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Satellite System Developed for Monitoring Insulator Pollution Levels on Transmission Lines in Brazil


UTILITY PRACTICE & EXPERIENCE

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Pollution accumulation on the surfaces of glass insulator strings in an environment of almost constant high humidity can lead to formation of conductive paths and resulting partial discharges. The intensity and rate of such discharges can quickly rise and eventually reach the point where the insulation is overwhelmed and there is a full line-to-ground discharge. Indeed, engineers at the Companhia Hidro Elétrica do São Francisco (CHESF), who are responsible for electricity generation and supply in northeast Brazil, estimate that pollution flashovers cause about a third of all power disruptions experienced on their overhead transmission network each year. Classifying the amplitude and time rate of short pulses arising in insulator leakage current waveforms is one way for power utilities to monitor contamination levels since these correlate directly with partial discharge activity. At any given level of humidity, these parameters will yield a good indication of the degree of pollution and allow maintenance staff to schedule cleaning in advance of the start of widespread problems. A locally developed system for monitoring pulses within the leakage current signal already exists. Now, this measurement system has been integrated into a satellite-based communication system as part of a new strategy proposed for remote sensing of pollution. To this effect, a special sensor system was developed and installed on 230 kV and 500 kV towers spread across four northeastern Brazilian states – each with differing microclimates. Basically, the system is comprised of leakage, humidity and temperature sensors, an electronic processing module and a satellite communication link. This article, contributed by Eduardo Fontana, J. F. Martins-Filho, R. A. Lima, G. O. Cavalcanti, T. L. Prata and R. B. Lima at the Federal University of Pernambuco in Recife as well as S. C. Oliveira at the University of Pernambuco and F. J. M. M. Cavalcanti from CHESF, summarizes what has been achieved and presents an analysis of initial real time field results.

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have had to be washed as often as once or twice a month, especially during the summer. In the case of one especially problematic line – the 55 km Paulo Alfonso-Xingó 500 kV line – agricultural and dust pollution once required washing the line’s porcelain insulators four times each year before they were ultimately replaced by silicone types with higher specific creepage. Another example of the impact of pollution on the CHESF network is an important substation located near the coastal city of Fortaleza and where high levels of maritime contamination have triggered problems of recurring flashovers. This has required such remedial measures as applying RTV silicone coatings.

On-Line Pollution Monitoring System

It is against such a backdrop of a severe pollution environment affecting service performance of its network that CHESF sponsored a project, led by researchers of the Photonics Group at Federal University of Pernambuco, to develop an effective means to monitor for incipient problems due to leakage current on line insulators. The diagnostic tool developed in this regard consists of an optical sensor made up of an 840 nm LED in a pigtail type configuration and terminated in an FC connector. The LED terminals are then connected in parallel with the insulator nearest to the tower’s ground terminal so as to shunt the leakage current through this point (see schematic in Fig.1). The processing module comprises detection, gain and comparison stages with the last stage, along with temperature and humidity sensors, connected to a PIC 16F877A microcontroller. Two BNC female connectors then permit real time visualization of leakage current

Background

The Companhia Hidro Elétrica do São Francisco (CHESF) is based in the coastal city of Recife in northeast Brazil and operates an overhead transmission network comprising over 18,000 km of lines. CHESF has traditionally used mostly glass cap & pin strings and recurring past problems affecting these have included pin corrosion and vandalism. Pollution, however, and its impact on line insulation has been by far the dominant concern. For example, insulators on some lines Pollution map of the CHESF network.

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Tension strings in areas of high vandalism near Recife employ alternating glass-porcelain strings to minimize risk of power arcs that lead to line drops. Composite insulators (right photo) are increasingly being used to resolve such problems.

pulse occurrences in any range from N1 to N4 using 18bit temporary counters. After one hour of operation, the temporary registers are saved in a non-volatile memory. Counts N1 through N4, average humidity and average temperature are then all organized into two 64-bit packets with each packet transmitted via satellite every 30 minutes. Temporization routines are based on the universal time clock (UTC) obtained from the satellite modem and avoid any errors caused by deviations in crystal frequency.

