## 951 Impact on birds

The impact of wind energy developments on birds was particularly controversial in the 1990s because of concern over raptors colliding with wind turbines in the USA and in the south of Spain at Tarifa. Elsewhere in Europe the main concern was not collisions of birds with turbines but issues associated with disturbance and habitat loss (Colson, 1995, ETSU, 1996). Since then, there has been very considerable research into the impact of wind farms on birds and it would appear that by careful...

## C

Tangential flow induction factor at the blade tip two-dimensional lift curve slope, (dCi da) constant defining magnitude of structural damping upstream and downstream stream-tube cross-sectional areas face width of gear teeth blade chord Weibull scale parameter damping coefficient per unit length generalized damping coefficient with respect to the ith mode decay constant Theodorsen's function, where v is the reduced frequency C(v) F(v) + iG(v) sectional drag coefficient sectional force...

## 1052 Harmonics

Only variable-speed wind turbines inject significant harmonic currents into the network. Fixed-speed wind turbines, particularly those with power-factor correction capacitors, alter the harmonic impedance of the distribution network and, in some circumstance, create resonant circuits. This may be important if fixed- and variable-speed wind turbines are installed in the same wind farm. It is noted in IEC (2000b) that harmonic currents have been reported from a few installations of fixed-speed,...

## 425 Pitch regulation

Many of the shortcomings of fixed pitch passive stall regulation can be overcome by providing active pitch angle control. Figure 4.9 shows the sensitivity of power output to pitch angle changes. The most important application of pitch control is for power regulation but pitch control has other advantages. By adopting a large positive pitch angle a large Wind speed (m s) Figure 4.9 Effect on Extracted Power of Blade Pitch Set Angle Wind speed (m s) Figure 4.9 Effect on Extracted Power of Blade...

## 825 Influence of the controller on loads

As well as regulating the turbine power in high winds, and perhaps optimizing it in low winds, it is clear that the action of the control system can have a major impact on the loads experienced by the turbine. The design of the controller must take into account the effect on loads, and at least ensure that excessive loads will not result from the control action. It is possible to go further than this, and explicitly design the controller with the reduction of certain loads as an additional...

## 1 2 2 rU2 Ad

A problem arises for values of a > because the wake velocity, given by (1 2a)U1, becomes zero, or even negative in these conditions the momentum theory, as described, no longer applies and an empirical modification has to be made (Section 3.5). The variation of power coefficient and thrust coefficient with a is shown in Figure 3.3. The manner in which the extracted energy is converted into usable energy depends upon the particular turbine design. Most wind energy converters employ a rotor...

## Acknowledgements

A large number of individuals have assisted the authors in a variety of ways in the preparation of this work. In particular, however, we would like to thank David Infield for providing some of the content of Chapter 4, David Quarton for scrutinising and commenting on Chapter 5, Mark Hancock, Martin Ansell and Colin Anderson for supplying information and guidance on blade material properties reported in Chapter 7, and Ray Hicks for insights into gear design. Thanks are also due to Roger Haines...

## 1072 Reactive power charges and voltage control

Equation (10.8) also shows that it is desirable to operate as near to unity power factor as possible (i.e., q 0) if network losses are to be minimized. This preference to reduce reactive power flow is reflected in tariffs applied by many distribution companies to charge customers either for reactive energy (kVArh) or for the peak reactive power (kVAr) drawn during a period. Such tariffs are usefully applied to loads where both real and reactive power flows are in the same direction and so any...

## 95 Ecological Assessment

Wind farms will often be constructed in areas of ecological importance and the Environmental Statement will include a comprehensive assessment of the local ecology, its conservation importance, the impact of the wind farm (during construction and operation) and mitigation measures. A study of the site hydrology is also likely to be included because of its importance for the ecology. It is suggested by English Nature (1994) that when considering the ecological impact of renewable energy schemes...

## 1046 Connection of embedded wind generation

Distribution utilities have an obligation to operate the electrical distribution networks in such a way as to provide power to their customers at an agreed quality. At present, power quality requirements are based on national standards although a common European position is emerging described in British Standard (1995b). However, it should be noted that this document describes minimum standards of supply which a customer may expect and is not directly applicable to the connection of embedded...

## 551 Synthesis of fatigue load spectrum

The complete fatigue load spectrum for a particular wind turbine component has to be built up from separate load spectra derived for turbine operation at different wind speeds and from the load cycles experienced at start-up, normal and emergency shut-down and while the machine is parked or idling. First the cycle counts for each stress range for 1 h operation in a particular wind speed band are calculated and scaled-up by the predicted number of hours of operation in that band over the machine...

## H

Which is close to unity because the tower head mass dominates the integral. (5) Calculate the size reduction factor for the root bending moment quasistatic or background response, -Ksmb , which reflects the lack of correlation of the wind fluctuations along the blades and tower. Ksmb may be derived from a similar expression to that for the resonant size reduction factor given in Equation (5.125), but with the exponential function modified to exp s 0.3LJj .

