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Study on decay‐like fracture of 500 kV composite insulators: Infrared, ultraviolet and electric field distribution detection

Study on decay‐like fracture of 500 kV composite insulators: Infrared, ultraviolet and electric... INTRODUCTIONComposite insulators are widely used worldwide, especially in contaminated areas, owing to strong pollution resistance, lightweight, high strength, easy installation and no requirement to detect zero values [1–4]. More than 10 million composite insulators have been employed in transmission lines with a rated voltage of 66 kV and higher in China by 2021. High voltage composite insulators are required equipment for transmission systems and are critical to the system's operation safety and efficiency. As a result, composite insulators must meet stringent reliability standards.Composite insulators are generally composed of silicone rubber housings, core rod and end fittings. The sheds and sheath are made of organic materials such as high‐temperature vulcanized (HTV) silicone rubber. The core rod that mainly bears mechanical force is a composite material (glass fibre‐reinforced plastics or GFRP) with epoxy resin as a matrix and glass fibre as a reinforcement material. Carbon cast steel or structural steel with a hot‐dip galvanized layer on the exterior surface is commonly used for end fittings. The core rod fracture will result in significant accidents such as offline short circuits, putting the power grid's operation at risk or even causing a power outage.Normal fracture and brittle fracture were the most common fracture modes of composite insulators [5–7]. Several abnormally fractured composite insulators have been discovered in China and Korea in recent years, and researchers proposed the third core rod fracture form, decay‐like fracture [8–12]. The former two's fracture mechanism and preventive measures are transparent at home and abroad. Nevertheless, the mechanism and solution of the decay‐like fracture are still being studied. The existing research [9] believes that the intrusion of external liquid into the core–sheath interface causes hydrolysis and failure of the interface. The accumulation of liquid at the interface distorts the electric field, resulting in electrical discharge and interface current. The GFRP core rod deteriorates under the combined effects of liquid medium, discharge, interface current, and mechanical load, resulting in decay‐like fracture of composite insulator.Haris et al. [13, 14] found that the sheath breakdown in composite insulators was similar to the decay‐like fracture. The microscopic morphology of the sheath breakdown was studied. They proposed that failure and deterioration began at the fittings’ junction and extended along with the interface. Due to the heating of the internal conductive area, resin ablation led to electrochemical damage, and the punctures formed at the weak sheath.The operation and maintenance staff discovered abnormal temperature rises near the high voltage end of insulators through infrared (IR) imaging during daily inspection. Since 1998, many abnormal fracture accidents of alternating current (AC) 500 kV composite insulators have took place in China. Using an infrared thermal imager, local temperature rises at the high voltage side of these insulators were observed before fracture [15–19]. Due to internal defects in the core rod–silicone rubber interface, partial discharge, polarization after water penetration, and insulation aging occurred, resulting in abnormal temperature rises. And internal heat could not dissipate in time because of wrapped sheath. Therefore, abnormal temperatures are observed on the surface of the composite insulator, which can be utilized to evaluate internal defects [20–23]. The operation experience shows that the position and size of internal defects can also be judged by ultraviolet (UV) imaging method to identify discharge or by electric field detection method to research the electric field of insulators. Decay‐like fracture of composite insulators is a small probability accident. Due to the rare samples and the difficulty of sampling, characteristics and mechanism of decay‐like fracture are not precise. This paper provides experimental data and technical support for the decay‐like fractured insulators.Literature [8–12] carried out experiments on the decay‐like fractured insulators and obtained the hydrophobicity, morphology, and physicochemical properties. Literature [11, 24–26] carried out IR and UV imaging tests on the decay‐like fractured insulators in the laboratory. Based on the current status and conclusions, electric field distortion and electrical discharge play an important role in the decay‐like formation and abnormal temperature rise during operation. However, typical characteristics and mechanism of decay‐like fracture are not precise due to the rare samples (decay‐like fractured insulators). This paper studies the heating, discharge and electric field of three types of insulators (decay‐like fractured, aging, and new) in the laboratory, expecting to reveal the typical characteristics of decay‐like fracture. The results provide support for judging decay‐like fracture of composite insulators.MATERIALS AND METHODSSamplesThe test samples come from a B‐phase (neutral line) V‐shaped composite insulator string of a 500 kV (AC) operating transmission line, and the type is FXBW‐500 kV/180 kN. The decay‐like fractured composite insulator, unbroken insulator in the same string, and new insulator are examined, respectively, and the corresponding numbers are No. 1, No. 2, and No. 3. The insulator sheds include large, medium, and small sheds. The specific geometric parameters of the insulator are shown in Table 1.1TABLEDimensions of the decay‐like fractured composite insulatorStructural heightInsulation distanceCore rod diameterShed spaceShed diameter4,450 mm4,160 mm24 mm126 mm190 mmAccording to the appearance of the fractured composite insulator, core rod has seriously deteriorated (rough macroscopic section, crisp texture, chalking core rod, separated glass fibre and epoxy resin matrix, and the shape is like rotten wood). Moreover, core–sheath interface near the core rod fracture is invalid, with decay‐like characteristics. The appearance of No. 2 is intact, and there is no sign of damage or failure. Figure 1 shows the field fracture diagram of the composite insulator.1FIGUREFracture of 500 kV composite insulator stringExperiment procedureVisual observationThe samples' fracture morphology and contamination distribution were initially detected on the spot. And then, the samples were transported back to the laboratory after protection measures. In this paper, insulator sheds are used to describe the position, starting from the grouding ends. There are 33 large sheds, including 32 group sheds plus an additional large shed, and four sheds are a group.Experiment setupThe experiments under diverse conditions were carried out in the high voltage hall of Chongqing University. A valid high voltage of 500 kV × 1.1/1.732 = 318 kV was applied to the insulators during the AC test. In the direct current (DC) test, the high voltage value was 449 kV, the peak value of the AC test. According to the temperature rise measuring experience of composite insulators, each experiment lasted for 30 min. The insulators' temperature rises and discharge were measured by the infrared thermal imager and ultraviolet imager at 10 m from the insulator, and the electric field distribution was at 3.83 m. The decay‐like fractured insulator was bonded with insulation tape and rope to prevent falling.Infrared imaging methodThe FLIR T1044 infrared thermal camera was used to measure temperature rise.Ultraviolet imaging methodThe CoroCAM®6D ultraviolet camera was employed to observe the insulator discharge during the research process.Electric field distribution methodDuring the experiment process, ultra wide‐band electromagnetic radiation analyser (SEM‐600) and low‐frequency electromagnetic field probe (LF‐04) were used to detect electrical field distribution.The above detection steps are roughly similar, as follows:Firstly, after the instrument was turned on, parameters were set (such as detection environment correction parameters, temperature range, and gain), and the optimal shooting point was selected. The voltage was applied to the composite insulator using the uniform step‐up method. The voltage at the predetermined voltage was maintained for about 30 min.Secondly, the temperature, discharge, and electric field strength of the insulator were measured point by point and the data was