What Is Casing Pressure?
High-voltage bushings are devices that provide insulation or support for one or several conductors to pass through such partitions as walls or boxes, and are important equipment in power systems. During the manufacturing, transportation, and maintenance of high-voltage bushings, there may be latent defects due to various reasons; during long-term operation, it is affected by the effect of electric field and conductor heating, mechanical damage and chemical corrosion, and atmospheric conditions. Defects also gradually develop.
High-voltage bushings are mainly used for the insulation of the incoming and outgoing lines of power equipment such as transformers, reactors, circuit breakers, and high-voltage circuits through walls. There are three types of high-pressure bushings: single dielectric bushing, composite dielectric bushing and capacitive bushing. The main insulation of the capacitive sleeve is composed of a coaxial cylindrical series capacitor group formed by interlayer winding of a layered insulating material and a foil-shaped metal electrode on a conductive rod. According to different insulation materials, it is divided into adhesive tape and oil-paper capacitor bushing.
In the operation of high-voltage bushings, the main insulation must withstand the role of high voltage, and the conductive part must bear high current. The main faults include poor connection of internal and external electrical connectors, damp and deterioration of the insulation of the bushing, lack of oil in the bushing, partial discharge of the capacitor core and discharge of the final screen to ground.
High-pressure casing can be divided into oil-filled casing and capacitive casing.
The cable paper in the oil-filled bushing is similar to the voltage equalizing plate in the capacitor bushing. The capacitor core in a capacitive bushing is a series of coaxial cylindrical capacitors. In an oil-filled bushing, the dielectric constant of the insulating paper is higher than that of oil, which can reduce the field strength there.
Oil-filled casing can be divided into single oil gap and multi-oil gap casing. Capacitive casing can be divided into adhesive tape and oil paper casing.
When the current-carrying conductor needs to pass through a metal box or wall with a different potential, a bushing is required. According to this application, bushings can be divided into transformer bushings, bushings for switches or combination appliances, and wall bushings. For this "plug-in" electrode arrangement, the electric field at the edge of the outer electrode (such as the middle flange of the sleeve) is very concentrated, and discharge often starts here.
High-voltage bushings are used for insulating and supporting high-voltage conductors through partitions with different potentials (such as walls and metal enclosures of electrical equipment). Due to the uneven electric field distribution in the bushing, especially the electric field at the edge of the intermediate flange is very concentrated, which can easily cause surface flash discharge. The internal insulation structure of the bushing with a higher voltage level is more complicated, and often combined insulation materials are used, and there are problems such as partial discharge. Therefore, the test of the casing must be strengthened. [1]
(1) The inspection process and quality requirements for single porcelain body and guide bushing with additional insulation (porcelain bushing in communication with the body oil) are shown in Table 1.
Table 1 Maintenance process and quality requirements for single porcelain body and guide bushing with additional insulation
| Overhaul process | Quality requirements |
| (1) Inspect the porcelain sleeve for damage | (1) Porcelain sleeves should be kept clean, no traces of discharge, no cracks, no cracks on the skirt damage |
| (2) When the casing is disassembled, the flange bolts should be loosened diagonally in turn. | (2) When loosening flange bolts, prevent uneven force from damaging the sleeve |
(3) Before removing the porcelain sleeve, shake it gently to make the space between the flange and the sealing pad Remove the porcelain sleeve after creating a gap | (3) Prevent the porcelain sleeve from cracking |
(4) Before removing the conductive rod and flange bolts, prevent the conductive rod from shaking and damaging Porcelain sleeve, the removed bolts should be cleaned, and the threaded fasteners damaged should be replaced Or trim | (4) The number of bolts and washers must be supplemented, and they must not be lost. new |
(5) Take out the insulation tube (including the conductive rod with cover layer) and wipe the oil The scale, insulation tube and the covering on the surface of the conductive rod should be properly stored (necessary Should be dry) | (5) Keep it in a safe place to prevent moisture and damage |
(6) Check the inside of the porcelain sleeve and wipe it with a white cloth; at the outer root of the sleeve Spray semiconductor paint as appropriate | (6) The interior of the porcelain sleeve is clean, free of oil stains, and the semiconductor paint is evenly sprayed |
(7) The removed porcelain sleeve and insulation should be sent to the drying room for light drying. Dry and then assemble | (7) The drying temperature is 70 80 , the time is not less than 4h, the heating rate is not Over 10 / h to prevent cracking of porcelain sleeve |
| (8) Replace the rubber pad with a new one. | (8) The rubber pad compresses evenly and seals well |
(9) Place the casing on the casing stand vertically, assembly and disassembly sequence in contrast | (9) Pay attention to the position of the insulation tube and the conductive rod with each other. Fixed loop to prevent movement, the conductive rod should be in the center of the porcelain sleeve |
(2) The inspection process and quality requirements of oil-filled casing are shown in Table 2.