## 1

Therefore, in terms of parameter p, from Equation (3.192) rf Rf 2)irUi (3 198) It should be noted that the axial acceleration distribution is axi-symmetric and independent of the yaw angle. The mean value of axial acceleration over the area of the disc is The non-dimensional form of the acceleration can be expressed as 97 Ur dt 256 Ctd (3 200)

## M rX c rX r d r2

Where the terms on the left-hand side are the loads on the element due to inertia, damping and flexural stiffness respectively. I( r) is the second moment of area of the blade cross section about the weak principal axis (which for this purpose is assumed to lie in the plane of rotation) and x is the out-of-plane displacement. The expressions m( r) and c( r) denote mass per unit length and damping per unit length respectively. The dynamic response of a cantilever blade to the fluctuating...

## 02 04 06 08

Figure 3.42 Angle of Attack Distribution for a Range of Tip Speed Ratios flow factor decreases with tip speed ratio. The angular velocity of the wake increases sharply with decreasing radius because it is determined by the root vortex. The importance of the outboard section of the blade is clearly demonstrated in Figure 3.44. The dramatic effect of stall is shown in the difference in torque distribution between the tip speed ratio of 4 and the tip speed ratio of 2. Note, also,

## Dc

Figure 5.43 Rotor Thrust During Operation in Steady, Uniform Wind Variation with Wind Speed for Similar Stall-regulated and Pitch Regulated Machines Figure 5.43 Rotor Thrust During Operation in Steady, Uniform Wind Variation with Wind Speed for Similar Stall-regulated and Pitch Regulated Machines and the perturbation of rotor thrust by AT ( pQ )wc( r) r d r (5.128) where the integral sign signifies that the integration is carried out over the whole rotor. Hence the following expression for the...

## 766 Lowspeed shaft brake design

The procedure for designing a low-speed shaft disc brake is much simpler than that for the high-speed shaft brake, because the limits on disc-rim speed, pad-rubbing speed, power dissipation per unit area and temperature rise do not influence the design, which is solely torque driven. The large braking torque required means that a brake placed on the low-speed shaft will be much bulkier than one with the same duty placed on the low-speed shaft. For example the design LSS braking torque of 1800...

## Info

The ratio of the steady moment at radius r* to that at the root is Jr* c(r) r _ r* dr JQR c(r)r dr, so the ratio of the standard deviation of the quasistatic fluctuations at radius r* to the steady value there is amb(r*) _ amb(r*) amb(0) M(0) amb(0) Ksmb(r*) (A5 59) M( r*) a mb (0) M(0) M( r*) M(0) V Ksmb(0) ( ' ) Generally, the square root will be close to unity, so a MB (r*) M( r*) will be nearly constant. In Section A5.5 it was shown that the standard deviation of the first mode resonant...

## W

Ul sin2 y + (Ui cos y - u)2 (3.104) Thrust is equal to the mass flow rate times the change in velocity in the direction of the thrust. Both F and u are assumed to be normal to the plane of the disc. The thrust coefficient is then The power developed is a scalar quantity and so is the scalar product of the thrust force and the resultant velocity at the disc W. Hence, the power coefficient is However, as some of the thrust is attributable to lift on the rotor disc acting as a circular wing that...

## Wind Turbine Installations and Wind Farms

For any wind turbine installation, there are certain additional activities (e.g., construction of foundations and access roads, electrical connections, site erection, as well as project development and management) that must be undertaken. For flat onshore sites, which might be found typically in Denmark or North Germany, the total investment cost is approximately 1.3 times the ex-works turbine cost (EUREC Agency, 1996). In the UK, where sites are often located in more remote, upland areas the...

## 49 Turbulence Effects

Because extended averaging periods are used in power performance testing (usually 10 min) a significant amount of wind energy is contained in the turbulence. Let the wind speed, U, at any instant be composed of a mean value U and the fluctuation U* about U. Then Energy is proportional to the cube of the wind speed and since we are interested in the mean energy, an expression is wanted for the mean cube wind speed, denoted U3. From Equation (4.10) 0 2 4 6 8 10 12 14 16 18 20 22 Figure 4.22...

## 612 Drivetrain Compliance

The rotational dynamics of the drive train can have a major effect on loading. The effect is very different in fixed- and variable-speed turbines, but in each case the consequence of ignoring drive-train dynamics at the design stage can be very severe. Figure 6.20 Direct-drive Generator Arrangement Figure 6.20 Direct-drive Generator Arrangement Figure 6.21 Integrated Gearbox on the Zond Z-750 turbine. (The gearbox is mounted on a circular nacelle bedplate, with the hub to the left and generator...

## 42 Constant Rotational Speed Operation

The majority of wind turbines currently installed generate electricity. Whether or not these turbines are grid connected they need to produce an electricity supply which is of constant frequency or else many common appliances will not function properly. Consequently, the most common mode of operation for a wind turbine is constant rotational speed. Connected to the grid a constant speed turbine is automatically controlled whereas a stand-alone machine needs to have speed control and a means of...

