1338 IEEE SENSORS JOURNAL, VOL. 15, NO. 3, MARCH 2015 Detection and Monitoring of Leakage Currents in Power Transmission Insulators Marcelo Martins Werneck, Daniel Moreira dos Santos, Cesar Cosenza de Carvalho, Fábio Vieira Batista de Nazaré, and Regina Célia da Silva Barros Allil Abstract— An optoelectronic sensor for real-time leakage current monitoring on high-voltage (500 kV) and mediumvoltage (13.8 kV) power line insulators was developed. The leakage current drives an ultrabright light-emitting diode producing an amplitude modulated light signal. The optically intensityencoded signal is coupled to a plastic optical fiber cable and transmitted from the high potential measurement point to the remote unit in ground potential. After the demodulation, the leakage current root mean square values are concentrated in a data logger and sent to a remote station 150-km away by general packet radio service technology. Field tests at real operational conditions on coastal regions have been performed; all data collected are stored in a structured database, which can be consulted from the Internet, while a serially produced head was developed and the sensor is ready for commercialization. Since leakage current on high-voltage insulators depends on local air pollution and microclimate changes, several sensors have to be used to cover the region monitored. For this reason, research has been conducted to determine the sensor representativeness, i.e., the actual area, which can be covered by only one sensor. Index Terms— Current, distribution, leakage, monitoring, optical, sensor, transmission lines. I. I NTRODUCTION I NSULATORS are key components in energy distribution systems, and, after being installed, remain in field for long periods of time. Insulator leakage current is the current flowing from high voltage conductor to ground over the outside surface of the insulator. Leakage current occurs in any high voltage insulator, either in transmission lines (TL) or in distribution lines installed outdoors due to the progressively coating of conductive deposit from environment pollution. These pollutants can be dust, ashes, smoke, clay powder, chemicals from nearby industries or salt-spray on seashore areas. In the presence of wet atmospheric conditions, particles Manuscript received July 1, 2014; revised October 3, 2014; accepted October 3, 2014. Date of publication October 8, 2014; date of current version December 11, 2014. This work was supported by the Ampla Energia e Serviços S.A., Niterói. The associate editor coordinating the review of this paper and approving it for publication was Prof. Aime Lay-Ekuakille. M. M. Werneck, D. M. dos Santos, C. C. de Carvalho, and F. V. B. de Nazaré are with the Photonics and Instrumentation Laboratory, Federal University of Rio de Janeiro, Rio de Janeiro 21941-901, Brazil (e-mail: werneck@lif.coppe.ufrj.br; daniel@lif.coppe.ufrj.br; cesar@lif.coppe. ufrj.br; fabio@lif.coppe.ufrj.br). R. C. da Silva Barros Allil is with the Biological Defense Laboratory, Brazilian Army Technology Center, Rio de Janeiro 20031-144, Brazil (e-mail: regina@lif.coppe.ufrj.br). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2014.2361788 on the insulator surface will dissolve with the water droplets and will provide an alternative path from high-voltage to ground potential [1], [2]. Although these currents are around a few miliamperes, when multiplied by the total number of insulators located in that particular transmission line, the total current can reach high values that can engage protection devices, leading to electrical power line interruptions, thus leading to financial and operational losses to distribution companies and unsatisfied customers. Additionally, night dew or light rain increases the conductance of the polluted layer leading to arcs. Depending on the conductivity of the layer, these arcs can develop and cause what is known as a flashover, when the air around the insulator ionizes becoming a conductor, leading occasionally to the destruction of the whole insulator followed by outages. In general, this phenomenon happens in the following stages: • Settling of pollutants on the insulator surface • Compounding soluble pollutants with rainwater or dew and formation of a conductive layer • Starting of leakage currents • Surface of the insulator gets hot followed by the formation of dry areas • Partial discharges • Occurrence of flashover. Consequently, the insulator leakage current monitoring as an indicative parameter of a possible flashover incident, enabling a proper planning of insulator predictive maintenance, becomes significant. II. L EAKAGE C URRENT E VALUATION Superficial leakage current is the result of the interaction between the climate and the contaminants deposited over the surface of the insulator, an approximate expression for the leakage current is given by U πγm D U = Eπγm D = (1) I L E AK AG E = Rm l where I L E AK AG E is the leakage current, U is the distribution/ transmission line voltage, Rm represents the insulator surface resistance (which is related to the contaminant density), l is the insulator