Balance of plant (BOP) is the term used to describe all the infrastructure and facilities of a wind turbine installation except for the turbine itself. It, therefore, encompasses all aspects of the project not covered in the turbine supply contract. This includes project management, ground condition surveys, crane pads / hard standings, foundations, substation and electricals as well.
Project Management and CDM
Under the BOP contract issued by the manufacturer or the client, Spectrum provides a full project management facility and handles the project from order to sign off. We are the lead project team, as defined by the CDM regulations and take on the role of Principle Contractor and take full responsibility for the coordination and delivery of the works.
Ground Condition Surveys
Ground condition surveys include boreholes, trial pits, soil samples and pressure calculations. Usually, these are done at each wind turbine location, the substation and along the access roads prior to award of the BOP contract. Sometimes additional ground investigation work may be required during the construction phase if the ground conditions are particularly complex. Should the ground conditions be considered as non-standard for a foundation, a newly designed rebar cage may be required as well as a piled foundation design.
Crane Pads / Hard Standings
Crane Pads are prepared on each turbine location, to facilitate and accommodate the heavy lifting operations of tower, turbine and blade installation. Following a ground soil assessment, these results 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 of 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 roadways will be constructed in consultation with the crane and turbine suppliers.
Where hard rock pathways are not possible, these hard standings are often replaced with temporary roadway which are either of aluminium or railway sleeper construction dependent upon the ground conditions.
Foundations are the key element in terms of both cost and materials for any wind turbine installation. Every turbine manufacturer has its own preferred foundation type but the most commonly used are the pad and pile reinforced types. A turbine foundation can be described as a mass of concrete and reinforcement that is cast accurately to connect to the turbine tower. It is designed to carry the loadings indicated by the turbine manufacturer. These loadings are confirmed by soil test analysis or ground condition surveys.
The foundation should be strong enough to provide the required base support for the rest of the structures above it. The quality of the foundation is therefore of the utmost importance to the wind turbine structure. The design specifications must be strictly adhered to. The pouring of concrete has to be done carefully to prevent surface and thermal cracking to ensure the final foundation is of the required quality and standard.
Spectrum can also provide the design modifications required when foundation designs need to be amended to suit both ground conditions and loading requirements.
Road Upgrades and Construction
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 wind turbine 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 traffic from cranes, diggers and delivery vehicles.
The roads should be able to provide safe and reliable access for the rest of the lifetime of the wind turbine in all weather conditions. These are usually unpaved roads made of compacted layers of crushed rock, gravel and high-density polythene ground layer.
Substation Civil and Electrical
Cabling to Substation
The power generated by each turbine is either exported wholly back to the national grid or is consumed locally with a small amount of export. This is done by means of a cable network connecting the turbine with the substation at either Low voltage (LV) or High voltage (HV). The cables can be overhead but underground trenched cabling is the most common. The overall length of a cable depends on the distance from the turbines to the substation and the network layout.
Cabling to Grid
A quotation from the local District Network Operator (DNO) will have been received prior to Spectrum’s installation commencing and will contain contestable and non-contestable elements and these may be high or low voltage connections. The characteristics and cost will depend on the power capacity of the connection and the singularities of the distribution network in the area.
This cabling connects the wind turbine substation to the closest distribution or transmission line. It is generally underground to reduce the visual impact. The length of the line depends on the distance from the wind turbine to the point of connection to the main grid. The cable characteristics will depend on the total wind turbine rated power and the transmission voltage on the downstream of the wind turbine substation. Losses of less than 2% voltage drop over distance are common.
Most 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. These step ups and step downs would be up to 100v or direct to grid at 11,000v dependent upon the grid connection offer from the DNO.
Some DNOs include transformers and some don’t. If supplied with the grid offer, they are not part of the BOP supply, but the connection of the LV equipment would be included by Spectrum. When the DNO does not offer one, Spectrum will include it in the scope of supply.