NG TR(E) 453 PDF

PURPOSE AND SCOPE

Wind turbines have become an increasingly common feature of the landscape in many countries. It is estimated that, at the beginning of 2007 there were 90,000 turbines installed worldwide with a total generating capacity of 70 GW. By the end of 2008, according to the World Wind Energy Association’s figures, the installed capacity is estimated to have risen to 120 GW. In the UK, the Government has now adopted a target of 15% of UK generating capacity to be provided from renewable resources by 2020. To meet this target it is envisaged that some 34 GW of wind power capacity will be installed, 14 GW of which are to be structed at onshore sites. Currently, the installed onshore capacity is 2.6 GW, with 772 MW under construction and a further 3.4 GW awaiting planning approval. Since the capacity of a typical modern turbine is about 2 to 3 MW, it is clear that wind turbines are also set to become a common feature of the UK landscape.

By their nature, wind turbines are installed in open country and it is inevitable that they will share some locations with National Grid’s established 275 and 400 kV transmission lines which traverse England and Wales. In addition, new National Grid routes will be required in some locations to provide connections for these installations to the transmission system. Wind farms have a fundamental effect on the local wind environment downstream of them. Since transmission lines are ‘wind-sensitive’ structures, the question has been raised as to whether the proximity of a wind turbine can have an adverse effect on the service performance of a line. This report considers the various aspects of the way in which the natural wind affects transmission lines and how this may be changed by the presence of a turbine. The availability of information to complete the assessment is examined and how best to acquire further information.

National Grid’s current policy is that the distance between an overhead line (OHL) and a wind turbine should be not less than five rotor diameters. This represents a significant constraint on land use for wind farms. A particular focus for the report, therefore, is the wind environment within a few diameters downstream of the turbine to see whether a reduction in this recommended separation between an OHL and turbine can be justified.

In general terms, the effect of an on-load, operational wind turbine on the downstream wind environment is to reduce the mean wind speed compared to the incident upstream value, to increase the turbulence levels and to inject some rotational flow components. When not operating, turbines are essentially no different to the variety of industrial and commercial structures with which National Grid’s on towers and conductors in the wake of a turbine are reduced as a result of the reduced mean wind speed, such loadings are not considered.

The steel members of transmission towers can be subject to wind-induced vibrations. In general, this is significant only for towers with long, unbraced elements (length-to-width ratio of some 250 or more). Some towers, internationally, have been designed with few bracings in the cross-arms, especially in climates where ice loading does not occur and where high wind loads are rare. Vibration-induced fatigue, particularly of the unbraced, sloping member at the top of each cross-arm, has then occasionally occurred. National Grid’s transmission tower designs are not of this type and they have not suffered from a general wind-induced vibration problem. However, National Grid has identified a specific problem with the J L Eve L6 tower design which utilises a 'notched' steel bracing that is known to be susceptible to failure; this is believed to be due to wind loading causing fatigue over time, the notch acting as a stress raiser (EMI 925, Issue 3, refers). Consideration should be given to replacing all such bracings if the location of a wind turbine is considered likely to increase the buffeting of a tower with notched bracings. No further consideration is given to tower vibration in this report.

This report starts with a review of the known types of wind-induced motion of National Grid’s transmission lines. It is followed by some basic considerations of the likely features of a wind turbine wake and how these might affect an overhead line’s response to the wind. A review is then undertaken of papers which deal directly with this question, followed by papers which provide turbine wake data relevant to a consideration of the problem. In order to understand the implications of these data, they are employed in a modal response model of an OHL subject to a turbulent wind.