creepage distance, γm is the conductivity of the dewy and contaminated surface, D is the insulator diameter, and E is the electric field intensity through the creepage path [3]. The variables described in (1) are shown in Fig 1. The accumulation of contaminants over the insulator surface, combined with the environmental high humidity, 1530-437X © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. WERNECK et al.: DETECTION AND MONITORING OF LEAKAGE CURRENTS IN POWER TRANSMISSION INSULATORS Fig. 1. Representation of the variables from (1). causes the increase of the conductivity (γm ) that enables the leakage current emergence. The electric companies need to maintain a continuous monitoring of leakage current and for this they rely in parameters that help establishing a preventive strategy of water-jet-cleaning or substitution of insulators placed on the area with favorable conditions to flashover occurrence. In recent years, many studies have been carried out to improve pollution assessment, such as [1], [4], and [5]. There are also many standards that help technicians to improve insulator performance, such as the IEC60815 [6], which defines parameters for insulator selection, for instance, pollution severity level, creepage or arcing distance. Particularly, the in-field tests which were carried out for this work showed that the quantity of deposited contaminants and environmental humidity are directly related to the leakage current value. The usual method to assess insulator pollution is the ESDD, meaning Equivalent Salt Deposit Density. The ESDD is by definition the amount of sodium chlorine deposited over the insulator surface that would produce the same conductivity at a given natural pollution. In order to measure ESDD it is necessary to wash the polluted insulator with distilled water, carefully collect the water and then measure its conductivity. Now, the amount of salt that would produce the same conductivity divided by the surface of the insulator area will give the ESDD factor in mg/cm2. The washing is performed on what is known as “sacrifice insulator”, that is, an insulator of the same type as those used on that specific location but without electric wires. This insulator is hoisted to the top of the tower and left there for a specific amount of time until it gets polluted as much as the others. Then, this insulator is lowered down and washed. Leakage current assessment is not a new concern of the electric company personnel. Accidents involving flashover events in insulators employed in transmission/distribution lines, especially near seashore areas, have been a major issue for energy companies for a long time. Consequently, several sensor types and on-line monitoring techniques have been developed and widely used for protection systems [7]. Also, current literature presents several techniques to monitor insulator pollution. Classic techniques apply resistive dividers [8], [9], induction coil, as described in patent [10] and a magnetic toroid around the insulator [11]. Optical methods 1339 have also been employed by [12] and, more recently, by [7], [13], and [14]. Particularly, in [14], the authors describe the first development stages of the employment of plastic optical fibres in leakage current detection. However, in order to be useful for the electric company, a leakage current monitoring system needs to present, at least, four basic characteristics: ease of use (i.e., be practical), low cost, safety and reliability. In this paper an optoelectronic system is described, being capable of measuring, storing and radio transmit by GPRS (General Packet Radio Service) data link the insulator leakage current data as well as local temperature and air relative humidity using plastic optical fiber (POF) technology. Comparing with other solutions available in literature, this system presents some advantages, such as immunity to electromagnetic interference; low cost; lightweight, easy installation at the power line without the need to turn it off, and does not need a power supply close to high voltages. Other benefits can also be mentioned, that is: optimization and monitoring of energy losses, increase of the distribution network reliability yielding a reduction of intervals between turn-offs, optimization of the insulators washing routine and easy way to compare different kinds of anti-pollution insulators. III. T HE L EAKAGE C URRENT M ONITORING S YSTEM An optoelectronic transducer intended for measuring leakage currents from both high voltage transmission line (TL) and medium voltage distribution line was developed. The working principle of the sensor is very simple, based on the fact that an LED output light is proportional to its direct current. So, a high efficient LED is installed inside the transducer so as to the leakage current flowing from high voltage line to ground potential over the insulator surface drives the LED supplying enough energy to make it lighten. Then the LED output light, modulated by the leakage current, is fed into a POF that transmits the signal to the Remote Unit (RU) installed at ground potential. Amplitude