## 04123

For aboor > fy 2 and ou 0.75abocr for aboor < fy 2 The buckling strength of a mild-steel cylinder in bending as a fraction of the yield strength is plotted against the radius to wall-thickness ratio for imperfection ratios (w L) of 1 percent and 2 percent on Figure 7.39. The effect of the choice of tower base diameter on total tower weight is best illustrated by reference to a concrete example. Consider the design of a 50 m hub Figure 7.39 Buckling Strength in Bending of Thin-walled...

## M1

The expression in square brackets corresponds to the formula for the impact factor j in Annex B of DS 472 It is often necessary to express the maximum moment in terms of the quasistatic moment due to the 50 year return gust speed, Ue50. In order to do this, we equate the latter quantity to the quasistatic component of Equation (5.13), obtaining c( r) r d r M 1 + g0 . 2 OjV SMB) (5 . 15) Here the peak factor, g takes a lower value, g0, corresponding to the lower frequency of the quasistatic root...

## 6102 Directdrive generators

There is considerable interest in the application of generators driven directly by the wind-turbine rotor without a speed increasing gearbox and a number of manufacturers offer such wind turbines. However, the power output of any rotating electrical machine may be generally described by (Laithwaite and Freris, 1980) where D is the rotor diameter, L is the length, n is the rotational speed, and K is a constant. Thus it may be seen that if the rotational speed is reduced then it is necessary...

## 917 Preparation and submission of the planning application

Wind farms are recognized as having significant environmental impact and it is usual for an Environmental Statement to be required as a major part of the application for planning permission. The preparation of the Environmental Statement is an expensive and time-consuming undertaking and usually requires the assistance of various specialists. The purpose of a wind farm Environmental Statement may be summarized to describe the physical characteristics of the wind turbines and their land-use...

## T

Which is the area under the curve of Figure 4.17 times the time period T. Unfortunately, the integral does not have a closed mathematical form in general and so a numerical integration is required, such as the trapezoidal rule or, for better accuracy, Simpson's rule. For a time period of 1 year T 365 X 24 h. Therefore, for the 10 data points shown in Figure 4.18, the energy capture will be, using the trapezoidal rule, E 0.98T (Pmf (ui+i) + Ptf (ui)) (U+12 U) (4.8) Even though the upper limit of...

## 41 The Performance Curves

The performance of a wind turbine can be characterized by the manner in which the three main indicators power, torque and thrust vary with wind speed. The power determines the amount of energy captured by the rotor, the torque developed determines the size of the gear box and must be matched by whatever generator is being driven by the rotor. The rotor thrust has great influence on the structural design of the tower. It is usually convenient to express the performance by means of...

## 92 Visual and Landscape Assessment

Modern wind turbines are large structures, sometimes over 100 m to the blade tip, and must be sited in exposed locations of high mean wind speed to operate effectively. The individual turbines must be spaced at at least 3-5 rotor diameters and hence large wind farms are extensive. There is generally a trade-off between energy yield and visibility of wind turbines and so visual and landscape assessment is a key aspect of the Environmental Statement required for any wind-farm development. The...

## 2

The form of the resulting normalized cross-correlation function, pU(r1, r2, r) k0u(r1, r2, r) o 1U, is illustrated in Figure 5.18 for the case considered in Example 5.2, taking r1 10 m and r2 20 m. In Figure 5.21, the rotationally sampled cross spectrum for this case is compared with the rotationally sampled single point spectra or 'autospectra' at these radii. It can be seen that the form of the cross spectrum curve is similar to that of the autospectra with a pronounced peak at the rotational...

## 424 Effect of blade pitch angle change

Another parameter which affects the power output is the pitch setting angle of the blades fis. Blade designs almost always involve twist but the blade can be set at the root with an overall pitch angle. The effects of a few degrees of pitch are shown in Figure 4.9. Small changes in pitch setting angle can have a dramatic effect on the power output. Positive pitch angle settings increase the design pitch angle and so decrease the angle of incidence. Conversely, negative pitch angle settings...

## 103 Lightning Protection

It is now recognized that lightning is a significant potential hazard to wind turbines and that appropriate protection measures need to be taken. Some years ago, it was thought that as the blades of wind turbines are made from non-conducting material (i.e., glass reinforced plastic (GRP) or wood-epoxy) then it was not necessary to provide explicit protection for these types of blades provided they did not include metallic elements for the operation of devices such as tip brakes. However, there...

## R

P (ri, r2, 0)c(ri)c(r2) n r2 n r2 dridr2 In the case of 40 m diameter turbines with TR blades operating in wind with a turbulence length scale of 73.5 m, the standard deviation of shaft moment due to stochastic loading for a three-bladed machine is 82 percent of that for a two-bladed, fixed hub machine rotating at the same speed. This ratio would rise to 3 2 if the effect on the cross correlation function of the 120 blade spacing were ignored. It is worth noting that, for a three-bladed...

