About transformer bushing

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The purpose of transformer bushing is to lead the leads of transformer coils separately to the insulation device outside the oil tank. It is not only the insulation of the leads to the oil tank, but also the fixing device of the leads. 

The external structure of the casing includes: wiring board, lead connector, rain cover, oil gauge, oil plug, oil conservator, upper porcelain sleeve, end screen, lifting ring, oil valve, nameplate, air release plug, connecting casing, lower porcelain sleeve, and equalizing ball. Internal structure: 

1. A multi-layer cylindrical capacitor core composed of oil impregnated cable paper and aluminum foil pressure equalization electrodes serves as the main insulation, while ceramic components serve as external insulation and containers for transformer oil.

2.  The casing is a fully sealed structure, and the transformer oil inside is an independent system that is not affected by the atmosphere.

3.  The overall connection of the casing adopts strong spring mechanical fastening, which not only ensures sealing but also compensates for the length changes of various components caused by temperature changes.

The oil pillow at the head of the casing is used to regulate the volume change of oil caused by temperature changes, so as to prevent large pressure inside the casing. The oil gauge on the conservator is used to monitor the oil level during operation. The function of the tail pressure equalization ball is to improve the distribution of the electric field, thereby reducing the insulation distance between the tail of the casing and the grounding part and the coil. The small bushing led out from the end screen of the oil paper capacitive bushing is used for bushing dielectric loss test and transformer partial discharge test. During normal operation, the small bushing should be reliably grounded. When disassembling the end screen small bushing, it is necessary to prevent the small bushing guide rod from rotating and pulling out to avoid wire breakage or damage to the copper skin on the electrode plate.

 Fault and defect of transformer bushing:

 1. After the surface of the bushing is dirty and absorbs moisture, it will reduce the insulation resistance, and the consequence is that it is prone to flashover, causing tripping. Meanwhile, flashover can also damage the surface of the casing. After the dirt absorbs moisture, the conductivity increases, which not only causes surface flashover, but also may cause insulation sleeve to heat up and cause porcelain damage or even breakdown due to increased leakage current; 

2. The sealing of the sleeve gasket is ineffective, and the top sealing of the oil paper capacitive sleeve is poor, which may cause water ingress and insulation breakdown. The poor sealing at the bottom causes oil leakage in the sleeve, resulting in a decrease in oil level. There are two main reasons for the failure of casing sealing: firstly, due to the lack of experience of maintenance personnel and insufficient bolt tightening force; The second reason is due to over cycle operation, quality issues with the rubber pad, aging of the rubber pad, etc; 

3. The structure of the casing itself is unreasonable and has defects. For example, some 220kV main transformer sleeves are soldered with tin due to the use of lead and lead head welding. The 220kV A-phase sleeve pressure tube is made of aluminum, the wire head is made of copper, and the rainproof phase is made of aluminum. 

This copper aluminum connection increases the contact resistance, making the connection prone to heating and sintering, leading to accidents; Local oil leakage in the casing, unqualified insulation oil, and mild dampness caused by water ingress into the casing; Loose and broken grounding small sleeve on the flange cylinder in the middle of the sleeve; The fault of the grounding small sleeve causes the suspension potential of the sleeve bundle screen to occur, resulting in partial discharge;

 The casing oil gauge is dirty and the oil level cannot be clearly seen, resulting in oil loss after annual pre test oil sampling. During the overhaul of the casing, incomplete vacuum pumping resulted in residual air between the screens. After operation, under the action of a high electric field, partial discharge occurred, even leading to insulation layer breakdown and causing accidents.