modulated fiber optic sensors have been abandoned some decades ago due to several side-effects such as optical power drift due to temperature and ageing, modal instability and macro-curvature losses. However, with the availability of POFs at low cost, these sensors were re-visited and some examples appeared in the literature lately [11], [12]. For the reasons stated above, when using an amplitude modulated fiber optic sensor it is necessary to address all the mentioned conditions. For instance, when dealing with plastic optical fiber waveguides, all rays in a bent fiber are leaky, that is, they can traverse the core-cladding interface and never return. At successive reflections the guided light inside the fiber loses power. However, some rays reflect back in the cladair interface returning to the core [15], and become guided again. The higher the numerical aperture (NA) of the fiber, the more these returning rays are allowed to penetrate back into the core. This effect happens with POFs that possess millions of guided modes and very large NA. Indeed, for a bending radius of 32 mm, a standard 1-mm-diameter POF loses only about 0.1 dB per turn [16]. Therefore, bend losses 1340 IEEE SENSORS JOURNAL, VOL. 15, NO. 3, MARCH 2015 Fig. 2. Temperature dependence for Nichia LEDs. (a) NSPB300 Blue, (b) NSPG310 Green and (c) NSPR510 Red (graphs available at [17]). are negligible in a POF cable connecting the transducer and the receiver. The other issue to account for is the temperature dependence of the LED, whose output light fades as temperature rises. This thermal drift depends on semiconductor material and dopants, that is, for different LED colors (wavelength), different drifts apply. In order to find the LED type which best complies with transmission requirements, three different LEDs were investigated, namely: blue, green and red LEDs. In reality, the best wavelength for a silicon photodetector is around 850 nm, which corresponds to its peak response. However, we decided to use visible LEDs for the project because it facilitates maintenance since the operator can see whether the transducer is working or not just by looking into the POF output connector. Fig. 2 shows three temperature dependence graphs for three LEDs manufactured by Nichia Corporation: NSPB310 Blue, NSPG310 Green and the NSPR510 Red [17], [18], [19]. Notice the smaller dependence of the blue and green LEDs with temperature variation. However, since the wavelength of all three LEDs fall outside the peak sensitivity of the photodetector, one has also to check the wavelength drift with temperature, since for different wavelengths there will be different gains. Fig 3 shows the wavelength performance for the three LEDs with the same current in different temperatures. As it can be seen in Fig. 3 (a), (b) and (c), the peak wavelength (that is, the wavelength at which the maximum value of the optical power is obtained) of both blue and green LED drifts are negligible with temperature, i.e. the peak wavelength does not considerably vary, whereas red LED presents a large drift. From these results the green LED was chosen since it provides lower optical losses when employed in POF light transmission systems while its temperature dependence characteristic is similar to the one presented by the blue LED (Figs. 2 and 3); its drift under temperature was recorded in Fig. 3. Temperature drift in the spectra of three different Nichia LEDs: (a) NSPB310 Blue, (b) NSPG310 Green and (c) NSPR510 Red. φ is the LED diameter. order to be compensated by the software. The circuit diagram of the sensor is shown in Fig 4. Since the LED only conducts in one direction, the full wave rectifier allows the detection of an alternate leakage current signal. The two transient voltage suppressors (transorbs) protect the sensor from transients which may occur in the high voltage line. The transducer, shown in Fig. 5, is comprised of a ceramic cup in which the circuits are located, a polymeric insulator and a POF cable to transmit the current signal to the receiver system. For a high voltage TL application, the sensor taps the leakage current from the top skirt of the insulator string as shown in Fig. 6 (a). The medium voltage distribution line version is intended to be installed on the top of the insulator WERNECK et al.: DETECTION AND MONITORING OF LEAKAGE CURRENTS IN POWER TRANSMISSION INSULATORS Fig. 4. 