Realistic Lightning Exposure System For Optimized Wind Turbine Reliability

High current impulse testing of full-scale wind turbine components like blades or nacelles is the optimal tool to verify the proper functionality of a Lightning protection system (LPS) during lightning exposure. The continuous development of the wind power industry increases the physical size of wind turbine components, leading to a higher load for the impulse generator. Due to this reason, a new lightning exposure system was developed which is able to inject realistic lightning currents into full-scale wind turbine components.
Lightning current measurement data from wind turbines and tall structures were gathered in order to define the actual current parameters of a realistic lightning flash. Downward and upward lightning flashes are characterized by significant differences between the current parameters. Testing both types of flashes is necessary to validate the impact of lightning to wind turbines.
The data revealed further that certain wind power plants and towers are exposed to frequent upward lightning attachment. It is investigated why certain wind power plants are affected and it is concluded that four factors increase the upward lightning probability which are the height of the structure, the elevation, the meteorological properties, and the nearby flash density. Special attention is attributed to the meteorological properties of sites and a dedicated section refers to winter thunderstorms and their effects on wind turbines.
The work investigates further the difference between a static tower and a rotating wind turbine in terms of lightning exposure. Space charges created by ionization of air due to high electric field from thunderclouds may influence the likelihood of upward lightning attachment. A Finite Element Analysis (FEA) simulation and laboratory experiments validate the impact of air flow to space charges.
Finally, the design of the novel exposure system is documented. Wind turbine blades are characterized by the largest expected impedance due to their long slender structure. Resistances and inductance are calculated for blades with different LPS solutions and return conductor arrangements. The limitations of high current testing are defined. Finally, the design, control, and validation of the exposure system are described. It is divided into three physical generators that represent the first return-stroke, continuous currents, and the subsequent return-stroke. One of the key elements for the generator is modularity in order to adapt the generator to the load. Selected topics related to the development process and the challenges in the design are discussed