Fig. 1: - Schematic of field experiments set up.

waveforms in two different ranges. There is also an RJ-12 connector, responsible for in-circuit serial programming. This allows for quick software updating as well as a DB-09 serial interface connector to a satellite modem. Based on previous laboratory and field testing, pulse amplitudes (Ip) on the leakage current signal related to partial discharges are classified into ranges N1 through N4, defined as follows: (1) N1, for Ip > 5 mA, (2) N2, for Ip > 10 mA, (3) N3, for Ip > 20 mA,

The satellite communication link, for its part, uses a modem with built-in planar antenna. This high level of integration minimizes the complexity of installing it on a typical transmission tower. The processing module and satellite modem are both powered by a photovoltaic system, with the module and part of the power system placed inside a custom built cabinet to protect them against the environment (see Fig. 2). The satellite modem is placed on top of the cabinet while four handles inserted into the cabinet facilitate attachment to towers. A database and a web-based graphical user interface (GUI) application were developed to download

(4) N4, for Ip > 40 mA. The time rate of pulses recorded in each range depends on the degree of pollution deposited on the surfaces of insulator strings being monitored as well as on ambient relative humidity. Interruption based routines register

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Fig. 2: Photograph of the sensor system developed for field experiments.

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above were installed on towers distributed across the northeastern Brazil. Two of these towers were located near substations while the remaining four were located at selected points along the line (see Fig. 3). The decision whether to place any given leakage current sensor on a suspension versus a tension tower was made by CHESF's own maintenance staff taking into account the complexity of the installation procedure. Below is a summary of preliminary findings from each tower monitored. These illustrate the wealth of information offered in the area of how pollution and climatic variables together impact discharge activity of line insulators. A. 500 kV Tower in Angelim The 500 kV Angelim substation is located in the state of Pernambuco, which has a mostly semi-humid tropical microclimate. The optical sensor in this case was installed on a glass suspension string and Fig. 4 shows the activity recorded over a 30-day period.

Fig. 3: Monitoring system installed on transmission tower.

the information from the satellite link provider website. They also permit it to be stored while allowing both graphical and/or tabular access to data. There is even the possibility of logging information and recording any comments about observed measurements. In order to study the overall impact of pollution, temperature and humidity on insulators on CHESF transmission lines, six of the sensor systems described

At the start of monitoring, occurrences in the range N1 were observed with more than 75K pulses per hour. This suggests that some pollution was already present on insulator surfaces. Beginning the 7th day, pulses in the N2 range were first registered and, from the 10th day on, such occurrences appear almost every night, indicating an increase in pollution deposition. Between the 22nd and the 24th day, the highest partial discharge activity was observed and once all levels were activated no further activity was

Fig. 4: Activity registered in the 500 kV Tower – Angelim. The circle in the plot indicates the occurrence of rain.

Fig. 5: Activity registered in the 230 kV Tower - Barra de SĂŁo Miguel Tower. The circle in the plot indicates the occurrence of rain.

Monitoring Leakage Currents

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observed until the conclusion of the experiment. Over this same period, the temperature and relative humidity diverged sharply from normal (as indicated in the circular areas of Figure 4), i.e. there was no increase in temperature during the day and humidity remained near 100%. This was likely due to rainfall, as confirmed by data from the Brazilian Institute of Space Research, measured by a precipitation sensor 30 km from the monitored tower.

Fig. 6: Activity registered in the 500 kV Pitaguary Tower. The circle in the plot indicates the occurrence of rain.

Future work in this area will be focused on longer observation periods in order to determine typical annual pollution deposition cycles at each site.

During rainfall, water droplets create a path for leakage current, thereby increasing partial discharge activity and resulting in high current pulse counts on all levels. The rain then naturally washes the insulator strings and, once these become clean and dry, activity diminishes significantly (also clearly evident in Fig. 4). B. 230 kV Tower in S達o Miguel dos Campos This 230 kV tower is located in a sugarcane field in S達o Miguel dos Campos, in the state of Alagoas, where the climate is typically hot and humid. The only access to the field is a dirt road and truck traffic as well as seasonal burning of sugarcane are the main sources of pollutants accumulating along insulator strings. The sensor system in this case was installed on a porcelain tension string and Figure 5 shows activity recorded over a 30-day period. At the beginning, there was activity in the N1 range with more than 200K pulses recorded per hour as well as in the N2 range with more than 7K such pulses per hour. As humidity levels are the same as in the first example above (Angelim), the insulator string may be more polluted but this could also be due to different contamination behavior of porcelain and glass. With long-term study, it will be possible to better assess the risk levels for both types of insulator strings.

Fig. 7: Activity registered in the 500 kV Fortaleza Tower. The circle in the plot indicates the occurrence of rain.

