Use Wind Energy To Generate Electricity Engineering Essay

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The wind is a clean, free, and continuous energy source. Humankind uses this energy source to propel ships and driving wind turbines to grind grain and pump water for many centuries and it serves well. Denmark is the first country to use wind energy to generate electricity. The Danes used the 23m diameter wind turbine to generate electricity in 1890. Until 1910, several hundred units wind turbine generators of 5 to 25 kW were built to generate electricity in Denmark (Johnson, 1985). Commercial wind power electronic generation plants which use two and three-bladed propellers appeared on the American farm and market until about 1925

Wind energy is the most rapidly expanding renewable energy source in the world today. The worldwide installed capacity of wind energy generation has grown at an average rate of over 28% per year over the past 10 years. At the end of 2004 installation nameplate capacity of about 48,000 MW which is enough to power about 16 million American homes. In January 2005, Germany had installed about 16,600 MW wind power generation which was the world leader in wind energy installations, followed by Spain which installed 8300, the US installed 6700, Denmark was 3100, India was 3000, Italy with 1100, The Netherlands with 1100, the United Kingdom was 900, Japan with 900, and China was 800. Although wind power supplies only about 0.6% of the world electricity demand today, the size of that contribution is growing rapidly. In Germany, the contribution of wind power to electricity consumption is over 5%, in Spain it is about 8%, and in Denmark it is approximately 20%.[] The cost to generate wind energy has decreased dramatically from more than 30 cents (U.S.) per kilowatt-hour (¢/kWh) in the early 1980s to under 4¢/kWh (at the best sites) in 2004. Although the technology improved continuously, the cost has actually increased because of worldwide increases in steel, concrete, and transportation costs which have led to increases in the prices of wind turbines. The large increases in the cost of natural gas and other fossil fuels have made wind-generated electricity a lower-cost option than natural gas for many utilities adding generating capacity. In fact, the demand for wind energy in the United States has been so strong that wind-power developers are demanding and receiving significantly higher prices in their new long-term power purchase agreements with utility companies than they did 2 years ago.[]

After World War II, we entered the era of cheap oil imported from the Middle East. Interest in wind energy died and companies making small turbines folded. The oil embargo of 1973 served as a wakeup call, and oil-importing nations around the world started looking at wind again. The two most important countries in wind power development since then have been the U.S. and Denmark (Brower et al., 1993). []

The U.S. immediately started to develop utility-scale turbines. It is reasonable that the large turbines had the potential that produce cheaper electricity than the small turbines. The Department of Energy decided that only large aerospace companies which had the enough engineering and manufacturing capability were chosen to build utility-scale turbines. At the same time the small companies which may have good ideas can not have the income flow for survival. But there is problem with the aerospace companies that they had no wish to manufacture utility-scale wind turbines.

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Table 1.1 Wind Power Installed Capacity

They gladly took the government's money to build test turbines, but when the money ran out, they were looking for other research projects. Under this situation, government only funded a number of test turbines which is from the 100 kW to the 2500 kW.

On the other hand, Denmark established a plan which a landowner can buy a turbine and sell the electricity which the turbine generates to the local company at the current market price. The early turbines were larger than what a farmer would need for himself, but not what we would consider utility scale. This provided an income flow for small companies. They could try new ideas and learn from their mistakes. Under this situation many people joined into this new market. There were 25 wind turbine manufacturers in Denmark in 1986. "The Danish market gave them a base from which they could also sell to other countries. It was said that Denmark led the world in exports of two products: wind turbines and butter cookies!"[] There has been consolidation in the Danish industry which makes some companies becoming large since 1986. Vestas as an example, has more installed wind turbine capacity worldwide than any other manufacturer.

Prices have dropped substantially since 1973. It is now commonplace for wind power plants to be able to sell electricity for fewer than four cents per kilowatt hour.

1.1 Applications

There are perhaps four definite categories of wind power. These are

1. Small, non-grid connected

2. Small, grid connected

3. Large, non-grid connected

4. Large, grid connected

By small, we mean a size appropriate for an individual to own, up to a few tens of kilowatts. Large refers to utility scale. []

1.1.1 Small, Non-Grid Connected

The one that electricity in a location not serviced by a utility, with batteries to level out supply and demand. This might be a vacation home, a remote antenna and transmitter site, or a Third-World village. The costs will be high, on the order of $0.50=kWh, but if the total energy usage is small, this might be acceptable. The alternatives, photovoltaic, micro-hydro, and diesel generators, are not cheap either, so a careful economic study needs to be done for each situation. []

1.1.2 Small, Grid Connected

The small, grid connected turbine is usually not economically feasible. The cost of wind-generated electricity is less because the utility is used for storage rather than a battery bank, but is still not competitive.

