The Purpose of Transformer Bushings: It is an insulating device that leads the leads of the transformer coils to the outside of the oil tank. It serves as both the insulation between the leads and the oil tank and a fixing device for the leads. During the operation of the transformer, the bushing passes the load current for a long time, and when there is an external short circuit, it passes the short-circuit current. Therefore, the following requirements are imposed on the transformer bushing: ① It must have the specified electrical strength and sufficient mechanical strength. ② It must have good thermal stability and be able to withstand the instantaneous overheating during a short circuit. ③ It should have a small appearance, light weight, good sealing performance, strong versatility, and be easy to maintain.
The external structure of the bushing includes: terminal board, lead joint, rain cover, oil gauge, oil plug, conservator, upper porcelain bushing, end screen, lifting ring, oil sampling valve, nameplate, gas release plug, connecting bushing, lower porcelain bushing, and grading ball.
Internal Structure:
The main insulation is composed of a multi-layer cylindrical capacitive core made of oil-impregnated cable paper and aluminum foil grading electrodes, and the porcelain part serves as the external insulation and the container for the transformer oil.
The bushing has a fully sealed structure, and the transformer oil inside is an independent system that is not affected by the atmosphere.
The overall connection of the bushing adopts strong spring mechanical fastening, which not only ensures the sealing but also compensates for the length changes of each component caused by temperature changes. The conservator at the head of the bushing is used to regulate the volume change of the oil caused by temperature changes, so that the inside of the bushing is protected from excessive pressure. The oil gauge on the conservator is used to monitor the oil level during operation. The grading ball at the tail serves to improve the distribution of the electric field, thereby reducing the insulation distance between the tail of the bushing and the grounded part and the coil.
The small bushing led out from the end screen of the oil-paper capacitive bushing is used for the bushing dielectric loss test and the partial discharge test of the transformer. During normal operation, the small bushing should be reliably grounded. When removing the small bushing of the end screen, care should be taken to prevent the guide rod of the small bushing from rotating and being pulled out to avoid the disconnection of the lead wire or the damage of the copper sheet led out from the plate.
The Arrangement of the Labels of the Bushings of a Three-phase Transformer: When viewed from the side of the high-voltage bushing of the transformer, the label sequence from left to right is: High voltage: O, A, B, C; Medium voltage: Om, Am, Bm, Cm; Low voltage: O, a, b, c.
Bushings can be classified into three types according to the insulating materials and insulating structures: ① Single insulation bushings: They are further divided into two types, pure porcelain and resin bushings; ② Composite insulation bushings: They are further divided into three types, oil-filled, glue-filled, and gas-filled bushings; ③ Capacitive bushings: They are further divided into two types, oil-paper capacitive and glue-paper capacitive bushings.
The capacitive core of the oil-paper capacitive transformer bushing can be generally classified into cable-passing type and conduit current-carrying type according to the current-carrying structure. Among them, the conduit current-carrying type can be divided into direct type and rod-passing type according to the connection mode between the terminal in the oil and the bushing. Cable-passing type and direct conduit current-carrying type bushings are widely used in the power system, while the oil-paper capacitive bushings with the rod-passing structure are relatively few.
The capacitive core of the capacitive bushing is made by tightly wrapping an insulating layer of a certain thickness with 0.08 to 0.12mm thick cable paper outside the hollow conductive copper tube, and then wrapping a layer of aluminum foil with a thickness of 0.01mm or 0.007mm as the capacitive screen. After that, the cable paper and aluminum foil are alternately wrapped until the required number of layers and thickness are achieved. In this way, a multi-layer series capacitor circuit is formed, the potential of the conductive tube is the highest, and the outermost aluminum foil is grounded (ground screen). According to the principle of series capacitor voltage division, the voltage between the conductive tube and the ground should be equal to the sum of the voltages between each capacitive screen, and the voltage between the capacitive screens is inversely proportional to their capacitances, so that the entire voltage is evenly distributed on the entire insulation of the capacitive core, thus making the bushing small in size and light in weight.
The Model and Its Meaning of the Transformer Bushing: B is for transformers; F is for composite insulation type; D is for single insulation type; J is for those with additional insulation; R is for capacitive type; Y is for oil-filled type; L is for cable-passing type; Q is for enhanced type; L is for those that can be equipped with current transformers; Numbers/Numbers represent the rated voltage (KV)/rated current (A).
If the top of the oil-paper capacitive bushing has poor sealing, it may lead to water ingress and insulation breakdown, and poor sealing at the bottom will cause oil leakage of the bushing and a drop in the oil level.
The two necessary conditions for the pollution flashover of the porcelain insulation of the bushing are the presence of dirty dust on the surface and the wetness of the surface. The dirty surface of the bushing is prone to flashover, resulting in false tripping. At the same time, the flashover will also damage the surface of the bushing. Another harm of the dirty surface of the bushing is that after the dirt absorbs moisture, its conductivity increases, which not only causes surface flashover but may also cause the insulating bushing to heat up and crack due to the increase in leakage current, and finally lead to breakdown.
The core of the oil-paper capacitive bushing is an integral body rolled from multi-layer cable paper boxes and aluminum foil. If the oil is injected according to the conventional method, air is likely to remain between the screens. Under the action of a high electric field, partial discharge will occur, and even the insulation layer will be broken down, causing an accident. Therefore, it is necessary to immerse it in oil under high vacuum to remove the remaining air.
The reason why the two ends of the core of the oil-paper capacitive bushing are wound into a cone shape: In order to make the voltage between each plate approximately equal, make the electric field tend to be uniform, and improve the electrical strength, the capacitance between each plate must be approximately equal. However, the capacitance is proportional to the area of the plate. Therefore, as the radial size of the electrode increases, the axial size should be correspondingly reduced. So, it is necessary to make the two ends of the core of the oil-paper capacitive bushing form a cone shape.
The Maintenance of the Oil-paper Capacitive Bushing:
① Check that the porcelain part of the bushing is intact without cracks, and the surface is smooth and clean; there are no discharge and flashover marks, the flange iron parts are intact and firmly combined with the porcelain part, and the packing is complete.
② Check that the end screen is well grounded.
③ Check that the oil level of the bushing is appropriate and there is no oil leakage.
④ Conduct dielectric loss and insulation tests on the bushing. If it fails, disassemble and repair it.
Matters Needing Attention When Installing the Oil-paper Capacitive Bushing: First, check whether the model of the bushing is correct, and whether the electrical test and oil test are qualified. Check whether the oil level of the bushing is appropriate, and whether there is oil leakage or damage to the porcelain sleeve. Check whether the length of the lead is appropriate. Wipe the bushing clean and check the welding condition of the lead head. When lifting the bushing, comply with the lifting operation regulations to prevent damage to the porcelain sleeve. The inclination of the bushing during lifting should be determined according to the angle of the bushing lifting seat of the transformer. The thin rope for pulling the lead should be strong and hung in an appropriate position. As the porcelain sleeve is installed, gradually pull out the lead head. Pay attention that the lead should not be twisted or knotted. If the lead cannot be pulled out, find out the cause, and do not pull it forcefully or pull it with a lifting tool. After the bushing is in place, the conical insulation at the root of the lead should extend into the grading ball of the bushing. If the root insulation is damaged, it should be re-wrapped. The lead and the terminal should have sufficient contact area and contact pressure. The terminal should be reliably sealed to prevent water ingress.
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