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On the second day of measurement, activity in the N3 range appears and following the humidity trend, recurs almost every night. On the 17th day (as seen in Fig. 5) activity in the N1, N2, N3 and N4 ranges with 120K, 40K, 7.5K and 500

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pulses per hour, respectively. Since activity is becoming higher, pollution deposition is likely increasing. On the 22nd day, a similar reversion to normal behavior for humidity and temperature occurred. Again, it is possible that this was due to rainfall and, indeed, over the following days no activity was registered, probably because of natural washing. Yet, within only 6 days, the sensor started to register activity in the N1 range, indicating the start of a new pollution deposition cycle. C. 500 kV Tower in Pitaguary The 500 kV Pitaguary tower is located near a lake in the state of Cear叩, about 26 km from its capital, Fortaleza, and in a semi-arid tropical climate. The optical sensor in this case was installed on a glass suspension string and Figure 6 depicts activity record over a 25-day period. During the experiment, activity in the N2 and N3 ranges was observed, but not in the N1 range, possibly due to malfunction in the processing module or another as yet unknown cause. As can be seen in Fig. 6, the system recorded about 1.5K pulses per hour in the N2 range on the 5th day and 81 pulses in the N3 range on the 7th day. These records demonstrate that the string already had some accumulation of pollutants over its surface. Along the experiment there was no record of current pulses in the N4 range. On the 17th day (as observed in the highlighted region of Fig. 6), the temperature-humidity reversion indicates rainfall. Afterwards, there was no significant activity registered until the end of the observation period. D. 500 kV Tower in Fortaleza This 500 kV tower, located near Fortaleza II substation, had activity monitored on a glass suspension string for a period of 25 days. As shown in Fig. 7, at the start there were about 20K pulses per hour in the N1 range and about 1K in the N2 range, indicating pollutants were already present on the insulators. On the 6th day, when the humidity went up to the same levels observed in the beginning of the experiment, the highest activity was recorded for this

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Fig. 8: Activity registered in the 500 kV Carcará Tower. The circle in the plot indicates the occurrence of rain.

Fig. 9: Activity registered in the 230 kV Mossoró Tower.

string, with more than 100K, 16K and 30 pulses per hour in the N1, N2 and N3 ranges respectively. It is evident that the string was becoming increasingly polluted.

pulses in the N2 range, were recorded. During 25 days of monitoring two rain patterns were observed, as highlighted. Insulators were likely being cleaned due to these rain events.

Figure 7 also illustrates possible rainfall between the 17th and 18th days, with reversion of typical temperature and relative humidity waveforms. Then, from the 18th day on, until the end of the experiment, almost no activity was observed. Isolated activity of only 2 pulses per hour was observed in the N4 range.

F. 230 kV Tower in Mossoró The 230 kV Mossoró tower is located in the state of Rio Grande do Norte, in a region with a semi-arid tropical climate and alongside a dirt road. Here, a glass tension string was monitored for a period of 25 days.

From data presented in Fig. 9, it can be concluded that the string was clean E. 500 kV Tower in Caracará at the start of the experiment and that The 500 kV Caracará tower is located pollution deposition increased until in the state of Ceará with its usual the region highlighted on the 15th day. hot, semi-arid tropical climate and the At that point, rainfall is detected from optical sensor here was installed on a the reversion in temperature pattern glass suspension string. with no further activity recorded since the insulators became clean. Erratic As shown in Fig. 8, no significant behavior of the relative humidity activity of leakage current was detected pattern over the test period is probably until the 17th day, when 27K pulses related to connection problems with the per hour in the N1 range and 2.7K humidity sensor or to the sensor itself.

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Summary & Conclusions

A network of six sensor systems was deployed across regions exhibiting six different microclimates in northeast Brazil. The systems are already transmitting information on leakage current activity via satellite. Analysis of preliminary data, combined with rainfall occurrence, has made it possible to infer the level of pollution accumulated on insulator strings on each monitored tower. Further research in this area will be focused on longer observation periods in order to determine typical annual pollution deposition cycles at each site. Given such longer time frames to collect and analyze data, scheduling preventive maintenance on the overhead transmission system will be made more efficient in terms of being done only when needed. Even more important, it will also become more effective in terms of overcoming historic problems of pollution flashover. 

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Satellite System Developed for Monitoring Insulator Pollution Levels on Transmission Lines in Brazil  

INMR Article Satellite System Developed for Monitoring Insulator Pollution Levels on Transmission Lines in Brazil

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