In order to be financial breakeven for the small, grid connected turbine the turbine owner needs to close to the retail price for the wind-generated electricity. The way the owner does is making an arrangement with the utility which is called net metering. With this system, the meter runs backward when the turbine is generating more than the owner is consuming at the moment. The owner pays a monthly charge for the wires to his home, but it is conceivable that the utility will sometimes write a check to the owner at the end of the month, rather than the other way around. The utilities do not like this arrangement. They want to buy at wholesale and sell at retail. They feel it is unfair to be used as a storage system without remuneration. []

"For most of the twentieth century, utilities simply refused to connect the grid to wind turbines. The utility had the right to generate electricity in a given service territory, and they would not tolerate competition. Then a law was passed that utilities had to hook up wind turbines and pay them the avoided cost for energy. Unless the state mandated net metering, the utility typically required the installation of a second meter, one measures energy consumption from the home and the other energy production by the turbine. The owner would pay the regular retail rate, and the utility would pay their estimate of avoided cost, usually the fuel cost of some base load generator. The owner might pay $0.08 to $0.15 per kWh, and receive $0.02 per kWh for the wind-generated electricity. This was far from enough to economically justify a wind turbine, and had the effect of killing the small wind turbine business." []

1.1.3 Large, Non-Grid Connected

These machines would be installed on islands or in native villages in the far north where it is virtually impossible to connect to a large grid. Such places are typically supplied by diesel generators. One or more wind turbines would be installed in parallel with the diesel generators, and act as fuel savers when the wind was blowing. This concept has been studied carefully and appears to be quite feasible technically. It would be helpful if the diesel maintenance companies would also carry a line of wind turbines so the people in remote locations would not need to teach another group of maintenance people about the realities of life at places far away from the nearest hardware store.[]

1.1.4 Large, Grid Connected

We might ask if the utilities should be forced to buy wind-generated electricity from these small machines at a premium price which reflects their environmental value. Many have argued this over the years. A better question might be whether the small or the large turbines will result in a lower net cost to society. Given that we want the environmental benefits of wind generation, should we get the electricity from the wind with many thousands of individually owned small turbines, or should we use a much smaller number of utility-scale machines?

If we could make the argument that a dollar spent on wind turbines is a dollar not spent on hospitals, schools, and the like, then it follows that wind turbines should be as efficient as possible. Economies of scale and costs of operation and maintenance are such that the small, grid connected turbine will always need to receive substantially more per kilowatt hour than the utility-scale turbines in order to break even. There is obviously a niche market for turbines that are not connected to the grid, but small, grid connected turbines will probably not develop a thriving market. Most of the action will be from the utility-scale machines.

Sizes of these turbines have been increasing rapidly. Turbines with ratings near 1MWare now common, with prototypes of 2 MW and more are being tested. This is still small compared to the needs of a utility, so clusters of turbines are placed together to form wind power plants with total ratings of 10 to 100 MW.

2. Wind turbine

There is considerable anecdotal evidence that the first wind machines may have been built over 2000 years ago, perhaps in China, but there is no firm evidence to support this conjecture. However, there is considerable written evidence that the windmill was in use in Persia by AD 900, and, perhaps, as early as AD 640. Figure 22.1 illustrates the main features of this type of mill. The center vertical shaft was attached to a millstone and horizontal beams or arms were attached to the shaft above the millstone. Bundles of reeds attached vertically to the outer end of the arms acted as sails, turning the shaft when the wind blew. The surrounding structure was oriented so that the prevailing wind entered the open portion of the structure and pushed the sails downwind. The closed portion of the structure sheltered the sails from the wind on the upwind pass. The primary applications of these machines were to grind or mill grain and to pump water; they became generally known as windmills. The wind turbines of today may look much different than those first machines, but the basic idea remains the same-use the power in the wind to generate useful energy. Modern wind machines, called wind turbines, tend to have a small number of airfoil-shaped blades, in contrast to the older windmills that usually had several flat or slightly curved blades (such as the American multi-blade water pumper shown in Figure 22.2). The reasons for this difference in blade number will be examined later in this chapter.