recorded.Thirdly, composite insulator was replaced and the procedure described above was repeated.Finally, different insulators' temperature rise, discharge and field strength were compared and analyzed.RESULTS AND DISCUSSIONAppearance inspectionThe overall morphology of No. 1 composite insulator is shown in Figure 2. The insulator has 33 large sheds, and the fracture spot is sited in the 31st group sheds (between the 6th and 7th sheds close to the high voltage end).2FIGUREThe overall morphology of decay‐like fractured 500 kV composite insulatorCore rod characteristics of decay‐like fractured insulatorThe macro‐characteristics of decay‐like fractured composite insulator can be summarized in the following aspects:Core rod has seriously deteriorated, and the macroscopic section is coarse. The fracture length and deterioration degree of the core rod at different positions vary. The core rod colour deepens from the outside to the inside, including black, grey and yellow. Internal damage of the core rod is more serious than the outside.Core rod has a rough and chalking surface, severe fuzzing, crisp texture, and is similar to rotten wood. The core rod surface layer fell off together when removed the sheath, and the core–sheath interface partially or entirely failed, resulting in poor adhesion.Carbonized channels appear on the core rod surface, extending from the high voltage end to the fracture position and extending several sheds to the grounding end.Glass fibre in the core rod is partially exposed and separated with epoxy resin matrix, and epoxy resin matrix degrades.In accordance with the above characteristics and FTIR (Fourier transform infrared spectroscopy) properties, the ‘gully and fracture’ on the core rod surface confirms the presence of acid and heat [9, 10, 26, 27]. The ‘crisp’ and ‘decay’ of the core rod are due to the epoxy resin modification by acid and heat, resulting in disappearance of its internal components in the form of gas. And then, glass fibre inside core rod is exposed and becomes ‘dry and messy’, indicating that the epoxy resin matrix has been severely oxidized and ablated.The core rod is showing signs of deterioration near the fracture. The core rod degrades severely along the direction of the fracture to the grounding end. The detailed fracture, interface failure phenomena, and holes on the sheath are shown in Figures 3 and 4.3FIGUREField pictures of the decay‐like fractured insulator4FIGUREThe sheath near the fracture of the decay‐like fractured insulatorSheds and sheath characteristics of decay‐like fractured insulatorThe insulator surface is heavily contaminated, especially the high voltage side. After cleaning with ethanol, the surface faded from red to pink or white, and several punctures have appeared.In addition to the transverse punctures, sheath cracks on the surface.Cracks and punctures initiate from the sheath–core interface and appear in the sheath surface.The sheath near the fracture position deteriorates, fades and is powdering seriously. The closer to the high‐voltage end, the worse the degradation and cracks become.The sheath–core interface has poor adhesion. Silicone rubber housings are easily torn and separated, and the sheath becomes brittle and hard. Black high‐temperature burning appearances occur on the surface. Different sheath colours represent various failure time.The aforementioned phenomena indicate that surface cracks are due to the partial discharge, internal penetrating current, temperature rise, nitric acid, and other factors. The local arc burns the core–sheath silicone rubber, and develops outward, causing cracks on the sheath surface. It can be seen from Figure 4 that the sheds also cracked.Figures 5 and 6 indicate that when the sheath goes closer to the high voltage end, the breaking worsens, and there are more cracks. The sheath's degradation is concentrated on the side with heavy pollution, with no notable occurrence on the other side. The degree of degradation diminishes along the direction of the grounding end. As a result, sheath deterioration is strongly linked to leakage current, the heating generated by leakage current, and other factors related to leeward sides.5FIGUREThe appearance of the wingward/leeward side in the 30th group sheds of the decay‐like fractured insulator6FIGUREThe appearances of 29, 28 and 27 sets of sheds of the decay‐like fractured insulatorThe ozone produced by the corona discharge is unavoidable in the air. The ozone odour proved the oxidation of epoxy resin matrix during the experiment. This trait is also seen in the macro‐morphology, with the colour becoming dark and the epoxy resin content reducing. Figure 7 shows the results after laboratory tests. The high‐temperature burning phenomenon is more visible than previously, and the cracking degree increases, indicating that epoxy resin matrix degradation is caused by internal factors of the composite insulator.7FIGUREObservation of FRP rod at fracture after the testExisting studies have shown that conductive interface defects may cause sheath cracks and punctures [11]. Therefore, local leakage current with high density may occur in punctures near the high voltage end.In accordance with the above phenomena, leeward side of the composite insulator is easy to contaminate, and surface leakage current increases under the action of rain. After a long‐term operation, insulator sheath ages, and the water absorption ability is enhanced. The sheath dielectric constant increases after water absorption. And then, electric field and polarization loss increase. Under the effect of high field strength, current, and heat, surface moisture easily penetrates the core–sheath interface. The air‐filled defects in the core–sheath interface or inside the core rod are filled with water. Due to the enhanced conductivity of the moisture defect, leakage current increases, electrical field strength increases and the aging process accelerates. The above aging process is a chain reaction.Temperature rise characteristicsTemperature rise characteristics of No. 1 composite insulatorIn this paper, the temperature difference between the maximum and minimum temperature measured on the insulator surface is defined as the insulator temperature rise (or ΔT). Ambient temperature and humidity were measured by thermo‐hygrometer. 318 kV AC voltage was applied on composite insulators for 30 min. The results are shown in Figure 8 (the upper end is the grounding terminal).8FIGUREThe temperature of No. 1 insulator after 30 minIt can be concluded from Figure 8:The temperature rises of the insulator middle area are 6.3 K and 5.1 K, respectively, and the high voltage side exceeds 50 K. Water will heat due to dielectric loss in AC electric field. Since the electric field strength is different at each position of the insulator, different positions have different dielectric losses. Therefore, the difference in temperature rise between various parts of the same insulator indicates that core rod heating is positively correlated with electric field and dielectric loss (or the degree of material ageing).Temperature rises change significantly with the voltage values. When the voltage reaches 318 kV, 80–100°C was detected at the high voltage side due to the violent discharge and aging core rod. The increasing internal penetrating current causes the deterioration on the high voltage side, which aggravates electric field distortion.The length of the abnormal temperature rise area at the high voltage side is about 50–120 cm, including multiple sheds, and the highest temperature rise area is centralized at the fracture and 1 to 2 group sheds near the fracture.The length of the abnormal temperature rise area at the high voltage side and the middle part grows as humidity rises.No. 1 composite insulator is tested at various ambient temperatures and humidity, and the temperature rises are recorded in Table 2.2TABLEThe temperatures and temperature rises of No. 1 insulator under diverse conditionsTest ConditionsT/°CRH%AC/DCLV side T/°CMid‐part ΔT/KHV side T/°CHV side ΔT /K3069AC33.96.395.565.52785AC29.95.183.756.72389AC24.02.882.359.31792AC17.14.570.453.41792DC17.12.323.86.8Abbreviations: AC, alternating current; DC, direct current; HV side, the area of the high voltage side; LV side, the vicinity of the ground side; RH, humidity.It can be seen from Table 2:Temperature rises at the low‐voltage side and middle part under AC test are both less than 7 K. The temperature rises of high voltage side exceed 50 K. Hence, ambient temperature and humidity have a certain influence on the temperature rise.During the DC tests, temperature rises of the low