Technology Conference. Warsaw, Poland, May 1-3, 2007. Integration of a Distributed Fiber Optic Current Sensor Setup for Lightning. Detection in Wind Turbines.
IMTC 2007 - IEEE Instrumentation and Measurement Technology Conference Warsaw, Poland, May 1-3, 2007
Integration of a Distributed Fiber Optic Current Sensor Setup for Lightning Detection in Wind Turbines S. G. M. Kramer1, F. Puente Leon1, Y. N. Mendez Hernandez2, B. Lewkel 1 Technische Universitat Miinchen, Fachgebiet Verteilte Messsysteme, D-80290 Miinchen, Germany Phone: +49 (0)89 289 25107, Fax: +49 (0)89 289 23348, E-mail:
s.kraemerOtum.de.
2GE Global Research Europe, Hybrid and Renewable Systems Lab, Freisinger Landstr. 50, D- 85748 Garching, Germany Phone: +49 (0)89 5528 3437, Fax: +49 (0)89 5528 3180, E-mail: yaru.mendezoresearch.ge.com. Abstract - With increasing height and rated power of wind turbines (WTs), the potential number oflightning strikes rises to the square of the height as well as the average lightning current peak value [1]. To prevent consequential damages due to lightning on such structures, lightning impact should be monitored. Therefore a new method for lightning measurements on WTs usingfiber optic current sensors (FOCS) has been developed. FOCS are robust with respect to electromagnetic interference (EMI), as the magneticfieldproduced 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 over-current 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
Keywords - lightning measurement, opticalfiber sensor, current sensor
systems implemented in WTs, where weight and cost represent key factors. With increasing height and rated power of WTs, the potential number of lightning strikes rises to the square of the height as well as the average lightning current peak value [1]. The most endangered components of the wind turbine are the control systems in the hub and the rotor blades. Based on these arguments, a lightning detection system for WTs could allow a better understanding in the prevention of consequential damages due to the destructive effects caused by lightning strikes. This system should measure the lightning parameters and also the lightning impact point (localization). With this information, a lifetime model and a damage assessment of the blade can be computed subsequently. A new lightning detection system, based on fiber optic current sensors, to measure and quantify these data has been developed. As environmental and operating conditions in WTs can be characterized by high temperature changes and vibrations, the integration of this system needs some additional requirements.
II. STATE OF THE ART
I. INTRODUCTION Lightning discharge events are random in nature. Their peak current, registered on high structures like wind turbines, can vary from some kA to over 200 kA and feature a mean value of approx. 30 kA [2]. The average current rise time for such currents can have a mean value of 24 kA/,us. Due to possible magnetic couplings, this effect can induce high voltages in non-shielded wired systems. State-of-the-art lightning current measurement systems used on structures with elevated height (100 m and above) work with optical transducers to convert the measured analog signal to an optical signal [3]. These elements typically observe heavy weight and elevated costs and therefore, they are not suitable for lightning current monitoring
1-4244-0589-0/07/$20.00 ©2007 IEEE
For lightning detection (LD) in WTs, two commercial stateof-the-art systems are currently available. One of them works like the magnetic links. It consists of magnetic cards positioned in the blade or on other parts of the structure through which the lightning current passes. After a lightning impact, these cards can be read out manually by a card reader unit to get the current peak value. Originally developed for lightning monitoring in buildings, this system is not able to detect the number of lightning sequences striking the building between two card reading periods. An integration of this detection application into a condition monitoring system, existing in most WTs, is also not possible because of the manual card reading process.
Field mill Temperature
Sensor
Receptor
Blade Position
Fig. 1. Lightning detection system setup.
Another LD sensor system, developed especially for WTs in a project headed by DEFU (The Association of Danish Energy Companies), is used or tested in several WTs over the world [4]. This system will detect the lightning strikes by using antennas fixed on WT towers, where the lightning current passes and induces an electromagnetic field. The signals from the antennas are transformed from an electrical signal to an optical signal which is transferred to a control box via an optical fiber. The output ofthe control box indicates a lightning impact immediately. However, before another strike can be detected, the system has to be reset by an acknowledge signal, and the parameters of lightning strikes cannot be detected. III.
DEVELOPED LIGHTNING DETECTION SYSTEM
Fiber optic current sensors, which rely on the magneto-optic Faraday effect, have received considerable attention as an alternative to conventional current-transformers [5-7]. The important advantages of these sensors are their simple potential separation, their robustness against EMI, and their high dynamic range and bandwidth. Due to their lightweight sensor head they may be installed as well in aerodynamic composite structures, like wind turbine rotor blades. During a lightning strike in a WT, the lightning current induces a magnetic field. When polarized light propagates across an optical medium (fiber or crystal) and the direction of propagation of the light wave is parallel to the direction of propagation ofthe induced magnetic field wave, the Faraday effect induces a rotation of the plane of polarization of the optical wave. This angle of rotation is given by:
O(A, T) = V(A,T)
JHdl,
(1)
where is the measured angle of rotation of the field, A is the free-space wavelength of light, T is the ambient temperature,
2
V is the Verdet's constant of the magneto-optic material, and H is the magnetic field intensity along the propagation path 1. Therefore, the accuracy of the fiber optic sensor depends also on the environmental conditions, such as i.e. temperature changes and mechanical vibrations. Even if the sensor is designed for a certain maximum peak current, the induced magnetic field of a possible over-current will not destroy the fiber optic sensor but can possibly yield to incorrect outputs. Due to the linear nature of the interaction, fiber optic current sensors using the Faraday effect can be applied to AC current measurement within a spectral range from 50 Hz to 100 MHz [8]. However, a possible limiting factor of the complete system performance is the sample rate of the data acquisition units. To extract the lightning parameters, one sensor at the bottom of the blade would suffice. To achieve a higher reliability and localize the point of impact, a sensor network with a simultaneous sampling of the sensors is needed (Fig. 1). A minimum sampling rate of 2 MHz is necessary to obtain enough data for an accurate localization and determination of the lightning impulse current steepness. To obtain the lightning parameters, the digitized impulses can accurately be represented by mathematical functions, and the variables of interest can then be derived mathematically [9]. However, to improve the accuracy of the calculated parameters and to determine the impact point, data fusion models are implemented in the data processing unit. Therefore, the data acquisition unit has to sample all sensors in each blade simultaneously, and the position of each blade has to be known. IV. EXPERIMENTAL RESULTS AND DISCUSSION
The performance of the lightning detection system has been verified on a single sensor setup (Fig. 2) as well as on a twosensors setup (Fig. 3) for the sensor specifications. The accu-
racy, the temperature dependence and mechanical effects on the sensors' performance have been tested. A multi-sensor simulation has been used to evaluate the data fusion accuracy. Current I
Light Source, SLD Module
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