Balance of Plant

Balance of plant is the cost of all infrastructural and facilities of a windfarm with an exception of the turbine and all its elements. The balance of plant therefore mainly comprises of the following items.

Crane pads/ Hard standings


Substation Civil and Electrical

Road upgrades and Construction

Cabling to substation and Grid



Miscellaneous costs


Crane pads / Hard Standings

Crane Pads are prepared on each turbine location so as to facilitate and accommodate the heavy lifting operations of tower, turbine and blade installation. The type of ground conditions will determine how intensive or costly the pad construction is going to be. Pads are rated by their carrying capacity which is the amount of load they can withstand. Typical ratings will range between 200KN/m2 to 500KN/m2 mainly to provide a strong solid base for heavy cranes to do the lifting the tower and wind turbine generator. Crane Pads are usually located adjacent to the turbine foundations to facilitate turbine components lifting during turbine erection and future maintenance or repairs. These are usually unpaved, compacted layers of crushed rock.

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Foundations are the key and critical BoP element in terms of both cost and materials for any windfarm development project. Every turbine manufacturer has its own preferred foundation type but the most commonly used are the gravity and pile reinforced types. A turbine foundation can be described as a mass of concrete and reinforcement cast accurately to connect to the turbine tower. The basic components of a foundation are:

  • Concrete: Gravel, sand, cement.

  • Reinforcement: Steel.

  • Can/Basket Assembly.

The foundation should be strong enough to provide the required base support for the rest of the structures above it. Most of the turbine foundations have generally been designed to be founded on the weathered schist or dense gravel with a minimum allowable bearing capacity of 500 KPa for a 2.3MW wind turbine.
The quality of the foundation is of the utmost importance to the wind turbine structure. The design specifications have to be strictly adhered to. The pouring of concrete has to be done carefully to prevent surface and thermal cracking to ensure final foundation is of the required quality and standard.

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Substation Civil and Electrical

Substation Civil:

This encompasses the building which houses the other electrical, safety and operational equipment. The building is usually constructed to the required acceptable standards as determined by the electrical providers and legislation. All elements of the substation structure from foundations, plumbing for cables, walls and roofs are usually designed and constructed as per the required specifications and quality requirements. The substation building usually consists of a single storey building with shallow foundations (if ground conditions permit).  The size could be around 10m by 25m for a 20MW wind farm, plus external transformer area which requires plinths for the equipment and an earthed fencing.  The building may also be facilitated with toilet, sink and possibly shower facilities and so aseptic tank may be required.

Substation Electricals:

The electrical components found in the substation are mainly for facilitation of the operational aspects of the windfarm. This usually includes components such as switchgear, power protection systems and the electric meter. A transformer may also be part of the substation components to enable step up or step down of the power output. The size of these elements are dependent on the windfarm power capacity.

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Road Upgrades and Construction

Roads are essential in providing access to the wind farm and specifically to the turbine location sites within the wind farm. During construction, access is required to move in material and equipment to where they are required within the site. Roads are also useful during the lifetime of the windfarm as they are used for operational maintenance purposes. It is necessary to construct site access roads which are strong and durable enough to withstand the heavy traffic during construction.The roads should be able to provide safe and reliable access for the rest of the lifetime of the wind farm in all weather conditions. These are usually unpaved roads made of compacted layers of crushed rock, gravel and high density polyethelene geogrid layer. The roads of a windfarm may be classified into two, namely:

Site access roads:

These connect the windfarm site to the nearest public road network. The length of which depends on the closest accessible public road network beyond the wind farm. In Scotland, this may often involve the upgrade of forestry or farmers track. The upgrade exercise may comprise reconstructing several kilometres of an existing road, re-building a bridge, or more commonly moving a few lamp posts.  

Windfarm network roads:

These are the roads which connect each turbine location to other elements of BoP within the windfarm. They are mainly used for access and transport of materials and equipment around the site, between the turbine locations, substation, and meteorological mast. In most wind farms the road length within the wind farm is normally between 0.4 and 0.5 km per turbine, but this length may vary depending on the characteristics of the terrain.

All roads have to be constructed prior to the erection of turbines. In some cases there may exist some roads which would require some upgrading in order to meet the standard needed for trafficking of the turbine and other windfarm components. The general requirements for all types of roads are 4.5 metres width but slightly wider at turns and a load bearing capacity to withstand the heaviest of the windfarm components to be trafficked through.

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Cabling to substation and Grid


Cabling to Substation

The power generated on each turbine is transmitted to the wind farm substation. This is done by means of a cable network connecting every of the turbines with the substation at a medium voltage (11-36kV). The cables can be overhead but underground trenched cabling is the most common. The overall length of cable depends on the distance from the turbines to the substation and the network layout. Normally the underground cables are buried by the road side as this makes installation and future maintenance simpler. As a rule of thumb the length of cable per turbine is approximated to between 0.4 to 0.5 km.

