Abstract: To detect insulation defects in composite insulators and effectively prevent through-breakdown and brittle fracture accidents, this study investigates the heating phenomena and their patterns caused by various factors, including partial discharge, dielectric loss due to moisture infiltration into defects, and resistive loss due to severe aging of the sheath leading to decreased insulation resistance. The results show that these three heating phenomena are representative, with the third being the dominant heating mechanism in transmission lines. Keywords : Composite insulator; Infrared thermography; Insulation defect; Online detection; Breakdown; Heating mechanism Three kinds of abnormal heating principle and phenomena were studied, such as the heat caused by partially internal discharge, the heat caused by medium loss with infiltration of humidity and the heat caused by insulation resistance decrease with severely aged insulator ring． Based on the infrared images of the composite insulators containing simulated defects and those containing real defects． These images with both electric Held distribution curves and dissection results were compared and analyzed． The research results show that the three heating phenomena are ordinary and representative． and the third one is the main heating mechanism of the composite insulators used in transmission lines． The general reason that the composite insulators produce heat is explained． The investigation result is help for inspecting the electrified composite insulator． Source: http://tede.cn Key words: composite insulator; infrared thermal image; insulator defects; on-line inspection; breakdown; heating Mechanism 0 Introduction With the increasing number of applications of composite insulators in our country, the amount of economic losses caused by the failure of composite insulators is also increasing D-s3. Several power companies in my country have experienced multiple incidents of through-breakdown and brittle fracture caused by internal insulation defects in composite insulators. Many operating composite insulators exhibit abnormal overheating, and numerous core rod fracture incidents have occurred in 500kV lines. Statistics show that by early 1999, the failure rate of all composite insulators in operation nationwide was 5.3 × 10⁻⁶, with a total of 231 failures, resulting in an annual failure rate of < 1 × 10⁻⁶. Internal insulation failures accounted for 6.9% of these. In recent years, with the increase in the number of insulators and their service life, the number of failures has increased. For example, on November 4, 2001, through-breakdown at the interface of the composite insulator at pole #24 of the 220kV Luofu line and on March 19, 2002, at pole #60 of the 220kV Funan line, caused line tripping; on November 23, 2002, a composite insulator fracture in phase A of tower #57 of the 500kV Luobei Jia line caused a major conductor grounding accident. From November 2001, statistics were collected on composite insulators with a voltage of ≥220kV in the Foshan system. Infrared thermography was used for pole-mounted spot checks, and thermal images of 304 insulators were analyzed. 54 insulators were found to exhibit overheating, resulting in a degradation rate of 17.8%. 257 insulators using the extruded sheath process were inspected, and 58 of them were found to be defective, resulting in a degradation rate of 22.6%. During operation, changes in the insulating material of composite insulators are often related to heat generation. Partial discharge and leakage current flowing through insulating materials can cause local temperature rise in insulators[1]. Based on the heating phenomenon, local damage and discharge defects of composite insulators were found, but the degree of heating was small and could not be detected without appropriate instruments. Infrared thermography uses infrared thermometry to convert the temperature of an object into an image and display it on the screen of a thermometer. Different gray levels or colors represent different temperatures, and the temperature information is displayed intuitively[1]. This method does not require operators to touch the composite insulator. The temperature of the insulator can be photographed on the ground or tower using an infrared imaging instrument, and it is simple and safe. At present, the understanding of the heating mechanism of composite insulators is still relatively vague, and it is difficult to explain many abnormal heating phenomena. In addition, there is a certain correspondence between the abnormal heating phenomenon of composite insulators and insulation defects, but no detailed reports have been found. Therefore, it is necessary to study the heating mechanism of composite insulators and the correspondence between the abnormal heating phenomenon of composite insulators and insulation defects, and to form a set of technical solutions for live detection of composite insulators by infrared thermography to guide the actual operation on site. 1 Partial discharge causes heating In theory, when the outer sheath or core rod of an insulator is damaged, moisture can seep in, and under the influence of an electric field and chemical action, it corrodes the core rod, forming carbonization channels, reducing the effective insulation distance of the insulator, and ultimately leading to core rod breakage. During this process, a localized strong electric field often exists at the damaged area, triggering partial discharge. Each partial discharge is accompanied by a tiny current pulse or electron avalanche, which not only damages the chemical structure of the material itself, causing it to carbonize, but also raises the temperature of the insulating material, exacerbating