Estimation of Contact Resistivity in Lightning Protection Equipotential Bonding Joints of Wind Turbine Blades

Abstract

Modern lightning protection systems for wind turbine blades with conducting structural elements, e.g., carbon fibre reinforced polymer (CFRP) spar caps, contain equipotential bonding joints to prevent sparking during strikes. Significant current levels are experienced through the joints and the characterization of the electrical contact at the bonding regions is essential for reliable protection. Therefore, this article aims to characterize the contact resistivity of several equipotential bonding joints. The proposed methodology first measures the total resistance of the samples, and then the bulk resistance of the conductive elements is computed using the finite-element method. The latter is required to predict the spreading effects in CFRP components due to the strong anisotropic nature of such materials. After that, the contact resistance is calculated by subtracting the predicted bulk resistances from the measured total resistances. The developed procedure was applied to three typical equipotential bonding materials: expanded copper foil (ECF), biaxial (BIAX) CFRP and unidirectional (UD) CFRP. Both ECF and BIAX CFRP showed superior contact quality than UD CFRP, with one to two orders of magnitude smaller contact resistivity.

Numerical simulation of lightning strike damage to wind turbine blades and validation against conducted current test data

“…The anisotropic electrical conductivities of both UD CFRP and BIAX CFRP were experimentally measured according to the procedures presented in [28]. The values of contact resistivity for BIAX EB joints are taken from [18]. The relative permittivity was assumed equal to 1 for both copper and…”

“…This is due to the fact that the spar conducts the lightning current for a short interval [13,14,46], as well as because the test sample presents EB layers over the spar chamfered surfaces. This allows the current to be injected through the entire spar cross-section, which reduces the Joule heating due to the CFRP anisotropy [18]. As the spar maximum temperature is considerably below the temperature dependence range of CFRP electrical and thermal conductivities [40,74], the weak coupling between the electromagnetic and heat transfer problems is a valid approach to model lightning strikes to wind turbine blades.…”

“…However, the increase in the solution accuracy thanks to its implementation would not justify the significant growth of the computational time. The second one is represented by the employed contact resistivity values, which have been determined by room temperature low voltage tests while it is known that they decrease with increasing temperatures [18].…”

“…Because of the presence of resin-rich areas, high values of contact resistance and temperatures are often found at the joint interfaces. These in turn lead thermal degradation of some of the resin around the EB points, sparking events, and potential delamination within the composite [14][15][16][17][18].…”

“…• The electrical contact resistivity values at the EB interfaces to predict accurate current density and temperature distributions, which were characterised and presented in [18].…”