Thermal Inspection Program Finds Failing Dead-End Polymeric Insulators
UTILIT Y PRACTICE & EXPERIENCE Changes in the temperature profile along an insulator are increasingly seen as one of the reliable methods to monitor for possible degradation or deterioration during service. This is done using a range of different thermography instruments, some ground-based, others better adapted for helicopter inspection. This article, contributed by INMR Columnist, Dr. William Chisholm, reports on how a large Canadian utility has benefited from thermographic imaging
Thermal Inspection Program Finds Failing Dead-End Polymeric Insulators Thermographer Derrick Brydges inspects thousands of insulators every year, both from the ground at substations and then again from the air during helicopter patrols. The utility he works for, Hydro-One in the Canadian province of Ontario, then follows up every thermal anomaly he detects with an on-site visit to make any necessary repairs, whether the heat rise indicates a bad joint in a conductor or some hidden defect in a non-ceramic insulator. It was a cold day last November when Brydges and Len Gunson, the senior helicopter pilot at Hydro-One, flew over a critical junction in North Bay. Outside it was about –11° C with a
wind of about 15-20 km/h and a mix of clouds and snow showers. The steady wind from the southwest made Gunson’s flying job easier by giving him something to ‘push against’. However, the wind was also making the thermography inspection more difficult. This was because forced convection (or wind chill) was rapidly dissipating heat energy and all of Brydges’ many years of experience were much needed to set up the thermal camera correctly for such conditions. Thermal inspection of transmission conductors using infrared imaging cameras has had a long history of success. As early as 1971, a survey
inspection. Whereas infrared works well at relative humidity values above 80%, ultrasonic inspection is generally superior under dry conditions. Overcast, cloudy days are better for reducing the effects of solar radiation, in much the same way that thermometers read too high when placed in direct sunshine. Whereas infrared inspection is more sensitive to circumferential and head cracking, ultrasonic inspection is usually better when it came to at detecting radial cracks.
F igure 1 : Typic a l de fe c ts to be monitore d in the c a s e of porc e la in c a p & pin dis c ins ula tor. of 8000 km of transmission circuits in England turned up more than 515 defective joints – a rate of one problem per 47 km of conductor. This, along with the proven ease and safety of helicopter inspection, has made thermal cameras an important line and station reliability assessment tool at many utilities the world over.
fully shorted or in good condition. For this reason, the best advice considering that most of the insulators in a string are usually sound is that any insulators showing either hot or cold during thermographic inspections should be reported for follow-up. An effective complement to infrared analysis has been ultrasonic
The use of thermal cameras to detect defective porcelain insulators, however, has proven more difficult than detecting problems in metallic joints. The resistance of breakdown channels, such as those between the top of the pin and the cap (as in Figure 1), can be high. Even under a service voltage stress of about 10 kV per disc, the internal power dissipation of a failing porcelain disc insulator leads to only a small temperature rise (less than 1° C) when compared to a disc that is either
In recent years, the power industry has moved increasingly towards the use of corona cameras that detect the ultraviolet emission of discharge activity rather than the ultrasonic noise signature itself. Under the same conditions favorable to ultrasonic inspection, the overlay of the detected corona activity on a visual image of the insulator can help pinpoint the exact locations of any problems. At present, most corona inspection cameras are used from the ground. Special optical housings with quartz glass are needed for helicopter installation and there is a considerable additional investment to set up the ‘helicopter office’ shown in Figure 2, set up for both highresolution visual and Thermovision.
inspections to detect insulators in need of replacement.
