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Reference

Generating for the UK Electricity System

Wind power is a clean, safe source of energy for generating electricity. There are many potential sites for wind farms, clusters or single wind turbines throughout the UK. Although some people are interested in providing power purely for their own use, in the great majority of cases the intention is to sell some or all of the electricity generated, by exporting it via the public electricity system.

This guide has three aims:

  • to highlight the technical factors that have to be considered when connecting wind turbines to the public electricity system
  • to give an outline of the administrative procedures necessary
  • to give guidance on the likely costs of a connection

This last point may be the most important, as connection costs can render small projects far from the existing system completely uneconomic. This is best discovered before much effort has been expended.

Wind turbine design is still evolving, and many different designs are available. From the point of view of the electricity network, the most important factors are:

  • the rating, which is the maximum continuous output in kilowatts (kW)
  • whether the turbine operates at fixed (single or 2 speeds) or variable-speed
  • whether the turbine is stall-regulated or pitch-regulated

In this guide, the term Public Electricity Supplier (PES) is used for the company that operates the local electricity network (in England and Wales this is synonymous with the term REC, or Regional Electricity Company). The PES is responsible for the safe and economic operation of its system and has obligations to maintain satisfactory quality of supply to users of its system. The PES is not necessarily the purchaser of the electricity generated by the wind turbines.

The Public Electricity Transmission & Distribution System

Wind turbines are usually located in rural or upland areas, where the electrical connection to the nearest electricity substation can be weak, and where local demand for electricity may be much less than the wind generation capacity. One way of defining the "strength" of the electricity network is by the fault level. The fault level is a measure of the current that will flow when there is a fault on a network. The fault level at the end of a long electricity circuit is much lower than at the centre of an interconnected network, for example, in a town or industrial development. At a low fault level site, the impact of the wind turbines can be great enough to disturb other local consumers. For this reason, it is sometimes necessary to reinforce the network, or connect the wind turbines to a higher voltage or stronger part of the network further away. This will increase costs.

Higher-voltage systems such as the 400 kV or 275 kV transmission systems have high fault levels. In general, the lower the voltage, the weaker the system. For most of the UK in rural areas, the distribution system voltages are 132, 33 and 11 kV. The 11 kV system is the most extensive but in rural areas is unlikely to support more than one to three megawatts (MW) of generation.

In rural or upland areas, it is most likely that the nearest point on the local electricity network is an overhead line, rather than underground cable. You may be able to find a number on a pole or tower (pylon): this will help the PES engineers locate the proposed site on their system maps, and they will then be able to tell you the voltage of the line. Ordnance Survey grid references can also be used.

Any overhead line with only two wires is carrying a single-phase system and will normally require reinforcement if generators are to be installed.

The Effects of Wind Turbines on Electricity Systems

The factors which need to be considered are listed below. Their relative importance will depend on the local electricity network, and the choice of wind turbine.

Network Voltage Range

Electricity networks were designed to transfer power from large generators on higher voltage systems to customers distributed on lower voltage systems. If power is transported in the other direction, the voltage experienced by consumers nearby on the network could go outside the statutory limits. To ensure there are no problems for consumers, the PES engineers will need to know the proposed maximum continuous output, and the power factor. For larger projects, additional power-factor correction capacitors or static VAr compensators may be worthwhile, to permit some control of power factor and voltage. Some variable-speed turbines provide control of their power factor, and may be more acceptable in this respect.

Voltage Fluctuations

When they are running, the output power of wind turbines varies second by second, depending on the strength and turbulence of the wind. The effect of the tower as the blades rotate past it also introduces a periodic disturbance in the power output, which is greater at high wind speeds. These power fluctuations cause voltage variations on the local electricity network, termed flicker. Limits on the flicker any connected equipment can cause are defined in reference 2 and are set to avoid disturbance to other consumers. Flicker is only likely to be a problem for small groups or single turbines, especially large machines connected at lower voltages. Stall-regulated wind turbines should produce less disturbance than pitch-regulated turbines. Variable-speed turbines should have very little effect.

The PES engineers may be concerned that wind turbines with induction generators (as is most common) will also cause disturbances when starting. This is largely a problem of the past, as "soft-start" units are fitted to most designs. However, the voltage step that will occur when a wind turbine shuts down from full output, perhaps due to high winds, must also be considered. Limits are set in reference 2. It is usually accepted that, under normal conditions, it is very unlikely that more than one or two turbines will shut down simultaneously.

Harmonics

Variable-speed wind turbines can cause harmonic voltages to appear on the network, which can cause equipment to malfunction or overheat, and disturb other consumers. The PES will work to the limits set out in Reference 3 which will avoid disturbance to other consumers. The problem can be reduced by fitting filters, at additional cost. Fixed-speed wind turbines will generate harmonics for the short periods when their soft-start units are in use, but this generally is not significant.

Thermal Ratings

Overhead lines, cables and transformers are designed to transport power up to a certain maximum rating. This sets an upper limit on the power that may be "exported" from a wind farm, without expensive system reinforcement.

Fault Contribution

The fault current which will flow from wind turbines to a fault on the network will generally be very much less than will flow from within the network itself, and is not usually a constraint.

Voltage Unbalance

This is a problem caused by the network, rather than by wind turbines. It is known that in some UK locations, the voltages on each of the three phases are unequal by more than the recommended amount (see reference 4). This can cause wind turbines to shut down, to protect against generator overheating, thus reducing income to the wind turbine owner.

Consulting the Utility Company

The PES should be advised of the proposed scheme at an early stage. They will carry out initial studies to ascertain the technical feasibility of the project and can then quote a cost for connection, which may determine whether or not the project will proceed.