Although there are many different configurations of wind turbines, most of them can be classified as either horizontal-axis wind turbines (HAWTs), which have blades that rotate about a horizontal axis parallel to the wind, or vertical-axis wind turbines (VAWTs), which have blades that rotate about a vertical axis. Figure 22.3 illustrates the main features of these configurations. They both contain the same major components, but the details of those components differ significantly.

According to Sheppard [1], the terms "horizontal" and "vertical" associated with these classifications are a potential source of confusion. Although they now refer to the driving shaft on which the rotor is mounted, in the past these terms referred to the plane in which the rotor turned. Thus, the ubiquitous multi-bladed water-pumper windmill shown in Figure 22.2, now referred to as a horizontal-axis machine,

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FIGURE 22.1 Illustration of ancient Persian wind mill

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FIGURE 22.2 Typical American multiblade windmills.(Courtesy of Nolan Clark, U.S. Department of Agriculture).

had a rotor that turned in a vertical plane, and was therefore at one point known as a vertical mill. Likewise, the earliest windmills, such as the one illustrated in Figure 22.1, had rotors that turned in a horizontal plane, and were known as horizontal windmills.

As shown in Figure 22.3, HAWTs and VAWTs have very different configurations. Each configuration has its own set of strengths and weaknesses. HAWTs usually have all of their drive train (the transmission, generator, and any shaft brake) equipment located in a nacelle or enclosure mounted on a tower, as shown. Their blades are subjected to cyclic stresses due to gravity as they rotate, and their rotors must be oriented (yawed) so the blades are properly aligned with respect to the wind. HAWTs may be readily placed on tall towers to access the stronger winds typically found at greater heights. The most common type of modern HAWT is the propeller-type machine, and these machines are generally classified according to the rotor orientation (upwind or downwind of the tower), blade attachment to the main shaft (rigid or hinged), maximum power control method (full or partial-span blade pitch or blade stall), and number of blades (generally two or three blades).

VAWTs, on the other hand, usually have most of their drive train on the ground; their blades do not experience cyclic gravitational stresses and do not require orientation with respect to the wind. However, VAWT blades are subject to severe alternating aerodynamic loading due to rotation, and VAWTs cannot readily be placed on tall towers to exploit the stronger winds at greater heights. The most common types of modern VAWTs are the Darrieus turbines, with curved, fixed-pitch blades, and the "H" or "box" turbines with straight fixed-pitch blades. All of these turbines rely on blade stall (loss of lift and increase in drag as the blade angle of attack increases) for maximum power control. Although there are still a few manufacturers of VAWTs, the overwhelming majority of wind turbine manufacturers devote their efforts to developing better (and usually larger) HAWTs.

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FIGURE 22.3 Schematic of basic wind turbine configurations

Although the fuel for wind turbines is free, the initial cost of a wind turbine is a very large contributor to the cost of energy (COE) for that turbine. To minimize that COE, wind turbine designs must be optimized for the particular site or wind environment in which they will operate. Trial and error methods become very expensive and time-consuming when used to design and/or optimize turbines, especially larger ones. A large optimized wind turbine can be developed at a reasonable cost only if the designers can accurately predict the performance of conceptual machines and use modeling to investigate the effects of design alternatives. Over the past two decades, numerous techniques have been developed to accurately predict the aerodynamic and structural dynamic performance of wind turbines. These analytical models are not, in general, amenable to simple approximations, but must be solved with the use of computer codes of varying complexity. Several of these models are summarized below.

22.4 Wind Turbine Structural Dynamic Considerations

Input loads and dynamic interactions result in forces, moments, and motions in wind turbines, phenomena referred to as structural dynamics. By applying various analytical methods, the impact of changes in turbine configurations, controls, and subsystems on the behavior of the turbine can be predicted. General wind turbine structural dynamic concerns and methods of analysis are discussed in the following paragraphs.