F igure 2 : He lic opte r ‘flying offic e ’ with c ombine d highre s olution vis ua l a nd the rma l ima ge re c ording. Photo: INMR ©
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UTILIT Y PRACTICE & EXPERIENCE Changes in the temperature profile along an insulator are increasingly seen as one of the reliable methods to monitor for possible degradation or deterioration during service. This is done using a range of different thermography instruments, some ground-based, others better adapted for helicopter inspection. This article, contributed by INMR Columnist, Dr. William Chisholm, reports on how a large Canadian utility has benefited from thermographic imaging
Thermal Inspection Program Finds Failing Dead-End Polymeric Insulators Thermographer Derrick Brydges inspects thousands of insulators every year, both from the ground at substations and then again from the air during helicopter patrols. The utility he works for, Hydro-One in the Canadian province of Ontario, then follows up every thermal anomaly he detects with an on-site visit to make any necessary repairs, whether the heat rise indicates a bad joint in a conductor or some hidden defect in a non-ceramic insulator. It was a cold day last November when Brydges and Len Gunson, the senior helicopter pilot at Hydro-One, flew over a critical junction in North Bay. Outside it was about –11° C with a
wind of about 15-20 km/h and a mix of clouds and snow showers. The steady wind from the southwest made Gunson’s flying job easier by giving him something to ‘push against’. However, the wind was also making the thermography inspection more difficult. This was because forced convection (or wind chill) was rapidly dissipating heat energy and all of Brydges’ many years of experience were much needed to set up the thermal camera correctly for such conditions. Thermal inspection of transmission conductors using infrared imaging cameras has had a long history of success. As early as 1971, a survey
inspection. Whereas infrared works well at relative humidity values above 80%, ultrasonic inspection is generally superior under dry conditions. Overcast, cloudy days are better for reducing the effects of solar radiation, in much the same way that thermometers read too high when placed in direct sunshine. Whereas infrared inspection is more sensitive to circumferential and head cracking, ultrasonic inspection is usually better when it came to at detecting radial cracks.
F igure 1 : Typic a l de fe c ts to be monitore d in the c a s e of porc e la in c a p & pin dis c ins ula tor. of 8000 km of transmission circuits in England turned up more than 515 defective joints – a rate of one problem per 47 km of conductor. This, along with the proven ease and safety of helicopter inspection, has made thermal cameras an important line and station reliability assessment tool at many utilities the world over.
fully shorted or in good condition. For this reason, the best advice considering that most of the insulators in a string are usually sound is that any insulators showing either hot or cold during thermographic inspections should be reported for follow-up. An effective complement to infrared analysis has been ultrasonic
The use of thermal cameras to detect defective porcelain insulators, however, has proven more difficult than detecting problems in metallic joints. The resistance of breakdown channels, such as those between the top of the pin and the cap (as in Figure 1), can be high. Even under a service voltage stress of about 10 kV per disc, the internal power dissipation of a failing porcelain disc insulator leads to only a small temperature rise (less than 1° C) when compared to a disc that is either
In recent years, the power industry has moved increasingly towards the use of corona cameras that detect the ultraviolet emission of discharge activity rather than the ultrasonic noise signature itself. Under the same conditions favorable to ultrasonic inspection, the overlay of the detected corona activity on a visual image of the insulator can help pinpoint the exact locations of any problems. At present, most corona inspection cameras are used from the ground. Special optical housings with quartz glass are needed for helicopter installation and there is a considerable additional investment to set up the ‘helicopter office’ shown in Figure 2, set up for both highresolution visual and Thermovision.
inspections to detect insulators in need of replacement.
F igure 2 : He lic opte r ‘flying offic e ’ with c ombine d highre s olution vis ua l a nd the rma l ima ge re c ording. Photo: INMR ©
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F igure 5 : De ta il of the rma l ins pe c tion for ne a rby nonc e ra mic ins ula tor with te mpe ra ture a noma ly ne a r line e nd.
Comfortable in his flying office last November, Derrick Brydges was viewing an image of apparently cold conductors and warmer wood poles at a distance of about 20 meters (as shown in Figure 3). The 12-degree lens on the thermal camera used was gyro-stabilized with a Kelvin 111 Polytech Gimbal on the belly of the helicopter. The images were obtained with a fixed emissivity setting of 0.83, meaning temperature differences rather than the absolute values are of greatest interest. Figure 4 shows that there was nothing apparently wrong in the visual image taken at the same time. F igure 3 : T he rma l s c a n a nd photogra ph of powe r s ys te m junc tion from he lic opte r.
