Utilities throughout the world use infrared cameras to inspect electrical distribution components such as overhead conductors, disconnect switches and bus work. While infrared scanning is effective at revealing poor connections, which cause components to operate at higher temperatures than surrounding components, the application of infrared technology on underground distribution systems poses unique challenges.
In Con Edison's underground system, arc proofing is done on all medium-voltage cables, splices and terminations operating at 2,400 V or greater in distribution manholes and vaults to protect cables from arc flashes resulting from faults on adjacent cables. Arc proofing is applied in the form of non-adhesive tape wrapped around an entire cable and splice, from duct edge to duct edge. Because arc proofing provides some degree of thermal insulation, which may mask the heat generated by defects, it has historically been blamed for the poor results obtained with infrared scanning on Con Edison's underground system.
The utility recently conducted both lab and field studies to understand the potential effectiveness of infrared technology to identify defective underground cables and splices. The laboratory study showed that an infrared camera could identify improperly made splices operating at elevated temperatures. The effectiveness of the technology was dependent on the severity of the defect, the type of splice and levels of current. The results of the laboratory study were used to guide field scans of operating splices, and two defective splices were successfully identified. These results showed promise for using infrared technology in the underground system.
For infrared scanning to effectively identify defects in underground distribution components, temperature differentials must be detected on the surface of the arc proofing. The amount of heat generated by a defect is a function of current. Therefore, it is critical to understand how the current level affects the heat produced by a defect, because distribution feeder loads vary by time of day, year and distance from the substation.
In order to develop a full understanding of the ability of infrared technology to identify defects in underground cables and splices, Con Edison's Cable and Splice Center for Excellence, a facility dedicated to advancing the understanding of cable systems and components, conducted an infrared lab study. The study compared the temperatures and thermal signatures of normal and defective splices, with and without arc proofing.
Normal and defective samples of three different types of splices — pre-molded, cold shrinkable and heat shrinkable — were constructed and tested. These splices use either bolted or compression connections in their construction. Both the cold- and heat-shrinkable splices used only compression connectors, while the pre-molded splices used a combination of both.
Compression connectors are crimped using a compression gun and die specific to the size and type of cable. If an improper gun or die is used, the connector can be incorrectly crimped. Insufficient compression results in high resistance and also may allow the conductor to move or slide out of position. Excessive compression may cause the connector to crack, reducing the effective contact area and increasing the connection resistance.
Bolted connections allow splices to be disassembled when required and rely on torque to achieve low connection resistance at the bolt. Insufficient torque may result in low contact pressure and cause a high resistance. Thermocouples were embedded within splices during construction to record the actual temperatures. For the pre-molded splices, thermocouples were placed on the connectors, bolts and yoke. For cold- and heat-shrinkable splices, thermocouples were placed on the connectors.
In the study, defective and normal splices were placed side by side so thermal scans would reveal the differences in a single picture. Because ambient light created the appearance of hot spots in the lab setting, a tarp was hung over the entire setup to shield the splices. In addition, the overhead lights were turned off while infrared pictures were taken to minimize light interference.
During the study, the splices were energized at low voltage and supplied with equal currents, which were gradually increased. The internal thermocouple temperatures were continuously recorded on a data recorder, and arc proofing surface temperatures were measured periodically by a FLIR T400 camera.
The study's results showed that with sufficient current flow, the infrared camera was able to identify defective splices operating at elevated temperatures. For all three splice types, the defective splices' connector and surface temperatures were higher than the respective normal splices. The pre-molded splice connector and surface temperatures were lower than the other splice types, owing to the greater mass of the central bus of the splice. In addition, the pre-molded splices required a greater current flow so a detectable surface temperature differential could occur.
The study also showed that at lower current levels, the surface temperatures of the normal and defective pre-molded splices may be close enough to make it difficult to differentiate them through infrared scanning. At current levels below about 350 A, the surface temperature difference of the normal and defective pre-molded splices is less than 1°C (33.8°F). This is within the margin of error for the infrared camera and, hence, such a small temperature difference would not permit the identification of a defective splice.
Based on the results of the lab study, engineers conducted infrared scans of underground cables and splices in manholes. The objective was to determine the effectiveness of infrared scanning under actual field conditions. Manholes with splices carrying higher currents were chosen, because defective splices in those structures would more likely be operating at elevated temperatures. In addition, manholes with multiple splices were chosen over manholes with fewer splices to maximize efficiency. A total of approximately 230 splices in 50 manholes were examined. Scans were conducted during the summer, when overall loads were higher.
Engineers took both infrared and visible pictures of operating cables and splices in each manhole. The infrared camera software allowed engineers to conduct detailed analysis back at the lab and determine the temperature at any point within an infrared image.
During the field study, a hot spot was found on a medium-voltage pre-molded splice that had been in service for approximately three years. One phase was visibly hotter than the other two, as seen through the infrared camera. The temperature differential between the hottest and coolest phases was more than 40°C (104°F). The splice was subsequently removed and examined. Upon examination at Con Edison's Cable & Splice Center for Excellence, it was determined there was an insufficient number of indents on the connector, and the depth of the indents was too shallow. In addition, the cross-linked polyethylene cable insulation showed signs of electrical tracking, which eventually would have led to failure.
The splicer who had constructed the defective splice was present for the forensic analysis and counseled on proper work practices. Con Edison also conducts periodic refresher training for all distribution splicers to reinforce proper work practices and introduce new developments in splicing technology.
The results of the lab study, field scans and identification of a defective splice have confirmed the effectiveness of infrared scanning on Con Edison's underground system. The study has implications in the design of an infrared inspection program aimed at identifying defective underground components.
The Con Edison system has more than 80,000 manholes. Conducting infrared scans in every manhole would not be cost effective. In order to effectively use infrared scanning, such a program should select manholes in which defects are most likely to be identified, consider the impact of system load, and reflect the design basis and reliability of individual networks.
An infrared camera's ability to detect a defect in a splice depends on the severity of the defect, the splice type and the amount of current flowing through the splice. While the severity of the possible defects is not known when selecting locations, infrared scans can be conducted during higher load periods when current flows are higher. Also, scanning locations closer to the substation can be selected since feeders in those locations will carry higher current.
Manholes containing more feeders would be more time and cost effective to scan. Infrared scans also would have more benefit on feeders whose reliability needs to be improved. Con Edison's future work on this project will include developing thermal models of various splices in order to predict how the surface temperatures of splices vary with loading and ambient temperature, in order to be able to accurately identify defective splices.
(uncompressed connector, loose bolt)
Neil Weisenfeld ([email protected]) has been with Con Edison of New York for 26 years and is currently department manager of distribution cable systems. He has previously worked in the areas of power generation, system operations, and substation and distribution engineering. He earned a BSEE degree from the City College of New York, and is a senior member of the IEEE and a registered professional engineer in New York.
Joseph Watts ([email protected]) has been with Con Edison for 21 years and is an associate engineer in the failure analysis and diagnostics section of the utility's Cable & Splice Center for Excellence. He has previously worked in the areas of energy services and steam operations. He earned a bachelor's degree in chemical engineering from the Polytechnic Institute of New York University.
Erica LeCount ([email protected]) is in her second year with Con Edison and recently completed the utility's management development program with rotations in distribution engineering and construction management. LeCount has managed projects involving electric distribution as well as solar and renewable technologies. She earned a bachelor's degree in civil and environmental engineering from Cornell University.
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