insulation failure. A good insulator will not experience partial discharge within the insulation material under normal operating voltage. However, under polluted conditions, partial discharge may occur on the surface. The heat generated by this discharge is easily dissipated; only partial discharge within the insulation material can accumulate sufficient heat. When a composite insulator develops defects, a strong local field can easily form at the defect location, leading to partial discharge. In a laboratory setting, AC voltage was applied to three insulators containing defects. Their models were: Foshan Electric 1 (with a 1.2 mm diameter, 15 cm long thin copper wire at the interface between the sheath and the core rod, connected to a high-voltage metal connector), Foshan Electric 3 (with a 15 cm long wire embedded between the sheath and the core rod). The thin copper wire (9cm from the high-voltage end metal connector) and the sheath between the first umbrella on the high-voltage side and the high-voltage metal connector of Fodian 4 cracked after being placed in the laboratory for a long time. It is estimated that the core rod expanded and opened the sheath after moisture seeped into the partially damaged sheath. A thermal image was taken after applying a 60kV AC voltage to these three insulators for 10 minutes (see Figure 1). In the figure, SP represents a point on the normal part of the insulator, and AR represents the area of ​​the defective part of the insulator. As shown in Figure 1, the heating point of insulator 1 is at the end of the copper wire, insulator 3 has no abnormal heating point, and the heating point of insulator 4 is at the defect location. The defects of insulators 1 and 4 are connected to or close to the high-voltage end, and a strong discharge sound is heard when pressure is applied. The defect of insulator 3 is in the middle of the insulator and has no corona. To further confirm that these heatings are caused by partial discharge, a higher voltage is applied to insulator 4, and the discharge pulse is measured from the grounding wire 50012 resistor. After increasing the voltage, the maximum amplitude of the partial discharge pulse increases from the original 50mV to 400mV, the discharge sound increases many times, and the heating temperature also increases accordingly (see Figure 1(d)). The first and second sheds of the high-voltage side of the previously non-heating insulator 5 are short-circuited with a copper wire and connected to the high-voltage fitting. A strong corona sound is heard after pressure is applied, and its thermal image is shown in Figure 2(a). The heating point is still at the defect end. When only the second shed is short-circuited, the wire is at a floating potential and has no corona, so it does not heat up (see Figure 2(b)). Therefore , internal discharge or strong external discharge of the insulator can cause heating at the partial discharge location. Water Dielectric Loss Leads to Heating Water is a highly polar substance with a relatively high dielectric constant. When placed in an alternating electric field, water molecules continuously change direction with the change of the electric field, i.e., they repeatedly polarize. The friction between water molecules during this polarization generates energy loss (dielectric loss) and is converted into heat, causing the water temperature to rise. Therefore, water in the cracks of the damaged area will generate heat due to dielectric loss in an alternating electric field. Injecting salt water into the defective area of ​​the Foshan Electric 4 insulator (its thermogram is shown in Figure 3(a)) resulted in more significant heating than when no water was injected. Injecting more water resulted in even more significant heating (see Figure 3(b)). This conclusion can explain the significant differences in the degree of heating detected in some insulators at different times, but the evaporation of water will lower its temperature. In Foshan Electric 4... Water was injected through holes drilled in the insulator sheath, and a thermal image was taken (see Figure 3(b)). The center of ARO2, where water was injected, had a lower temperature than the surrounding area. This is because the water seeping into the defect has both heating and cooling effects; when the heating effect is less than the cooling effect, the temperature drops. Therefore, the impact on the insulator surface temperature is complex. 3. Insulation Aging and Resistance Heating Under normal circumstances, the dielectric resistance of composite insulators is very high, and the leakage current flowing through the insulator is only in the A-level range. When the resistance of the insulating material drops to a certain range, the previously uniformly distributed leakage current concentrates on that point, causing the resistance loss there to be greater than elsewhere, resulting in localized heating. When the leakage current flows through the surface resistance of the insulator, it generates Joule heat, raising its surface temperature. To achieve prominent localized heating, the resistance at that point must meet certain conditions. The optimal heating resistance value varies for insulators with different surface conditions. A 40Mfl resistor was placed between the 5th and 6th umbrellas of the Foshan No. 5 insulator. Copper wire was wrapped around the insulator core rod at both ends of the resistor to ensure that all surface leakage current flowed through the resistor. Slight heating occurred after applying pressure, as shown in Figure 4(a). If the resistor was wrapped with tape, the heat was not easily dissipated, and the heating was more pronounced than without tape, as shown in Figure 4(b). Simultaneously, the same tape