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 - Awr + Gwr (9.23) where ATW is...

References

C., (1959). 'Water turbine driven induction generators', Proc. IEE, Paper No. 3140S. ALSTOM, (1987). 'Protective relays application guide'. ALSTOM Protection and Control, Stafford, UK. ANSI IEEE, (1986). 'IEEE Guide for safety in AC substation grounding'. Standard 80. Bossanyi, E., Saad-Saoud, Z. and Jenkins, N., (1998). 'Prediction of flicker caused by wind turbines', Wind Energy, 1,1, 35-50. British Standard, (1999). 'Code of practice for protection of structures against...

Modelling and prediction of EMI from wind turbines

There are two fundamental interference mechanisms for EMI from wind turbines, back-scattering and forward-scattering (Moglia, Trusszi and Orsenigo, 1996). These are shown in Figure 9.14. Forward-scattering occurs when the wind turbine is located between the transmitter and receiver. The interference mechanism is one of scatter or refraction of the signal by the wind turbine and, for TV signals, it causes fading of the picture at the rotational speed of the blades. Back-scattering occurs when...

Measurement and assessment of power quality characteristics of grid connected wind turbines

Determination of the power quality of wind turbines and prediction of their performance in service is not straightforward and IEC 614200-21 (IEC, 2000b) has been written to provide guidance. There are a number of difficulties when assessing the power quality of wind turbines as their performance will depend on the design of the entire wind turbine (including the aerodynamic rotor and control system), the conditions of the electrical network to which it is connected, and the wind conditions in...

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...

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...

Electrical Protection

All parts of a high-voltage power system are protected by relays which detect abnormal conditions and circuit breakers which then open to isolate the faulty circuits. Some lower-voltage circuits are protected by fuses, as these are cheaper but do not give the degree of control offered by relays and circuit breakers. However, fuses do have the advantage of operating very rapidly and so limiting the energy transferred into the fault. On a distribution network the protection system is designed...

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...

The Method of Acceleration Potential Introduction

An aerodynamic model that is applied to the flight performance of helicopter rotors, and which can also be applied to wind turbine rotors that are lightly loaded, is that based upon the idea of acceleration potential. The method allows distributions of the pressure drop across an actuator disc that are more general than the, strictly, uniform pressure distribution of the momentum theory. The model has been expounded by Kinner (1937), inspired by Prandtl, who has developed expressions for the...

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...

Interface protection

Section 10.6.1 considered the protection of the wind turbines from the effects of insulation failure and subsequent high fault currents, which were predominantly supplied by the distribution network. However, protection is also required to ensure that the wind farm does not feed into faults on the distribution network or attempt to supply an isolated section of network. The problem is illustrated in Figure 10.24. For faults on the network, the difficulty is that wind turbines are not a reliable...

Germanischer Lloyd rules for certification

Germanischer Lloyd's Regulation for the Certification of Wind Energy Conversion Systems, commonly referred to as the GL rules, adopts the same classification of wind turbines as IEC 61400-1, but specifies a single value of hub-height turbulence intensity of 20 percent. A larger number of load cases are specified, but many of them parallel cases in IEC 61400-1. However, the GL rules also provide a simplified fatigue spectrum for aerodynamic loading and simplified design loads for turbines with...

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...

Optimal feedback methods

The controller design methods described above are based on classical design techniques, and often result in relatively simple PI or PID algorithms together with various filters in series or in parallel, such as phase shift, notch or bandpass filters, and sometimes using additional sensor inputs. These methods can be used to design fairly complex high-order controllers, but only with a considerable amount of experience on the part of the designer. There is, however, a huge body of theory (and...

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...

Aerodynamics of Horizontal Axis Wind Turbines

Aerodynamics Wind Turbines

To study the aerodynamics of wind turbines some knowledge of fluid dynamics in general is necessary and, in particular, aircraft aerodynamics. Excellent text books on aerodynamics are readily available, a bibliography is given at the end of this chapter, and any abbreviated account of the subject that could have been included in these pages would not have done it justice recourse to text books would have been necessary anyway. Some direction on which aerodynamics topics are necessary for the...