There is an established procedure for making the connection to the grid. The steps are:

Notification:
Informing the PES of the proposed project, and providing outline details, to enable them to establish the feasibility of the project, and prepare budget costs. The PES may charge for this service. Costs will depend on the size of the development, the distance from the nearest connection point, and the connection voltage. You should impress on the PES any areas where you may have some flexibility, for example, you may be willing to reduce the generation capacity slightly if this gave a significant reduction in connection costs.

Technical Submission:
This includes more detailed technical information, as required by the PES. If the PES needs to carry out design studies, the costs will be included in the final connection charge. Some PES's may agree to carry out design studies before a decision is made to proceed with the project, but will charge for this.

Connection Quotation:
When details are agreed the PES will make a formal connection quotation.

Planning and Wayleaves:
If a new connection is required, planning permission and wayleaves must be obtained. This is usually done by the PES, sometimes with assistance from the developer. The project proposers must, of course, also obtain planning permission for the wind turbines, site roads and buildings and other structures.

Connection, Supply and Meter Operator Agreements:
These are drawn up between the PES and the project operator, and include all aspects of the interface between the wind farm and the grid, including protection, earthing and metering. They also define the limits of responsibility of both parties, and charges for imported real and reactive energy. Until these agreements are signed, the PES may allocate the capacity to another project.

For very large projects, a generation licence is required, which is obtained from the Office of Electricity Regulation (OFFER). The costs and benefits of this option cannot be explained in a short factsheet. The circumstances under which a generation licence is required are complex, and differ in N. Ireland, Scotland, and England & Wales. OFFER will be able to assist.

Costs

The costs given here are very approximate and should be taken as a guide only (they could easily double in some circumstances). They are classified by the voltage level at the point of connection to the PES system. The costs are only for PES equipment, and exclude the switchgear, cables, transformers and other equipment within the wind farm. Costs for reinforcement of the network at remote locations are also excluded from these estimates. The costs include capitalised charges to cover future O&M (operation and maintenance) of the PES equipment provided specifically for the project. PES's often insist this is paid as a capitalised charge, typically 25% of the capital cost. Others allow this to be paid as an annual charge.

  • Low Voltage This is only feasible for very small generators connecting directly to the existing network. Costs will vary so widely that it would be misleading to state any here.
  • 11kV Grid connection equipment: £20,000 - £60,000
  • Overhead line: £15,000 - £30,000/km
  • 33 kV Grid connection equipment: £120,000 - £150,000
  • Overhead line: £20,000 - £35,000/km
  • 132 kV Grid connection equipment: £800,000 - £1,000,000
  • Overhead line: Insufficient information

These costs are for a single connection only - in fault conditions or during maintenance of the network, the wind farm may not be able to generate.

Design

The design of the connection to the electricity network requires an experienced electrical engineer. This may be provided by the wind turbine manufacturer, an electrical contractor, or an independent consultant. They will design the electrical connections within the wind farm, as well as the interface with the local network and can negotiate technical decisions with the PES, for example, on the appropriate connection voltage.

A number of issues need to be considered, including:

  • Connection voltage - This has a significant effect on the cost of the connection. For example, a new 33 kV line will cost considerably more than connecting into an existing 11 kV circuit: however, a new 11 kV line can cost more than a new 33 kV line, for the same wind farm output.
  • Distribution voltage within the wind farm.
  • Arrangement of transformers and wind turbines.
  • Earthing - The electrical installation must be adequately earthed to ensure that people or equipment are not harmed by electrical faults or lightning strikes, and to comply with the Electricity Supply Regulations and Electricity at Work Regulations (references 5 & 8). This is a complex issue and should not be underestimated.
  • Protection - Equipment must be provided to ensure that the wind turbine or wind farm is automatically disconnected when there is a fault on the network. Similarly, the network must be protected from the effects of a fault within the wind farm. Reference 6 defines the protection needed and is used as part of the connection agreement with the PES.
  • Metering - The electricity is metered at the point of connection to the local network. Meters are required for both exported and imported power (which can occur in very low winds), and for reactive power. Metering accuracy and cost increases with the size of the wind farm.

These design decisions are based on minimising electrical losses and capital cost, while ensuring that the system is safe and reliable. It is often worth spending more money initially on an efficient electrical design which, by reducing losses, will increase the total energy output.

Although the details may appear complex, the process of connecting wind farms to the grid is now well established. The growing numbers of grid-connected wind turbines in the UK are proof of the benefits of harnessing the wind.

References

1. The Distribution Code
Separate but similar documents apply to England and Wales, Scotland and Northern Ireland.
These documents set the quasi-legal framework. Available from the PES.
2. Engineering Recommendation P28, Electricity Association
Planning Limits for Voltage Fluctuations Caused bu Industrial, Commercial and Domestic Equipment in the UK
3. Engineering Recommendation G5/3, Electricity Association
Limits for Harmonics in the UK Electricity Supply System
4. Engineering Recommendation P29, Electricity Association
Voltage Unbalance
5. Electricity Supply Regulations (l989), as amended
6. Engineering Recommendation G59/1 (Amendment 2, l995), Electricity Association
Recommendations for the Connection of Embedded Generating Plant to the Public Electricity Suppliers' Distribution System
7. D J Milborrow, Grid Connection, Review, No 23, Nov. 1994
Department of Trade & Industry
(available from Renewable Energy Enquiries Bureau, ETSU, Harwell, Oxfordshire OX11 0RA)
8. Electricity at Work Regulations l989, SI l989/635, HMSO, ISBN 0 11 096635 X

Acknowledgements

This Guide has been produced by members of the British Wind Energy Association working group on electrical integration and regulatory issues. The principal contributors were Lucy Craig, David Milborrow and Paul Gardner.