1341 Circuit diagram of the sensor. Fig. 5. The optoelectronic transducer. Left - Ceramic head and optoelectronics, Right - Bottom view after epoxy filling, Inset: Optoelectronic printed circuit board. under test and then energized from the high voltage line, as shown in Fig 6 (b). Notice that in both cases the leakage current from high voltage to ground is forced to traverse the transmitter LED. The optical fiber from the current transducer goes from the high voltage line all the way down to a mid-tower location that can be easily accessed by the operators. The Remote Unit receives the optical fiber and converts the optical amplitude modulated signal to a voltage one, which is proportional to the monitored current. The block diagram of the RU is shown in Fig. 7. The RU’s interface section demodulates the optical signal into digital data that is stored in memory together with local temperature and air relative humidity. After been codified, the data are transmitted by GPRS to the cellular communication network. The electronic hardware of the RU is composed of a CPU board associated with a datalogger system, the sensor interrogation system, power supply (which can be implemented by a battery, for instance) and a GPRS modem transmitter. Figure 7 shows the block diagram of the overall system. As mentioned before, the RU can communicate with the data server through GPRS cell phone network. In this way all RUs can send data to the electric company’s Internet server where data is stored and analyzed. Any authorized user can access the web page to check for the data in order to observe, for instance, the trend line of the leakage current in any specified location. In a medium voltage application, the RU is energized by the 127 VAC available on the poles. In this case the RU box is compact as shown in Fig. 8. In high voltage applications there is no AC voltage available to powering the RU, therefore we Fig. 6. Installation scheme of the high voltage TL sensor (a) and medium voltage distribution lines sensor (b). In both cases the leakage current from high voltage to ground is forced to traverse the sensor. Fig. 7. Block diagram of the Remote Unit. used batteries with enough ampere-hour capacity to power the transmitter for one year in continuous operation. In this case the RU was accommodated in a larger box as will be seen later (in Fig. 12). 1342 IEEE SENSORS JOURNAL, VOL. 15, NO. 3, MARCH 2015 TABLE I U NCERTAINTY OF L EAKAGE C URRENT M EASUREMENT Fig. 8. The remote unit box. Fig. 9. Calibration of the sensor. The fitting shows a third order polynomial that is used by the software for calculating the leakage current. IV. P ERFORMANCE T ESTS Three tests have been applied to the sensor in order to calibrate it and collect data to perform software compensations where necessary. They are: input/output relation (sensitivity); uncertainty at fixed temperature and temperature drift of the sensor/receiver outside the case (ceramic cup) for different currents. measurements: First, the standard deviation (SD) of the output voltage was calculated by applying a current source to the input and covering all input range, in this case, from zero to 60 mArms . This procedure was done ten times for each value shown in the first column of Table I, resulting in the second column of the table (Averaged Output Voltage). The next step was to evaluate the SD of the output voltage for each value, resulting in the third column of the table (Standard Deviation) which is the uncertainty of the voltage measurement. The output voltage SD results obtained so far were produced for calibration purpose. In field applications, when leakage currents are to be measured, there is a need to know the uncertainty of the input current measurements. In order to calculate the uncertainty of the input current, the first step was analyze the regression curve and evaluate its derivative at each current value shown in the first column of Table I (Input Current). The SD of current is the projection of the SD in voltage over the x-axis by the slope of the curve at that point. This can be found by dividing the SD of the output voltage by the derivative of the regression curve shown in Fig. 9 at each point. The results are shown in the fourth column of the table (Uncertainty). Table I shows the measurements, the averages, the standard deviations obtained and the calculated accuracy. A. Input/Output (Sensitivity) The LED output optical power responds parabolically to the input current, whereas photodiodes in photoconductive configuration responds linearly to the input light. Therefore we must expect a parabolic sensitivity of output voltage against input current for the transducer chain. In order to measure the input current by observing output voltage, a calibration curve must be obtained. This is shown in Fig 9 where each dot in the curve is the result of an average of 50 measurements at constant temperature. The regression line drawn over the input/output plot is a third order polynomial with regression coefficient R2 = 0.999908. This polynomial is used by the software to calculate the leakage current from the output voltage. B. Uncertainty of Leakage Current Measurement Accuracy of measurement can be done by sweeping the input range several times, from zero to 60 mA and calculating the standard deviation for each value. The steps below were used in order to calculate the uncertainty of the input current C. Thermal Drift Tests LED optical power fades as temperature increases. This is clear by observing in Fig. 2 the output power of the green LED which drifts about 5% for a temperature range of 20 to 50 °C. Therefore, it is expected to observe this drift in the transducer chain. The transducer thermal drift evaluation was performed by inserting both transmitter and receiver into an oven and varying the temperature covering the temperature range expected in the field. Figure 10 shows the results of this test. As expected, for all input currents, the output voltage decreases as temperature rises, particularly for 60 mA at 65 °C. However, as it will be seen, maximum currents observed in real applications rarely exceed 20 mA and local temperatures are limited to the range of 25 to 40 °C. In this range the error is about 3%, quite acceptable for this kind of application, when the idea is to monitor leakage current increase rather than to know it precisely. WERNECK et al.: DETECTION AND MONITORING OF LEAKAGE CURRENTS IN POWER TRANSMISSION INSULATORS Fig. 10. Temperature drift of the set transmitter-receiver. Fig. 11. Installation sequence. 