## 8wJ1 w2

As before, the pressure in Equation (3.203) is non-dimensionalized by the free-stream dynamic pressure (1 2) . Applying the differential operator given in Equation (3.196) to Equation (3.203), from Equation (3.192) we get at the rotor disc, where w 0, aT CmzDV sin (3.204) In terms of non-dimensional time and velocity Similarly, if there is a tilting moment then the corresponding acceleration is dr I CmyD V cos (3.206) The radial variation is linear and so no linearization adjustment is...

## Conceptual Design of Horizontal Axis Turbines

Within the general category of horizontal axis wind turbines for grid applications there exists a great variety of possible machine configurations, power control strategies and braking systems. This chapter looks at the different areas where design choices have to be made, and considers the advantages and disadvantages of the more conventional options in each case. Inevitably there are situations in which decisions in one area can impact on those in another, and some of these are noted....

## 32

In which the pressure is normalized by (1 2) . The term Ctd is the contribution to the total thrust coefficient of the dynamic acceleration u t. Note that, as explained at the end of Section 3.11.1, the pressure level just upwind of the rotor disc, as given by Equation (3.197), is half the magnitude of the pressure drop across the disc given by Equation (3.166). By means of Equation (3.195), at the rotor plane, where y 0, the pressure gradient is found to be

## 932 Windturbine noise

Noise from wind turbines is partly mechanical, and partly aerodynamic. Mechanical noise is generated mainly from the rotating machinery in the nacelle particularly the gearbox and generator although there may also be contributions from cooling fans, auxiliary equipment (such as pumps and compressors) and the yaw system. Mechanical noise is often at an identifiable frequency or tone (e.g., caused by the meshing frequency of a stage of the gearbox). Noise containing discrete tones is more likely...

## 471 Field testing methodology

Although a few years ago, the International Energy Agency (IEA) recommended practices for wind turbine testing which were the nearest thing to an agreed procedure for wind turbine evaluation. Now, as mentioned, an IEC standard is available IEC 61400-12 Wind-turbine Generator Systems, Part 12 - Wind-turbine Power Performance Testing (1998). It is interesting to contrast it with Volume 1 of the IEA recommendations (1982) and subsequent editions (1990), which deal with power performance testing....

## A31 Definition of Drag

The drag on a body immersed in an oncoming flow is defined as the force on the body in a direction parallel to the flow direction. In a very slow-moving fluid the drag on a body may be directly attributable to the viscous, frictional shear stresses set up in the fluid due to the fact that, at the body wall, there is no relative motion. This type of flow is known as Stokes' flow after Sir George Stokes. Two centuries before Stokes, Isaac Newton showed that that the shear stress t at a boundary...

## 7

Regulated turbine with the same rating. The rotational speed of the stall-regulated turbine has been reduced in order to limit the power to the same rated level. Therefore, although the stall-regulated turbine generates slightly more energy at very low wind speeds, as the blades approach stall above 8 m s there is a large loss of output compared to the pitch-regulated machine. (In practice of course, if the turbine was designed to operate in stall, the blade design, solidity and rotor speed...

## 6112 Highspeed shaft and generator mounting

The generator is normally mounted to the rear of the gearbox on an extension of the nacelle bedplate and the connecting drive shaft - the 'high speed shaft' - is fitted with flexible couplings at each end, to cater for small misalignments between the generator and gearbox. The generator axis is normally offset from the low-speed shaft axis. This is because, except in the case of machines fitted with a mechanical brake acting on the rotor, access is required to the rear end of the low-speed...

## 749Integrated gearboxes

As noted in Section 6.11.1, the cases of integrated gearboxes must be very robust, in order to transmit the rotor loads to the nacelle structure without experiencing deflections which would impair the proper functioning of the gears. In view of the complex shape of the casing, stress distributions due to each load vector usually have to be determined using finite-element (FE) analysis - these can then be superposed in line with the different extreme load combinations. The fatigue analysis will...

## 16

The maximum achievable value of the power coefficient is known as the Betz limit after Albert Betz the German aerodynamicist (119) and, to date, no wind turbine has been designed which is capable of exceeding this limit. The limit is caused not by any deficiency in design, for, as yet, we have no design, but because the stream-tube has to expand upstream of the actuator disc and so the cross section of the tube where the air is at the full, free-stream velocity is smaller than the area of the...

## 837 Variablespeed stall regulation

Figure 8.6 shows two power curves for the same rotor, one running as a 600 kW fixed-speed pitch-regulated turbine and one adjusted to run as a fixed-speed stall- Figure 8.6 shows two power curves for the same rotor, one running as a 600 kW fixed-speed pitch-regulated turbine and one adjusted to run as a fixed-speed stall-

## Qrt mrTJrMt crTJrfT t

Figure 5.31 Fundamental Mode Shapes of Blade and Tower Figure 5.31 Fundamental Mode Shapes of Blade and Tower where the coupled terms have been transferred to the right hand side. Multiplying through by r) and integrating over the length of the blade gives mi fj( t) + ci j( t) + mi 2 j( t) By analogy with Equation (5.70), the equation of motion of the tower is mri t( t) + cti t( t) + mTi T( t) Here ,mt is the tower first mode shape and mT1 is the generalized mass of the tower, nacelle and rotor...