voltage side and middle part of the insulator are meagre, and temperature rise of the high voltage side is 6.7 K, which is far less than the temperature rises under AC circumstances. The polarization loss at the decay‐like fractured insulator plays a crucial role in the temperature rise. In the alternating electric field, the heat generated by the water steering polarization is the main reason for insulator heating under high humidity conditions.With regard to the above phenomena, insulating materials' properties mainly determine the insulator's temperature rise under low humidity conditions. For instance, there are quality defects in their internal interface. With increasing humidity, insulating materials' surface state, such as water polarization and leakage current, will lead to the aggravation of insulator heating. When the environmental humidity is high, the convective heat dissipation coefficient between the silicone rubber surface and the air will increase, and the high humidity will inhibit the temperature rise. Therefore, water is a crucial heating factor.Therefore, under high humidity conditions, abnormal temperature rise does not necessarily reflect the internal insulation defects of composite insulators. Low humidity conditions are conducive to improving the detection accuracy of the infrared detection technology.Temperature rise characteristics of No. 2 and No. 3 insulatorsUnder the same conditions, experiments were performed on the same V‐string unbroken insulator (No. 2 sample) and new insulator (No. 3 sample) to investigate the heating characteristics of decay‐like insulators. The applied voltage was 318 kV (AC). The other test conditions were the same as above. The results are shown in Figure 9 (left, middle, and right are the grounding end for 30 min, the high voltage end for 30 s and 30 min, respectively).9FIGUREThe temperature of No. 2 and No. 3 insulators (T = 30°C, RH = 69%)It can be seen from Figure 9:After removing the grading ring of high voltage end, temperature rises of No.2 insulator are 5.1 K, 7.2 K, and 11.1 K, respectively, and the temperature rises increase by 0.8 K, 3.1 K, and 6.5 K. This change indicates that electric field of the high‐voltage end increases significantly without the grading ring, resulting in great losses and serious heating at the high‐voltage end. The reducing effect of the grading ring on the electric field strength at the high voltage end will effectively inhibit the temperature rise of the insulator.There are also temperature rises for No. 2 and No. 3 composite insulators during the research. The temperature rise of high voltage end is greater than the low‐voltage side and the middle area. The temperature rises from high to low are the high voltage side, low‐voltage side, and middle part of the insulator, respectively, indicating that the electric field is one of the causes of the temperature rise.Temperature rise of No. 2 insulator is slightly higher than that of No. 3 insulator, indicating that insulator's performance has changed due to its degradation and deterioration in operation. Therefore, temperature rise can judge the degree of insulator degradation during the daily inspection.Temperature rise of high voltage ends of No. 2 and No. 3 insulators is much lower than that of No. 1 insulator. The temperature rise of the operating composite insulator on the transmission line is measured, and the temperature rise generally does not exceed 30 K [10, 24–26]. Different temperatures and temperature rises indicate different degradation degrees of the core rod. Therefore, conforming to the test results, when composite insulators' temperature and temperature rise are abnormal (For example, T > 50°C, ΔT > 30 K) during the daily inspection, the core rod may be severely decay‐like and should be take immediate measures. The relationship between decay‐like status and abnormal temperature rise value can be further studied in the future.UV discharge characteristicsUV discharge characteristics of No. 1 insulatorThe discharge distribution characteristic of No. 1 composite insulator was researched. Power frequency voltage of 318 kV (AC) was applied on composite insulators for 30 min. The discharge results of the high voltage end are shown in Figure 10.10FIGUREDischarge at the high voltage end of No. 1 insulator during the AC testIt can be seen from Figure 10 that the high voltage side of No. 1 composite insulator has an apparent discharge, but the middle and low voltage sides have no visible discharge. There is no evident difference between high humidity and low humidity in the discharge form and position of the high voltage end of No. 1 composite insulator. The main characteristics are as follows: On the grading ring, there are numerous discharges. The phenomenon is white purple arc breakdown air discharge, and the shed's discharge at the shelter of the grading ring creates the possibility of forming nitric acid around it. The phenomenon is a white, purple edge arc connected to the bare core rod at both ends of the shed's discharge.It can be seen that the discharge occurs in the high field strength area, resulting in a local temperature rise on the core rod's surface. Surface discharge in a wet environment is also the possible cause of acid medium in the composite insulator. Figure 11 shows the discharge of No. 1 insulator during the DC test.11FIGUREDischarge of No. 1 insulator during the DC test (T = 17°C, RH = 92%)The number of photons at the major discharge position under various conditions is compared, as shown in Table 3, and the shooting parameters are consistent. The ordinate and abscissa are test number and test conditions, respectively. Test condition includes ambient temperature (°C), humidity (RH%), and voltage type of the experiments. The unit of photons was a thousand.3TABLEPhoton number of different insulators under diverse test conditions (Unit: Thousand)Number of photons30°C 69% AC27°C 85% AC23°C 89% AC17°C 92% AC17°C 92% DCNo. 1120–130190–200150–160130–1400.6–0.7No. 2110–120130–140130–140130–1500.3–0.4No. 3110–120120–130130–140140–1500.1–0.2It can be seen from Table 3 that the photon numbers of different insulators under various conditions show the following characteristics:The photon number of No. 1 insulator shows a tortuous upward trend under different humidity conditions. The maximum value appears at 27°C (Temperature) and 85% (RH), indicating that the discharge of decay‐like insulator is affected by humidity.The photon numbers of No. 2 and No. 3 insulators increase with humidity, but the difference is not apparent when humidity is greater than 85% (RH). Therefore, the observation of insulators' discharge should carry out under low humidity.The photon number of the same insulator under DC conditions is much smaller than in AC. The discharge intensity is also the same, so the possibility of decay‐like generation under DC conditions is low. Under the same experimental conditions, the order of photon number is No. 1 > No. 2 > No. 3, and the degree of sample deterioration is the same.The reason for the discharge at the fracture site of the decay‐like fractured insulator is that the core rod has defects such as erosion, where the sheath and sheds have fallen off, resulting in core rod being exposed to the air. The defect site is filled with air to form an air gap. The dielectric constant of the air is smaller than that of the organic material, so the air gap withstands a large voltage and is easy to discharge. High field strength and discharge at defects enhance organic material ageing, leading to the decay‐like occurrence of core rod, sheath and interface, which may cause breakage when it continues to run.When the insulator's insulation damage or penetrating channel occurs, discharge situation is similar to the above figures. Temperature rises and nitric acid are formed due to partial discharge and conduction current, which continuously destroy the internal insulating structure and eventually cause decay‐like ageing and even fracture.The insulator surface is generally contaminated by conductive compounds in different states. The surface becomes partially or completely damp as the humidity rises, and the soluble matter in the contamination layer dissolves, transforming it into a conductive layer. There is a leakage current flowing under the operating voltage. The leakage current increases as the wetness level rises, causing the conductive layer to heat up. Then, it may be dried to reduce the conductivity of the damp layer or heated to cause the positive temperature coefficient electrolyte to decompose, increasing the conductivity. The final result is to generate