22.4.1 Horizontal-Axis Wind Turbine Structural Dynamics

Small-horizontal-axis turbine designs usually use fairly rigid, high-aspect-ratio (the blade length is much greater than the blade chord) blades, cantilevered from a rigid hub and main shaft. As turbine size increases, the flexibility of the components tends to increase, even if the relative scales remain the same, so the blades on larger turbines tend to be quite flexible, and the hub and main shaft tend to be far less rigid than corresponding components on small turbines. The entire drive-train assembly is mounted on and yaws about a tower that may also be flexible. These structures have many natural vibration mode-sand some of them may be excited by the wind or the blade rotation frequency to cause a resonance condition, amplifying vibrations and causing large stresses in one or more components. Operating at a resonance condition can quickly lead to component failure and result in the destruction of the turbine.

Careful structural analysis during the turbine design may not guarantee that the turbine will not experience a resonance condition, as analysis techniques are not infallible, but ignoring the analysis altogether or failing to properly conduct parts of it may dramatically increase the probability that the turbine will experience one or more resonance conditions, leading to early failure. Although the relatively rigid small turbines are not likely to experience these resonance problems, the very flexible, highly dynamic, larger turbines may well experience resonance problems unless they are very carefully designed and controlled. Turbines that operate over a range of rotational speeds (variable-speed turbines) are especially challenging to design. The designer will usually try to minimize the number of resonances that occur within the operational speed range, and then implement a controller that will avoid operating at those resonance conditions. The actual resonances typically depend on the rotor speed, and the severity of the resonance depends on the wind-speed, so the controller logic can become quite complicated.

The large relative motion between the rotor and the tower frequently precludes the use of standard commercial finite-element analysis codes and requires the use of a model constructed specifically for analysis of wind turbines. Development of such a model can be a rather daunting task, as it requires the formulation and solution of the full nonlinear governing equations of motion. The model must incorporate the yaw motion of the nacelle, the pitch control of the blades, any motion and control associated with hinged blades, the time-dependent interaction between the rotor and the supporting tower, etc. If the full equations of motion are developed with either finite-element or multi-body dynamics formulations, the resultant models contain moderate numbers of elements and potential motions (or degrees of freedom, DOF), and significant computer resources are required to solve the problem. On the other hand, a modal formulation utilizing limited DOF may be able to yield an accurate representation of the wind turbine, resulting in models that do not require large computing resources.

The development of the modal equations of motion may require somewhat more effort than do the finite-element or multi-body equations, and the equations are apt to be more complex. The modal degrees of freedom must include, at a minimum, blade bending in two directions, blade motion relative to the main shaft, drive train torsion, tower bending in two directions, and nacelle yaw. Blade torsion (twisting about the long axis) is not normally included in current models, but it may become more important as the turbine sizes continue to increase and the blades become more flexible. The accuracy of some modal formulations is limited by their inability to model the direction-specific, nonlinear variation of airfoil lift with angle of attack that occurs as a result of aerodynamic stall. However, this is not an inherent limitation of the technique, and some formulations are free of these limitations. The modal formulation is the most widely used HAWT analysis tool today, but, as computer resources become more readily available, the more accurate finite-element (such as Abaqus [26]) and multi-body dynamic codes (such as Adams [27]) will certainly become more widely used. It is possible to develop techniques in the frequency domain to analyze many aspects of the turbine dynamics. The frequency domain calculations are fast, but they can only be applied to linear, time invariant systems and, therefore, cannot deal with some important aspects of wind turbine operations such as aerodynamic stall, start-up and shutdown operations, variable speed operation, and nonlinear control system dynamics. In spite of these limitations, frequency domain solutions of modal formulations are frequently used in the preliminary design of a wind turbine, when quick analysis of many configurations is required. Regardless of the methods used in preliminary design, the state of the art in wind turbine design today is to use highly detailed modal, lumped-mass, or finite-element-based equations of motion, coupled with time-accurate solutions, to analyze the turbine behavior for the detailed final design calculations. Malcolm and Wright [28] and Molenaar and Dijkstra [29] provide reviews of some of the available HAWT dynamics codes that have been developed, together with their limitations. Buhl et al. [30] compare some of the HAWT dynamics codes that have been extensively verified and that are widely used today, and Quarton [31] provides a good history of the development of HAWT wind turbine analysis codes. More general finite-element dynamics codes are described by others [32-35].

