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...

2

6.4.2 Influence of rotational speed on blade weight The effect of rotational speed on blade weight can be explored with reference to the family of blade designs just described. As in Section 6.3.1, it is assumed that the blade design is governed by out-of-plane bending moments in fatigue and that the moment fluctuations are proportional to the product of the wind speed fluctuation, the rotational speed and the chord scaling factor (see Equation (5.25) in Section 5.7.5). By Equation (6.8) the...

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,...

Info

For a circular tower, the stress ranges would have to be computed at several points around the circumference in order to identify the location with respect to the nacelle axis where the fatigue damage was maximum. A simpler but potentially cruder approach to the combination of the two fatigue spectra is to use the 'Damage Equivalent Load' method. This involves the calculation of constant amplitude fatigue loadings, MX.Del and My.Del, of, say 107 cycles each, that would respectively produce the...

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

Van Der Hoven Spektrum

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...