Modelling of lightning streamer formation and propagation in wind turbine blades

The positioning of lightning air terminations along a wind turbine blade is a complex issue to consider when designing the lightning protection of wind turbine blades. According to the IEC 61400-24 on lightning protection of wind turbines, the interception efficiency depends on the effectiveness of the air termination in enhancing the electric field and attracting the lightning discharge, thus shielding the blade surface and preventing electrical breakdown of the blade material. However, the number and location of the discrete receptors may be difficult to establish, since their performance is significantly influenced by the presence of conducting materials inside the blade. The design and validation of the lightning air termination system of a blade, as well as the evaluation of the effects of internal conductive components, involve high voltage tests, which are expensive and usually require complex setups. Furthermore, the tests may need to be repeated when a new conducting element is included in the blade with unpredictable effects for the lightning protection system. Numerical methods to determine the areas of a structure more likely to be struck by lightning have proved to be a useful tool to establish the preliminary design of the lightning protection of wind turbines. However, these methods mainly concern the lightning exposure on a macroscopic level while more detailed models containing the blade internals with multiple streamer initiations will add great value to the detailed design process. The present paper presents a method to investigate the origin and propagation of streamers from different conductive elements of the blade when exposed to a high electric field. The calculations are performed using dynamic simulations with the finite element method, and the results have been correlated with high voltage tests in the laboratory. The algorithms developed are intended to be a new and improved tool for the design of the blade lightning protection system, in particular to assess the effectiveness of the air termination system and the effects of internal conductive materials. The simulation models can involve a high level of detail and therefore be used in the detailed positioning of air terminations in blades equipped with conductive elements such as carbon fiber or electrical monitoring systems (load, temperature, etc.).