Satellite Telemetry System for Pollution Detection on Insulator Strings of High-Voltage Transmission Lines Ricardo A. de Lima, Eduardo Fontana, Joaquim F. Martins-Filho, Thiago L. Prata, Gustavo O. Cavalcanti and Renato B. Lima Grupo de Fotônica, Departamento de Eletrônica e Sistemas Universidade Federal de Pernambuco. Recife – PE, Brazil
[email protected]
Sérgio Campello Oliveira Departamento de Sistemas e Computação Universidade de Pernambuco. Recife – PE, Brazil
[email protected]
Fernando J. M. M. Cavalcanti Companhia Hidroelétrica do São Francisco CHESF. Recife – PE, Brazil
Abstract — Leakage current monitoring is a promising method of indirect estimation of pollution levels on insulators strings of high voltage transmission lines. This paper describes the development of a complete real time leakage current monitoring sensor system. This system is composed of an optical sensor with a fiber link, a processing microcontrolled module, a satellite link and a solar panel with battery. The most important characteristics obtained from the leakage current signal are internally stored in the PIC16F877A microcontroller and transmitted through the satellite communication link. The information is downloaded from the website of the satellite link provider and inserted in a database with a web-based graphical user interface. Index Terms — Optical Fiber Transducers, LEDs, Flashover, Insulators, Leakage current, Power Transmission Lines, satellite telemetry.
I.
INTRODUCTION
When insulators strings are exposed to polluted environments a pollutant layer is deposited over the insulators surfaces. In the presence of high humidity this layer becomes conductive allowing a leakage current, which heats the conductive layer creating dry bands. The high electric field near the dry band breaks the dielectric resistance of the surrounding air, creating a partial discharge near the insulator surface [1], [2]. The partial discharges (PD) rate and intensity may increase until a complete discharge known as flashover breaks the insulation between the high voltage cable and the grounded structure. To measure pollution levels deposited over the insulator string Equivalent Salt Deposit Density (ESDD) and Non Soluble Deposit Density (NSDD) techniques can be used [3], [4]. One way to indirectly estimate the pollution level is by monitoring the leakage current flowing in the conductive pollutant layer. The leakage current signal has short pulses superposed to the sinusoidal waveform correlated with the partial discharges [5], [6]. Measurement of the rate and intensity of these short pulses is a way to indirect measure the PD rate and intensity. In previous publications we have reported on the first versions of the fiber optic based leakage current monitoring
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and partial discharge detection system, which comprised an optical sensor with fiber link connected to a processing microcontrolled module [7], [8]. We also reported on the results obtained from this system installed in transmission lines in the field [9]. In this previous version of the system the data were stored in the microprocessor memory for on site collection, which represented a costly operation involving a few workers and also a delay of about a month before the data could be analyzed. This paper describes the development of a complete real time partial discharge detection system. This system is composed of an optical sensor with an optical fiber link similar to the previous one, connected to a new processing microcontrolled module and a satellite link. The choice of a satellite communication approach for the system developed in this work is due to the fact that most high voltage transmission line towers that require surveillance are located in remote regions in the field, not usually covered by other wireless communication technologies. This system is electrically powered by a battery and a photovoltaic solar panel. The most important characteristics obtained from the leakage current signal are internally stored in the microcontroller and transmitted by the satellite communication link. The information is downloaded from the website of the satellite link provider and inserted in a database with a web-based Graphical User Interface (GUI). THE MONITORING SYSTEM A. Optical Sensor II.
The optical sensor used in our system is the same reported in [7]. It consists of a LED connected in parallel with the insulator nearest to the ground level, as illustrated in Fig. 1. The LED electric terminals are connected to the metal caps of the insulators so that all leakage current is captured by the sensor. Just prior to a flashover occurrence, high amplitude current peaks appear within the leakage current waveform, with a high possibility of permanent damage to the LED, so the use of expensive LEDs must be avoided.