22.4.2 Vertical-Axis Wind Turbine Structural Dynamics

Darrieus turbine designs normally use relatively slender, high-aspect-ratio structural elements for the blades and supporting tower. As with large HAWTs, the result is a very flexible, highly dynamic structure, with many natural modes of vibration that again must be carefully analyzed to ensure that the turbine will avoid structural resonance conditions under all operating environments. The guy cables and turbine support structure can typically be analyzed with commercial or conventional finite-element codes, but the tower and blades require a more refined analysis, usually requiring the use of a finite-element code possessing options for analyzing rotating systems. With such a code, the blades and tower of a VAWT are modeled in a rotating coordinate frame, resulting in time-independent interaction coefficients. The equations of motion must incorporate the effects of the steady centrifugal and gravitational forces, the aerodynamic forces due to the turbulent wind, and the forces arising from rotating coordinate system effects. Detailed information on finite-element modeling of VAWTs may be found in Lobitz and Sullivan [36].

22.7.5 Environmental Concerns

Although wind turbines generate electricity without causing any air pollution or creating any radioactive wastes, like all man-made structures, they do cause an impact on the environment. Wind turbines require a lot of land, but only about 5% of that land is used for turbine foundations, roads, electrical substations, and other wind-farm applications. The remaining 95% of the land is available for other uses such as farming or livestock grazing. Wind turbines do generate noise as well as electricity, but noise is seldom a problem with newer, large wind turbines. Some small turbines are quite noisy, but many of the newer ones are very quiet. Current industry standards call for characterization of turbine noise production and rate of decay with distance as part of the turbine testing process; therefore, noise information is readily available. Noise decreases quickly with distance from the source, so placing wind turbines appropriate distances from local homes has proven to be an effective means of eliminating noise as a problem. The noise level due to a typical, large, modern wind turbine, 300 m distant, is roughly comparable to the typical noise level in the reading room of a library.

22.7.5.1 Visual Impact

The visual impact of wind turbines is extremely subjective. What one person considers highly objectionable, another might consider as attractive or, at least, not objectionable. The relatively slow rotation of today's large wind turbines is viewed by most people as far less intrusive than the fast rotation of the early, small turbines. Visual impact can be minimized through careful design of a wind farm. The use of a single model of wind turbine in a wind farm and uniform spacing of the turbines helps alleviate concerns in this area. Computer simulation can be very helpful in evaluating potential visual impacts before construction begins.

22.7.5.2 Bird and Bat Collisions

One of the greatest environmental issues that the wind industry has had to face is the issue of bird deaths due to collisions with wind turbines. Concerns about this issue were, in large part, the result of relatively high numbers of raptor deaths in the Altamont Pass wind farms east of San Francisco, California in the 1980-1985 time frame. Dozens of studies of this issue have been conducted during the past 20 years.

Sinclair and Morrison [53] and Sinclair [54] give overviews of the recent U.S. studies. One conclusion of the Altamont Pass studies is that the Altamont Pass situation is a worst-case scenario, due in large part to bad siting, and to the presence of overhead power lines that led to a large number of bird electrocutions. Colson [55] and Wolf [56] provide summaries of ways to minimize the impact of wind farms on birds.

Among the specific recommendations are:

·Avoid bird migration corridors and areas of high bird concentrations (micro habitats or fly zones)

·Use fewer, larger turbines

·Minimize number of perching sites on turbine towers

·Bury electrical lines

·Conduct site-specific mitigation studies

In spite of the ongoing bird collision problems at Altamont Pass, the impact of wind energy on birds is very low compared with other human-related sources of bird deaths. According to the National Wind Coordinating Committee (NWCC), bird collisions with wind turbines caused the deaths of only 0.01%-0.02% of all the birds killed by collisions with man-made structures across the U.S. in 2001 [57]. Extrapolating their estimate of roughly two bird deaths per turbine per year to a scenario where 100% of U.S. electricity is provided by wind (and assuming a turbine size of 1.5 MW), yields an estimate of deaths due to collisions with wind turbines of 0.5%-1% of all bird deaths caused by collisions with structures. In contrast, bird collisions with buildings and windows account for about 55% of structure related bird deaths, whereas collisions with vehicles, high-tension power lines, and communication towers account for about 17%.

