Power transformer is mainly composed of iron core, winding, oil tank, conservator, insulating sleeve, tap changer and gas relay.
1. Iron core
The iron core is the magnetic circuit part of the transformer. During operation, hysteresis loss and eddy current loss should be produced to generate heat. In order to reduce the heat loss and volume and weight, the core is made of cold-rolled grain oriented silicon steel sheet with magnetic conductivity less than 0.35mm. According to the arrangement of winding in the iron core, there are two types: iron core type and iron shell type.
In the large capacity transformer, in order to make the heat generated by the core loss be fully taken away by the insulating oil during circulation, so as to achieve good cooling effect, cooling oil channel is often set in the iron core.
Winding and iron core are the core components of transformer. Because of the resistance of the winding itself or the contact resistance at the joint, it can be known from Joule's law that heat should be generated. Therefore, the winding cannot pass the current higher than the rated current for a long time. In addition, a large electromagnetic force will be generated on the winding when the short-circuit current passes through, which will damage the transformer. There are two kinds of basic windings: concentric winding and overlapping winding.
The main faults of transformer winding are inter turn short circuit and short circuit to shell. Turn to turn short circuit is mainly caused by insulation aging, or due to overload of transformer and mechanical damage of insulation during through short circuit. When the oil level in the transformer drops, the turn to turn short circuit can also occur when the winding is exposed to the oil surface; in addition, when there is a through short circuit, the winding will be deformed due to the action of over-current, which will cause mechanical damage to the insulation and cause turn to turn short circuit. In case of inter turn short circuit, the current in the short circuit winding may exceed the rated value, but the current in the whole winding may not exceed the rated value. In this case, the gas protection acts, and when the situation is serious, the differential protection device will also act. The short circuit to the enclosure is also caused by insulation aging, oil moisture, oil level drop, or lightning and switching overvoltage. In addition, in the case of through short circuit, the winding deformation caused by over-current will also produce the phenomenon of short circuit to the shell. When the shell is short circuited, the gas protection device and the grounding protection device usually act.
3. Fuel tank
The body (winding and iron core) of oil immersed transformer is installed in the oil tank filled with transformer oil, and the oil tank is welded with steel plate. The oil tank of medium and small-sized transformers is composed of a tank shell and a cover. The transformer body is placed in the tank shell, and the tank body can be lifted out for maintenance by opening the tank cover.
Insulation cooling classification
It can be divided into oil immersed transformer and dry-type transformer. In order to strengthen the insulation and cooling conditions, the core and winding of the transformer are immersed together in the oil tank filled with transformer oil. In special cases, such as street lamps and mine lighting, dry-type transformers are also used.
Oil immersed type
1. Non enclosed oil immersed transformer: mainly S8, S9, S10 series products, widely used in industrial and mining enterprises, agriculture and civil buildings.
2. Enclosed oil immersed transformer: mainly S9, S9-M, s10-m and other series of products, mostly used in the oil and chemical industry, where there are more oil and chemical substances.
3. Sealed oil immersed transformer: bs9, S9 -, S10 -, s11-mr, SH, sh12-m and other series products can be used for power distribution in industrial and mining enterprises, agriculture, civil buildings and other places.
a. The low-voltage winding of oil immersed transformer generally adopts the cylinder structure of copper foil winding shaft, except for small capacity copper wire; the high-voltage winding adopts multi-layer cylindrical structure, which makes the distribution of ampere turns of winding balance, small magnetic leakage, high mechanical strength and strong short-circuit resistance.
b. The iron core and winding adopt fastening measures respectively. The fastening parts such as the height of the device and the low-voltage lead are equipped with self-locking lock nuts. The structure of no hanging core is adopted, which can withstand the bumping of transportation.
c. The coil and core are dried in vacuum, and the transformer oil is filtered and injected with vacuum to minimize the moisture inside the transformer.
d. The oil tank adopts corrugated sheet, which has breathing function to compensate for the volume change of oil caused by temperature change. Therefore, the product has no oil conservator and obviously reduces the height of transformer.
e. Because the corrugated sheet replaces the oil conservator, the transformer oil is isolated from the outside, which effectively prevents the oxygen and moisture from entering into the oil conservator, which leads to the decline of insulation performance.
f. According to the above five performances, the oil immersed transformer does not need to be changed in normal operation, which greatly reduces the maintenance cost of the transformer and prolongs the service life of the transformer.