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EU lightning study offers best practice guide

A better understanding of the interaction of wind turbines and lightning has been reached with the completion of a research project under the EU's Joule program. Entitled "Lightning Protection of Wind Turbines," the best practice guide has been produced with the aim of providing assistance to manufacturers and operators of wind turbines by suggesting means of reducing the annual incidence of lightning damage and dealing with the associated safety issues.

The mechanisms causing lightning strikes of wind turbines are not fully understood, making it difficult to provide adequate protection. Lightning can and does damage blades, bearings, control systems and other components and-with wind turbines increasing in size-lightning strikes are growing more frequent.

The new guide to protection was produced following research undertaken by the project partners. Britain's University of Manchester Institute for Science and Technology (UMIST) led a team comprising the National Technical University of Athens, AEA technology in Oxford, England, and the British blade manufacturer Aerolaminates in Southampton.

Data was taken from WindStats, a Windpower Monthly supplement, containing information from Denmark spanning 8000 turbine years. The data base identifies the actual components affected and the resultant loss in turbine availability and energy production for every lightning damage incident. A German data set supplied by the Institute for Solar Energy Technology covered 6000 turbine years and included details of the repair costs of lightning damage in addition to information on outage time and energy losses.

Overall, the analysis reveals a similarity between the frequency of lightning strikes in Denmark and Germany, with both recording 6.3 lightning faults per 100 turbine years. Lightning faults result in an average downtime of 110 hours and 3200 kWh of lost generation. The component most often affected by lightning is the control system (in about 45% of cases) followed by the electrical systems (10-25%) and then blades (10-20%). The costs of blade repairs, however, are far higher than those of any other components, averaging about i15,000 per incident.

Survey results

The results of a data survey detailing lightning damaged wind turbines were used to focus the project work towards specific areas. Information from wind farm operators enabled analyses of the circumstances surrounding lightning strikes. The study also looks at regional and local variations in the incidence of lightning and finds, for example, that the number of annual thunderstorms in northern Greece is about double those on the island of Crete.

For wind turbine designers, the most useful section in the report is the one dealing with key components likely to be susceptible to lightning strike and which steps are likely to be most successful to minimise damage in the blades, bearings and electronics systems. The blade tips and the meteorological instrument support mast are the most vulnerable items. A study of lightning safety issues and methods for earthing a turbine or wind farm are also included. Guidance is provided on methods of lightning protection and the research includes testing of protection systems in a high voltage laboratory. A wealth of detail on the necessary circuitry for lightning protection is included, and it is clear that the understanding of the science of lightning strikes and wind turbines has moved from being a "black art" to one which now rests on analytical foundations. Copies of the report are available from UMIST: ian.cotton@umist.ac.uk

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