an arc and discharge along the surface or reduce the discharge intensity.UV discharge characteristics of No. 2 and No. 3 insulatorsThe discharge distribution characteristics of No. 2 and No. 3 insulators are tested by the same method, including insulators without or with grading rings. The test results are shown in Figures 12 and 13.12FIGUREDischarge of No. 2 and No. 3 insulators with grading ring (T = 30°C, RH = 69%)13FIGUREDischarge of No. 2 and No. 3 insulators with grading ring (T = 30°C, RH = 69%)When No. 2 and No. 3 insulators are researched, the main discharge characteristics are as follows:Discharge position is at the grading ring or near the high voltage side. The discharge on the ring is intense, but discharge in other areas can be ignored. The high voltage sides of the three insulators have an evident discharge, and the discharge degree is 1 > 2 > 3. There is no noticeable discharge in other parts. It can be seen that the electrical field strength at both ends of the composite insulator, especially the high voltage end, is significantly affected by the size of the grading ring and the installation position.When the grading rings are installed improperly or the type is inappropriate, field strength at the sheath‐fittings adherent point increases, increasing the possibility of nitric acid formation around them. When there is a defect in this part, water in a high humidity environment may enter the defect through silicone rubber to make the electric field distorted at this place, resulting in partial discharge, ageing, and even decay‐like.The discharge results of No. 1, No. 2 and No. 3 samples are compared and analysed, including discharge position, shape, and intensity. No. 1 insulator discharges at the exposed core rod of the fractured position. In addition, the remaining positions are the same as No. 2. There is no apparent difference between No. 2 and No. 3 samples, so it is not easy to judge whether the composite insulator is decay‐like or not in the transportation inspection.Electric field strength distributionThe electric field strength of the insulator was measured using an electromagnetic radiation analyser (SEM‐600) and a low‐frequency electromagnetic field probe (LF‐04) during the test. The electric field intensity was measured at 3.83 m from the composite insulator. The first measurement point was measured at 1.30 meters from the insulator's low voltage end (upper end), and each subsequent measurement point was 0.50 m lower for seven measuring points. Figures 14 and  15 are the measurement diagram and the test results, respectively.14FIGURESchematic diagram of electric field measurement15FIGUREElectric field distribution of three insulators under different humidityThe electric field distribution of decay‐like fractured insulator is approximately linear with the measurement distance, especially from the middle part to the high voltage end. Slope is approximately the same and increases, respectively, as shown in the diagram. From the electric field measurement data and Figure 15 curve:Under the same humidity conditions, the electric field strength around the decay‐like fractured insulator is significantly higher than that of the intact insulator, indicating that the insulation performance of the core rod and sheds/sheath decreases during the decay‐like ageing of the insulator.Comparing the field strength curves under various test conditions, it is clear that the grading ring, insulator and humidity have apparent influences. The influence of test conditions should be considered in field strength analysis.Optimizing the grading ring may reduce the voltage and surface field strength towards the high voltage end, and the distribution uniformity can be improved.Conforming to the above results, electrical field strength of decay‐like fractured composite insulator is higher than that of other insulators under the same conditions. Nevertheless, electrical field strength curves have no noticeable distortion. Different field strength curves have a similar variation trend, and the field strength is easily affected by environmental factors such as humidity. Therefore, it is not easy to determine whether the composite insulator is decay‐like using electric field distribution under natural conditions.CONCLUSIONThe observations of decay‐like fractured composite insulator are examined in depth in this research. The temperature, discharge, and electric field distribution during the experiment of three composite insulators (decay‐like fractured, aging, new) in the laboratory are detected using the ultra‐wideband electromagnetic radiation analyser SEM‐600, ultraviolet imager CoroCAM®6D, and infrared thermal imager FLIR T1044.The characteristics of the decay‐like fractured composite insulator are as follows:According to the findings, core rods undergo continuous degradation till fracture due to a combination of internal (core‐sheath interface failure, water, electrical discharge, electrical field strength distortion, oxidation and acidic medium) and external (pollution accumulation and leakage current) factors. The epoxy resin matrix is mainly destroyed by discharge, current and heating. Acidic medium and mechanical stress mainly lead to stress corrosion fracture of glass fibre. The leading cause of the formation of acidic medium is electrical discharge in the humid environment.Decay‐like part of composite insulators has the following characteristics: evident degradation phenomenon of glass fibre and epoxy resin matrix, separation between them, fracture of glass fibre, degradation of epoxy resin and carbonized channel penetration on the surface of core rod. The surface damage of the sheath is only concentrated on the side with heavy contamination, and the deterioration degree decreases along the grounding end direction. Degradation of the sheath is closely related to surface contamination or leakage current. Internal discharge and heating of composite insulators are the causes of epoxy resin matrix damage. Electrical discharge, current, and heating mainly destroy the epoxy resin matrix.The ambient temperature and humidity have an impact on the temperature rise of composite insulators, but the decay‐like composite insulators have the characteristics of significant high‐temperature rise. Therefore, one of the most obvious characteristics of decay‐like is abnormal temperature and temperature rise (such as T > 50°C, ΔT > 30 K). The values can be used to judge the decay‐like of composite insulators during the daily inspection, and the use of infrared temperature measurement detection technology under low humidity conditions (For example, RH < 85%) is conducive to improving the detection accuracy of composite insulators. The temperature rise changes in different periods during the decay‐like fracture process of composite insulators need to be further studied.Under the same conditions, discharge strength of decay‐like insulators is much higher than that of other insulators, and the discharge at bare rod is intense. Insulator discharge is affected by humidity and should be observed under low humidity (e.g. < 85%). Under DC conditions, photon number, as well as discharge intensity, is much lower than that under AC conditions for the same insulator.The electric field strength around the decay‐like fractured insulator is significantly higher than that of the intact insulator under the same conditions. Compared with the field strength curves under different test conditions, the influence of grading ring, insulator condition and humidity is apparent. Electric field distribution and ultraviolet measurement are difficult to judge whether the composite insulator is decay‐like.ACKNOWLEDGEMENTThis work was supported by the Science and Technology Project of State Grid Corporation (5500‐202024073A‐0‐0‐00).CONFLICT OF INTERESTThe author declares that there is no conflict of interest.DATA AVAILABILITY STATEMENTThe datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.REFERENCESLiang, X., Gao, Y., Wang, J., Li, S.: The rapid development of silicone rubber composite insulators in China. High Voltage Eng. 42(9), 2888–2896 (2016)Cherney, E.: 50 years in the development of polymer suspension‐type insulators. IEEE Electr. Insul. Mag. 29(3), 18–26 (2013)Papailiou, K.O., Schmuck, F.: Silicone Composite Insulators Materials, Design, Applications, 1st ed., pp. 1–7. 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Study on decay‐like fracture of 500 kV composite insulators: Infrared, ultraviolet and electric field distribution detection

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© 2022 The Institution of Engineering and Technology.
