Molinar, Gabriela

Stork, Wilhelm

English

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Infrared spectral imaging for damage detectionand prevention of overhead power linesGabriela Molinar and Wilhelm StorkKarlsruhe Institute of Technology,Institute for Information Processing Technologies,Engesserstraße 5, 76131 Karlsruhe, GermanyAbstract A proposal for an overhead line monitoring system us-ing infrared imaging is presented as a thermal characterizationof a target far from the imaging sensor. The thermal image isused to measure the clearance from conductor to ground, whichallows the transmission system operators to regulate the powertransmission to prevent possible dangerous events. If the line istoo elongated due to high temperatures, the possibility of electri-cal sparks with objects near the line increases. Additionally, thissystem can detect hot spots in the connectors and ice loads overthe line if the position of the field of view is changed. The designof this system is described in detail, as well as some simulationsand experimental results are shown to illustrate the capabilitiesof the proposed system.Keywords: Overhead line monitoring system, infrared camera,clearance measurement, hot spot, ice detection.1 IntroductionElectricity has become a basic need for the modern society and the elec-tric network is used to transport it from power plants to consumers.This medium must be safe and employed in an efficient way for eco-nomic reasons. Therefore the transmission system operators (TSO) usemonitoring systems to maintain the state of the transmission lines un-der supervision. Using sensors and optimal control, the capacity of theexistent network can be increased up to 20 % [2], reducing the needto construct new lines [3]. Moreover, these monitoring systems help160 G. Molinar and W. Storkprevent premature conductor aging due to high temperatures or anelectrical discharge with objects below or near the line (clearance toground too low).The state-of-the-art of overhead line monitoring systems (OLMS)shows different ways to measure the conductor parameters: Indirectmethods, which use only weather data to estimate the conductor tem-perature, or direct methods, which include sensors installed on thelines, suffering from electromagnetic interference and lacking of energydue to the need to use induction from the line as power source [2, 4].This paper proposes a new idea for the OLMS, to overcome the inac-curacy of the indirect methods and the problems of sensors installedon a high voltage line: the use of an infrared camera installed on thetransmission tower and a passive target attached to the conductor.The IR-camera will be installed pointing to the target, which musthave high thermal conductivity to follow the conductor temperaturealong time. The target must also have a high emissivity in order to bevisible for the camera and to increase the accuracy of the temperaturemeasurement. Based on these conditions, the material and geometryof the target as well as the camera selection are discussed in this paper.In section 2 the thermal behavior of the overhead lines and its rela-tionship with the elongation of the conductor is explained. In section3 the proposed design is described in detail and the criteria for theselection of a target and a camera are discussed. Moreover, some simu-lations and experimental results are shown in section 4 to illustrate thecapabilities of the proposed system.2 Decrease of clearance for high conductor temperaturesThe conductor temperature increases proportional to the currentsquare value but also depends on ambient factors as eq. 16.1 shows[2, 5]. The conductor heat losses are on the left side of this equation:convection qc, which depends on the wind speed and direction; radi-ation qr, whose value depends on the emissivity ε of the conductor;and the conductor heat storage, where Cp is the heat capacity of theconductor material and m the mass per unit length of the conductor.Infrared imaging for overhead power line monitoring 161The heat gains are on the right side of eq. 16.1: solar radiation qs andthe ohmic heat gain depending on the current flowing through the lineand its temperature dependent resistance.qc + qr + mCpdTavgdt= qs + I2R(Tavg) (16.1)The convection heat loss is the greatest term in eq. 16.1. This meansthat the wind plays an important role in the value of the conductortemperature. The wind direction and velocity normally changes alongthe line, i.e. a homogeneous conductor temperature cannot be expected.To illustrate this, the study made in [6] demonstrates how a punctualtemperature measurement can deviate from the mean conductor tem-perature. This demonstration was done using punctual temperaturesensors installed at several points of a line during a period of 12 hours.The result was a deviation of around ±40◦C from the mean conduc-tor temperature value [6]. This means that a single point temperaturemeasurement cannot represent the temperature of the whole line.On the other hand, as the conductor temperature increases so does itslength. The relationship between them is linear and is shown in eq. 16.2[7]. The constant α is the thermal expansion coefficient of the materialand means how easy the conductor elongates for a change of 1K fromthe reference temperature Tc,re f . Greater values of α correspond tomore elongation of the conductor from the same reference length Lre f .L = Lre f (1 + α(Tc − Tc,re f )) (16.2)If the conductor length increases, the clearance from the line to grounddecreases. The distance from the span (S, straight line between con-nectors) and the minimum point of the conductor curve is called thesag D. The sag plus the clearance to ground corresponds to the totalheight of the conductor connectors The relationship between sag andconductor temperature is shown in the eq. 16.3 [7] as