usually, the aging of insulation materials is mostly caused by the complex interaction of various main factors such as electrical, mechanical, thermal, and environmental factors, so the aging presented is also diverse. these abnormal phenomena can be detected during maintenance and inspection, and should be listed as important items for inspection in order to prevent accidents in advance.
1. cracking
when cracks are found on porcelain insulators, insulation sleeves, and epoxy resin products, whether in terms of electrical or mechanical properties, they are dangerous and must be replaced as soon as possible. local skirt edge or flange defects, although not necessarily causing accidents, should be replaced as soon as possible as they can expand into cracks.
for insulators and insulation sleeves made of porcelain and polymer materials, there are several reasons for cracking:
(1) reasons for cracking of porcelain insulators and insulating sleeves
a. there are small defects on the surface and inside of porcelain parts that occur during the manufacturing process. due to repeated external forces, they are subjected to mechanical stress, which then develops into cracks, skirt fractures, and so on.
b. flashover caused by overvoltage or contamination can cause damage to porcelain components due to arc or local overheating.
c. applying silicone grease on insulators is generally used as a measure to prevent contamination and damage. when not repainting silicone grease for a long time and continuing to use it, leakage current and partial discharge may occur due to the aging of silicone grease, as well as the peeling of glaze on the surface of porcelain insulators, skirt defects, and cracks.
d. due to the tightening of the hardware, certain parts of the porcelain component are subjected to excessive stress.
e. due to negligence during operation, the insulator may be damaged due to unexpected external force strikes or damage caused by external forces such as throwing stones.
f. porcelain sleeves used on equipment, such as poor coordination of internal equipment, can sometimes cause damage to the interlayer connection of porcelain sleeves.
(2) reasons for cracking of insulators and sleeves made of polymer materials
a. the residual internal stress generated during the solidification and shrinkage of materials during the manufacturing process can cause cracking.
b. the thermal cycle caused by repeated operation and shutdown of equipment can cause cyclic thermal stress on products due to differences in thermal expansion coefficients of different materials, leading to metal peeling and cracking embedded in the resin.
c. due to the decrease in mechanical strength of insulation materials or fatigue caused by repeated stress during long-term operation, cracking can also occur.
d. excessive tightening of the fastening part leads to excessive mechanical stress and cracking.
2. creepage marks
when the surface of organic insulation materials is stained and wet, leakage current flowing through the surface will form a local, high insulation resistance dry zone, causing the voltage applied to this part to increase, resulting in small discharges. as a result, the insulation surface is carbonized to form a conductive pathway, which is known as creepage marks. if insulators that have generated creepage traces are left intact, they will gradually develop and eventually cause a grounding short circuit accident due to flashover.
when replacing insulators with creepage marks, it is necessary to strengthen the management of issues such as dirt and moisture, and to use materials with excellent resistance to creepage marks, in order to prevent the recurrence of creepage marks.
3. oil leakage
insulating sleeves with insulating oil inside can cause porcelain tube misalignment due to cracking, excessive bending load, or oil leakage due to aging of sealing materials. when oil leakage is severe, it not only causes insulation breakdown of the casing, but also may cause significant damage to the equipment installed with the casing, such as transformers, reactors, oil circuit breakers, etc. therefore, in case of oil leakage, the severity should be immediately investigated and necessary measures should be taken according to the situation, such as stopping operation or replacing.
by observing the position of the oil level and checking the condition around the installation site of the casing, oil leakage can be monitored. the method (structure) for monitoring the position of the oil level varies slightly from manufacturer to manufacturer, and attention should be paid when the oil level is below the visible range of the oil level gauge.
furthermore, the sealing material of the casing is made of organic materials such as nitrile copolymer cork and synthetic rubber, which inevitably leads to aging over time. therefore, it is necessary to conduct regular inspections and replace sealing materials at appropriate intervals.
4. corona sound
corona discharge from protruding parts of terminal hardware and surface discharge from contaminated insulation surfaces can cause audible sounds. however, cracks and internal defects in insulators and sleeves can also be the cause of corona sound. when hearing the sound of corona, it is necessary to identify the cause early and take appropriate measures. in addition, the stray electromagnetic waves generated by such corona discharges can interfere with radio and television.
5. terminal overheating
the central part of the insulating sleeve runs through a conductor that carries current, which is connected to the busbar through the terminal hardware at the head of the sleeve. if the connection of the terminal is poor, it will overheat and cause discoloration of the terminal, shorten the lifespan of the insulation, and other faults.
therefore, while monitoring the temperature of the conductor connection using temperature indicating coatings or temperature indicating plates, it is necessary to regularly check the fastening status of various bolts here.