Unexpectedly large numbers of bats have been killed by some wind farms in the eastern U.S. in the past few years. The wind industry has joined with Bat Conservation International, the U.S. Fish and Wildlife Service, and the National Renewable Energy Laboratory to identify and quantify the problem and to explore ways to mitigate these deaths. Several wind-energy companies are providing a portion of the funding for the cooperative effort that includes hiring a full-time biologist to spend three years coordinating the research effort. Efforts to resolve this issue are ongoing.

3. Wind Farm Expansion

There is now general acceptance that the burning of fossil fuels is having a significant influence on the global climate. Effective mitigation of climate change will require deep reductions in greenhouse gas emissions, with UK estimates of a 60-80% cut being necessary by 2050 (Stern Review, UK HM Treasury, 2006). The electricity system is viewed as being easier to transfer to low-carbon energy sources than more challenging sectors of the economy such as surface and air transport and domestic heating. Hence the use of cost-effective and reliable low-carbon electricity generation sources, in addition to demand-side measures, is becoming an important objective of energy policy in many countries (EWEA, 2006; AWEA, 2007).

Over the past few years, wind energy has shown the fastest rate of growth of any form of electricity generation with its development stimulated by concerns of national policy makers over climate change, energy diversity and security of supply.

Figure 1.1 shows the global cumulative wind power capacity worldwide (GWEC, 2006). In this figure, the 'Reference' scenario is based on the projection in the 2004 World Energy Outlook report from the International Energy Agency (IEA). This projects the growth of all renewable including wind power, up to 2030. The 'Moderate' scenario takes into account all policy measures to support renewable energy either under way or planned worldwide. The 'Advanced' scenario makes the assumption that all policy options are in favor of wind power, and the political will is there to carry them out.

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Figure 1.1 global cumulative wind power capacities (GWEC, 2006)

1.1 Wind Farms Classification

Numerous wind farm projects are being constructed around the globe with both offshore and onshore developments in Europe and primarily large onshore developments in North America. Usually, sites are preselected based on general information of wind speeds provided by a wind atlas, which is then validated with local measurements. The local wind resource is monitored for 1 year, or more, before the project is approved and the wind turbines installed.

Onshore turbine installations are frequently in upland terrain to exploit the higher wind speeds. However, wind farm permitting and sitting onshore can be difficult as high wind-speed sites are often of high visual amenity value and environmentally sensitive.

Offshore development, particularly of larger wind farms, generally takes place more than 5 km from land to reduce environmental impact. The advantages of offshore wind farms include reduced visual intrusion and acoustic noise impact and also lower wind turbulence with higher average wind speeds. The obvious disadvantages are the higher costs of constructing and operating wind turbines offshore, and the longer power cables that must be used to connect the wind farm to the terrestrial power grid.

In general, the areas of good wind energy resource are found far from population centers and new transmission circuits are needed to connect the wind farms into the main power grid. For example, it is estimated that in Germany, approximately 1400 km of additional high-voltage and extra-high-voltage lines will be required over the next 10 years to connect new wind farms (Deutsche Energie-Agentur GmbH, 2005).

Smaller wind turbines may also be used for rural electrification with applications including village power systems and stand-alone wind systems for hospitals, homes and community centers (Elliot, 2002).

Table 1.1 illustrates typical wind turbine ratings according to their application.

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Table 1.1 Wind turbine applications (Elliot, 2002)

4. Wind Turbine Architectures

There are a large number of choices of architecture available to the designer of a wind turbine and, over the years, most of these have been explored (Ackermann, 2005; Heier, 2006). However, commercial designs for electricity generation have now converged to horizontal axis, three-bladed, upwind turbines. The largest machines tend to operate at variable speed whereas smaller, simpler turbines are of fixed speed.

Modern electricity-generating wind turbines now use three-bladed upwind rotors, although two-bladed, and even one-bladed, rotors were used in earlier commercial turbines. Reducing the number of blades means that the rotor has to operate at a higher rotational speed in order to extract the wind energy passing through the rotor disk. Although a high rotor speed is attractive in that it reduces the gearbox ratio required, a high blade tip speed leads to increased aerodynamic noise and increased blade drag losses. Most importantly, three-bladed rotors are visually more pleasing than other designs and so these are now always used on large electricity-generating turbines.