Relying on his trained eye, Brydges turned pilot Gunson back since he suspected that there was something wrong based on the thermal image he had obtained (Figure 3). The problem
Thermal inspections such as this one have been identifying a few defective polymeric units every year since about 2004. was not the glints of ‘cold’ from the end fittings of the non-ceramic insulator – these are normal. The conductor end fittings and joints also show colder temperature than the bare wire because they have a larger diameter and tend to run cooler. Similarly, the stripes of heat down the sides of the wood poles could also be ignored since the sun is low at these northern latitudes during this time of year. Rather, it was the temperature rise in the center of the insulator (about 2° C higher than the rest of the unit) that caught his attention. A closer look revealed that a second nearby insulator had a similar temperature rise near the line end, where electric stress is usually highest. F igure 4 : High re s olution vis ua l s till ima ge of pa rtia lly de fe c tive de a d-e nd ins ula tor.
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These indications of heat rise tell utility maintenance staff that both
insulators should be replaced and that an outage should be planned to take them off potential prior to such work. This was not the first time that Brydges has used heat rise to identify problems with non-ceramic insulators. In fact, thermal inspections such as this one have been identifying a few defective polymeric units every year since about 2004. But what makes this case remarkable is that the heat rise was detected outdoors, under cold and windy conditions, and during a helicopter patrol rather than in the relative comfort of a high voltage laboratory. In the Kinectrics test lab in Toronto, when an internal ‘flashunder’ degradation process was active, arcing and carbonization were found to occur between the polymer rubber and the fiberglass core of the insulator. Such activity was basically invisible to both
corona cameras and image intensifiers since the arcing activity was covered by the polymer rubber. However, the heat from the arcing showed up clearly as a distinctive pattern, reaching temperatures exceeding 200°C in Figure 6. Once such a flashunder track is formed, a non-ceramic insulator can have permanent zones of heat rise at line voltage, such as the ones in Figure 7. Or it may recover to a state that shows no temperature rise at line voltage. It seems that oil from the silicone rubber material can diffuse back into the flashunder track and to some extent repair it electrically. When this happens, the electrical strength of the insulator recovers and this can make the defect hard to find in subsequent inspections. Nevertheless, the identified defective
insulator should still clearly be replaced. To help this process, research reported to IEEE ESMOL insulator inspection groups noted that: • Any non-ceramic insulator showing a heat rise on the shaft is tracking internally; • Partially failed 115 kV insulator showing any temperature rise could not withstand switching overvoltages and could not be replaced with liveline methods such as that shown in Figure 5; • Partially failed 115 kV non-ceramic insulators showing no temperature rise still retained sufficient switching surge strength for live work under the same set-up. Manufacturers of corona cameras have
F igure 6 : “ F la s hunde r” de ve lopme nt proc e s s unde rwa y on ove rs tre s s e d 1 1 5 k V nonc e ra mic ins ula tor (from E S M OL 2 0 0 3 ).
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F igure 5 : De ta il of the rma l ins pe c tion for ne a rby nonc e ra mic ins ula tor with te mpe ra ture a noma ly ne a r line e nd.
Comfortable in his flying office last November, Derrick Brydges was viewing an image of apparently cold conductors and warmer wood poles at a distance of about 20 meters (as shown in Figure 3). The 12-degree lens on the thermal camera used was gyro-stabilized with a Kelvin 111 Polytech Gimbal on the belly of the helicopter. The images were obtained with a fixed emissivity setting of 0.83, meaning temperature differences rather than the absolute values are of greatest interest. Figure 4 shows that there was nothing apparently wrong in the visual image taken at the same time. F igure 3 : T he rma l s c a n a nd photogra ph of powe r s ys te m junc tion from he lic opte r.
