April 02, 2009 | 5 Comments
Brussels, Belgium [Renewable Energy World Magazine] The initial design of a wind farm can have profound implications for its future profitability. Based on onshore wind farms, though also relevant for offshore, this extract from a new EWEA book reveals some of the key considerations.
Once a site has been identified and a decision taken to invest in its development, the wind farm design process begins. The fundamental aim is to maximize energy production, minimize capital and operating costs, and stay within the constraints imposed by the site. As the constraints and costs are all subject to some level of uncertainty, the optimization process also seeks to minimize risk. A first task is to define the constraints on the development:
These constraints may change as discussions and negotiations with various parties progress, so this is inevitably an iterative process.
When the likely constraints are known, a preliminary design of the wind farm can be produced. As a rough guide, the installed capacity is likely to be of the order of 12 MW/km2, unless there are major restrictions that affect the efficient use of available land.
For the purpose of defining the preliminary layout, it is necessary to define approximately what sizes of turbine are under consideration. The selection of a specific turbine model is often best left to the more detailed design phase, when the commercial terms of potential turbine suppliers are known. Therefore, at this stage it is either necessary to use a ‘generic’ turbine design, defined in terms of a range of rotor diameters and a range of hub heights, or to proceed with several layouts, each based on specific wind turbines.
The factors most likely to affect turbine location are optimization of energy production, visual influence, noise and turbine loads.
Optimization of energy production
Once the wind farm constraints are defined, the layout of the wind farm can be optimized – also called wind farm ‘micro-siting’. For most projects, the economics are substantially more sensitive to changes in energy production than infrastructure costs. It is therefore appropriate to use energy production as the dominant design parameter. The detailed design of the wind farm is facilitated by the use of wind farm design tools (WFDT). There are several commercially available, and others that are research tools. Once an appropriate analysis of the wind regime at the site has been undertaken, a model is set up that can be used to design the layout, predict the energy production, and address environmental issues.
For large wind farms it is often difficult to manually derive the most productive layout. For such sites a computational optimization using a WFDT may result in substantial gains in predicted energy production. Even a 1% gain in energy production from improved micro-siting is worthwhile, as it may be achieved at no increase in capital cost. The optimization process will usually involve many thousands of iterations and can include noise and visual data.
WFDTs conveniently allow many permutations of wind farm size, turbine type, hub height and layout to be considered quickly and efficiently. Financial models may be linked so that returns from different options can be directly calculated. Examples of typical WFDT outputs are presented in Figure 1, below.
‘Visual influence’ is the term used for the visibility of the wind turbines from the surrounding area.
In many countries the visual influence of a wind farm on the landscape is an important issue, especially in regions with high population density. The use of computational design tools allows the zone of visual influence (ZVI), or visibility footprint, to be calculated. It is usually necessary to agree a number of cases with the permitting authorities or other interested parties, such as locations from which 50% of turbine hubs can be seen, or from which at least one hub can be seen, or from which at least one blade tip can be seen. It is also common to generate ‘visualizations’ of the appearance of the wind farm from defined viewpoints; these can take the form of ‘wireframe’ representations of the topography, see Figure 2, below. With more work, photomontages can be produced in which turbines are superimposed on photographs taken from the defined viewpoints.
Other factors also affect the visual appearance of a wind farm. Larger turbines rotate more slowly than smaller ones, and a wind farm of fewer larger turbines is usually preferable to a wind farm of many smaller ones. In some surroundings, a regular area or straight line may be preferable compared to an irregular layout.
In densely populated countries, noise can sometimes be a limiting factor for the capacity that can be installed on any particular site. Noise produced by operating turbines has been significantly reduced in recent years by manufacturers, but is still a constraint. This is for two main reasons. Firstly, unlike most other generating technologies, wind turbines are often located in rural areas where background noise levels can be very low, especially overnight. In fact the critical times are when wind speed is at the lower end of the turbine operating range, because then the wind-induced background noise is lowest. Secondly, the main noise sources (blade tips, the trailing edge of the outer part of the blade, the gearbox and generator) are elevated, and so are not screened by topography or obstacles.
Turbine manufacturers may provide noise characteristic certificates. The internationally recognized standard typically referred to is ‘Wind turbine generator systems: Acoustic noise measurement techniques’ (IEC 61400 Part 11 of 2003). Standard techniques, taking into account noise propagation models, are used to calculate the expected noise levels at critical locations, usually the nearest dwellings. The results are then compared with the acceptable levels, often defined in national legislation. The standard for such calculations is ‘Acoustics – Attenuation of sound during propagation outdoors; Part 2: General method of calculation’ (ISO 9613-2). Turbine manufacturers could also provide a warranty for the noise produced by the turbines.
In order to ensure that the turbines are not being used outside their design conditions, the minimum acceptable turbine spacing should be obtained from the turbine supplier and adhered to. This is strongly dependent on the nature of the terrain and the wind rose for a site. If turbines are spaced closer than five rotor diameters (5D) it is likely that unacceptably high wake losses will result. For areas with predominantly unidirectional or bidirectional wind roses greater distances between turbines in the prevailing wind direction and tighter spacing perpendicular to the prevailing wind will prove to be more productive.