1) The tower at left was chosen for the installation; 2) Equipment being hoisted to the top; 3) Transmitter box being fixed; 4) Arrows indicate the position of the sensor and the transmitter box; 5) Transmitter box in place and being tested by technician. V. F IELD I NSTALLATION AND DATA A SSESSMENT As stated above, the system was designed for monitoring currents in both high voltage TL and medium voltage distribution lines. Two Brazilian electric companies allowed the installation of the system in their operational grids: one in a 500 kV TL and another one in a 13.8 kV distribution line. The first prototype was installed in a single 500 kV TL tower and was kept there for several months while transmitting data. In a second row of experiments, several sensors were installed at a medium voltage distribution line. This section describes the installation of both systems. A. Installation in 500 kV Transmission Line The TL chosen for installation of the 500 kV monitoring system was located in the island of São Luis, at the Brazilian northern state of Maranhão where strong winds blowing from the seashore bring severe pollution to the insulators. Fig. 11 shows the installation sequence. The transmitter box (Fig.11-5) contains two 45 Ah batteries, enough for powering the receptor and GPRS transmitter for as much as 12 months. 1343 Fig. 12. Satellite picture showing the location of the measurement units (adapted from Google Maps). Fig. 13. Pictures of the installation sequence in a 13.8 kV line. 1) The pole chosen for the installation; 2) Equipment being hoisted to the top of insulator; 3) Complete system installed on the pole; 4) Indication of the sensor position and the high voltage line. B. Installation in 13.8 kV Distribution Line The medium voltage system was installed in a seashore area where the local electric company has had many problems due to flashovers and electric shortages. We installed five units in different locations of a large area so as to map and identify similar meteorological conditions. Fig. 12 shows a satellite picture of the area under study with the location of the remote units (RU). The installation in the field was performed without de-energizing the high voltage line using appropriated highvoltage procedures, Fig. 13 shows the installation sequence. After installation, the transducer starts acquiring data to the RU which are sent in real time to a specially designed web page where each RU can be accessed separately in order to check for the actual status of each insulator. VI. R ESULTS AND D ISCUSSION A. Real Time Analysis After installation, in both high- and medium-voltage lines, the RU started transmitting data immediately so it was possible 1344 IEEE SENSORS JOURNAL, VOL. 15, NO. 3, MARCH 2015 Fig. 14. Data collected from sensor installed on an insulator at 500 kV. Upper trace is the temperature; lower trace the leakage current. Each peak in the upper trace represents one complete day cycle in which the temperature rises from 25 °C during the night to about 40 °C at noon. Fig. 15. Data collected from 500 kV insulator show leakage currents in excess of 12 mA. to check locally on the Internet the adequate operation of the system. The high voltage version transmits to its web page the data acquired; in this case the leakage currents together with local temperature. Fig. 14 shows one of these graphs. Note that the temperature fluctuates from 25 °C during the night to about 40 °C at noon. On the other hand, the leakage current is nearly zero during the hotter part of the day but rises during the night when temperature drops and eventually, reaches the dew point. Notice also that the leakage current keeps a tendency of increasing from day to day, starting on Day 1 and increases progressively up to Day 7. Then, suddenly at Day 8 it is low again. This is because there was a tropical rain that day (notice the drop in temperature on Day 7) and the insulator was washed out from the deposited salt. After that day the current starts to rise again until the next raining event. Strong rain washes the insulator, fine rain or dew, on the other hand, makes the salt conducting, increasing the leakage current. Another example of information that can be extracted from these data is shown in Fig. 15, where very high peaks of currents occur in dry seasons without rains. This graph shows several peaks in excess of 12 