Explanation of Transformer Bushing Basics: Understand Everything About Bushings in One Article!

1. They must have the specified electrical strength and sufficient mechanical strength.
2. They must possess good thermal stability and be able to withstand instantaneous overheating during short circuits.
3. They should be compact in size, lightweight, have excellent sealing performance, versatility, and ease of maintenance.

The external structure of the bushing includes

Terminal board, lead connection, rain shield, oil gauge, oil plug, oil reservoir, upper porcelain bushing, end screen, lifting ring, oil sampling valve, nameplate, air vent, connecting bushing, lower porcelain bushing, and voltage equalizing sphere.

Internal structure

1. The main insulation consists of a multi-layer cylindrical capacitor core made of oil-impregnated cable paper and aluminum foil equalizing electrodes, with porcelain components serving as external insulation and containers for the transformer oil.
2. The bushing features a fully sealed structure, with its internal transformer oil being an independent system unaffected by atmospheric conditions.
3. The overall connection of the bushing utilizes strong spring mechanisms to ensure sealing and compensate for component length changes caused by temperature variations. The oil reservoir at the top of the bushing adjusts for oil volume changes due to temperature fluctuations, preventing excessive internal pressure. An oil gauge on the oil reservoir allows monitoring of the oil level during operation. The equalizing sphere at the bottom improves the electric field distribution, reducing the insulation distance between the bushing's tail and the grounding parts and coils. The small bushing on the capacitor-type bushing's end screen is used for measuring dielectric loss in the bushing and partial discharge tests in the transformer. During normal operation, this small bushing should be reliably grounded. When disassembling the small bushing from the end screen, care must be taken to prevent rotation or removal of the conductor rod inside to avoid wire breakage or damage to the copper strip on the electrode plate.

1. Single-insulation bushings: These are further divided into two types: pure porcelain and resin-impregnated bushings.
2. Composite-insulation bushings: These are further divided into three types: oil-filled, resin-filled, and gas-filled bushings.
3. Capacitive bushings: These are further divided into two types: oil-paper capacitive and resin-paper capacitive bushings.

Transformer Bushing Models and Their Meanings:

• B: For transformer use
• F: Composite insulation type
• D: Single insulation type
• J: With additional insulation
• R: Capacitive type
• Y: Oil-filled type
• L: Cable-through type
• Q: Reinforced type
• L (second L): Can be equipped with a current transformer
• Numbers/Numbers: Rated voltage (kV) / Rated current (A)
• Poor sealing at the top of oil-paper capacitive bushings can lead to water ingress, potentially causing insulation breakdown. Poor sealing at the bottom can cause oil leakage, resulting in a drop in oil level.
• Two necessary conditions for surface pollution flashover on porcelain insulators are the presence of dirty dust on the surface and surface moisture. Dirty surfaces on bushings are prone to flashover, which can result in unintended tripping.

Additionally, flashover can also damage the surface of the bushing. Another hazard of a dirty bushing surface is that dirt absorbs moisture, increasing its conductivity. This not only causes surface flashover but can also lead to increased leakage current, causing the insulating bushing to heat up and develop cracks, ultimately resulting in breakdown.

Oil-paper capacitive bushing cores are made from multiple layers of cable paper and aluminum foil rolled into an integrated unit. If oil is injected using conventional methods, air can remain trapped between the layers. Under high electric field conditions, this trapped air can lead to partial discharges and may even result in insulation breakdown, causing accidents. Therefore, it is essential to use high-vacuum impregnation to remove any residual air.

The reason why the cores of oil-paper capacitive bushings are tapered at both ends: To ensure that each pair of plates bears approximately equal voltage, making the electric field more uniform and improving the dielectric strength, it is necessary for the capacitance between each pair of plates to be approximately equal. However, capacitance is directly proportional to the plate area. Therefore, as the radial dimensions of the electrodes increase, the axial dimensions must correspondingly decrease. Consequently, the cores of oil-paper capacitive bushings must be tapered at both ends.

Maintenance of Oil-Paper Capacitive Bushings:

1. Inspect the porcelain components of the bushing to ensure they are intact without any cracks, with a smooth surface. There should be no signs of discharge or flashover, the flange metal parts should be intact and securely bonded to the porcelain components, and the filler material should be complete.
2. Check that the end screen grounding is good to ensure proper electrical continuity.
3. Inspect the oil level of the bushing to ensure it is appropriate and there are no signs of oil leakage.
4. Perform dielectric loss (tan delta) and insulation resistance tests on the bushing. If the results are unsatisfactory, a full disassembly and inspection should be carried out.

Considerations for Installing Oil-Paper Capacitive Bushings:

1. Verify the bushing model to ensure it is correct, and confirm that electrical tests and oil analysis have been performed and passed.
2. Check the oil level of the bushing to ensure it is appropriate, and look for any signs of oil leakage or damage to the porcelain casing.
3. Verify the length of the lead wires to ensure they are suitable.
4. Clean the bushing thoroughly and inspect the welding condition of the lead wire ends.
5. Follow lifting procedures carefully when hoisting the bushing to prevent damage to the porcelain casing. The angle of inclination during hoisting should be adjusted according to the angle of the transformer's bushing riser.
6. Use a sturdy rope to pull the lead wire and attach it at an appropriate position. As the porcelain casing is inserted, gradually pull out the lead wire end. Ensure that the lead wire does not twist or knot.
7. If the lead wire cannot be pulled out, identify the cause and do not force it by pulling hard or using lifting equipment to forcibly pull it.
8. Once the bushing is in place, ensure that the conical insulation at the root of the lead wire extends into the equalizing sphere inside the bushing. If there is any damage to the root insulation, it should be rewrapped.
9. Ensure that the lead wire and terminal connections have sufficient contact area and pressure. The terminal connections must be reliably sealed to prevent water ingress.