However, tight spacing means that turbines are more affected by turbulence from the wakes of upstream turbines. This will create high mechanical loads and will require approval by the turbine supplier if warranty arrangements are not to be affected.
Separately from the issue of turbine spacing, turbine loads are also affected by ‘natural’ turbulence caused by obstructions, topography, surface roughness and thermal effects, as well as extreme winds.
The wind farm infrastructure consists of civil works – such as roads and drainage, wind turbine, met mast foundations and buildings housing electrical switchgear – and electrical works such as equipment at the point of connection (POC), underground cable networks and/or overhead lines forming radial ‘feeder’ circuits to strings of wind turbines, switchgear for protection and disconnection of the feeder circuits, and transformers and switchgear associated with individual turbines (although this is now commonly located within the turbine and is supplied by the turbine supplier).
The civil and electrical works, often referred to as the ‘balance of plant’ (BOP), are often designed and installed by a contractor or contractors separate from the turbine supplier. The turbine supplier usually provides the SCADA system.
As discussed, the major influence on the economic success of a wind farm is the energy production. However, the wind farm infrastructure is also significant as it constitutes a significant part of the overall project cost. The civil works also present significant risks to the project costs and programme. It is not unknown for major delays and cost overruns to be caused by poor understanding of ground conditions, or the difficulties of working on sites that, by definition, are exposed to the weather and may have difficult access. Major electrical items such as transformers and switchgear also have long lead times, possibly several years. Grid connection works may present a significant risk to the programme as it is likely that works will need to be undertaken by the network operator, effectively out of the control of the wind farm developer.
The foundations must be adequate to support the turbine under extreme loads. Normally the design load condition for the foundations is the extreme ‘once in 50 years’ wind speed. In Europe this wind speed is characterized by the ‘three-second gust’. For most sites this will lie between 45–70 m/s. At the lower end of this range it is likely that the maximum operational loads will be higher than those generated by the extreme gust.
The turbine supplier will normally provide a complete specification of the foundation loads as part of a tender package. As the turbine will typically be provided with reference to a generic certification these loads may also be defined with reference to the generic classes, rather than site specific load cases.
Although extremely important, the foundation design process is a relatively simple civil engineering task.
A typical foundation will be perhaps 13 metres across, a hexagonal form and might be 1–2 metres deep. It will be made from reinforced concrete cast into an excavated hole. The construction time can easily be less than a week. However, for wind farms sited on peat or bogs, it is necessary to ensure that the roads, foundations and drainage do not adversely affect the hydrology.
For upland sites, it is often beneficial to locate the control building and substation in a sheltered location. This also reduces visual impact.
The turbines are interconnected by a medium voltage (MV) electrical network, in the range 10–35 kV. In most cases this network consists of underground cables, but in some locations and some countries overhead lines on wooden poles are adopted. This is cheaper but creates greater visual influence. Overhead wooden pole lines can also restrict the movement and use of cranes.
The turbine generator voltage is normally classed as ‘low’, in other words below 1000 V, and is often 690 V. Some larger turbines use a higher generator voltage, around 3 kV, but this is not high enough for economical direct interconnection to other turbines. Therefore, it is necessary for each turbine to have a transformer to step up to medium voltage, and associated switchgear. This equipment can be located outside the base of each turbine, sometimes termed ‘padmount transformers’.
However, many turbines now include a transformer as part of the turbine supply. In these cases the turbine terminal voltage will be medium tension and can connect directly to the farm network.
The network takes the power to a central point (or several points, for a large wind farm) and a typical layout is shown in Figure 3, above. The medium voltage electrical network consists of radial ‘feeders’ as, unlike industrial power networks, there is no economic justification for providing ring arrangements. Therefore a fault in a cable or at a turbine transformer will result in all turbines on that feeder being disconnected.
Definitions of the point of connection (POC) vary from country to country and are either called delivery point, point of interconnection or similar. The definitions are similar: it is the point at which responsibility for ownership and operation of the electrical system passes from the wind farm to the electricity network operator. The meters for the wind farm will usually be located at or close to the POC. In some cases, where the POC is at high voltage, the meters may be located on the medium voltage system to save costs and in such cases it is usual to account for transformer losses.
The ‘point of common coupling’ (PCC) – at which other customers are (or could be) connected – is the point at which the effect of the wind farm on the electricity network should be determined. These effects include voltage step changes, voltage flicker and harmonic currents. Often the PCC coincides with the POC.
The wind farm electrical system must meet local electrical safety requirements and be capable of being operated safely, should achieve an optimum balance between capital cost, operating costs and reliability and must ensure that the wind farm satisfies the technical requirements of the electricity network operator.
SCADA and instruments
A vital element of the wind farm is the SCADA system. This system acts as a ‘nerve centre’ for the project. It connects the individual turbines, the substation and meteorological stations to a central computer. This computer and the associated communication system allow the operator to supervise the behaviour of all the wind turbines and also the wind farm as a whole. It keeps a record on a 10 minute basis of all the activity, and allows the operator to determine what corrective action, if any, needs to be taken. It also records energy output, availability and error signals – which acts as a basis for any warranty calculations and claims. The SCADA system also has to implement any requirements in the connection agreement to control reactive power production, to contribute to network voltage or frequency control, or to limit power output in response to instructions from the network operator.
