a new power class of large-scale wind energy converters (7 MW WECs) into the market with potential to significantly contribute to higher market penetration levels for wind electricity, especially in Europe. On the other hand, sea-based wind farms are likely to mean bigger turbines than on land, with models that produce up to three times the power of standard on-shore models. Series production of offshore wind turbines can reach to date up to 5 MW or more, being the largest onshore wind turbine presently under development a 10 MW unit. At least four companies are working on the development of this "giant power class” 10 MW turbine for sea-based applications, namely American Superconductors (U.S.), Wind Power (U.K.), Clipper Windpower (U.K.) and Sway (Norway). Even more, it is likely that in the near future, power rating of wind turbines will increase further, especially for large-scale offshore floating wind turbine applications. A wind turbine is a rotary engine that captures power from a fluid flow (the wind) using aerodynamically designed blades and convert it into useful mechanical power. The available power depends on the wind speed but it is important to be able to control and limit the power at higher wind speeds so as to avoid the damage of the unit. The power limitation may be done by some of the three following methods, namely stall control (the blade position is fixed but stall of the wind appears along the blade at higher wind speed), active stall (the blade angle is adjusted in order to create stall along the blades) or pitch control (the blades are turned out of the wind at higher wind speed). Essentially, three types of typical wind generator systems are the most widely spread. The first type is a constant-speed wind turbine system with a standard squirrel-cage induction generator (SCIG) directly connected to the grid. The second type is a variable speed wind turbine system with a doubly fed induction generator (DFIG). The power electronic converter feeding the rotor winding has a power rating of approximately up to 30% of the rated power; the stator winding of the DFIG is directly connected to the grid. The third type is a variable speed wind turbine with full- rated power electronic conversion system and a synchronous generator or a SCIG. A multi- stage gearbox is usually used with the first two types of generators. Synchronous generators, including permanent magnet synchronous generator (PMSG), may be direct driven though a low-ratio gear box system; one or two stage gearbox, becomes an interesting option. Fig. 2 summarized the major parts included in the mechanical and electrical power conversion of a typical wind turbine system (Chen & Blaabjerg, 2009). drive, etc. The wind turbines are not only installed dispersedly on land, but also combined as wind farms (or parks) with capacities of hundreds MWs which are comparable with modern power generator units. Consequently, their performance could significantly affect power system operation and control (Hansen, et al. 2004). Wind turbines can either be designed to operate at fixed speed (actually within a speed range about 1%) or at variable speed. Many low-power wind turbines built to-date were constructed according to the so-called “Danish concept” that was very popular in the eighties, in which wind energy is transformed into electrical energy using a simple squirrel- cage induction machine directly connected to a three-phase power grid (Qiao et al., 2007). The rotor of the wind turbine is coupled to the generator shaft with a fixed-ratio gearbox. At any given operating point, this turbine has to be operated basically at constant speed. On the other hand, modern high-power wind turbines in the 2-10 MW range are mainly based on variable speed operation with blade pitch angle control obtained mainly by means of power electronic equipment, although variable generator rotor resistance can also be used. These wind turbines can be mostly developed using either a direct-in-line system built with a direct-driven (without gearbox) PMSG grid-connected via a full-scale power converter, or a doubly-fed induction generator (DFIG) system that consists of a DFIG with a partial-scale power converter connected to the rotor windings. Based on these concepts, the most commonly applied wind turbine designs can be classified into four wind turbine concepts, as ...

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