Control of fixedspeed pitchregulated turbines

Fixed Speed Pitch Regulated Wind Turbine

A fixed-speed pitch-regulated turbine usually means a turbine that has an induction generator connected directly to the AC network, and which therefore rotates at a nearly constant speed. As the wind speed varies, the power produced will vary roughly as the cube of the wind speed. At rated wind speed, the electrical power generated becomes equal to the rating of the turbine, and the blades are then pitched in order to reduce the aerodynamic efficiency of the rotor and limit the power to the...

Variablespeed generators

There are two fundamental approaches to electrical variable-speed operation. Either all the output power of the wind turbine may be passed through the frequency converter to give a broad range of variable speed operation or a restricted speed range may be achieved by converting only a fraction of the output power. Figure 7.32 shows in schematic form how a broad range, variable-speed generation system may be configured. Early broad range variable-speed wind turbines used a diode rectifier bridge...

Unsteady flow Dynamic inflow Introduction

Natural winds are almost never steady in either strength or direction and so it is seldom that the conditions for the momentum theory apply. It takes a finite time for the wind to travel from far upwind of a rotor to far downwind and in that time wind conditions will change so an equilibrium state is never achieved. Even if the 'average' wind speed changes only slowly small-scale turbulence will cause a continuous unsteadiness in the velocities impinging on a rotor blade. Several approximate...

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...

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 because...

Individual pitch control

Although individual or cyclic pitch control has been proposed many times, it has yet to find a place in commercial wind turbines. Asymmetrical loadings across the rotor are responsible for a significant contribution to fatigue loads, and in principle it should be possible to reduce these loads by controlling the pitch of each blade separately according to the conditions experienced by each blade. This may become particularly important for large wind turbines. However, in order to achieve any...

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...

Control of tower vibration

Vibration Lines

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...

Nacelle Bedplate

The functions of the nacelle bedplate are to transfer the rotor loadings to the yaw bearing and to provide mountings for the gearbox and generator. Normally it is a separate entity, although in machines with an integrated gearbox, the gearbox casing and the nacelle bedplate could, in principle, be a single unit. The bedplate can either be a welded fabrication consisting of longitudinal and transverse beam members or a casting sculpted to fit the desired load paths more precisely. One fairly...

Braking loads

Braking Torque

Most turbines have the mechanical brake located on the high-speed shaft, with the result that braking loads are transmitted through the gearbox. If, as is usually the case, the mechanical brake is one of the two independent braking systems required, then it must be capable of decelerating the rotor to a standstill from an overspeed, e.g., after a grid loss. This typically requires a torque of about three times rated torque. The mechanical brake is only required to act alone during emergency...

Constraints on firstmode natural frequency

Dynamic Magnification

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...

Definition of Drag

Picture Wind Flow Around Cylinder

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...

Drivetrain Mounting Arrangement Options Lowspeed shaft mounting

Drive Train Nacelle

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...

Rotor Diameter

The issue of what size of turbine produces energy at minimum cost has been fiercely debated for a long time. Protagonists of large machines cite economies of scale and the increase in wind speed with height in their favour. From the other camp, the 'square-cube law', whereby energy capture increases as the square of the diameter, whereas rotor mass and therefore cost increases as the cube, is advanced as an argument against. In reality, both arguments are correct, and there is a trade-off...

Earthing Grounding of Wind Farms

Horizontal Electrode

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...

Active stall control

Active stall control achieves power limitation above rated wind speed by pitching the blades initially into stall, i.e., in the opposite direction to that employed for active pitch control, and is thus sometimes known as negative pitch control. At higher wind speeds, however, it is usually necessary to pitch the blades back towards feather in order to maintain power output at rated. A significant advantage of active stall control is that the blade remains essentially stalled above the rated...

Axial Momentum Theory

Blade Momentum Theory With Wake

Across the disc caused by the rate of change of axial momentum as developed in Section 3.2.1 Equation 3.9 is additional to the pressure drop associated with the rotation of the wake and is uniform over the whole disc. If the wake did not expand as it slows down the rotational wake structure together with the rotational pressure gradient would not change as the wake develops whereas the pressure loss caused by the change of axial momentum will gradually reduce to zero in the fully-developed...