mA. These currents represent a power of more than 5.000 W over the insulator. In an LT of 400 km, with each insulator burning such power, an enormous amount of energy is lost. These currents not only decrease the lifetime of the insulator but can also trigger the TL protection devices if many peaks occur at same time. The data obtained from RUs installed in medium voltage distribution lines can be seen in the system web page. Figure 16 shows the main page which can be accessed by authorized users. In this page there is a list of all the Fig. 16. Fig. 17. Web page screens of the system. Graph from field data obtained from the system web page. units installed. When a unit is chosen, its parameters are displayed in the screen as shown in Figure 16. Figure 17 shows the data collected from two RUs installed in the field. It is possible to create graphs of all parameters WERNECK et al.: DETECTION AND MONITORING OF LEAKAGE CURRENTS IN POWER TRANSMISSION INSULATORS such as temperature, dew point, humidity and leakage current of any location. During the generation of the graph in Fig. 17 (upper) all monitored parameters were selected: leakage current, humidity and temperature. However, just two or one type of data can be selected such as in Fig 17 (lower), where only the leakage current is shown. The data has been analyzed for two months in order to establish relationships between leakage current and climate parameters, and between two different sites of installation. It is possible to observe that the leakage current is strongly related to the humidity. One can note in Fig 17 (upper) that larger humidity (green) leads to larger leakage current (orange), that is, high humidity increases the conductance of the polluted layer leading to arcs. It can also be observed that as the ambient temperature approaches the dew point the leakage current increases. Observing the two graphs of Fig. 17, a slightly similarity can be seen in the leakage current behavior. Despite the fact that these two RUs were installed about 3 km away from each other, their leakage current increasing patterns are similar. The intensity difference, however, can be explained by the fact that the RU 52237 (Fig. 17 (lower) is sheltered from pollutants by large trees and buildings. This analysis is important to determine the maximum distance that the RUs can be installed from each other. 1345 page proving, therefore, to be reliable. The small thermal drift observed in the sensor-transmitter set was unimportant for the data analysis. The LED/POF technology applied for the leakage current sensor presented some advantages over other techniques adopted previously: efficiency, easy installation, robustness and reliability. The objectives of this project were reached. However, in order to this technique be of any usefulness to electric companies, it is necessary to transform the data into information. This means to establish parameters which can determinate the real status of the insulator regarding the leakage current that flows to the ground. After such parameters have been established, it will be possible to issue the “washing warnings” meaning that if the set of insulators were not washed immediately, a flashover may occur. The establishment of these parameters will produce the logistic insertion of this activity in companies, reducing the risk of a failing insulator causing an energy shortage. To create these parameters it will be necessary to analyze a great diversified amount of data from different critical points of maritime pollution inside the area under study. Thus, it will be possible to determinate the optimal time to intervene, i.e., to carry out the washing of insulators. The data obtained from the sites of installation will be evaluated in order to be possible to increase the distance between them. All external parameters have to be taken into account, such as the buildings and trees around, the distance from the sea and roads. B. Statistical Analysis of the Data The resulting data of the leakage current monitoring performed by five RUs installed in field at the coastal region of Rio de Janeiro –Brazil (Fig. 12), were submitted to statistical analysis in order to obtain the climatic parameters necessary for the occurrence of the critical levels of leakage current and subsequent occurrence of flashover. The leakage current is considered critical if its root mean square values become higher or equal 5 mA. On the other hand, the occurrence of leakage current is related to a wet environment, in the presence of dew. The dew occurs when the local temperature is close enough to the dew point temperature, so that to select the data it was considered the points where the difference between the local temperature and the dew point temperature was smaller or equal than 1 °C. Respecting these boundary conditions, the temperature and humidity figures related with the occurrence of critical levels of leakage current were isolated from the raw data collected in a 12 months interval. Then, a statistical analysis was performed using the selected data, from which, averages (AVG) and standard deviations (STD) were calculated: Temperature: AVG = 20.3 °C; STD = 1.7 °C. Humidity: AVG = 98.1%; STD = 1.8% These calculated values establish the boundary climatic conditions necessary for occurrence of critical levels of leakage current. In these cases it would be indicated the intervention of the maintenance team for washing procedure of the isolators. VII. C ONCLUSIONS The system has been operating for several months in the field with continuous monitoring and sending data to the web R EFERENCES [1] L. H. Meyer et al., “A study of the correlation of leakage current, humidity and temperature of 25 kV insulators in urban and rural areas,” in Proc. Annu. Rep. Conf. Elect. Insul. Dielectr. Phenomena (CEIDP), Oct. 2011, pp. 398–402. [2] I. Ramirez, R. Hernandez, and G. Montoya, “Measurement of leakage current for monitoring the performance of outdoor insulators in polluted environments,” IEEE Elect. Insul. Mag., vol. 28, no. 4, pp. 29–34, Jul./Aug. 2012. [3] Z. Tingtao, L. Tianyu, C. Ke, and H. Xiaoguang, “On-line monitoring system of insulator leakage current based on ARM,” in Proc. 6th IEEE Conf. Ind. Electron. Appl. (ICIEA), Jun. 2011, pp. 365–369. [4] A. G. Kanashiro and G. F. Burani, “Leakage current monitoring of insulators exposed to marine and industrial pollution,” in Proc. IEEE Int. Symp. Elect. 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Durana, J. Zubia, J. Arrue, G. Aldabaldetreku, and J. Mateo, “Dependence of bending losses on cladding thickness in plastic optical fibers,” Appl. Opt., vol. 42, no. 6, pp. 997–1002, 2003. [16] W. Daum, J. Krauser, P. E. Zanzow, and O. Ziemann, POF— Polymer Optical Fiber for Data Communication. Berlin, Germany: Springer-Verlag, 2002. [17] Nichia Corp. (Jun. 2, 2014). Specifications for Blue LED. [Online]. Available: http://www.nichia.co.jp [18] Nichia Corp. (Jun. 2, 2014). Specifications for Green LED. [Online]. Available: http://www.nichia.co.jp [19] Nichia Corp. (Jun. 2, 2014). Specifications for Red LED. [Online]. Available: http://www.nichia.co.jp Marcelo Martins Werneck was born in Petrópolis, Brazil. He received the Degree in electronic engineering from the Pontifícia Universidade Católica of Rio de Janeiro, Rio de Janeiro, Brazil, in 1975, the M.Sc. degree from the Biomedical Engineering Program, Federal University of Rio de Janeiro (UFRJ), Rio de Janeiro, in 1977, and the Ph.D. degree from the University of Sussex, Brighton, U.K., in 1985. He has been with UFRJ since 1978, where he is currently a Lecturer and Researcher. He is also the Coordinator of the Instrumentation and Photonics Laboratory at the Electrical Engineering Program, UFRJ. His research interests include fiber optics sensors, transducers, and instrumentation. Daniel Moreira dos Santos was born in Rio de Janeiro, Brazil. He received the Degree from the Federal Centre of Technology Education Celso Suckow da Fonseca, Rio de Janeiro, in 2010, as an Electronic Engineer. He is currently pursuing the M.Sc. degree at the Photonics and Instrumentation Laboratory, Electric Engineering Program, Federal University of Rio de Janeiro (UFRJ), Rio de Janeiro. He is also a Researcher with the Institute for Post-Graduate Studies and Research in Engineering, UFRJ. His research interests include sensors, instrumentation, and fiber optics. IEEE SENSORS JOURNAL, VOL. 15, NO. 3, MARCH 2015 Cesar Cosenza de Carvalho was born in Rio de Janeiro, Brazil, in 1966. He received the B.Sc., M.Sc., and D.Sc. degrees in electronic engineering from the Federal University of Rio de Janeiro (UFRJ), Rio de Janeiro, in 1989, 1994, and 2000, respectively. He is currently a Researcher with the Instrumentation and Photonics Laboratory, Electrical Engineering Program, UFRJ. His research interests include fiber optics, sensors, transducers, and optoeletronic instrumentation. Fábio Vieira Batista de Nazaré was born in Maceió, Brazil, in 1984. He received the Degree in electronic engineering from the Universidade Federal de Pernambuco, Recife, Brazil, in 2007. He was a Research Engineer with the Nuclear Instrumentation Laboratory, Regional Center of Nuclear Sciences, Recife. He received the M.Sc. degree in electrical engineering from the Institute for Post-Graduate Studies and Research in Engineering, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil, in 2010, where he is currently pursuing the D.Sc. degree at the Instrumentation and Photonics Laboratory, Electrical Engineering Program, and is also an Electronic Engineer. Regina Célia da Silva Barros Allil was born in Rio de Janeiro, Brazil. She received the Degree in electronic engineering from the Faculdade Nuno Lisboa, Rio de Janeiro, in 1988, and the M.Sc. degree from the Biomedical Engineering Program, Federal University of Rio de Janeiro (UFRJ), Rio de Janeiro, in 2004, and the Ph.D. degree from the Electronic Engineering Program, Instrumentation and Photonics Laboratory, UFRJ, in 2010. She is currently a Researcher with the Brazilian Army Technology Center, Rio de Janeiro. Her research interest lies in fiber optics sensors and optoelectronic instrumentation.
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