1.2.2.1 Fixed-speed Wind Turbines

Fixed-speed wind turbines are electrically fairly simple devices consisting of an aerodynamic rotor driving a low-speed shaft, a gearbox, a high-speed shaft and an induction (sometimes known as asynchronous) generator. From the electrical system viewpoint they are perhaps best considered as large fan drives with torque applied to the low-speed shaft from the wind flow.

Figure 1.6 illustrates the configuration of a fixed-speed wind turbine (Holdsworth et al., 2003; Akhmatov, 2007). It consists of a squirrel-cage induction generator coupled to the power system through a turbine transformer. The generator operating slip changes slightly as the operating power level changes and the rotational speed is therefore not entirely constant. However, because the operating slip variation is generally less than 1%, this type of wind generation is normally referred to as fixed speed.

Squirrel-cage induction machines consume reactive power and so it is conventional to provide power factor correction capacitors at each wind turbine. The function of the soft-starter unit is to build up the magnetic flux slowly and so minimize transient currents during energization of the generator. Also, by applying the network voltage slowly to the generator, once energized, it brings the drive train slowly to its operating rotational speed.

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Figure 1.6 Schematic of a fixed-speed wind turbine

1.2.2.2 Variable-speed Wind Turbines

As the size of wind turbines has become larger, the technology has switched from fixed speed to variable speed. The drivers behind these developments are mainly the ability to comply with Grid Code connection requirements and the reduction in mechanical loads achieved with variable-speed operation. Currently the most common variable-speed wind turbine configurations are as follows:

• doubly fed induction generator (DFIG) wind turbine

• fully rated converter (FRC) wind turbine based on a synchronous or induction generator.

Doubly Fed Induction Generator (DFIG) Wind Turbine

A typical configuration of a DFIG wind turbine is shown schematically in Figure 1.7. It uses a wound-rotor induction generator with slip rings to take current into or out of the rotor winding and variable-speed operation is obtained by injecting a controllable voltage into the rotor at slip frequency (Muller et al., 2002; Holdsworth et al., 2003). The rotor winding is fed through a variable-frequency power converter, typically based on two AC/DC IGBT-based voltage source converters (VSCs), and linked by a DC bus. The power converter decouples the network electrical frequency from the rotor mechanical frequency, enabling variable-speed operation of the wind turbine. The generator and converters are protected by voltage limits and an over-current 'crowbar'.

A DFIG system can deliver power to the grid through the stator and rotor, while the rotor can also absorb power. This depends on the rotational speed of the generator. If the generator operates above synchronous speed, power will be delivered from the rotor through the converters to the network, and if the generator operates below synchronous speed, then the rotor will absorb power from the network through the converters.

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Figure 1.7 Typical configuration of a DFIG wind turbine

Fully Rated Converter (FRC) Wind Turbine

The typical configuration of a fully rated converter wind turbine is shown in Figure 1.8. This type of turbine may or may not include a gearbox and a wide range of electrical generator types can be employed, for example, induction, wound-rotor synchronous or permanent magnet synchronous. As all of the power from the turbine goes through the power converters, the dynamic operation of the electrical generator is effectively isolated from the power grid (Akhmatov et al., 2003; Heier, 2006). The electrical frequency of the generator may vary as the wind speed changes, while the grid frequency remains unchanged, thus allowing variable-speed operation of the wind turbine.

The power converters can be arranged in various ways. Whereas the generator-side converter (GSC) can be a diode rectifier or a PWM voltage source converter (VSC), the network-side converter (NSC) is typically a PWM VSC. The strategy to control the operation of the generator and the power flows to the network depends very much on the type of power converter arrangement employed. The network-side converter can be arranged to maintain the DC bus voltage constant with torque applied to the generator controlled from the generator-side converter. Alternatively, the control philosophy can be reversed. Active power is transmitted through the converters with very little energy stored in the DC link capacitor. Hence the torque applied to the generator can be controlled by the network-side converter. Each converter is able to generate or absorb reactive power independently.

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Figure 1.8 Typical configuration of a fully rated converter-connected wind turbine

1.3.2.2 Reactive Power and Voltage Support

The voltage on a transmission network is determined mainly by the interaction of reactive power flows with the reactive inductance of the network. Fixed-speed induction generators absorb reactive power to maintain their magnetic field and have no direct control over their reactive power flow. Therefore, in the case of fixed-speed induction generators, the only way to support the voltage of the network is to reduce the reactive power drawn from the network by the use of shunt compensators.