## Ct

P0 . 214 + 0 . 1- -4m(1 - PO . 134 + 0 . 124m) (1 V0.214 + 0.144mV0.134 + 0.124m is the growth rate contribution due to ambient turbulence, dxj m (1 + m)9.76 is the contribution due to shear-generated turbulence, and is the contribution due to mechanical turbulence, where B is the number of blades and is the tip speed ratio. Deep inside a wind farm, the reduction in wind speed and the increase in turbulence intensity are the result of the superposition of wakes from many upwind turbines....

## 824 Yaw control

Turbines whether upwind or downwind, are generally stable in yaw (Section 3.10) in the sense that if the nacelle is free to yaw, the turbine will naturally remain pointing into the wind. However, it may not point exactly into wind, in which case some active control of the nacelle angle may be needed to maximize the energy capture. Since a yaw drive is usually required anyway, e.g. for start-up and for unwinding the pendant cable, it may as well be used for active yaw tracking. Free yaw has the...

## MyCFr mrq2r[pr fir dr5

(6) calculate combined bending moment distribution (7) calculate new deflected profile resulting from this bending moment distribution (8) calculate revised estimate of natural frequency from Tip deflection calculated for new deflected profile (9) repeat steps (2)-(8) with revised mode shape and frequency until calculated mode shape converges. It is important to note that the lateral loads and deflections of a centrifugally loaded beam do not conform to Betti's Law, so, as a consequence, the...

## Ervin Bossanyi

Garrad Hassan & Partners, Bristol, UK Chichester New York Weinheim Brisbane Singapore Toronto Copyright 2001 by John Wiley & Sons, Ltd Baffins Lane, Chichester West Sussex, PO19 1UD, England National 01243 779777 International (+44) 1243 779777 e-mail (for orders and customer service enquiries) cs-books wiley.co.uk Visit our Home Page on http www.wiley.co.uk or http www.wiley.com All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system, or...

## Smb

Where KsMB is a size reduction factor to take account of lack of correlation of wind fluctuations along the blade. As shown in Section A5.6 of the Appendix, KsMB is usually only slightly less than unity because the blade length is small compared with the integral length scale of longitudinal turbulence measured in the across wind direction. The variance of the total root bending moment fluctuations is equal to the sum of the resonant and background response variances, i.e.,

## 714 Form of blade structure

A hollow shell corresponding to the defined blade envelope clearly provides a simple, efficient structure to resist flexural and torsional loads and some blade manufacturers adopt this form of construction (see Figure 7.1). However, in the case of small and medium size machines, where the out-of-plane loads dominate, there is greater benefit in concentrating skin material in the forward half of the blade, where the blade thickness is a maximum, so that it acts more efficiently in resisting...

## 1042 Embedded generation

In the early days of electricity supply, each town or city had its own generating station supplying the local load. Thus all generation was local and embedded into the distribution networks. This arrangement suffered from two major problems (1) the generating sets were rather small and hence of low efficiency, and (2) each station had to keep an additional generator running in case of breakdowns. Then in the 1930s it was found to be technically possible and cost-effective to interconnect these...

## Extreme loading during operation pitchregulated machines

The characterization of extreme operational loadings on pitch-regulated machines is inevitably more complicated than for stall-regulated machines, although at the same time it should be more accurate because of the avoidance of uncertainties associated with stall. It is instructive to focus comparisons on the blade bending moment about the weak axis at 60 percent radius once again. This time it is referred to as the flapwise bending moment rather than the out-of-plane (of rotation) moment...

## 8

Figure 5.7 Blade In-plane and Out-of-plane Aerodynamic Bending Moment Distributions for Typical 40 m Diameter Stall-regulated Machine Operating in a Steady, Uniform 10 m s Wind Figure 5.7 Blade In-plane and Out-of-plane Aerodynamic Bending Moment Distributions for Typical 40 m Diameter Stall-regulated Machine Operating in a Steady, Uniform 10 m s Wind aerodynamic blade bending moments. The variation of these moments with radius is shown in Figure 5.7 for the example above. The blade bending...

## 835 Control of tower vibration

For both fixed and variable-speed machines the influence of the pitch controller on tower vibration and loading, described in Section 8.2.1, is one of the major constraints on the design of the control algorithm. The first tower fore-aft vibra-tional mode is essentially very lightly damped, exhibiting a strong resonant response which can be maintained at quite a high level even by a small amount of excitation which is naturally present in the wind. The strength of the response depends...

## 921 Landscape character assessment

The fundamental step in minimizing the visual impact of a wind farm is to identify an appropriate site and ensure that the proposed development is in harmony with the location. Many exposed upland areas are likely to be of high amenity and have been designated as areas of significant landscape value or even as National Parks. Figure 9.3 Windfarm of Six 660 kW Turbines in Flat Terrain (Reproduced by permission of Cumbria Wind Farms Ltd, Paul Carter) Figure 9.3 Windfarm of Six 660 kW Turbines in...