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1751-8695
DOI
10.1049/gtd2.12584
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Abstract

INTRODUCTIONComposite insulators are widely used worldwide, especially in contaminated areas, owing to strong pollution resistance, lightweight, high strength, easy installation and no requirement to detect zero values [1–4]. More than 10 million composite insulators have been employed in transmission lines with a rated voltage of 66 kV and higher in China by 2021. High voltage composite insulators are required equipment for transmission systems and are critical to the system's operation safety and efficiency. As a result, composite insulators must meet stringent reliability standards.Composite insulators are generally composed of silicone rubber housings, core rod and end fittings. The sheds and sheath are made of organic materials such as high‐temperature vulcanized (HTV) silicone rubber. The core rod that mainly bears mechanical force is a composite material (glass fibre‐reinforced plastics or GFRP) with epoxy resin as a matrix and glass fibre as a reinforcement material. Carbon cast steel or structural steel with a hot‐dip galvanized layer on the exterior surface is commonly used for end fittings. The core rod fracture will result in significant accidents such as offline short circuits, putting the power grid's operation at risk or even causing a power outage.Normal fracture and brittle fracture were the most common fracture modes of composite insulators [5–7]. Several abnormally fractured composite insulators have been discovered in China and Korea in recent years, and researchers proposed the third core rod fracture form, decay‐like fracture [8–12]. The former two's fracture mechanism and preventive measures are transparent at home and abroad. Nevertheless, the mechanism and solution of the decay‐like fracture are still being studied. The existing research [9] believes that the intrusion of external liquid into the core–sheath interface causes hydrolysis and failure of the interface. The accumulation of liquid at the interface distorts the electric field, resulting in electrical discharge and interface current. The GFRP core rod deteriorates under the combined effects of liquid medium, discharge, interface current, and mechanical load, resulting in decay‐like fracture of composite insulator.Haris et al. [13, 14] found that the sheath breakdown in composite insulators was similar to the decay‐like fracture. The microscopic morphology of the sheath breakdown was studied. They proposed that failure and deterioration began at the fittings’ junction and extended along with the interface. Due to the heating of the internal conductive area, resin ablation led to electrochemical damage, and the punctures formed at the weak sheath.The operation and maintenance staff discovered abnormal temperature rises near the high voltage end of insulators through infrared (IR) imaging during daily inspection. Since 1998, many abnormal fracture accidents of alternating current (AC) 500 kV composite insulators have took place in China. Using an infrared thermal imager, local temperature rises at the high voltage side of these insulators were observed before fracture [15–19]. Due to internal defects in the core rod–silicone rubber interface, partial discharge, polarization after water penetration, and insulation aging occurred, resulting in abnormal temperature rises. And internal heat could not dissipate in time because of wrapped sheath. Therefore, abnormal temperatures are observed on the surface of the composite insulator, which can be utilized to evaluate internal defects [20–23]. The operation experience shows that the position and size of internal defects can also be judged by ultraviolet (UV) imaging method to identify discharge or by electric field detection method to research the electric field of insulators. Decay‐like fracture of composite insulators is a small probability accident. Due to the rare samples and the difficulty of sampling, characteristics and mechanism of decay‐like fracture are not precise. This paper provides experimental data and technical support for the decay‐like fractured insulators.Literature [8–12] carried out experiments on the decay‐like fractured insulators and obtained the hydrophobicity, morphology, and physicochemical properties. Literature [11, 24–26] carried out IR and UV imaging tests on the decay‐like fractured insulators in the laboratory. Based on the current status and conclusions, electric field distortion and electrical discharge play an important role in the decay‐like formation and abnormal temperature rise during operation. However, typical characteristics and mechanism of decay‐like fracture are not precise due to the rare samples (decay‐like fractured insulators). This paper studies the heating, discharge and electric field of three types of insulators (decay‐like fractured, aging, and new) in the laboratory, expecting to reveal the typical characteristics of decay‐like fracture. The results provide support for judging decay‐like fracture of composite insulators.MATERIALS AND METHODSSamplesThe test samples come from a B‐phase (neutral line) V‐shaped composite insulator string of a 500 kV (AC) operating transmission line, and the type is FXBW‐500 kV/180 kN. The decay‐like fractured composite insulator, unbroken insulator in the same string, and new insulator are examined, respectively, and the corresponding numbers are No. 1, No. 2, and No. 3. The insulator sheds include large, medium, and small sheds. The specific geometric parameters of the insulator are shown in Table 1.1TABLEDimensions of the decay‐like fractured composite insulatorStructural heightInsulation distanceCore rod diameterShed spaceShed diameter4,450 mm4,160 mm24 mm126 mm190 mmAccording to the appearance of the fractured composite insulator, core rod has seriously deteriorated (rough macroscopic section, crisp texture, chalking core rod, separated glass fibre and epoxy resin matrix, and the shape is like rotten wood). Moreover, core–sheath interface near the core rod fracture is invalid, with decay‐like characteristics. The appearance of No. 2 is intact, and there is no sign of damage or failure. Figure 1 shows the field fracture diagram of the composite insulator.1FIGUREFracture of 500 kV composite insulator stringExperiment procedureVisual observationThe samples' fracture morphology and contamination distribution were initially detected on the spot. And then, the samples were transported back to the laboratory after protection measures. In this paper, insulator sheds are used to describe the position, starting from the grouding ends. There are 33 large sheds, including 32 group sheds plus an additional large shed, and four sheds are a group.Experiment setupThe experiments under diverse conditions were carried out in the high voltage hall of Chongqing University. A valid high voltage of 500 kV × 1.1/1.732 = 318 kV was applied to the insulators during the AC test. In the direct current (DC) test, the high voltage value was 449 kV, the peak value of the AC test. According to the temperature rise measuring experience of composite insulators, each experiment lasted for 30 min. The insulators' temperature rises and discharge were measured by the infrared thermal imager and ultraviolet imager at 10 m from the insulator, and the electric field distribution was at 3.83 m. The decay‐like fractured insulator was bonded with insulation tape and rope to prevent falling.Infrared imaging methodThe FLIR T1044 infrared thermal camera was used to measure temperature rise.Ultraviolet imaging methodThe CoroCAM®6D ultraviolet camera was employed to observe the insulator discharge during the research process.Electric field distribution methodDuring the experiment process, ultra wide‐band electromagnetic radiation analyser (SEM‐600) and low‐frequency electromagnetic field probe (LF‐04) were used to detect electrical field distribution.The above detection steps are roughly similar, as follows:Firstly, after the instrument was turned on, parameters were set (such as detection environment correction parameters, temperature range, and gain), and the optimal shooting point was selected. The voltage was applied to the composite insulator using the uniform step‐up method. The voltage at the predetermined voltage was maintained for about 30 min.Secondly, the temperature, discharge, and electric field strength of the insulator were measured point by point and the data was recorded.Thirdly, composite insulator was replaced and the