First, the classification of power transformer insulation
The insulation of power transformers can be divided into two categories: external insulation and internal insulation. Generally, the insulation other than the transformer fuel tank (for oil-immersed transformers) is called external insulation, and the insulation inside the fuel tank is called internal insulation (including insulating oil). And paper and cardboard soaked in oil, etc.). The outer insulation consists of the casing itself and the in-phase or out-of-phase casing and the casing to the ground. Internal insulation includes winding insulation and tap changer insulation. The winding insulation is divided into main insulation (to ground and high and low voltage windings and phase insulation) and longitudinal insulation (interlayer and interturn insulation).
Second, the structural type of the main insulation of the power transformer
Power transformers can be divided into oil-immersed transformers (cooled with transformer oil), dry transformers (with air cooling) and SF6 transformers (cooled with SF6 gas) depending on the cooling medium. At present, most of the power systems are oil-immersed transformers, while dry-type transformers and SF6 transformers account for only a small portion. To this end, this section is represented by an oil-immersed transformer, and the insulation of the transformer is explained. The model and the SF6 transformer are only briefly introduced.
It can be seen from the above that the insulation between the transformer windings and the windings between the iron core columns, the insulation of the windings against the iron sills, and the interphase insulation between the high and low voltage windings belong to the main insulation of the transformer, and the structure thereof is shown in Figure 1-2.
1. Insulation between windings and windings on the core column
The main insulation of oil-immersed transformers is the insulation of oil and barrier synthesis. It is made of insulating paperboard pressed mats, cylinders, corner rings, etc. (bonded with insulating varnish); longitudinal insulation is covered by cable paper on the wires. It is also immersed; the lead insulation is often composed of a yellow wax cloth tape. It can be seen from Figure 1-2 that the higher the voltage level, the more the number of paper tubes, the finer the oil gap is, and the higher the electrical strength.
2. Winding insulation of the iron shovel
This part of the insulation is the weak link of the main insulation. Since the electric field at the end of the winding has both vertical and tangential components on the insulating surface, corona and sliding discharge are prone to occur, which will cause the winding to break down the shovel. Therefore, the insulation distance of this part is much larger, and the oil passage is separated by a paper corner ring (see Figure 1-2).
Third, fully insulated transformer and graded insulation transformer
The "pre-regulation" stipulates that power transformers are divided into two types: fully insulated transformers and graded insulated transformers. Their AC withstand voltage test methods are different. The so-called fully insulated transformer (such as 10, 35kV distribution transformer) means that the insulation level of the transformer from the first to the end of the winding is the same; and the power transformer of 110kV and above, due to the direct grounding method of the neutral point, The insulation level requirement of the neutral point is reduced, and the insulation level from the first to the end of the winding is also different. This type of transformer is called a graded insulation transformer.
Fourth, dry type transformer and SF6 transformer insulation
The "pre-regulation" mentioned the test problems of dry-type transformers and SF6 transformers. Because of the difference in structure and cooling medium between the two transformers and the immersed transformer, the test items are different.
Modern dry-type transformers are made using more advanced structures and processes. The high voltage windings are usually wound with insulated copper wire and cast with epoxy under vacuum. The windings have high dielectric properties and low partial discharge levels. The low-voltage winding is wound with high-quality copper foil to achieve zero axial short-circuit stress during short-circuit, and the winding layers and ends are insulated with F-grade thermosetting epoxy prepreg. Dry-type transformers typically have a maximum capacity of 15,000 kVA and a maximum voltage of 35 kV due to the cooling medium.
The SF6 transformer is a new type of transformer characterized by a variety of pie and foil windings. Between the high and low voltage windings, the insulation strength of the main insulation between the windings and the ground mainly depends on the insulation strength of the SF6 gas, and the high-sealing plastic film with high mechanical strength and insulation capacity is used as the insulation cloth of the copper wire.
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