## 913The Measurecorrelatepredict technique

The MCP approach is based on taking a series of measurements of wind speed at the wind farm site and correlating them with simultaneous wind speed measurements made at a meteorological station. The averaging period of the site-measured data is chosen to be the same as that of the meteorological station data. In its simplest implementation, linear regression is used to establish a relationship between the measured site wind speed and the long-term meteorological wind speed data of the form Usite...

## References

C. and Hancock, M., (1991). 'Feasibility study of teetered, stall-regulated rotors'. ETSU Report No. WN 6022. Bossanyi, E. A. and Gamble, C. R., (1991). 'Investigation of torque control using a variable slip induction generator' ETSU WN-6018, Energy Technology Support Unit, Harwell, UK. Burton, A. L., Mill, P. W. and Simpson, P. B., (1990). 'LS1 post-synchronization commissioning'. Proceedings of the 12th BWEA Conference, pp 183-193. Mechanical Engineering Publications, Bury St...

## 792 Constraints on firstmode natural frequency

As noted in Section 6.14, it is important to avoid the excitation of resonant tower oscillations by rotor thrust fluctuations at blade-passing frequency or, to a lesser extent, at rotational frequency. Dynamic magnification impacts directly on fatigue loads, so the further the first-mode tower natural frequency is from the exciting frequencies, the better. Unfortunately, it is generally the case that the natural frequency of a tower designed to be of adequate strength for extreme loads is of...

## 0 577

Here, v, is the mean zero-upcrossing frequency (i.e., the number of times per second the moment fluctuation changes from negative to positive) of the root bending moment fluctuations, which will be intermediate between that of the quasistatic wind loading and the blade natural frequency, n1 (see Section A5.7 of the Appendix). (Note that, as g varies relatively slowly with frequency, it is a reasonable approximation to set g at an upper limit of 3.9, which corresponds to a frequency of about 1.9...

## 963 Support mechanisms for wind energy

Historically, electrical energy from wind turbines was not competitive in commercial markets with other forms of generation, particularly the use of a combined cycle gas turbine (CCGT) plant burning natural gas. Hence, in order to take account of external costs, and to meet commitments to reduce CO2 emissions, various support mechanisms have been used by governments to encourage the development of wind power as well as other forms of renewable energy. These support mechanisms, together with the...

## 15

Figure 3.45 Axial Force Coefficient and the Variation of the Actual Force with Wind Speed remembered that the actual thrust force increases with wind speed as is demonstrated in Figure 3.45. 3.10 The Aerodynamics of a Wind Turbine in Steady The rotor axis of a wind turbine rotor is usually not aligned with the wind because the wind is continuously changing direction the rotor is not capable of following this variability and so spends most of its time in a yawed condition. The yawed rotor is...

## A311 Cambered Aerofoils

Cambered aerofoils, such as the NACA4412, shown in Figure A3.23, have curved chord lines and this allows them to produce lift at zero angle of attack. Generally, cambered aerofoils have higher maximum lift drag ratios than symmetrical aerofoils for positive angles of attack and this is the reason for their use. The classification of the NACA four-digit range of aerofoils, which were commonly used on wind turbines, is very simple and is illustrated in Figure A3.24 from left to right, the first...

## 1061 Wind farm and generator protection

Figure 10.20 shows a typical protection arrangement for a wind farm of fixed-speed wind turbines with generator voltages of 690 V and with a collection circuit voltage of 11 kV. The 11 kV circuit is fed from a 33 11 kV Delta Star wound transformer with the 11 kV neutral grounded either directly or through a resistor. The 11 0.69 kV transformers are also wound Delta Star and so the 690 V neutral points of each circuit may be directly grounded. The neutral point of the generators is not connected...

## 94 Electromagnetic Interference

Wind turbines have the potential to interfere with electromagnetic signals that form part of a wide range of modern communication systems and so their siting requires careful assessment in respect of electromagnetic interference (EMI). In particular, wind energy developments often compete with radio systems for hilltops and other open sites that offer high energy outputs from wind farms and good propagation paths for communication signals. The types of system that may be affected by EMI, and...

## 1073 Connection charges deep and shallow

When a wind farm owner wishes to connect a project to the distribution network there are clearly costs associated with doing so and it is entirely reasonable that all the appropriate connection costs are borne by the wind farm. This is a similar situation to the connection of any large load. There are two main philosophies in charging for connection of either generation or load to a power system, i.e., 'Deep' or 'Shallow' charging. In deep charging the embedded wind-farm project will pay for...

## 5

The spectrum of turbulence describes the frequency content of wind-speed variations. According to the Kolmogorov law, the spectrum must approach an asymptotic limit proportional to n 5 3 at high frequency (here n denotes the frequency, in Hz). This relationship is based on the decay of turbulent eddies to higher and higher frequencies as turbulent energy is dissipated as heat. Two alternative expressions for the spectrum of the longitudinal component of turbulence are commonly used, both...

