Passive Optical Sensor for Lightning Detection on Overhead Power Lines J. B. Rosolem*, C. F. Barbosa, C. Floridia, E. L. Bezerra CPqD, Rod. Campinas-Mogi Mirim, km 118, Campinas, SP, 13086-902, Brazil, *
[email protected]; phone 55 19 3705-6796; fax 55 19 3705-6119; cpqd.com.br ABSTRACT In this work we present the results of a passive optical sensor for monitoring lightning strikes on overhead power lines, which can also be used for several other applications. The optic sensor is very simple and cheap and basically consists in the use of an antenna connected directly to a semiconductor laser. No batteries and solar panels are necessary to implement this sensing system in power lines towers. It was tested in laboratory and showed a good performance. Keywords: optical lightning sensor, lightning, overhead power lines, OPGW.
1. INTRODUCTION Overhead power lines are affected for many natural causes like wind, rain, snow, birds, earthquakes and lightning. Lightning damages are the most serious causes of power outages in overhead power lines. Although some protection circuits like ground wire and devices like surge arresters are used in overhead power lines, they are not totally effective. For maintenance purposes, it is very important nowadays the location of lightning strikes in the power lines, in order to provide fast power recovery or precise preventive maintenance in transmission line elements. Several lightning location methods using fiber optics sensors have already been proposed. In references [1-2] are proposed the use of Faraday Effect to detect lightning. In reference [3] is used a Rogowski coil coupled to the line isolators and fiber optic light transmitters to detect when lightning breaks an insulator string of a high voltage power transmission line. These methods are not appropriated to use in overhead power lines due to the high electromagnetic fields present in the environment and also due the high cost associated with its installation in many towers of a transmission line because they have to use batteries and solar panels. In reference [4] is described one method for lightning detection, which is based on detecting the state of polarization (SOP) fluctuation of the light in fibers of an optic ground wire cable (OPGW). Despite of use this method in edge of transmission line the lightning detection based in SOP é very complex. In this work, we present the results of a passive optical sensor for monitoring lightning strikes on overhead power lines, which can also be used for several other applications. The optic sensor is very simple and cheap and basically consists in the use of a compact antenna connected directly to a semiconductor laser. No batteries and solar panels are necessary to implement this sensing system. The proposed system based on RF/optical integrated technology aims to obtain nonexpensive, flexible and non-intrusive characteristics from the adopted system design.
2. PASSIVE OPTICAL LIGHTNING SENSOR The cloud-to-ground lightning (Figure 1(a)) phenomenology is extensively studied nowadays. Concerning to overhead power lines, there are three possible discharge paths that can cause surges on the power line. In the first discharge path, the leader core of the lightning strikes the earth near the power line and generates a transient electromagnetic field [5] which can induce overvoltages of significant magnitudes on overhead power line, so that a voltage is developed across the insulator string. The second discharge path is between the lightning and the overhead earth wire. The discharge comes down the tower and raises the potential of the tower top to a point where the difference in voltage across the insulation is sufficient to cause flashover from the tower back to the conductor. The third mode of discharge is between the lightning and the phase conductor, due to the failure of the shielding provided by the overhead earth wire. The discharge injected into the phase conductor raises the voltage across the insulator string. At relatively low current, the insulation strength is exceeded and the discharge path is completed to earth via the tower. The sensor conception can detect any of these discharge paths. It detects the voltage induced by lightning electromagnetic fields in an antenna which immediately modulates a semiconductor laser coupled to an optical fiber. 20th International Conference on Optical Fibre Sensors, edited by Julian Jones, Brian Culshaw, Wolfgang Ecke, José Miguel López-Higuera, Reinhardt Willsch, Proc. of SPIE Vol. 7503, 75033Y © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.832302 Proc. of SPIE Vol. 7503 75033Y-1
None electrical power supply like batteries and solar panel are used and the sensor can be installed above the towers in order to detect any kind of stroke in the power line or in the in the neighborhoods of the line. The inset in Figure 1(a) shows a diagram of the passive lightning sensor conception. In one possible lightning sensing system, each sensor can transmit the lightning signal in a specific CWDM or DWDM wavelength to a remote receiver. The sensor output can be connected to one single fiber belong to OPGW cable using fix OADM (Optical Add-Drop Multiplexer). In such way the wavelength signature of each sensor demarks the position of lightning strike location. The passive optical lightning sensor used in our experiments is composed of a low cost Fabry-Perot semiconductor laser operating in 1300 nm, a dipole meander antenna, which is fixed to its electrodes, and a single mode optical fiber length connected to the laser to carry the detected signal to a remote optical receiver. Other passive devices like resistors and surge protective devices (SPD) can be used in order to protect the laser circuit. Figure 1(b) shows the laser characterization in terms of emitted optical power versus polarization current. An important concern about the sensor is the antenna design. The antenna chosen for the sensor must meet the following requirements: reduced dimensions suitable for installation on the towers of overhead power lines, resonance frequency within the lightning spectrum band, resistance of radiation adjusted for laser impedance matching and high directivity in order to detect lightning only in the power line direction. The final dimensions of the print circuit antenna plus laser are 240 x 120 x 1.6 mm. 0.0015
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Fig.1 - (a) Optical lightning sensor conception and (b) laser characterization
3. LABORATORIAL TESTS RESULTS In order to observe qualitatively the behavior of the proposal sensor to the lightning phenomena we first used one spark generator (Figure 2(a)) to produce one electrical field like the one produced by the lightning. The sensor was placed 3 meters from the spark generator. In this experience the gap between the rod electrode and the sphere was 30 mm and the voltage applied to generate the spark was 30 kVrms. In Figure 2(b) we can observe the optical waveform related to the spark generation detected by sensor and measured in optical receiver.