an approximatedversion only valid for level spans (both poles are at the same height).D ≈√3S8[Lre f (1 + α(Tc − Tc,re f ))− S] (16.3)As an example, the sag of an ACSR Drake 403 mm2 26/7 with 300m ofspan in the temperature range from −15◦C to 80◦C goes from 6.4 m to162 G. Molinar and W. Stork9.1 m, that means a change of 2.7 m. The conductor length, on the otherhand, increases 30 cm in the same temperature range. This incrementin length could mean a dangerous situation for a transmission line andits surroundings, depending on the line case and weather conditions.3 Concept descriptionThis article proposes a new design for OLMS: Infrared imaging for aremotely detection and characterization of a target to determine theelongation of a transmission line in real time. Ice load and hot spotsdetection can also be derived from the same infrared image. Knowingthe ambient temperature as well as the clearance to ground, ice loadscan be estimated [8]. On the other hand, hot spots on the connectionsbetween conductor and insulator can also be detected using servomo-tors to change remotely its point of view: from the target to the lineconnections and vice versa. Hot spots are produced by damages on theconductor connectors. They mean loss of energy through heating anda fast detection of them avoids a break of the line [9]. With this systemthe TSOs can perform remotely manual checks of the line connectionswhen considered appropriate, using the sensor unit already installed.The camera is mounted on the transmission tower, therefore the needto power-off the line for installation is completely eliminated (normallya disadvantage of state-of-the-art systems with electrical sensors di-rectly installed on the line). The passive target can be hanged onthe overhead line using a hot stick, which reduces the system instal-lation time. These advantages, the fact that there is just one sensorinstalled on the tower to measure clearance, ice loads and hot spotsand the possibility to calculate the conductor temperature (if the cur-rent is known1), make this system a competitive solution compared tothe state-of-the-art overhead line monitoring systems.Clearance measurement can be done through the detection of theposition of the target from the images produced by the IR camera.Since the target is following the conductor temperature, which is nor-mally well above ambient temperature, the target can be seen by theIR-camera even at night without the need of extra-illumination. The1 A current sensor can be also installed in the tower if the TSO is not able to give areal-time current value.Infrared imaging for overhead power line monitoring 163advantage of using a target hanged on the line is the increment of thedetection area from the conductor diameter (0.5 to 5 cm for AluminumConductor Steel Reinforced (ACSR)) to the target diameter, reducingthe required resolution of the camera, i.e. reducing costs.3.1 Target designTo design the target several parameters must be considered:• High thermal conductivity: to maintain the target at a highertemperature than the surrounding ambient.• High emissivity: To allow the detection of the target by the IRimaging device.• Type of the line: Single conductor or a bundle.• Low cost and weight.The architecture proposed for the target is shown in figures 16.1 and16.2. On the left side there is a target for the case of a single conduc-tor and at the right side the quad-bundle2. In both cases they have acircular and symmetric form.Figure 16.1: Target design for thesingle conductor case.Figure 16.2: Target design for thequad-bundle case.2 In practice there are also bundles of two or three conductors, but for simplicity is onlyconsidered the four conductors bundle (quad-bundle).164 G. Molinar and W. StorkThe materials with highest thermal conductivity are the diamond andthe copper. To keep the costs as low as possible, copper is the best op-tion to construct the target (with a thermal conductivity of 400 W/m°Cand a cost of around 4.2 Euro/kg). The disadvantage of it is its lowemissivity (high reflectivity instead), i.e. for an infrared camera it willbe invisible as long as it is not reflecting any IR-light coming fromother objects. In this case a high emissivity coating like the offered bythe company Aremco [10] can be used to cover it completely.To determine the size of the quad-bundle target, the fact that nor-mally the distance between conductors is 45 cm must be considered.In that case and supposing 3 cm for the conductor diameter, then thetarget diameter should be at least 68 cm. To reduce weight a meshedinner structure is proposed.3.2 Camera selectionTo select a appropriate camera the following criteria must be consid-ered:• Operating wavelength: Since normally the temperature of theconductor cannot exceed 80°C, the target temperature will be al-ways below that value and then the imaging device can work inthe long wavelength infrared range (LWIR, from 8 to 14 μm).• Cooled or uncooled camera: Since the imaging in this systemshould work in the LWIR range, to reduce costs and energy con-sumption an uncooled camera is chosen.• Number of pixels: The resolution of the clearance measurementdepends on this parameter. The number of pixels should beenough to have at least a pixel for a change of sag of 2 cm (equiv-alent to approximately ΔT = 1◦C). To cover 3 m of change inclearance with this resolution, at least 150 pixels are needed inthe vertical direction.