procedure described above was repeated.Finally, different insulators' temperature rise, discharge and field strength were compared and analyzed.RESULTS AND DISCUSSIONAppearance inspectionThe overall morphology of No. 1 composite insulator is shown in Figure 2. The insulator has 33 large sheds, and the fracture spot is sited in the 31st group sheds (between the 6th and 7th sheds close to the high voltage end).2FIGUREThe overall morphology of decay‐like fractured 500 kV composite insulatorCore rod characteristics of decay‐like fractured insulatorThe macro‐characteristics of decay‐like fractured composite insulator can be summarized in the following aspects:Core rod has seriously deteriorated, and the macroscopic section is coarse. The fracture length and deterioration degree of the core rod at different positions vary. The core rod colour deepens from the outside to the inside, including black, grey and yellow. Internal damage of the core rod is more serious than the outside.Core rod has a rough and chalking surface, severe fuzzing, crisp texture, and is similar to rotten wood. The core rod surface layer fell off together when removed the sheath, and the core–sheath interface partially or entirely failed, resulting in poor adhesion.Carbonized channels appear on the core rod surface, extending from the high voltage end to the fracture position and extending several sheds to the grounding end.Glass fibre in the core rod is partially exposed and separated with epoxy resin matrix, and epoxy resin matrix degrades.In accordance with the above characteristics and FTIR (Fourier transform infrared spectroscopy) properties, the ‘gully and fracture’ on the core rod surface confirms the presence of acid and heat [9, 10, 26, 27]. The ‘crisp’ and ‘decay’ of the core rod are due to the epoxy resin modification by acid and heat, resulting in disappearance of its internal components in the form of gas. And then, glass fibre inside core rod is exposed and becomes ‘dry and messy’, indicating that the epoxy resin matrix has been severely oxidized and ablated.The core rod is showing signs of deterioration near the fracture. The core rod degrades severely along the direction of the fracture to the grounding end. The detailed fracture, interface failure phenomena, and holes on the sheath are shown in Figures 3 and 4.3FIGUREField pictures of the decay‐like fractured insulator4FIGUREThe sheath near the fracture of the decay‐like fractured insulatorSheds and sheath characteristics of decay‐like fractured insulatorThe insulator surface is heavily contaminated, especially the high voltage side. After cleaning with ethanol, the surface faded from red to pink or white, and several punctures have appeared.In addition to the transverse punctures, sheath cracks on the surface.Cracks and punctures initiate from the sheath–core interface and appear in the sheath surface.The sheath near the fracture position deteriorates, fades and is powdering seriously. The closer to the high‐voltage end, the worse the degradation and cracks become.The sheath–core interface has poor adhesion. Silicone rubber housings are easily torn and separated, and the sheath becomes brittle and hard. Black high‐temperature burning appearances occur on the surface. Different sheath colours represent various failure time.The aforementioned phenomena indicate that surface cracks are due to the partial discharge, internal penetrating current, temperature rise, nitric acid, and other factors. The local arc burns the core–sheath silicone rubber, and develops outward, causing cracks on the sheath surface. It can be seen from Figure 4 that the sheds also cracked.Figures 5 and 6 indicate that when the sheath goes closer to the high voltage end, the breaking worsens, and there are more cracks. The sheath's degradation is concentrated on the side with heavy pollution, with no notable occurrence on the other side. The degree of degradation diminishes along the direction of the grounding end. As a result, sheath deterioration is strongly linked to leakage current, the heating generated by leakage current, and other factors related to leeward sides.5FIGUREThe appearance of the wingward/leeward side in the 30th group sheds of the decay‐like fractured insulator6FIGUREThe appearances of 29, 28 and 27 sets of sheds of the decay‐like fractured insulatorThe ozone produced by the corona discharge is unavoidable in the air. The ozone odour proved the oxidation of epoxy resin matrix during the experiment. This trait is also seen in the macro‐morphology, with the colour becoming dark and the epoxy resin content reducing. Figure 7 shows the results after laboratory tests. The high‐temperature burning phenomenon is more visible than previously, and the cracking degree increases, indicating that epoxy resin matrix degradation is caused by internal factors of the composite insulator.7FIGUREObservation of FRP rod at fracture after the testExisting studies have shown that conductive interface defects may cause sheath cracks and punctures [11]. Therefore, local leakage current with high density may occur in punctures near the high voltage end.In accordance with the above phenomena, leeward side of the composite insulator is easy to contaminate, and surface leakage current increases under the action of rain. After a long‐term operation, insulator sheath ages, and the water absorption ability is enhanced. The sheath dielectric constant increases after water absorption. And then, electric field and polarization loss increase. Under the effect of high field strength, current, and heat, surface moisture easily penetrates the core–sheath interface. The air‐filled defects in the core–sheath interface or inside the core rod are filled with water. Due to the enhanced conductivity of the moisture defect, leakage current increases, electrical field strength increases and the aging process accelerates. The above aging process is a chain reaction.Temperature rise characteristicsTemperature rise characteristics of No. 1 composite insulatorIn this paper, the temperature difference between the maximum and minimum temperature measured on the insulator surface is defined as the insulator temperature rise (or ΔT). Ambient temperature and humidity were measured by thermo‐hygrometer. 318 kV AC voltage was applied on composite insulators for 30 min. The results are shown in Figure 8 (the upper end is the grounding terminal).8FIGUREThe temperature of No. 1 insulator after 30 minIt can be concluded from Figure 8:The temperature rises of the insulator middle area are 6.3 K and 5.1 K, respectively, and the high voltage side exceeds 50 K. Water will heat due to dielectric loss in AC electric field. Since the electric field strength is different at each position of the insulator, different positions have different dielectric losses. Therefore, the difference in temperature rise between various parts of the same insulator indicates that core rod heating is positively correlated with electric field and dielectric loss (or the degree of material ageing).Temperature rises change significantly with the voltage values. When the voltage reaches 318 kV, 80–100°C was detected at the high voltage side due to the violent discharge and aging core rod. The increasing internal penetrating current causes the deterioration on the high voltage side, which aggravates electric field distortion.The length of the abnormal temperature rise area at the high voltage side is about 50–120 cm, including multiple sheds, and the highest temperature rise area is centralized at the fracture and 1 to 2 group sheds near the fracture.The length of the abnormal temperature rise area at the high voltage side and the middle part grows as humidity rises.No. 1 composite insulator is tested at various ambient temperatures and humidity, and the temperature rises are recorded in Table 2.2TABLEThe temperatures and temperature rises of No. 1 insulator under diverse conditionsTest ConditionsT/°CRH%AC/DCLV side T/°CMid‐part ΔT/KHV side T/°CHV side ΔT /K3069AC33.96.395.565.52785AC29.95.183.756.72389AC24.02.882.359.31792AC17.14.570.453.41792DC17.12.323.86.8Abbreviations: AC, alternating current; DC, direct current; HV side, the area of the high voltage side; LV side, the vicinity of the ground side; RH, humidity.It can be seen from Table 2:Temperature rises at the low‐voltage side and middle part under AC test are both less than 7 K. The temperature rises of high voltage side exceed 50 K. Hence, ambient temperature and humidity have a certain influence on the temperature rise.During the DC tests, temperature rises