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Off-the-shelf LEDs, such as those used for remote control of electronic appliances are very cheap and easily replaceable. Infrared LEDs can stand 300 mA of continuous current and up to 1 A on pulsed regime. This type of LED is not constructed for coupling light into optical fibers but with proper modification and simple fabrication strategies it can be adapted for current sensing and efficient coupling to optical fibers. To improve the LED to fiber light coupling the plastic lens of the LED is removed with a portable laboratory polishing system. The LED emitting surface is aligned with a multimode silica fiber and a UV curable epoxy is used to glue the LED and the multimode fiber. The other fiber optic end is terminated with a FC-PC connector. An optical sensor photograph is shown in Fig. 2. The details about this fabrication process are already published in previous papers [7], [8]. Note that the LED works as a transducer and also as the signal transmitter in the optical fiber link that connects the sensor to the processing module, as shown in Fig. 1. B. Processing Module A processing module was developed to detect, amplify, record the main information related to leakage current and forward this information to a satellite modem. The module uses a PIC16F877A microcontroller and includes two BNC female connectors for real time monitoring of the leakage current signal on two different ranges using an oscilloscope during
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installation or maintenance procedures. It also includes a DB-9 serial interface with the satellite modem and a RJ-12 In-circuit Serial Programming (ICSP) interface for in-loco control and fast firmware updating. Figure 3 shows the processing module block diagram. The detection system has a large area PIN silicon photodetector to ensure that all light guided by the multimode optical fiber is spotted inside the active region of the photodetector. This detection system has 20 μs of rise time. The amplification stage consists of four fast response operational amplifiers, each one with independently adjustable gain. The reason to use four distinct amplifiers is to reduce the inter-level interference introduced by the hysteresis on the comparison system. These comparators are responsible for the generation of proper TTL levels to the microcontroller I/O ports. Based on previous laboratory and field tests, the pulses on leakage current signal related to the partial discharges are classified into four ranges, which are: above 5, 10, 20 and 40 mA, namely N1, N2, N3 and N4 respectively. Levels N1 and N2 may be correlated with weakly polluted insulators or exposed to low humidity environments. Levels N3 and N4 can be correlated with severely polluted insulators exposed to high humidity environments. As the pulses are only present when polluted insulators are exposed to humidity, the processing module is equipped with a humidity sensor to permit the analysis of the partial discharges activity. A temperature sensor, not included on previous versions of the processing module [8] improves characterization of the environment. The peak detection system records the maximum pulse amplitude occurred. This peak detection system will be used to correlate the maximum pulse amplitude with humidity. A 10-bit resolution analog to digital converter is used to digitize humidity, temperature and maximum peak information. After one hour the average values of humidity and temperature together with the maximum peak value are stored in a non-volatile memory. Interruption based routines register any pulse occurred in any range from N1 to N4 in 18-bit temporary counters. After
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one hour operation the temporary registers are saved on nonvolatile memory. The N1 to N4 registers, average humidity, average temperature and maximum peak value are organized into two 64-bit packets. Each packet is transmitted via a satellite link every half-hour. Temporization routines are based on the Universal Time Clock (UTC) obtained from the satellite modem. The use of UTC avoids temporization errors caused by crystal frequency deviations. C. Satellite System Configuration Figure 1 shows a schematic diagram of the satellite system configuration. The processing module and the satellite modem are powered by a system that consists of a photovoltaic solar panel, a charge controller and a battery. The power system and the processing module were placed into a sealed cabinet as shown on Fig. 4, for field experiments. All parts were fixed for safe transportation and installation procedures. The satellite modem was placed on the cabinet top in order to ensure direct communication between modem and satellite. The modem comes with its own antenna in a compact sealed package as shown in Fig. 4. No extra satellite transmission device is needed. The satellite constellation is geostationary and the upload frequency ranges from 1525.0 to 1559.0 MHz, whereas the download frequency is between 1626.5 and 1660.5 MHz. Behind the cabinet there are four handles to be used to hold the cabinet on the grounded structure of the transmission line tower. On the right side of the picture on Fig. 4 there is a tube to expose the humidity and temperature sensors to the air around the cabinet, but avoiding direct contact with water from rain. The solar panel is also shown in Fig. 4. The optical signal obtained from the optical sensor is received and analyzed by the processing module as described in section II-B. The information is organized in 64-bit packets and transmitted every half-hour. Transmitted packets become
available for download from the website of the satellite link provider. Using a proprietary protocol a homemade webapplication downloads the packets and stores them in a database. Authorized system users can use a second web-based GUI application to access the database information as tables or graphs. The GUI application first screen lists operating sensors by partial discharge activity in descending order. When one sensor is selected the application shows a set of options for the visualization of the partial discharge activity registered as graphs or tables. The application allows users to insert text as observations, such as visual activity reports or washing dates. It also includes a configurable alert level, classified as low, moderate or high level partial discharge activity. Inspectors are able to configure the alert level based on their experience and on field inspections. III.