Table 2 Inspection process and quality requirements for oil-filled casing
| Overhaul process | Quality requirements |
| Replace casing oil | |
(1) Drain the oil in the casing (2) Recycle with hot oil (temperature 60 70 ) and release (3) Fill with qualified transformer oil | (1) Exhausting residual oil (2) Cycle at least 3 times to flush out residual oil and other impurities (3) The quality of the oil should comply with GB2536-90 and GB4109-97 Appendix B Provisions |
| Casing disintegration | |
(1) Drain the internal oil (2) Remove the upper terminal (3) Remove the upper gland bolt of the oil level gauge and remove the oil level gauge (4) Remove the upper porcelain sleeve and flange connection bolts, gently shake the upper porcelain parts, and remove Upper porcelain sleeve | (1) Exhausting residual oil (2) Keep it in a safe place to prevent loss (3) When disassembling, prevent damage to glass oil gauge (4) Be careful not to damage the porcelain sleeve |
Continued Table 2
| Overhaul process | Quality requirements |
(5) Take out the inner insulation tube (6) Remove the connecting bolt between the porcelain sleeve and the conductive rod, remove the conductive rod and the lower Porcelain sleeve | (5) Place it vertically, do not crush or deform it (6) When disassembling the flange bolt at the bottom of the conductive rod, prevent the conductive rod from shaking and damaging Bad porcelain sleeve |
| Maintenance and cleaning | |
(1) All dismantled parts should be properly stored and wiped before assembly Wipe clean | (1) Keep it in a safe place to prevent moisture |
(2) The insulation tube should be wiped clean. If the insulation is poor, it can be at 70 80 . Dry at the temperature of 24 48h (3) Inspect and clean the inner and outer surfaces of the porcelain sleeve, and check the joints of iron and porcelain Does the cement filler fall off? (4) In order to prevent oil deterioration, apply silver powder on the surface of the glass oil level gauge | (2) The insulation tube shall be clean and free from lift-off, paint film peeling and traces of discharge, and insulation good (3) The inner and outer surfaces of the porcelain sleeve should be clean, free of grease, impurities, and crack-free porcelain. Grain, cement filler does not fall off (4) The silver powder should be painted evenly, leaving a 30mm wide transparent along the longitudinal direction. Belt to monitor oil level |
| (5) Replace each flange rubber pad | (5) The rubber pad is compressed evenly, and the parts are well sealed. |
| Casing assembly | |
| (1) The order of assembly and disassembly is reversed | (1) The conductive rod should be at the center of the porcelain sleeve, and the gap between the porcelain sleeve is even to prevent Partially stressed porcelain sleeve crack |
| (2) Fill with qualified transformer oil after assembly | (2) The oil quality complies with the specifications of Appendix B of GB2536-90 and GB4109-97 set |
| (3) Conduct insulation test | (3) According to preventive test standards |
(3) Maintenance of oil-paper capacitor casing.
During the standard project overhaul of transformers, oil-paper capacitor bushings are generally not disassembled and repaired. When the test results show that the capacitor core is slightly wet, the hot oil circulation method can be used to dry and drive the moisture slightly to make the tg value meet the requirements. The specific operation method is: Connect the oil delivery pipe to the oil plug hole at the top of the casing, and the oil return pipe to the oil discharge hole at the end of the casing. Through the hot oil circulation not higher than 80 ± 5 , make the casing tg The value reaches the acceptable value. Generally this processing time does not exceed 10h.
However, when the sleeve itself is deeply wet or the capacitor core has serious defects, or the capacitor core in the sleeve has been found to have dendritic discharge traces, it needs to be performed in an inspection yard with special processing equipment or in the manufacturing plant Repair or replace. [2]
