Yaru Najem Mendez Hernandez

Fusion of a FBG-based health monitoring system for wind turbines with a fiber optic lightning detection system

Wind turbine blades are made of composite materials and reach a length of more than 42 meters. Developments for modern offshore turbines are working on about 60 meters long blades. Hence, with the increasing height of the turbines and the remote locations of the structures, health monitoring systems are becoming more and more important. Therefore, fiber-optic sensor systems are well-suited, as they are lightweight, immune against electromagnetic interference (EMI), and as they can be multiplexed. Based on two separately existing concepts for strain measurements and lightning detection on wind turbines, a fused system is presented. The strain measurement system is based on a reflective fiber-Bragg-grating (FBG) network embedded in the composite structure of the blade. For lightning detection, transmissive &fiber-optic magnetic field sensors based on the Faraday effect are used to register the lightning parameters and estimate the impact point. Hence, an existing lightning detection system will be augmented, due to the fusion, by the capability to measure strain, temperature and vibration. Load, strain, temperature and impact detection information can be incorporated into the turbines monitoring or SCADA system and remote controlled by operators. Data analysis techniques allow dynamic maintenance scheduling to become a reality, what is of special interest for the cost-effective maintenance of large offshore or badly attainable onshore wind parks. To prove the feasibility of this sensor fusion on one optical fiber, interferences between both sensor systems are investigated and evaluated.

A first approach in the modelling of failures in rotor blades caused by lightning strikes

Recently the efforts of working groups from academia, industry and certification organizations around the globe in standardization and testing of wind turbine lightning protection systems have increased significantly, with the result that the expected damage caused by lightning strikes, especially in modern rotor blades with a length of 35 meters and above, has been decreased. As part of this effort, the following research approach is presented in this paper: a theoretical model of different rotor blade lightning receptor configurations with finite element analysis has been developed. In order to validate the results of simulations, experimental tests with a special video system were conducted in a high voltage lab. Also a method of determining the possible breakdown path was assessed. An acceptable correlation between the theoretical results and the experiments in the lab was found.

Integration of a Distributed Fiber Optic Current Sensor Setup for Lightning Detection in Wind Turbines

With increasing height and rated power of wind turbines (WTs), the potential number of lightning strikes rises to the square of the height as well as the average lightning current peak value. To prevent consequential damages due to lightning on such structures, lightning impact should be monitored. Therefore a new method for lightning measurements on WTs using fiber optic current sensors (FOCS) has been developed. FOCS are robust with respect to electromagnetic interference (EMI), as the magnetic field produced by the lightning current is directly converted into an optical signal in a device with small dimensions. Another advantage is the broad bandwidth, allowing the transducer to measure high current steepness. Furthermore, the sensor cannot be damaged by overcurrent coming from an unexpected surge caused by a lightning stroke. However, the accuracy of current measurements with FOCS is affected by the environmental perturbations, such as mechanical vibration and temperature changes. To prove the feasibility and accuracy of the new fiber optic measurement system for WT application, simulations and practical experiments were undertaken and are presented in this paper.

An experimental approach of the effects of lightning currents on rotor blade tips of wind turbines

Recent editions of standards for lightning protection of wind turbines recommend that air termination systems of rotor blades shall withstand severe requirements in terms of lightning current. An exception to this recommendation can be suggested, if a risk analysis shows that a lower lightning protection level - LPL can be achieved in the geographical location, where the wind park will be erected. In order to comply with the latest edition of these standards and the highest LPL, an experimental assessment in the high-current lab at the university was conducted; different configurations of air terminations systems where tested, which aim to reproduce “short duration strokes” and “long duration strokes”. The classification of “passed” or “not-passed” was chosen for this validation.