Fig.2 - (a) Spark generation and (b) optical waveform related detected by sensor and measured in the optical receiver.
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This test proves the sensor ability to detect lightning but unfortunately is not possible to make controlled measurements concerning the electrical field generation. Other test done using this set-up was about Corona detection. The Corona effect is very common in overhead power lines and it can be confused sometimes with lightning. The proposed sensor did not detect the corona effect generated by the spark generator. The test was carried out by energizing the set up shown in Fig. 2(a) with a voltage high enough to produce audible and visual corona, but not spark. In order to make controlled measurements, we used the set-up showed in Figure 3(a). In this set-up the spark generator was substituted by one overvoltage test generator which produce a standard lightning impulse of 1.2/50 μs according IEEE recommendations [6]. Figure 3(b) shows the photo of the set-up. The set-up consists in two large metallic plates in parallel spaced by 0.3 m. The overvoltage test generator applies standard lightning impulse of 1.2/50 μs with amplitudes from 0 to 5 kV. Figure 4(a) shows a standard lightning impulse with 5 kV amplitude.
Fig.3 - (a) Set-up utilized for standard lightning test on sensor and (b) set-up photo.
We submitted the sensor to different amplitudes of standard lightning impulse and simultaneously measured the waveforms in an oscilloscope connected to a PIN optical receiver and the current through the laser by means of a Hall Effect sensor. Figure 4(b) shows the waveforms measured in the optical receiver. We can observe that only the main peak of the standard lightning impulse is detected. In Figure 5 we shown the peak voltage measured in the optical receiver and the peak current generated by the laser versus the applied electrical field. It is observed that, for the maximum electric field applied, the maximum current through the laser was 3.7 mA, that is, the laser operated below its threshold, in spontaneous emission regime. We can observe also that the laser current is practically linear to the applied electrical field. The detected optical power in the receptor increases proportionally to the applied electric field. -0.015
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Fig.4 - (a) Standard lightning impulse with 5 kV amplitude and (b) waveforms of the detected optical signal for different peak values of applied electric field. .
One import issue about the sensor is how its sensitivity (induced voltage) drops as a function of distance from the lightning discharge. Assuming perfectly conducting ground and the model from Rusck [7], a lightning return stroke with 12 kA peak current, propagating at 150 m/μs and striking at 50 m from the sensor generates a vertical electric field with 28 kV/m peak. If the discharge strikes at 100 m, from the sensor, the peak of the vertical electric field drops to 14 kV/m, that is, the peak of the electric field decreases inversely proportional to the distance.
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Considering the comments above, it is important to mention that the behavior of our sensor can be non-linear because the laser has two different polarization regions (spontaneous emission and amplified emission). According to the inset in Figure 2(b) and Figure 5, the maximum current generated by the laser in spontaneous regime is 4 mA (or maximum electric field = 18 kV/m). One alternative is to limit the laser current using a resistor in order to use it only in the spontaneous emission regime or to use a LED device, which is more linear than a Laser. Other possibility is to use a laser device like an alarm in such way when its current is above the threshold level means that the lightning discharge struck very close to the sensor (less than 85 m, for example), situation where it is more dangerous for the power line. 12
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4. CONCLUSIONS In this work we presented the results of a passive optical sensor for monitoring lightning strikes on overhead power lines. The implemented optic sensor is very simple and cheap and aims to obtain non-expensive, flexible and non-intrusive characteristics from the adopted system design. No batteries and solar panels are necessary to implement this sensing system in overhead power lines towers. The good performance obtained in laboratory tests proves that the passive optical lightning sensor is a good solution to use in on overhead power lines providing fast power recovery and precise preventive maintenance in transmission line elements.
REFERENCES [1] W. J. Koshak, R. J. Solakiewicz, “Electro-optic lightning detector”, Applied Optics, Vol. 38, No. 21, pag. 4623-4634, 1999. [2] S. G. M. Krämer, F. P. León, “Fiber-Optic Current Sensors for Lightning Detection in Wind Turbines”, OFS Optical Fibre Sensors Conference, paper TuE53, 2006. [3] Yangchun Cheng1, Chengrong Li2, and Fei Zhang1, “Lightning current and flashover path measurement on high voltage transmission lines”, Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 2006. [4] Z. Qin, Z. Cheng, Z. Zhang, J. Zhu, and Feng Li, “New method for lightning location using optical ground wire”, Chinese Optics Letters, Vol. 4, No. 12, pag.712-714, 2006. [5] C.A. Nucci, F. Rachidi, “Lightning-Induced Overvoltages”, IEEE Transmission and Distribution Conference, Panel Session "Distribution Lightning Protection", New Orleans, April, 1999. [6] IEEE 1410, “Guide for Improving the Lightning Performance of Electric Power Overhead Distribution Lines”, 1997. [7] Rusck, S., “Induced lightning overvoltages on power transmission lines with special reference to the overvoltage protection of low voltage networks”, Ph.D. dissertation, Royal Institute of Technology, Stockholm, 1957.
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