Passive pitch control

An attractive alternative to active control of blade pitch to limit power is to design the blade and or its hub mounting to twist under the action of loads on the blades in order to achieve the desired pitch changes at higher wind speeds. Unfortunately, although the principle is easy to state, it is difficult to achieve it in practice, because the required variation in blade twist with wind speed generally does not match the corresponding variation in blade load. In the case of stand-alone wind...

Wind farm and generator protection

Ring Main Wind Farm

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...

Historical Development

Ireland Wind Speed Resource

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...

Info

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...

Islanding and selfexcitation of induction generators

Fixed-speed wind turbines use induction generators to provide damping in the drive train and, as there is no direct access to the field of an induction generator, the magnetizing current drawn from the stator leads to a requirement for reactive power. In order to reduce the reactive power supplied from the network it is conventional to fit fixed-speed wind turbines with local power factor correction PFC capacitors. As long as the induction machine is connected to a distribution network its...

Prandtl Tip Loss Factor

Tip Losses Wind Turbine

Figure 3.32 Prandtl's Wake-disc Model to Account for Tip-losses Figure 3.32 Prandtl's Wake-disc Model to Account for Tip-losses Rw r is a distance measured from the wake edge. Distance d between the discs should be that of the distance travelled by particle three between successive vortex sheets. Glauert 1935 takes d as being the normal distance between successive helicoidcal vortex sheets. The helix angle of the vortex sheets is the flow angle 0 s and so with N sheets intertwining from N...

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...

Yaw control

Pitch And Stall Power Curve

As most horizontal-axis wind turbines employ a yaw drive mechanism to keep the turbine headed into the wind, the use of the same mechanism to yaw the turbine out of wind to limit power output is obviously an attractive one. However, there are two factors which militate against the rapid response of such a system to limit power first, the large moment of inertia of the nacelle and rotor about the yaw axis, Figure 6.11 Specimen Pitch Angle Schedules for Active Pitch Control and Active Stall...

Betz Schraubenpropeller Mit Geringstem Energieverlust

Schraubenpropeller mit geringstem energieverlust. Gottinger Nachr., Germany. Coleman, R. P., Feingold, A. M. and Stempin, C. W., 1945 . 'Evaluation of the induced velocity field of an idealised helicopter rotor'. N.A.C.A. A.R.R,. No. L5E10. Glauert, H., 1926a . 'The analysis of experimental results in the windmill brake and vortex ring states of an airscrew'. ARCR R amp M No. 1026. Glauert, H., 1926b . 'A general theory of the autogyro'. ARCR R amp M No. 1111. Glauert, H.,...

Power Quality

Harmonic Wind Turbine

Power quality is the term used to describe how closely the electrical power delivered to customers corresponds to the appropriate standards and so operates their end-use equipment correctly Dugan, McGranaghan and Beaty, 1996 . Thus, it is essentially a customer-focused measure although greatly effected by the operation of the distribution and transmission network. There are a large number of ways in which the electrical supply i.e., current, voltage or frequency can deviate from the specified...

Highspeed shaft brake design

A key parameter to be chosen in brake design is the design braking torque. The coefficient of friction can vary substantially above and below the design value due to such factors as bedding in of the brake pads and contamination, so the design braking torque calculated on the nominal friction value must be increased by a suitable materials factor. Germanischer Lloyd specify a materials factor of 1.2 for the coefficient of friction, and add in another factor of 1.1 for possible loss of calliper...

Switching between torque and pitch control

In practice, acoustic noise, loads or other design constraints usually mean that the maximum allowable rotor speed is reached at a relatively low wind speed. As the wind speed increases further, it is desirable to increase the torque and power without any further speed increase, in order to capture more energy from the wind. The simplest strategy is to implement a torque-speed ramp line CD in Figure 8.3 Once rated power or torque is reached, pitch control is used to maintain the rotor speed at...

Calculation of brake disc temperature rise

The build up in temperature across the width of a brake disc over the duration of the stop can be calculated quite easily if a number of assumptions are made. First, the heat generated is assumed to be fed into the disc at a uniform intensity over the areas swept out by the brake pads as the disc rotates. This is a reasonable approximation for a high-speed shaft-mounted brake and for a low-speed shaft-mounted brake with several callipers until rotation has almost ceased, but the energy input by...