Interaction between electrical discharges and materials for wind turbine blades – particularly related to lightning protection

Background:
Due to increasing demands for more efficient wind power generation and new manufacturing technologies, the size of wind turbines is continuously increasing worldwide. This fact and the trend of wind farms being placed offshore, increases the probability of lightning strikes to wind turbines. When a lightning strikes a structure, it usually finds the electrically shortest path towards earth potential. The latest generation of wind turbines have towers with a height around 60-80 m, and wings of length 40-50 m. This means that a wind turbine placed at sea rises app. 120 m above sea level, resulting in relatively many direct lightning strikes. Lightning protection systems for wind turbine wings usually consists of a number of receptors and an inner down conductor inside the wing, giving some protection proven for wings of length 20-30 m. For longer wings, the risk of lightning not striking the receptors is higher and might instead break through the wing material, creating an arc between the puncture and the down conductor. The large pressure from the arc results in delamination of the entire wing struck by the direct lightning, in severe cases also affecting the two other wings.

Aim of project: 
The aim of the project is to investigate discharge formation in fibre-reinforced polymers, its dependency of a large number of parameters, and the materials resistance to discharges. Test methods and means of classifying materials used for wind turbine wings will be developed and applied. Within this class of materials we find Glass and Carbon fibre Reinforced Polymers constructed with different weaving techniques, different matrix materials and different kinds of polishing. The project also aims at improving the different materials in order to obtain better qualities within resistance against lightning.

Procedure: 
As a base for a comprehensive test programme the following theoretical subjects are concerned: Detailed studies of arcing theory, breakdown mechanisms in gases and solid materials, physics of discharges, lightning theory and statistics, ageing of polymers under influence of pollution, mechanical degradation of wind turbine wings, etc. all read with HV testing in mind.
Different materials provided by Fiberline Composites A/S are to be tested in the HV laboratory.

Organization: 
The work is a PSO-F&U -funded project, number 4517 with Eltra as responsible administrator and Fiberline Composites A/S and DEFU as participants. MSc.E.E. Søren Find Madsen is employed at DTU as Ph.D.-student under this project. 

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2003-05-21 sfm