was wrapped around the core rod between the 4th and 5th umbrellas to study the effect of the tape. The results showed that the tape did not cause heating. For the actual defective resistor heating phenomenon, the phase (B) insulator of the 10+2 tower of the 220kV Liangxiao line was studied. Its metal ends had many burrs, and corona sound was observed under normal operating voltage. The lower sheath of the 1st umbrella had severely aged and turned white due to long-term corona corrosion, and its insulation resistance per unit length decreased to 50 GΩ/m, while the normal part was >250 GΩ/m. Its thermal image is shown in Figure 5(a), where the aged sheath area showed significant heating. To determine that the heating was caused by the aging sheath rather than the core rod, this part of the sheath was removed, and a thermal image was taken under renewed pressure (see Figure 5(b)). The lower part of the first umbrella of the insulator no longer heated up, but the high-voltage metal end heated up abnormally. The above experiment shows that a suitable low resistance can cause local heating of the insulator, and the heating site is the part where the insulation resistance decreases. All five composite insulators from the field exhibited the same heating phenomenon as the insulator in phase (B) of the 10+2 tower of the Liangxiao line, so its heating mechanism is considered to be the main reason for the heating of the insulators in the field. In addition, under external conditions that are not conducive to heat dissipation, when the leakage current on the surface of the insulator is large enough, the insulator will heat up over a large area. In some cases, dirt on the surface of the insulator may form a low resistance in a certain place, causing resistive heating, making the temperature at that place higher than other places, forming a false heating phenomenon. 4 Conclusion a) When there is internal partial discharge in the defect of the insulator, it will cause the insulator to heat up. High-voltage end conduction defects are more likely to produce partial discharge at the defect end, causing heating at that point. Middle defects and some high-voltage end defects do not produce obvious partial discharge and will not heat up. b) The moisture seeping into the defect causes the insulator to heat up due to dielectric loss on the one hand, and evaporates and dissipates heat on the other hand. Therefore, the effect of moisture on the heating phenomenon of the insulator is more complicated. When moisture seeps into the high-voltage end conductivity defect, the heating factor is greater than the heat dissipation factor, and the temperature of the insulator rises. The more moisture, the more serious the heating. c) After the insulator sheath is severely aged, the resistance loss caused by the sharp drop in insulation resistance is the main heating mechanism of the insulator in the field. References [1] Yi Hui, Cui Jiang-liu, Su Zhi-yi. The application and prospect of composite insulators in my country[J]. High Voltage Engineering, 1999, 25(1): 78-81. [2] Gong Wen-quan, Zhong Lian-hong, Li Jin-fa. Operation and evaluation of composite insulators[J]. High Voltage Engineering, 2002, 28(5): 46-48. Lian-hong, LI Jin-fa. Operation and evaluation of composite insulator[J]. High Voltage Engineering, 2002, 28(5): 46-48. [3] Teng Guo-li, XU Li-xian, ZHANG Hai-an. Revelation about composite insulator used for ±500kV tession string[J]. High Voltage Engineering, 2005, 31(4): 90-91. [4] HU Yi. The analysis of characteristics of porcelain insulator, glass insulator and composite insulator in transmission line[J]. High Voltage Engineering, 2001, 27(2): 33-35. Engineering, 2001, 27(2): 33-35. [5] WU Xiong, WAN Bao-quan, JIANG Hong. Radio interference from insulator string on UHV lines[J]. High Voltage Engineering, 1999, 25(2): 128-29. [6] QIAN Zhi-yin, YE Zi-qiang, ZHANG Yu. The test and analysis of reason of breakdown of Siemens 500kV composite insulator. High Voltage Engineering, 2004, 30(6): 124-25. [7] ZHANG Ming, CHEN Mian. Analysis on core brittle fracture of composite insulators in a certain 500kV transmission line[J]. Power System Technology, 2003, 27(12): 51-53. [8] ZHANG Chang-sheng, WANG Xiao-gang, HUANG Li-hong. Analysis of composite insulator core rod breaking on 500kV Huishan line. Power System Technology, 2002, 26(6): 71-72, 75. [9] CHENG Yang-chun, LI Cheng-rong, MA Xiao-hua. Study on online detection of faulty composite insulators by electric field method[J]. High Voltage Engineering, 2002, 28(12): 8, 53. [10] SIMA Wen-xia, YANG Qing, SUN Cai-xin, et al. Optimization of corona ring design for EHV composite insulator using finite element and neural network method[J]. Proceedings of the CSEE. 2005, 25(17): 115-120. [11] WANG Bin, PENG Zong-ren. A finite element method for the calculation of the voltage distribution along the 500kV line insulators[J]. Insulators and Surge Arresters, 2003(1): 13-15. [12] XU Zhong, ZHONG Lian-hong, GU Li-li. The measurement method of voltage distribution on insulator strings of the UHV line[J]. High Voltage Engineering, 1998, 24(4): 59-61, 78. Engineering, 1998, 24(4): 59-51, 78. [13] DING Yi-zheng, ZHANG Jun-lan, CHEN Xiong-yi, et al. Field measurement and analysis of voltage distribution on 500kV transmission line insulator strings [J]. Electric Power, 2000, 33(2): 45-47. [14] Sima WX, Espino-Cortes FP, Cherney EA, et al. Optimization of corona ring design for long-rod insulators using FEM based computational analysis [C]. Proceedings of 2004 IEEE International Symposium on Electrical Insulation. Indianapo lis, USA, 2004: 480-483. [15] HU Shi-zheng. Heating mechanism and thermograph of degraded insulators[J]. Power Systern Technology, 1997, 21(10): 44-45.