of the low voltage side and middle part of the insulator are meagre, and temperature rise of the high voltage side is 6.7 K, which is far less than the temperature rises under AC circumstances. The polarization loss at the decay‐like fractured insulator plays a crucial role in the temperature rise. In the alternating electric field, the heat generated by the water steering polarization is the main reason for insulator heating under high humidity conditions.With regard to the above phenomena, insulating materials' properties mainly determine the insulator's temperature rise under low humidity conditions. For instance, there are quality defects in their internal interface. With increasing humidity, insulating materials' surface state, such as water polarization and leakage current, will lead to the aggravation of insulator heating. When the environmental humidity is high, the convective heat dissipation coefficient between the silicone rubber surface and the air will increase, and the high humidity will inhibit the temperature rise. Therefore, water is a crucial heating factor.Therefore, under high humidity conditions, abnormal temperature rise does not necessarily reflect the internal insulation defects of composite insulators. Low humidity conditions are conducive to improving the detection accuracy of the infrared detection technology.Temperature rise characteristics of No. 2 and No. 3 insulatorsUnder the same conditions, experiments were performed on the same V‐string unbroken insulator (No. 2 sample) and new insulator (No. 3 sample) to investigate the heating characteristics of decay‐like insulators. The applied voltage was 318 kV (AC). The other test conditions were the same as above. The results are shown in Figure 9 (left, middle, and right are the grounding end for 30 min, the high voltage end for 30 s and 30 min, respectively).9FIGUREThe temperature of No. 2 and No. 3 insulators (T = 30°C, RH = 69%)It can be seen from Figure 9:After removing the grading ring of high voltage end, temperature rises of No.2 insulator are 5.1 K, 7.2 K, and 11.1 K, respectively, and the temperature rises increase by 0.8 K, 3.1 K, and 6.5 K. This change indicates that electric field of the high‐voltage end increases significantly without the grading ring, resulting in great losses and serious heating at the high‐voltage end. The reducing effect of the grading ring on the electric field strength at the high voltage end will effectively inhibit the temperature rise of the insulator.There are also temperature rises for No. 2 and No. 3 composite insulators during the research. The temperature rise of high voltage end is greater than the low‐voltage side and the middle area. The temperature rises from high to low are the high voltage side, low‐voltage side, and middle part of the insulator, respectively, indicating that the electric field is one of the causes of the temperature rise.Temperature rise of No. 2 insulator is slightly higher than that of No. 3 insulator, indicating that insulator's performance has changed due to its degradation and deterioration in operation. Therefore, temperature rise can judge the degree of insulator degradation during the daily inspection.Temperature rise of high voltage ends of No. 2 and No. 3 insulators is much lower than that of No. 1 insulator. The temperature rise of the operating composite insulator on the transmission line is measured, and the temperature rise generally does not exceed 30 K [10, 24–26]. Different temperatures and temperature rises indicate different degradation degrees of the core rod. Therefore, conforming to the test results, when composite insulators' temperature and temperature rise are abnormal (For example, T > 50°C, ΔT > 30 K) during the daily inspection, the core rod may be severely decay‐like and should be take immediate measures. The relationship between decay‐like status and abnormal temperature rise value can be further studied in the future.UV discharge characteristicsUV discharge characteristics of No. 1 insulatorThe discharge distribution characteristic of No. 1 composite insulator was researched. Power frequency voltage of 318 kV (AC) was applied on composite insulators for 30 min. The discharge results of the high voltage end are shown in Figure 10.10FIGUREDischarge at the high voltage end of No. 1 insulator during the AC testIt can be seen from Figure 10 that the high voltage side of No. 1 composite insulator has an apparent discharge, but the middle and low voltage sides have no visible discharge. There is no evident difference between high humidity and low humidity in the discharge form and position of the high voltage end of No. 1 composite insulator. The main characteristics are as follows: On the grading ring, there are numerous discharges. The phenomenon is white purple arc breakdown air discharge, and the shed's discharge at the shelter of the grading ring creates the possibility of forming nitric acid around it. The phenomenon is a white, purple edge arc connected to the bare core rod at both ends of the shed's discharge.It can be seen that the discharge occurs in the high field strength area, resulting in a local temperature rise on the core rod's surface. Surface discharge in a wet environment is also the possible cause of acid medium in the composite insulator. Figure 11 shows the discharge of No. 1 insulator during the DC test.11FIGUREDischarge of No. 1 insulator during the DC test (T = 17°C, RH = 92%)The number of photons at the major discharge position under various conditions is compared, as shown in Table 3, and the shooting parameters are consistent. The ordinate and abscissa are test number and test conditions, respectively. Test condition includes ambient temperature (°C), humidity (RH%), and voltage type of the experiments. The unit of photons was a thousand.3TABLEPhoton number of different insulators under diverse test conditions (Unit: Thousand)Number of photons30°C 69% AC27°C 85% AC23°C 89% AC17°C 92% AC17°C 92% DCNo. 1120–130190–200150–160130–1400.6–0.7No. 2110–120130–140130–140130–1500.3–0.4No. 3110–120120–130130–140140–1500.1–0.2It can be seen from Table 3 that the photon numbers of different insulators under various conditions show the following characteristics:The photon number of No. 1 insulator shows a tortuous upward trend under different humidity conditions. The maximum value appears at 27°C (Temperature) and 85% (RH), indicating that the discharge of decay‐like insulator is affected by humidity.The photon numbers of No. 2 and No. 3 insulators increase with humidity, but the difference is not apparent when humidity is greater than 85% (RH). Therefore, the observation of insulators' discharge should carry out under low humidity.The photon number of the same insulator under DC conditions is much smaller than in AC. The discharge intensity is also the same, so the possibility of decay‐like generation under DC conditions is low. Under the same experimental conditions, the order of photon number is No. 1 > No. 2 > No. 3, and the degree of sample deterioration is the same.The reason for the discharge at the fracture site of the decay‐like fractured insulator is that the core rod has defects such as erosion, where the sheath and sheds have fallen off, resulting in core rod being exposed to the air. The defect site is filled with air to form an air gap. The dielectric constant of the air is smaller than that of the organic material, so the air gap withstands a large voltage and is easy to discharge. High field strength and discharge at defects enhance organic material ageing, leading to the decay‐like occurrence of core rod, sheath and interface, which may cause breakage when it continues to run.When the insulator's insulation damage or penetrating channel occurs, discharge situation is similar to the above figures. Temperature rises and nitric acid are formed due to partial discharge and conduction current, which continuously destroy the internal insulating structure and eventually cause decay‐like ageing and even fracture.The insulator surface is generally contaminated by conductive compounds in different states. The surface becomes partially or completely damp as the humidity rises, and the soluble matter in the contamination layer dissolves, transforming it into a conductive layer. There is a leakage current flowing under the operating voltage. The leakage current increases as the wetness level rises, causing the conductive layer to heat up. Then, it may be dried to reduce the conductivity of the damp layer or heated to cause the positive temperature coefficient