Relying on his trained eye, Brydges turned pilot Gunson back since he suspected that there was something wrong based on the thermal image he had obtained (Figure 3). The problem
Thermal inspections such as this one have been identifying a few defective polymeric units every year since about 2004. was not the glints of ‘cold’ from the end fittings of the non-ceramic insulator – these are normal. The conductor end fittings and joints also show colder temperature than the bare wire because they have a larger diameter and tend to run cooler. Similarly, the stripes of heat down the sides of the wood poles could also be ignored since the sun is low at these northern latitudes during this time of year. Rather, it was the temperature rise in the center of the insulator (about 2° C higher than the rest of the unit) that caught his attention. A closer look revealed that a second nearby insulator had a similar temperature rise near the line end, where electric stress is usually highest. F igure 4 : High re s olution vis ua l s till ima ge of pa rtia lly de fe c tive de a d-e nd ins ula tor.
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These indications of heat rise tell utility maintenance staff that both
insulators should be replaced and that an outage should be planned to take them off potential prior to such work. This was not the first time that Brydges has used heat rise to identify problems with non-ceramic insulators. In fact, thermal inspections such as this one have been identifying a few defective polymeric units every year since about 2004. But what makes this case remarkable is that the heat rise was detected outdoors, under cold and windy conditions, and during a helicopter patrol rather than in the relative comfort of a high voltage laboratory. In the Kinectrics test lab in Toronto, when an internal ‘flashunder’ degradation process was active, arcing and carbonization were found to occur between the polymer rubber and the fiberglass core of the insulator. Such activity was basically invisible to both
corona cameras and image intensifiers since the arcing activity was covered by the polymer rubber. However, the heat from the arcing showed up clearly as a distinctive pattern, reaching temperatures exceeding 200°C in Figure 6. Once such a flashunder track is formed, a non-ceramic insulator can have permanent zones of heat rise at line voltage, such as the ones in Figure 7. Or it may recover to a state that shows no temperature rise at line voltage. It seems that oil from the silicone rubber material can diffuse back into the flashunder track and to some extent repair it electrically. When this happens, the electrical strength of the insulator recovers and this can make the defect hard to find in subsequent inspections. Nevertheless, the identified defective
insulator should still clearly be replaced. To help this process, research reported to IEEE ESMOL insulator inspection groups noted that: • Any non-ceramic insulator showing a heat rise on the shaft is tracking internally; • Partially failed 115 kV insulator showing any temperature rise could not withstand switching overvoltages and could not be replaced with liveline methods such as that shown in Figure 5; • Partially failed 115 kV non-ceramic insulators showing no temperature rise still retained sufficient switching surge strength for live work under the same set-up. Manufacturers of corona cameras have
F igure 6 : “ F la s hunde r” de ve lopme nt proc e s s unde rwa y on ove rs tre s s e d 1 1 5 k V nonc e ra mic ins ula tor (from E S M OL 2 0 0 3 ).
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F igure 9 : Ultraviolet and infrared images of failing non-c eramic ins ulator us ing C S IR /E s k om multi-s pec tral imager (from S tolper et al). F igure 7 : T he rma l s igna ture of pa rtia lly de fe c tive non-c e ra mic ins ula tor a t line volta ge a fte r fla s hunde r de ve lopme nt (from E S M OL 2 0 0 3 ).
already recognized the complementary nature of thermal and ultraviolet inspection. For example, researchers at the High Voltage Research Institute of Tomsk Polytechnic University in Russia have been tracking the time evolution of heat rise from corona in the pre-breakdown phase when it is best to catch any possibility of polymeric insulator failures.
faster adoption for helicopter use. Industry working groups such as the IEEE Task Force for Guidelines for InService Classification of Non-Ceramic Insulator Damage (15.09.04.01) are interested in consolidating the findings of thermal inspection as a complement to other inspection methods. INMR readers are encouraged to report their own findings too.