The SCADA computer communicates with the turbines via a communications network, which almost always uses optical fibres. Often the fibre-optic cables are installed by the electrical contractor, then tested and terminated by the SCADA supplier.
The SCADA system is usually provided by the turbine supplier, for contractual simplicity. There is also a market for SCADA systems from independent suppliers.
The major advantages of this route are claimed to be:
In addition to the essential equipment needed for a functioning wind farm, it is also advisable, if the project size can warrant the investment, to erect some permanent meteorological instrumentation on met masts. This equipment allows the performance of the wind farm to be carefully monitored and understood. If the wind farm is not performing according to its budget, it will be important to determine whether this is due to poor mechanical performance or a less-than expected wind resource. In the absence of good quality wind data on the site, it will not be possible to make this determination. Large wind farms therefore usually contain one or more permanent meteorological masts, which are installed at the same time as the wind farm.
A wind farm may be a single machine or it may be a large number of machines, possibly many hundreds. The design approach and the construction method will, however, be almost identical whatever the size of project envisaged. The record of the wind industry in the construction of wind farms is generally good. Few wind farms are delivered either late or over budget.
Newcomers to the wind industry tend to think of a wind farm as a power station. There are, however, some important differences between these two types of power generation. A conventional power station is one large machine, which will not generate power until it is complete. It will often need a substantial and complicated civil structure, and construction risk will be an important part of the project assessment. However, the construction of a wind farm is more akin to the purchase of a fleet of trucks than to the construction of a single large asset. The turbines will be purchased at a fixed cost agreed in advance and a delivery schedule will be established exactly as it would be for a fleet of trucks. In a similar way, the electrical infrastructure can be specified well in advance, again probably at a fixed price. There may be some variable costs associated with the civil works, but this cost variation will be very small compared to the cost of the project as a whole. The construction time is also very short compared to a conventional power plant. A 10 MW wind farm can easily be built within a couple of months.
To minimize cost and environmental effects, it is common to source material for roads from on-site quarries or ‘borrow pits’.
Wind farm costs are largely determined by two factors: the complexity of the site and the likely extreme loads. The site may be considered complex if the ground conditions are difficult – hard rock or very wet or boggy ground, for example – or if access is a problem. A very windy site with high extreme loads will result in a more expensive civil infrastructure as well as a higher specification for the turbines.
The cost of the grid connection may also be important. Grid connection costs are affected by the distance to a suitable network connection point, the voltage level of the existing network, and the network operator’s principles for charging for connections and for the use of the electricity system.
Commissioning, operation and maintenance
Once construction is complete, commissioning will begin. The definition of ‘commissioning’ is not standardized, but generally covers all activities after all components of the wind turbine are installed. Commissioning of an individual turbine can take little more than two days with experienced staff.
Commissioning will usually involve standard tests for the electrical infrastructure as well as the turbine, and inspection of routine civil engineering quality records. Careful testing at this stage is vital if a good quality wind farm is to be delivered and maintained.
The long-term availability of a commercial wind turbine is usually in excess of 97%. This value is superior to values quoted for conventional power stations. However, it will usually take a period of some six months for the wind farm to reach full, mature, commercial operation, and hence, during that period, the availability will increase from a level of about 80%–90% immediately after commissioning to the long-term level of 97% or more.
It is normal practice for the supplier of the wind farm to provide a warranty for between two and five years. This warranty will often cover lost revenue, including downtime to correct faults, and a test of the power curve of the turbine. If the power curve is found to be defective, then reimbursement will be made through the payment of liquidated damages. For modern wind farms, there is rarely any problem in meeting the warranted power curves, but availability, particularly for new models, can be lower than expected in the early years of operation. During the first year of operation of a turbine some ‘teething’ problems are usually experienced. For a new model this effect is more marked. As model use increases, these problems are resolved and availability rises.
After commissioning, the wind farm will be handed over to the operations and maintenance crew. A typical crew will consist of two people for every 20 to 30 turbines in a wind farm. For smaller wind farms there may not be a dedicated O&M crew but arrangements will be made for regular visits from a regional team. Typical routine maintenance time for a modern wind turbine is 40 hours per year. Non-routine maintenance may be of a similar order.
There is now much commercial experience with modern wind turbines and high levels of availability are regularly achieved. Third party operations companies are well established in all of the major markets, and it is likely this element of the industry will develop very much along the lines associated with other rotating plant and mechanical/electrical equipment.
The building permits obtained in order to allow the construction of the wind farm may have some ongoing environmental reporting requirements, for example the monitoring of noise, avian activity, or other flora or fauna issues. Similarly there may, depending on the local regulations, be regulatory duties to perform in connection with the local electricity network operator. Therefore, in addition to the obvious operations and maintenance activity, there is often a management role to perform in parallel. Many wind farms are funded through project finance and hence regular reporting activities to the lenders will also be required.
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