electrolyte to decompose, increasing the conductivity. The final result is to generate an arc and discharge along the surface or reduce the discharge intensity.UV discharge characteristics of No. 2 and No. 3 insulatorsThe discharge distribution characteristics of No. 2 and No. 3 insulators are tested by the same method, including insulators without or with grading rings. The test results are shown in Figures 12 and 13.12FIGUREDischarge of No. 2 and No. 3 insulators with grading ring (T = 30°C, RH = 69%)13FIGUREDischarge of No. 2 and No. 3 insulators with grading ring (T = 30°C, RH = 69%)When No. 2 and No. 3 insulators are researched, the main discharge characteristics are as follows:Discharge position is at the grading ring or near the high voltage side. The discharge on the ring is intense, but discharge in other areas can be ignored. The high voltage sides of the three insulators have an evident discharge, and the discharge degree is 1 > 2 > 3. There is no noticeable discharge in other parts. It can be seen that the electrical field strength at both ends of the composite insulator, especially the high voltage end, is significantly affected by the size of the grading ring and the installation position.When the grading rings are installed improperly or the type is inappropriate, field strength at the sheath‐fittings adherent point increases, increasing the possibility of nitric acid formation around them. When there is a defect in this part, water in a high humidity environment may enter the defect through silicone rubber to make the electric field distorted at this place, resulting in partial discharge, ageing, and even decay‐like.The discharge results of No. 1, No. 2 and No. 3 samples are compared and analysed, including discharge position, shape, and intensity. No. 1 insulator discharges at the exposed core rod of the fractured position. In addition, the remaining positions are the same as No. 2. There is no apparent difference between No. 2 and No. 3 samples, so it is not easy to judge whether the composite insulator is decay‐like or not in the transportation inspection.Electric field strength distributionThe electric field strength of the insulator was measured using an electromagnetic radiation analyser (SEM‐600) and a low‐frequency electromagnetic field probe (LF‐04) during the test. The electric field intensity was measured at 3.83 m from the composite insulator. The first measurement point was measured at 1.30 meters from the insulator's low voltage end (upper end), and each subsequent measurement point was 0.50 m lower for seven measuring points. Figures 14 and  15 are the measurement diagram and the test results, respectively.14FIGURESchematic diagram of electric field measurement15FIGUREElectric field distribution of three insulators under different humidityThe electric field distribution of decay‐like fractured insulator is approximately linear with the measurement distance, especially from the middle part to the high voltage end. Slope is approximately the same and increases, respectively, as shown in the diagram. From the electric field measurement data and Figure 15 curve:Under the same humidity conditions, the electric field strength around the decay‐like fractured insulator is significantly higher than that of the intact insulator, indicating that the insulation performance of the core rod and sheds/sheath decreases during the decay‐like ageing of the insulator.Comparing the field strength curves under various test conditions, it is clear that the grading ring, insulator and humidity have apparent influences. The influence of test conditions should be considered in field strength analysis.Optimizing the grading ring may reduce the voltage and surface field strength towards the high voltage end, and the distribution uniformity can be improved.Conforming to the above results, electrical field strength of decay‐like fractured composite insulator is higher than that of other insulators under the same conditions. Nevertheless, electrical field strength curves have no noticeable distortion. Different field strength curves have a similar variation trend, and the field strength is easily affected by environmental factors such as humidity. Therefore, it is not easy to determine whether the composite insulator is decay‐like using electric field distribution under natural conditions.CONCLUSIONThe observations of decay‐like fractured composite insulator are examined in depth in this research. The temperature, discharge, and electric field distribution during the experiment of three composite insulators (decay‐like fractured, aging, new) in the laboratory are detected using the ultra‐wideband electromagnetic radiation analyser SEM‐600, ultraviolet imager CoroCAM®6D, and infrared thermal imager FLIR T1044.The characteristics of the decay‐like fractured composite insulator are as follows:According to the findings, core rods undergo continuous degradation till fracture due to a combination of internal (core‐sheath interface failure, water, electrical discharge, electrical field strength distortion, oxidation and acidic medium) and external (pollution accumulation and leakage current) factors. The epoxy resin matrix is mainly destroyed by discharge, current and heating. Acidic medium and mechanical stress mainly lead to stress corrosion fracture of glass fibre. The leading cause of the formation of acidic medium is electrical discharge in the humid environment.Decay‐like part of composite insulators has the following characteristics: evident degradation phenomenon of glass fibre and epoxy resin matrix, separation between them, fracture of glass fibre, degradation of epoxy resin and carbonized channel penetration on the surface of core rod. The surface damage of the sheath is only concentrated on the side with heavy contamination, and the deterioration degree decreases along the grounding end direction. Degradation of the sheath is closely related to surface contamination or leakage current. Internal discharge and heating of composite insulators are the causes of epoxy resin matrix damage. Electrical discharge, current, and heating mainly destroy the epoxy resin matrix.The ambient temperature and humidity have an impact on the temperature rise of composite insulators, but the decay‐like composite insulators have the characteristics of significant high‐temperature rise. Therefore, one of the most obvious characteristics of decay‐like is abnormal temperature and temperature rise (such as T > 50°C, ΔT > 30 K). The values can be used to judge the decay‐like of composite insulators during the daily inspection, and the use of infrared temperature measurement detection technology under low humidity conditions (For example, RH < 85%) is conducive to improving the detection accuracy of composite insulators. The temperature rise changes in different periods during the decay‐like fracture process of composite insulators need to be further studied.Under the same conditions, discharge strength of decay‐like insulators is much higher than that of other insulators, and the discharge at bare rod is intense. Insulator discharge is affected by humidity and should be observed under low humidity (e.g. < 85%). Under DC conditions, photon number, as well as discharge intensity, is much lower than that under AC conditions for the same insulator.The electric field strength around the decay‐like fractured insulator is significantly higher than that of the intact insulator under the same conditions. Compared with the field strength curves under different test conditions, the influence of grading ring, insulator condition and humidity is apparent. Electric field distribution and ultraviolet measurement are difficult to judge whether the composite insulator is decay‐like.ACKNOWLEDGEMENTThis work was supported by the Science and Technology Project of State Grid Corporation (5500‐202024073A‐0‐0‐00).CONFLICT OF INTERESTThe author declares that there is no conflict of interest.DATA AVAILABILITY STATEMENTThe datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.REFERENCESLiang, X., Gao, Y., Wang, J., Li, S.: The rapid development of silicone rubber composite insulators in China. High Voltage Eng. 42(9), 2888–2896 (2016)Cherney, E.: 50 years in the development of polymer suspension‐type insulators. IEEE Electr. Insul. Mag. 29(3), 18–26 (2013)Papailiou, K.O., Schmuck, F.: Silicone Composite Insulators Materials, Design, Applications, 1st ed., pp. 1–7. 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Published: Oct 1, 2022

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