CSIR and the utility, Eskom, in South Africa first described the daylight corona distance camera in 1997. This research group has also recognized the advantages of thermal inspection for transmission line insulators. One result of their recent collaboration is the multi-spectral imager, yielding images such as shown in Figure 9.
An instrument that combines both UV and infrared inspection would seem to provide the best of both techniques. Instruments such as the MultiCAM build on the ten years of research aimed at combining both technologies. This is a compact unit built from the ground up to combine the two inspection methods on the same screen and that advantage can lead to
In the meantime, utility thermographers should be encouraged to follow Brydges’ example, namely scanning polymeric insulators for any temperature rise and reporting anomalies for follow-up as part of their standard operating procedure. At this time, there is no amount of thermal activity that is considered safe. Therefore, until more experience is developed, any polymeric unit showing an internal temperature rise should be replaced and sliced open as shown in Figure 10 to verify the extent of internal defects. ?
F igure 8 : Verific ation of s witc hing s urge s trength for live-work method when replac ing partially failed non-c eramic ins ulator at the K inec tric s L aboratory, Toronto (from IE E E E S M OL 2 0 0 3 ).
F igure 1 0 : F ollow-up ins pec tion s hows depth of flas hunder trac k ing ac tivity found on polymer ins ulator pres enting abnormal thermal profile.
Editors Note: The author and INMR acknowledge the support of Hydro-One in preparing this article. Photo: INMR ©
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F igure 9 : Ultraviolet and infrared images of failing non-c eramic ins ulator us ing C S IR /E s k om multi-s pec tral imager (from S tolper et al). F igure 7 : T he rma l s igna ture of pa rtia lly de fe c tive non-c e ra mic ins ula tor a t line volta ge a fte r fla s hunde r de ve lopme nt (from E S M OL 2 0 0 3 ).
already recognized the complementary nature of thermal and ultraviolet inspection. For example, researchers at the High Voltage Research Institute of Tomsk Polytechnic University in Russia have been tracking the time evolution of heat rise from corona in the pre-breakdown phase when it is best to catch any possibility of polymeric insulator failures.
faster adoption for helicopter use. Industry working groups such as the IEEE Task Force for Guidelines for InService Classification of Non-Ceramic Insulator Damage (15.09.04.01) are interested in consolidating the findings of thermal inspection as a complement to other inspection methods. INMR readers are encouraged to report their own findings too.
CSIR and the utility, Eskom, in South Africa first described the daylight corona distance camera in 1997. This research group has also recognized the advantages of thermal inspection for transmission line insulators. One result of their recent collaboration is the multi-spectral imager, yielding images such as shown in Figure 9.
An instrument that combines both UV and infrared inspection would seem to provide the best of both techniques. Instruments such as the MultiCAM build on the ten years of research aimed at combining both technologies. This is a compact unit built from the ground up to combine the two inspection methods on the same screen and that advantage can lead to
In the meantime, utility thermographers should be encouraged to follow Brydges’ example, namely scanning polymeric insulators for any temperature rise and reporting anomalies for follow-up as part of their standard operating procedure. At this time, there is no amount of thermal activity that is considered safe. Therefore, until more experience is developed, any polymeric unit showing an internal temperature rise should be replaced and sliced open as shown in Figure 10 to verify the extent of internal defects. ?
F igure 8 : Verific ation of s witc hing s urge s trength for live-work method when replac ing partially failed non-c eramic ins ulator at the K inec tric s L aboratory, Toronto (from IE E E E S M OL 2 0 0 3 ).
F igure 1 0 : F ollow-up ins pec tion s hows depth of flas hunder trac k ing ac tivity found on polymer ins ulator pres enting abnormal thermal profile.
Editors Note: The author and INMR acknowledge the support of Hydro-One in preparing this article. Photo: INMR ©
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