Variable-speed wind turbines have the capability of reactive power control and may be able to support the voltage of the network to which they are connected. However, individual control of wind turbines may not be able to control the voltage at the point of connection, especially because the wind farm network is predominantly capacitive (a cable network).

On many occasions, the reactive power and voltage control at the point of connection of the wind farm is achieved by using reactive power compensation equipment such as static var compensators (SVCs) or static synchronous compensators (STATCOMs).

1.3.2.3 Frequency Support

To provide frequency support from a generation unit, the generator power must increase or decrease as the system frequency changes. Hence, in order to respond to low network frequency, it is necessary to de-load the wind turbine leaving a margin for power increase. A fixed-speed wind turbine can be de-loaded if the pitch angle is controlled such that a fraction of the power that could be extracted from wind will be 'spilled'. A variable-speed wind turbine can be de-loaded by operating it away from the maximum power extraction curve, thus leaving a margin for frequency control.

Power electronic systems are frequently used for electrical power conversion at a wind turbine generator level, wind farm level or both. Within the wind turbine generator, power electronic converters are used to control the steady-state and dynamic active and reactive power flows to and from the electrical generator.

2.3 Application of VSCs for Variable-speed Systems

As discussed in Section 1.2, wind turbines use power electronic converters for variable-speed operation. Table 2.1 summarizes the application of VSCs for different generator configurations.

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Table 2.1 Generators and power electronics in wind turbine applications

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Figure 2.19 Back-to-back VSCs

2.3.2 Back-to-Back VSCs

The back-to-back VSC is a bi-directional power converter consisting of two voltage source converters as shown in Figure 2.19.

The IGBTs on the generator-side VSC are controlled using a PWM technique (usually based on SVPWM). In variable-speed wind turbines, the frequency of the reference sinusoidal waveform is locked to the frequency of the generated voltage. Therefore, the frequency of the output voltage of the VSC contains a component at the frequency of the generated voltage, referred to as the fundamental and also higher-order harmonics. The magnitude of the VSC output voltage can be controlled by changing the amplitude modulation index and the phase angle can be controlled by controlling the phase angle of Vref with respect to the generated voltage.

In order to describe the operation of the VSC connected to the generator, it is assumed that the VSC produces a sinusoidal waveform (higher-order harmonic components are neglected). As the wind turbine generator can be represented by a voltage behind a reactance (Kundur, 1994), the generator-side connection of the VSC can be represented by the equivalent circuit shown in Figure 2.20, where VG is the magnitude of the generated voltage, VVSC is the magnitude of the VSC output voltage, δ is the phase angle between these two voltages and XG is the equivalent generator reactance.

6.jpg

Figure 2.20 Active and reactive power transfer between the generator and the VSC. (a)Equivalent circuit diagram; (b) phasor diagram (Kundur, 1994)

The active power, PG, and reactive power, QG, transferred from the generator to the VSC are defined as follows (Kundur, 1994):

7.jpg(2.11)

8.jpg(2.12)

If the load angle δ is assumed to be small, then sin δ≈ δ and cos δ ≈ 1. Hence Eqs (2.11) and (2.12) can be simplified to

9.jpg(2.13)

10.jpg(2.14)

From Eqs (2.13) and (2.14), it is seen that the active power transfer depends mainly on the load angleδand the reactive power transfer depends mainly on the difference in voltage magnitudes. As VVSC and δ can be controlled independently of the generator voltage, the VSC control facilitates control of the magnitude and the direction of the active and reactive power flow between the generator and the DC link. Similarly, as the other VSC is connected to the grid via a reactor or via a transformer, the power transfer between that VSC and the grid can also be described using the same principle.

Even though the active and reactive power transfers can be controlled by controlling VVSC and δ, in practice to control the power transfer other parameters may be used. For example, by maintaining the DC link voltage constant, it is possible to make sure that the generated power is transferred to the grid.

The main advantages of using back-to-back VSCs include the following: (a) it is a well-established technology and has been used in machine drive-based applications for many years; (b) many manufacturers produce components especially designed for this type of converter; and (c) the decoupling of the two VSCs through a capacitor allows separate control of the two converters.

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