• Field of view (FOV): Since the target will be at a distance around100 m to 150 m from the camera, the FOV must be narrow enoughto have the target covering the most part of the image. For 150m a FOV of 5◦, for 100 m a FOV of 7◦ (considering a camera of640x480 pixels and a resolution of 2cm per pixel).Infrared imaging for overhead power line monitoring 1654 Simulations and experimentsThe thermal behavior of the target was simulated using the simula-tion software ANSYS. A conductor temperature was specified as theboundary condition of the target holes where the overhead line shouldbe placed. The graphs in figures 16.3 and 16.4 show how the quad-bundle target follows the conductor temperature in the natural andforced convection cases3, respectively. In both cases the ambient tem-perature was fixed at 22°C and the beginning temperature of the targetis 50°C.Figure 16.3: Natural convection of the quad-bundle target.Figure 16.4: Forced convection of the quad-bundle target.3 Natural convection means almost no wind (less than 0.5 m/s) and forced convectioncorresponds to an effective wind velocity of 0.5 m/s or more [5].166 G. Molinar and W. StorkIn the natural convection case the target is able to follow better the con-ductor temperature as in the forced convection case. This is due to thefact that the target transfers more heat to the ambient because of forcedconvection. This result says that the camera will not be able to measuremost of the time an accurate conductor temperature, because the tar-get will not have a homogeneous temperature as long as the conductortemperature and the ambient conditions are changing. However, thegoal of this system is not to measure temperature but to do a targetrecognition from an infrared image and hence to determine the clear-ance from the transmission line to ground. The achievement of thisis possible as long as the mean target temperature is higher than theambient temperature, which will happen providing that the conductoris under electrical load.The automatic recognition of the target hanging from the conductorand its position measurement is made using image processing appliedto the captured infrared data. As a general idea, the program shouldrecognize the circular form of the target and return important param-eters as the center of the circle and its radius. Using this informationand knowing the camera parameters (FOV, number of pixels) as well asthe distance camera to target, the distance from conductor to groundcan be calculated.A first version of the image processing program for this applicationwas done making use of Matlab. Using this high level approach thecapabilities of target detection algorithms can be tested. After that thealgorithm can be programmed to be executed by the embedded systemused in the overhead line monitoring system (a microcontroller, a DSPor a FPGA, for example).The algorithm used to acquire the following results is based on thecircular Hough transform [11]. Several images of a hot target (around60°C) at different positions to the camera were captured with an in-frared camera FLIR A655sc with a 24,6 mm lens (FOV: 25°×19°) and640×480 pixels.From the captured images and knowing the radius of the target (7cm) the distance from the camera to the target was calculated by thesoftware and compared to the distance measured by hand. The man-ual measurement had a precision of 5 mm and the distance target tocamera was between 1.17 m and 1.19 m at each image. The mean er-ror obtained was 9.4 mm between calculated and measured distance,Infrared imaging for overhead power line monitoring 167Figure 16.5: Target partially outside of the camera FOV and still recognized bythe circular hough transform programmed in Matlab.which corresponds to a 0.8 % of error. This result is considered interest-ing because this error includes a target partially outside of the cameraFOV, which was still recognized by the software (see figure 16.5).5 SummaryIn this article a new approach for overhead line monitoring systemswas presented: the use of an infrared camera to measure the clearanceof the transmission line to ground, by detecting a target hanged on theconductor. With the results obtained was shown that the implementa-tion and use of the proposed system is possible. The next step is theimplementation of the idea on overhead lines, which implies new chal-lenges as harsh weather conditions and the proximity to high voltageconductors.References1. M. Vollmer, “Infrared thermal imaging: fundamentals, research and appli-cations,” Weinheim, 2010.168 G. Molinar and W. Stork2. D. Douglas, W. Chisholm et al., “Real-time overhead transmission-linemonitoring for dynamic rating,” IEEE Transactions on Power Delivery,vol. 31, no. 3, pp. 921–927, Dec. 2016.3. Tennet, “Netzausbau in Niederbayern: Stromversorgung zwischen Bayernund Österreich,” Deutschland, Apr. 2015.4. R. Stephen, “Real time monitoring,” Electra, vol. 197, Aug. 2001.5. Cigré, “Thermal behaviour of overhead conductors,” Cigré, techreport 207,Aug. 2002.6. G. Biedenbach, “Echtzeitmonitoring von Freileitungen: Leiterseiltempera-tur zuverlässig ermitteln.” EW Magazin für die Energiewirtschaft, vol. 7104,pp. 70–73, 2012.7. Cigré, “Sag-tension calculation methods for overhead lines,” Cigré, techre-port 324, Apr. 2016.8. G. Biedenbach, “Kapazitäten in Stromnetzen Optimal Ausschöpfen. Moni-toring macht den Betrieb von Freileitungen sicherer.” EW Magazin für dieEnergiewirtschaft, pp. 74–83, 2009.9. J. Frate, D. Gagnon et al., “Evaluation of overhead line and joint perfor-mance with high-definition thermography,” in Transmission and Distribu-tion Construction, Operation and Live-Line Maintenance Proceedings. IEEE,Oct. 2000.10. Aremco, “High temperature high emissivity coatings,” URL, 2016.[Online]. Available: http://www.aremco.com/high-emissivity-coatings/11. B. Jähne, “Digitale Bildverarbeitung und Bildgewinnung,” Berlin,Heidelberg, 2012. [Online]. Available: http://swbplus.bsz-bw.de/bsz377583960cov.htm

Infrared spectral imaging for damage detection and prevention of overhead power lines

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Infrared spectral imaging for damage detection and prevention of overhead power lines

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