## 16461

Courtesy of Windpower Monthly News Magazine Courtesy of Windpower Monthly News Magazine constructed in the 1980s and are now being re-equipped with larger modern wind turbines. Table 1.1 shows the installed wind-power capacity worldwide in January 2001 although it is obvious that with such a rapid growth in some countries data of this kind become out of date very quickly. The reasons development of wind energy in some countries is flourishing while in others it is not fulfilling the potential...

## A35 Laminar and Turbulent Boundary Layers

A boundary layer grows in thickness from the forward stagnation point, or leading edge. Initially, the flow in the layer is ordered and smooth (laminar) but, at a critical distance I from the stagnation point, characterized by Recrit pUI i, the flow begins to become turbulent (Figure A3.8). This turbulence causes mixing of the boundary layer with the faster moving fluid outside resulting in re-energization and delaying of the point of separation. The result is to reduce the pressure drag,...

## 717 Properties of wood laminates

Although laminated wood epoxy is classed as a composite, it is markedly different in form from GFRP. Individual plies are made up of large sheets of wood veneer (Figure 7.7) instead of a multiplicity of fibres laid up in a matrix, and the epoxy behaves as an adhesive rather than a matrix, bonding the sheets together at the longitudinal and transverse joints and bonding each ply to its neighbour. Thus the fibre volume fraction is close to 100 percent and the anisotropic properties of the wood...

## 76 Mechanical Brake 761 Brake duty

As indicated in Section 6.8.3, a mechanical brake can be called on to fulfil a variety of roles, according to the braking philosophy adopted for the machine in question. The minimum requirement is for the mechanical brake to act as a parking brake, so that the machine can be stopped for maintenance purposes. The brake will also be used to bring the rotor to a standstill during high wind shut-downs for the majority of machine designs, and during low-speed shut-downs as well in some cases....

## Extreme loading at standstill

The derivation of stationary blade loads is described in Section 5.6. Figure 7.12 shows the out-of-plane bending moment distribution for a TR blade under the action of a 60 m s wind, corresponding to the 50 year return extreme 3 s gust specified for a Class II wind turbine in IEC 61400-1. A uniform lift coefficient of 1.5 is assumed. Two curves are shown the lower curve is the quasistatic bending moment while the upper one incorporates dynamic magnification due to excitation of resonant...

## 923 Assessment of impact

A major part of the Environmental Statement is the assessment of visual impact. Two main techniques are used (1) visibility analysis using zones of visual impact (ZVI), and (2) viewpoint analysis using wire frames and photomontages. Zones of visual impact show those areas of the surrounding country, usually up to 10-20 km radius, from which a wind turbine, or any part of a wind turbine, in a wind farm is visible. The ZVI is generated using computer methods based on a digital terrain model and...

## Contents

1.1 Historical Development 1 1.2 Modern Wind Turbines 6 1.3 Scope of the Book 7 References 8 Bibliography 9 2.1 The Nature of the Wind 11 2.2 Geographical Variation in the Wind Resource 12 2.3 Long-term Wind-speed Variations 13 2.4 Annual and Seasonal Variations 14 2.5 Synoptic and Diurnal Variations 16 2.6.1 The nature of turbulence 17 2.6.2 The boundary layer 18 2.6.3 Turbulence intensity 21 2.6.4 Turbulence spectra 22 2.6.5 Length scales and other parameters 23 2.6.6 Cross-spectra and...

## 611 Drivetrain Mounting Arrangement Options 6111 Lowspeed shaft mounting

The functions of the low-speed shaft are the transmission of drive torque from the rotor hub to the gearbox, and the transfer of all other rotor loadings to the nacelle structure. Traditionally the mounting of the low-speed shaft on fore and aft bearings has allowed these two functions to be catered for separately the gearbox is hung on the rear end of the shaft projecting beyond the rear bearing and the drive torque is resisted by a torque arm. The front bearing is positioned as close as...

## 500

Figure 6.7 Power Curves for Different Pitch Angles 40 m Diameter Rotor Rotating at 33 r.p.m. A variety of pitch actuation systems have been adopted (see also Section 8.5). They are divided between those in which each blade has its own actuator and those in which a single actuator pitches all the blades. The former arrangement has the advantage that it provides two or three independent aerodynamic braking systems to control overspeed, and the disadvantage that it requires very precise control of...

## U

Where a is the standard deviation of wind speed variations about the mean wind speed U, usually defined over 10 min or 1 h. Turbulent wind speed variations can be considered to be roughly Gaussian, meaning that the speed variations are normally distributed, with standard deviation a, about the mean wind speed U. However, the tails of the distribution may be significantly non-Gaussian, so this approximation is not reliable for estimating, say, the probability of a large gust within a certain...