Cabling to Grid

This cabling connects the wind farm substation to the closest distribution or transmission line. It can be underground but overhead cabling is the most common. Although the underground option is mostly preferred due to its lack of visual impact, the overhead option is more economic. The length of the line depends on the distance from the windfarm substation to the point of connection on the main grid. The cable characteristics will depend on the total wind farm rated power and the transmission voltage on the downstream of the wind farm substation.

Point of Connection

This includes the commissioning of all the components necessary to complete the connection to the distribution network. Hardware’s such as switches, reinforcements, supports including protection and measuring elements. The functions of the different systems at the point of connection may be classified contestable and non-contestable. The non-contestable affect the security of the distribution line and must be carried out by the distribution network operator in the area. The characteristics and cost will depend on the power capacity of the connection and the singularities of the distribution network in the area.

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Supervisory Control And Data Acquisition(SCADA) is an industrial control system which enables users to monitor and control remote operations of wind turbines and other associated systems in real time. Users can access accurate real-time information including live weather and meteorological updates as well as fully configurable Key Performance Indicators (KPIs). Comparison of past production patterns against historical current and live information helps in fine tuning to keep equipment at optimal efficiency. Some windfarm SCADA systems provide users with rich 2D and 3D visualisations and reports with integrated real-time and historical geographical terrain maps and graphics. Turbine information including wind speed, wind direction, power blade position, temperature and vibration can be accessed on real-time. The cost of a SCADA system will depend on the complexity of the desired package.

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Most of the wind turbines generate electricity at a low voltage level from 600 to 1000 (V). In order to reduce the losses associated with electricity transmission, each wind turbine is equipped with a transformer to step up the voltage. Usually these transformers are situated at the base of the wind turbine as this makes it more accessible for operational monitoring and maintenance works. Having the transformer located in the nacelle, very close to the generator, has the advantage of reducing transmission losses as compared to the base location. If the transformers are supplied by the turbine manufacturer they won’t be considered as part of BoP. The transformer may also be positioned inside or adjacent to the tower and therefore it is treated as one of the turbine components. The characteristics of each transformer are fixed by the rated power generated by the turbine, the voltage level and the transformation ratio.

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Miscellaneous Costs

Overheads and others:

This is a classification of other supplementary but necessary facilities or services which form an essential part of BoP but cannot be particularly categorised in to any of the main items above. They are herewith categorised under the class of overheads and others as described below.

Borrow Pit:

This is a small, on-site quarry, typically used to obtain crushed rock for the construction of roads and crane pads.  Not always feasible if rock is not present or suitable, and not always allowed by planning, but can result in savings on road stone costs if a borrow pit is used.

Site wide drainage:

This may be considered as a separate cost, or could be included in the cost of the various ground works such as tracks and crane pads.

Ground Investigation:

This includes boreholes, trial pits, soil samples and laboratory tests etc. Usually this is done at each wind turbine location, the substation and along the access roads prior to award ofthe BoP contract. Sometimes additional ground investigation work may be required during the construction phase if the ground conditions are particularly complex.

Ground Risk:

There is always a risk of cost or delay relating to the ground characteristics because it is not feasible to investigate the ground in every part of the site before construction commences. Usually the contractor is expected to take the risk relating to the ground by assigning a price to it. The cost price of this risk will depend local ground conditions regarding soil type, location of bedrock or peatbogs for example, plus the cost of any groundwork associated with making the ground suitable for building on.


Transport of BoP construction materials is usually included in the price of materials. The price of transport will mainly depend on the distance and remoteness of the location, local fuel costs and the types of transport being utilized for delivery.

Topographic Survey:

This is usually required before and after the all ground works. The post construction topographical survey usually forms part of the BoP works but the initial one is sometimes omitted.

Design Costs:

Windfarms are typically let as "design and build", otherwise known as EPC-engineer, procure and construct contracts. The outline design comprises largely of the site layout, which is usually completed at the pre-tender stage. All detailed design calculations and drawings are the responsibility of the BoP contractor.

Site Facilities:

The BoP contractor is usually responsible for providing toilets, offices, and other requirements for the site as a whole for the duration of site construction.

Project Management:

Some wind farms are built as two contracts: BoP and Turbine Supplier Agreement. Some are constructed under one wrapped contract, with either the turbine supplier or the BoP contractor acting as the lead party and taking full responsibility for the coordination and delivery of the works. The wrapped contract is lower risk and less work for the client, but consequently more expensive. The BoP cost will depend on the option taken.

Meteorological mast:

Not all wind farms have a meterological mast, and the responsibility for the design and construction of the foundation/crane pad and the mast may or may not be the BoP contractor's responsibility.

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