RESULTS
One month laboratory experiments were conducted to test the system performance, specially the satellite modem control and communication routines. Different oscillators were used to emulate random leakage current activity. The leakage current emulator was connected to the optical sensor, the signal was fiber optic transmitted to the processing module and the 64-bit information packets were satellite transmitted every half-hour. Figure 5 shows a one month simulated activity downloaded from the website of the satellite link provider and registered by the GUI application. As can be seen in Fig. 5 the system can register up to 262143 pulses in one hour of operation. This is enough capacity to register activities as high as one pulse per 60 Hz current cycle, avoiding counting pulse saturation as reported on previous works [9]. The latency of the half hour spaced transmissions during approximately two days was measured. As shown in Fig. 6 the average latency was very low, around two minutes. But near the fifteenth transmission a very long latency of approximately 45 minutes was observed. In the next 15 transmissions, 7 other long latencies were observed. The satellite modem comes with Simulated random leakage current activity 250000
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Fig 4 -Photograph of the sealed cabinet used for equipment accommodation for field experiments.
Fig. 5 –Laboratory results for one month of simulated random leakage current activity.
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Latency, min
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a default setting that if a packet remains more than one hour in the modem stack, it is discarded. Longer experiments are necessary to determine if a change in the modem default configuration, to increment stack packet lifetime, will be needed.
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To determine the efficiency of the solar system power supply a 1 Ω precision resistor was serially connected to the battery positive terminal. Measuring the voltage on the resistor allowed to monitor the current flowing through the battery. Battery charge can be obtained by integrating this current on time. Figure 7 shows a three day battery charging cycle. As can be seen in Fig. 7 the total amount of charge entering battery during daylight is greater than the charge leaving the battery at nights. At the moment of this paper submission six systems are being tested on five transmission line towers on field, four in 230 kV transmission lines towers and two in a 500 kV tower. To generate preliminary reliable results, oscillators are used to emulate leakage current activity. This experiment will measure transmission latency, packet loss probability and solar power supply system reliability. IV.
CONCLUSIONS
A complete and autonomous system for real time monitoring of leakage current and partial discharge detection on insulators strings via an optical sensor using satellite telemetry was proposed and demonstrated. This system also has a web-based, user friendly software to download, store and process the data from several sensors, to give support for preventive maintenance procedures, reducing the time for decision. Future work will concentrate on analysis of long time leakage current activity to automatically generate alert levels allowing transmission companies to better schedule insulation string washing and maintenance procedures.
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Fig. 6 – Latency of half hour spaced 64 bits packets satellite transmission. Long time experiments will be able to identify any periodicity of long latencies observed, for example, on specific hours of the day.
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ACKNOWLEDGMENT This work was financed by Companhia Hidro Elétrica do São Francisco (CHESF), Contract CT-I-2008.0210.00. REFERENCES [1]
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