## Back Scattering

Figure 9.14 Interference Mechanisms of Wind Turbines with Radio Systems Rees, 1989) provide an analysis of the electromagnetic interference caused by a wind turbine. The useful carried signal received, C, is given by where PT is the transmitter power (dB), ATR is the attenuation between the transmitter and receiver (dB), and GTR is the receiver antenna gain in the direction of the required signal (dB). The interfering signal, I, is given by I Pt - Atw + 10 log 10 4_) - Awr + Gwr (9.23) where...

## 762 Factors governing brake design

The braking torque provided by callipers gripping a brake disc (Figure 7.34) is simply the product of twice the calliper force, the coefficient of friction (typically 0.4), the number of callipers and the effective pad radius. Callipers providing clamping forces of up to 500 KN are available. However, the brake design is also limited by centrifugal stresses in the disc, power dissipation per unit area of pad, and The nature of these constraints is described below. The critical stress generated...

## 682 Aerodynamic brake options

Blade pitching to feather (i.e., to align the blade chord with the wind direction) provides a highly effective means of aerodynamic braking. Blade pitch rates of 10 per second are generally found adequate, and this is of the same order as the pitch rate required for power control. The utilization of the blade pitch system for startup and power control means that it is regularly exercised with the result that the existence of a dormant fault is highly unlikely. In machines relying solely on...

## 836 Control of drive train torsional vibration

A typical drive train can be considered to consist of a large rotor inertia and a (smaller) high-speed shaft inertia (mainly the generator and brake disc), separated by a torsional spring which represents twisting of shafts and couplings, bending of gear teeth and deflection of any soft mountings. Sometimes it is important to consider also the coupling of the torsional mode of vibration with the first rotor inplane collective mode, in which case the drive train can be approximated by three...

## 512 Iec 614001

IEC 61400-1 Wind turbine generator systems - Part 1 Safety Requirements identifies four different classes of wind turbines to suit differing site wind conditions, with increasing class designation number corresponding to reducing wind speed. The wind speed parameters for each class are given in Table 5.1. The reference wind is defined as the 10 min mean wind speed at hub-height with a 50 year return period. To allow for sites where conditions do not conform to any of these classes, a fifth...

## Root bending moment

The standard deviation of tip displacement in combination with the blade mode shape yields an inertial loading distribution from which the standard deviation of the resulting bending moment at any position along the blade may be calculated. In particular, the standard deviation of the root bending moment may be expressed in terms of the mean root bending moment as follows -n -7 VRu niWKsx ni mi 0x1 Mi M U V2d xi See Section A5.5 in the Appendix for the derivation of the expression for mi. The...

## 514 Danish Standard DS 472

DS 472 bases the derivation of design-extreme wind speeds on four terrain classes, ranging from the very smooth expanses of water to the very rough e.g., built-up areas . The base wind velocity is taken to be the same all over Denmark, so the result is four alternative profiles of wind speed variation with height. The philosophy behind the selection of design load cases in the Danish standard is similar to that in IEC-1400 and the GL rules, although the number of load cases is fewer. Similarly,...

## 102 Earthing Grounding of Wind Farms

All electrical plant require a connection to the general mass of earth in order to minimize shock hazards to personnel and animals, establish a low-impedance path for earth-fault currents and hence satisfactory operation of protection, improve protection from lightning and retain voltages within reasonable limits, and prevent large potential differences being established which are potentially hazardous to both personnel and equipment. In the UK this subject is referred to as 'earthing' while in...

## 421 The Kp 1 curve

An alternative performance curve can be produced for a turbine controlled at constant speed. The CP - curve shows, non-dimensionally, how the power would vary with rotational speed if the wind speed was held constant. The KP - 1 curve describes, again non-dimensionally, how the power would change with wind speed when constant rotational speed is enforced. KP is defined as The CP - and KP - 1 curves for a typical fixed-pitch wind turbine are shown in Figure 4.6. The KP - 1 curve, as stated...

## The Wind Resource

The energy available in the wind varies as the cube of the wind speed, so an understanding of the characteristics of the wind resource is critical to all aspects of wind energy exploitation, from the identification of suitable sites and predictions of the economic viability of wind farm projects through to the design of wind turbines themselves, and understanding their effect on electricity distribution networks and consumers. From the point of view of wind energy, the most striking...

## Wind Turbine Concept In Civil Engineering

Windmills have been used for at least 3000 years, mainly for grinding grain or pumping water, while in sailing ships the wind has been an essential source of power for even longer. From as early as the thirteenth century, horizontal-axis windmills were an integral part of the rural economy and only fell into disuse with the advent of cheap fossil-fuelled engines and then the spread of rural electrification. The use of windmills or wind turbines to generate electricity can be traced back to the...

## 036

Where si and s are the inter-turbine spacings, normalized by rotor diameter, within a row and between rows. Since this does not apply below hub height, the average added turbulence intensity I is then calculated as However, no consensus has yet emerged on a sufficiently well-validated formula for turbulence intensity within a wind farm for use in wind turbine design calculations. Predicting the turbulence intensity and spectrum at a given point within an area of complex terrain is not...