[Oil-immersed transformer] Oil-immersed transformer structure Working principle of oil-immersed transformer
Oil-immersed transformer structure
Structural classification: It can be divided into iron core transformer and iron shell transformer. If the winding is wrapped around the iron core, it is an iron core transformer; if the iron core is wrapped around the winding, it is an iron shell type transformer. The two are only slightly different in structure, but there is no essential difference in principle. Power transformers are all iron core type.
The transformer is mainly composed of iron core, winding, oil tank, oil pillow, insulating bushing, 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 are generated and heat is generated. In order to reduce heating loss and reduce volume and weight, the iron core is composed of cold-rolled grain-oriented silicon steel sheets with a high magnetic permeability of less than 0.35mm. According to the arrangement of the windings in the iron core, there are iron core type and iron shell type.
In large-capacity transformers, in order to ensure that the heat emitted by the iron core loss can be fully taken away by the insulating oil during circulation to achieve a good cooling effect, cooling oil channels are often provided in the iron core. .
2. Winding
The winding and core are the core components of the transformer. Since the winding itself has resistance or there is contact resistance at the joint, heat is generated by I2Rt. Therefore, the winding cannot pass a current higher than the rated current for a long time. In addition, when a short-circuit current passes through, a large electromagnetic force will be generated on the winding and damage the transformer. Its basic windings are concentric and overlapping.
The main faults of transformer windings are short circuit between turns and short circuit to the shell. Inter-turn short circuit is mainly caused by insulation aging, or mechanical damage to the insulation caused by overload of the transformer and through short circuit. When the oil level in the transformer drops, causing the winding to expose the oil surface, an inter-turn short circuit can also occur; in addition, when a cross-circuit occurs, the winding is deformed due to overcurrent, causing mechanical damage to the insulation, and an inter-turn short circuit can also occur. When there is a short circuit between turns, the current in the short-circuited winding may exceed the rated value, but the current of the entire winding may not exceed the rated value. In this case, the gas protection device will operate. When the situation is serious, the differential protection device will also operate. Short circuits to the outer casing are also caused by aging insulation, moisture in the oil, drop in oil level, or due to lightning and operating overvoltage. In addition, when a through short circuit occurs, the winding is deformed due to overcurrent, and a short circuit to the casing can also occur. When the shell is short-circuited, the gas protection device and the grounding protection are generally activated.
3. Oil tank
The body (windings and iron core) of the oil-immersed transformer are installed in an oil tank filled with transformer oil. The oil tank is made of steel plates. Welded. The oil tank of medium and small transformers consists of a tank shell and a tank cover.The transformer body is placed in the box shell, and the box cover can be lifted out for inspection.
The working principle of oil-immersed transformer
The basic principle of the transformer is the principle of electromagnetic induction. Here, a single-phase double winding transformer is taken as an example to illustrate its basic working principle. : When voltage ú1 is applied to the primary winding, current í1 flows, and an alternating magnetic flux À1 is generated in the iron core. These magnetic fluxes are called main magnetic fluxes. Under its action, the windings on both sides induce electromotive force é1 respectively. , é2, the formula of induced electric potential is: E=4.44fN?m
In the formula: E–effective value of induced electric potential
f–frequency
N –Number of turns
m–Maximum value of main magnetic flux
Since the number of turns of the secondary winding and the primary winding is different, the magnitudes of the induced potentials E1 and E2 are also different. When omitting the inner After the impedance voltage drop, the voltages ú1 and ú2 are different in size.
When the secondary side of the transformer is unloaded, only the current (í0) of the main magnetic flux flows through the primary side. This current is called the excitation current. When a load is added to the secondary side and the load current í2 flows, magnetic flux is also generated in the iron core, trying to change the main magnetic flux. However, when the primary voltage remains unchanged, the main magnetic flux remains unchanged, and the primary side will flow through two Part of the current, part is the excitation current í0, and part is used to balance í2, so this part of the current changes as í2 changes. When the current is multiplied by the number of turns, it is the magnetic potential.
The above-mentioned balancing effect is essentially a magnetic potential balancing effect. The transformer achieves energy transfer on the primary and secondary sides through the magnetic potential balancing effect.
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