S. Krämer

Fiber Optic Sensor Network for Lightning Impact Localization and Classification in Wind Turbines

As wind turbines are increasing both in number and in height, they are exposed to a major threat in form of lightning strikes. The protection of these structures from the effects of lightning is an important issue in todays wind turbine development. However, as lightning is random in nature, a complete protection against its damages is not achievable. The presented method for lightning impact localization and classification using a fiber optic current sensor network helps to detect damages caused by lightning and to monitor the blades. The system is connected to the wind turbine control and monitoring system

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.

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.

EMC Analysis of a Wind Turbine Blade's Lightning Protection System

An electromagnetic analysis of a wind turbine blades lightning protection system will be given. Impedance measurements on a 16 m blade tip are performed for a frequency sweep up to 30 MHz. The influence of the blades position is analyzed. Measurement data is used to derive a method-of- moments model with a commercial tool. The method-of-moments model will be compared to a finite element model. Both models will be used to derive the electro-magnetic interference caused by the electro-magnetic fields radiated by the lightning protection system after a lightning strike.

MoM based EMI analysis on large wind turbines

Method-of-moments is used for all kinds of electromagnetic analysis. As being part of the integral equation techniques, it is especially well suited for the EMC/EMI analysis of large structures and distributed systems. In the present paper, a wind turbine will be analyzed concerning its EMC characteristics. The large range of application possibilities will be shown by choosing high- and low-frequent EMI sources as model excitations. For low-frequent excitation a lightning strike will be examined and for high-frequent analysis, GSM antenna placement will be shown.

Use of a fiber-optic sensor system to review distributed magnetic field simulation of a wind turbine

According to the Lightning Protection Institute in the USA, lightning damage is the single largest cause of unplanned downtime in wind turbines. The damages can either be caused by direct effects on the structure, or by indirect effects on the systempsilas electronics. Hence, enhanced efforts on the improvement of the lightning protection system of wind turbines have been undertaken to reduce both effects. Therefore, electromagnetic analysis of different elements of the turbine is performed. In order to verify these simulations and to specify the need for shielding inside the turbine elements, the lightning induced field inside the different elements has to be measured and reviewed. In this paper we present a novel approach for potential-free magnetic field measurements, using fiber optic magnetic field sensors based on magnetic garnets. These sensors feature a broad frequency range, an inherent galvantic isolation as well as a low weight.

Magnetic Garnets for Lightning Current Measurements

Magneto-optic phenomena are those in which the optical properties of certain materials are affected by external magnetic fields or the materials own magnetization. One of them is the Faraday effect, which occurs when linearly polarized light propagates through a material exposed to a magnetic field aligned parallel to the direction of propagation of the light. Ferromagnetic iron garnet crystals exhibit a magneto-optic sensitivity that is orders of magnitude higher than those of typical paramagnetic and diamagnetic materials. Based on magnetic garnets, a fiber-optic sensor for lightning current measurements has been developed that features reduced weight, a broad bandwidth, and immunity of the data transmission against electromagnetic interference (EMI).

Combined distributed temperature sensing and current monitoring

Power cables should be operated at an adequate temperature level. Therefore, numerous power utilities have installed optical distributed temperature sensing (DTS) systems to measure the temperature of underground cables. Protection and metering systems used in power systems require measurements of the current flowing in the high-power conductors as well. Optical current sensors achieve increasing attention and acceptance for this application due to their inherent electrical insulation, high bandwidth, and immunity to EMI. DTS systems are based on spontaneous Raman scattering or Brillouin scattering, which use spectral information in the reflected light, whereas optical current sensors are based on the Faraday effect, which changes the intensity of transmitted light. This paper proposes a novel design of a combined optical temperature and current measurement system, using both physical effects. A first measurement setup is described, and first results are discussed. Thereby, the specifications for the combined data acquisition and data processing unit are analyzed in order to optimize the accuracy and the reliability of each subsystem and the whole system.