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09
2022
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02
Infrared and ultraviolet diagnosis of defects in composite insulator core rods
Composite insulators are currently used in large quantities, but if the core rod process is not well controlled, defects may be left behind, which can develop during operation, leading to fractures and failures. To prevent failures, diagnosing defects in the core of composite insulators is crucial.
Composite insulators are currently used in large quantities, but if the core rod process is not well controlled, defects may remain, leading to fractures and failures during operation. Diagnosing defects in composite insulator core rods is crucial to eliminate failures. For operating units, electrical testing methods for core rods are a better means of balancing reliability and equipment status identification. Currently, effective and mature methods for electrical testing of composite insulator core rods include infrared and ultraviolet testing.
Infrared diagnosis of internal faults in the core rod:
Infrared characteristics of internal defects in the core rod:
The heat sources of composite insulators mainly include polarization loss, partial discharge, and leakage current. The first two arise from excessive concentration of the electric field caused by defects. Leakage current heating is often seen when the insulator's umbrella surface is heated; leakage current concentrates in deteriorated areas, resulting in a noticeable temperature difference with the surroundings. Generally speaking, air gaps and the ends of carbonized channels become 'hot spots' due to partial discharge conditions, while the middle part of carbonized channels tends to have lower temperatures due to weaker electric fields.
Characteristics: At AC high voltage, temperature rise under high humidity is around 4-5K, significantly higher than that under low humidity; during DC high voltage tests, temperature rise in insulators is almost unobservable. Due to differences in temperature rise caused by humidity, on-site inspections may show very high temperature rises or sometimes no observable temperature rise.
Carbonization channel
One tower on a 500kV line has a broken V-string composite insulator with multiple holes in the core rod; there are already electric channels inside. Insulators A and B from this batch were placed under operating voltage for infrared temperature measurement.
1) The high-voltage end temperatures of both Insulators A and B are around 15 degrees Celsius with little difference.
2) The middle temperature of Insulator A is 14.8 degrees Celsius, higher than Insulator B's 12.9 degrees Celsius. During the experiment, it was found that at units 45-46 (where unit one at low voltage end meets deep rod corrosion), Insulator A's temperature was higher by 1.9 degrees than that of Insulator B.
Core rod function:
1) The high-voltage end of the electric channel shows significant temperature rise compared to intact boundary areas.
2) Compared to thermal insulation materials, the degree of temperature rise may not be very high.
The temperature difference at different positions within the insulator itself is more pronounced.
(In fact, Insulator B also has internal defects due to poor adhesion.)
Core rod testing judgment:
The on-site infrared diagnostic judgment standard for composite insulators follows Appendix I.2 of standard DL/T 664 Application Specification for Infrared Diagnosis of Live Equipment; attention should be paid when the body temperature difference exceeds 0.5-1.0 K.
On-site infrared detection can easily be affected by external lighting reflections, angles, and focus clarity; thus errors in core rod testing are difficult to control. Therefore, general units including China Electric Power Research Institute and other local companies have relaxed these standards to around 2.0 K.
To assist judgment, multiple follow-up checks must be conducted on suspicious insulators along with regular auxiliary assessments.
Core rod testing precautions:
(a) Prevent sunlight and other visible light interference
(b) Composite insulators are heat-type devices caused by voltage; therefore, temperature field tests can easily be affected by external environments. When sunlight and visible light are strong enough, it can cause differences between double-sided or white surfaces leading to variations elsewhere on the core peak. On days with strong external sunlight, infrared measurement's highest point may be located on surfaces reflecting strong sunlight; measuring points (SP1) on both sides of the peak show temperatures of 27.9 and SP2 shows a reading of 25.0 respectively resulting in a difference reaching up to about two degrees Celsius likely caused by sunlight interference.
Therefore it’s best to conduct core rod tests on cloudy days.
(b) Reasonably set test distance parameters such as reflectivity and wind speed etc.; reflectivity must be set between .85-.95 range; tower climbing or ground tests should reasonably set test distances (distance measuring devices or anemometers can be carried).
(c) During core rod tests try choosing angles perpendicular to the peak while paying attention to focus clarity; discard overly blurry infrared images as necessary; hotspots on insulating cores should be compared against other distant locations along their axis for determining elevated temperatures.
(d) Record test time along with environmental humidity & temperatures during tests while providing infrared images for subsequent analysis.
(e) If parameters set during infrared photography tests aren’t correct generally software within infrared instruments has image inversion functions allowing adjustments made through software recalculating elevated temperatures which can somewhat mitigate impacts from incorrect parameter settings (however accuracy still declines compared against correctly set parameters during actual tests).
(f) If top height is too great causing long distances where limited resolution prevents accurate measurements consider adding focal lenses improving distance capabilities.
Core rod
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