The purpose of the compressor bushing: It is an insulating device that leads the leads of the transformer coil to the outside of the oil tank. It is not only the insulation of the lead wire to the oil tank, but also the fixing device of the lead wire.
In the operation of the transformer, the bushing passes the load current for a long time, and passes the short-circuit current when the external short-circuit occurs. Therefore, the following requirements are imposed on the transformer bushing:
① It must have the specified electrical strength and sufficient mechanical strength.
②It must have good thermal stability and be able to withstand instantaneous overheating when short-circuited.
③Small shape, light weight, good sealing performance, strong versatility and easy maintenance.
The external structure of the casing includes: wiring board, lead connector, rain cover, oil gauge, oil plug, oil pillow, upper porcelain sleeve, end screen, lifting ring, oil valve, nameplate, air bleed plug, connecting casing, lower Porcelain sleeve, equalizing ball.
Internal structure:
1. The multi-layer cylindrical capacitor core composed of oil-impregnated cable paper and aluminum foil voltage equalizing electrode is used as the main insulation, and the porcelain piece is used as the outer insulation and the container of transformer oil.
2. The bushing is a fully sealed structure, and the transformer oil inside is an independent system, which is not affected by the atmosphere.
3. The overall connection of the casing is mechanically fastened with a strong spring, which not only ensures the sealing, but also compensates for the change in the length of each component caused by the temperature change.
The oil pillow at the head of the casing is used to adjust the change in oil volume caused by temperature changes, so as to protect the inside of the casing from high pressure. The oil gauge on the oil pillow is used to monitor the oil level during operation. The function of the tail pressure equalizing ball is to improve the electric field distribution, thereby reducing the insulation distance between the bushing tail and the grounding part and the coil.
The small bushing drawn from the end screen of the oil-paper capacitive bushing is used for the dielectric loss test of the bushing and the partial discharge test of the transformer. The small bushing should be reliably grounded during normal operation. The rod rotates and pulls out to avoid wire breakage or damage to the lead-out copper on the pole plate.
The label arrangement of the three-phase transformer bushing: from the side of the transformer high-voltage bushing, the order of the labels from left to right is:
High pressure: O, A, B, C; medium pressure: Om, Am, Bm, Cm; low pressure: O, a, b, c.
The casing can be divided into three types according to the insulation material and insulation structure:
①Single insulating casing: it is divided into pure porcelain and resin casing;
②Composite insulating casing: It is divided into three types: oil-filled, rubber-filled and gas-filled casing;
③Capacitive bushing: It is divided into oil-paper capacitive type and adhesive-paper capacitive type. Oil-paper capacitive transformer bushings are classified from the current-carrying structure. Generally, they can be divided into cable type and conduit current-carrying type. The conduit current-carrying type can be divided into direct type and rod-through type according to the connection mode of terminal and bushing in the oil. Mode. Cable-through and direct-conduit current-carrying bushings are widely used in power systems, while oil-paper capacitor bushings with rod-through structures are rare.
The capacitor core of the capacitive bushing is made of a 0.08～0.12mm thick cable paper tightly wrapped with a certain thickness of insulating layer on the outside of the hollow conductive copper tube, and then wrapped with a layer of 0.01mm or 0.007mm thick aluminum foil as a capacitor screen , And continue to wrap cable paper and aluminum foil alternately until the required number of layers and thickness are reached.
In this way, a multi-layer series capacitor circuit is formed, the conductive tube has the highest potential, and the outermost aluminum foil is grounded (ground screen). According to the principle of series capacitor voltage divider, the voltage of the conductive tube to the ground should be equal to the sum of the voltages between the capacitor screens, and the voltage between the capacitor screens is inversely proportional to its capacitance, so that all voltages are more evenly distributed on all the insulation of the capacitor core. Therefore, the casing is small in size and light in weight.
The type of transformer bushing and its meaning: B for transformer; 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 can Equipped with current transformer; digital/digital rated voltage (KV)/rated current (A).
Poor sealing at the top of oil-paper capacitive bushing may cause insulation breakdown due to water ingress, and poor sealing at the bottom will cause oil leakage in the bushing and lower the oil level.
The two necessary conditions for the occurrence of pollution flashover of bushing porcelain insulation are dirt and dust on the surface and wet surface. Dirty casing surface is prone to flashover, causing false tripping.
At the same time, flashover will damage the surface of the casing. Another hazard of the dirt on the surface of the casing is: the conductivity of the casing increases after the dirt absorbs water, which not only causes surface flashover, but also may cause the insulating casing to heat up and produce cracks due to the increase of leakage current, and finally break down.
The oil-paper capacitive bushing core is a whole made of multi-layer cable carton aluminum foil. If oil is conventionally filled, air is likely to remain between the screens. Under the action of high electric fields, partial discharges will occur, and even the insulation layer may break down. In case of accidents, it is necessary to immerse the oil in a 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 tapered shape: In order to withstand approximately the same voltage between the plates, make the electric field more uniform, and improve the electrical strength, the capacitance between the plates must be similar equal. However, the size of the capacitance is proportional to the area of the electrode plate, so as the radial size of the electrode increases, the axial size should be reduced accordingly. Therefore, the two ends of the core of the oil-paper capacitive bushing must be tapered.
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