Knowledge Sharing | Transformer coil insulation

1►The type and classification of insulation structure

In order to ensure the long-term safe and reliable operation of the transformer, the insulation structure of the transformer coil must meet the following basic requirements.

① electrical strength (low local discharge)

② heat resistance strength (smooth oil flow, no local overheating)

③Mechanical strength (strong short-circuit resistance)

2► Basic requirements and principles of insulation design

01 Basic principles of insulation structure design

To get a more reasonable design, cost savings, safety and reliability to meet the requirements of the insulation structure, we must use the electric field analysis, leakage field analysis, temperature field analysis, electrodynamic analysis and other "field" analysis software to analyze and optimize the insulation structure, and the minimum safety margin of the insulation distance and the most economical insulation Layout.

The insulation structure of conventional products is fine-tuned with reference to the more mature structures that have been analyzed and optimized. For special products, where no experience is available, the insulation structure must be analyzed and verified using various "field" analysis software, especially electric field analysis software, in order to achieve cost savings and safety and reliability requirements.

The electrical strength of the coil, i.e. the level of insulation withstand voltage and the voltage difference between the coil and the other coils and headers, is the main design basis for deciding the insulation structure of the transformer.

The coil insulation design is based on the theory of "thin paper cylinder, small oil gap" theory. The layout and design of the insulation structure is aimed at reducing the field strength of the conductor surface and the attached oil gap so that the withstand field strength is less than the breakdown field strength, and to take design measures to reduce local discharge and prevent creepage along the surface as much as possible.

Angle ring, oil ring and paper ring, the enclosing screen with each other when the arrangement should pay attention to ensure that the oil path is smooth, do not form a dead oil area.

The main insulation of the coil is divided into the main airway insulation and end insulation, and the arrangement must be reasonably divided into the main airway and the oil gap at the end of the coil to improve the breakdown voltage, and the reasonable arrangement of the corner ring to prevent creepage along the coil end.

A typical main insulation arrangement and analysis diagram is shown below.

02 Arrangement of the main airway

The main airway of the coil is mainly composed of the oil gap divided by the enclosure and the support strip, the oil gap is divided according to the theory of "thin paper tube, small oil gap", except for the coil forming paper tube is 3mm thick, the general enclosure cardboard is 2mm thick, the oil gap between the enclosure and the support strip is generally 6~10mm, and the support strip next to the coil is generally 6mm. The number of layers is related to the voltage insulation level of the coil and the voltage difference between adjacent coils, and the number of outermost enclosing screen is related to the voltage insulation level of the outermost coil and the voltage difference between phases, and the distance between the lead, oil tank and core side yoke.

03 Arrangement of end insulation

Coil surface and electrostatic ring surface is the highest electric field strength, must be designed as a small oil gap, do not allow a large oil gap. For example: the pad at the end of the line cake is generally 4mm, due to the electrostatic ring and the first end of the coil isotropic, the field strength is higher, so the oil gap between the coil corner ring and the electrostatic ring is generally 6mm, the coil inside and outside the support strip oil gap is generally 6mm.

Electrostatic ring edge chamfer R is generally 1/4 round, note that the low-voltage coil and regulating coil electrostatic ring facing the high-voltage side of the potential and field strength should be increased chamfer R. Higher voltage coil electrostatic ring around the general large oil gap is not allowed, there is a large oil gap when the chamfer should be added to the package insulation to form a small oil gap, reducing the surrounding field strength.

Angle ring arrangement R angle size should be as perpendicular to the electric field line direction, similar to or overlap with the equipotential surface (the more up, the greater the R angle), and as far as possible, staggered arrangement of positive and negative angle ring to increase the creepage distance. The number of corner rings on both sides of the coil is related to the coil voltage insulation level and the voltage difference between adjacent coils.

The higher the end voltage, the smaller the oil gap between the paper ring and the corner ring split should be, in order to reduce the end breakdown field strength.

04 Handling of the headers

When the coil outlet voltage level is very high, it is necessary to take measures to reduce the surface field strength of the outlet. Such as adding copper tube to change the shape of the electrode, adding insulation, adding insulation molding parts such as outlet corner ring or groove guard, shielding cold pressing welding place, etc.

3►The components of coil insulation

Transformer coil insulation is mainly composed of various insulating parts.

Insulation parts are mainly divided into insulating paper, insulating paperboard and other insulation molding parts.

Insulation material: generally made of sulfate fiber paper, insulating cardboard is generally T4 cardboard, after densification treatment to increase the mechanical strength. Insulation parts are generally formed insulation parts, more conducive to reducing the surface field strength.

The basic requirements of insulating parts: smooth rounded edges, no burrs, no broken layers and cracks, densification treatment, dry without moisture, no metal dust and other foreign matter.

4►Layout of coils

The arrangement of transformer high and low voltage windings is determined by a variety of factors. But for most transformers, the low-voltage winding stage is arranged inside the high-voltage winding. This is mainly from insulation considerations. Theoretically, no matter how the high-voltage winding or low-voltage winding is arranged, it can play a role in voltage transformation. But because the core of the transformer is grounded, because the low-voltage winding is close to the core, it is easy to do from the insulation point of view. If the high-voltage winding is close to the core, it requires a lot of insulation material and a large insulation distance to achieve the insulation requirements because the high-voltage winding voltage is very high. This not only increases the volume of the winding, but also wastes the insulation material.

Furthermore, since the voltage regulation of the transformer is achieved by changing the tap of the high-voltage winding, i.e. changing its number of turns, it is easier to place the high-voltage winding on the outside of the low-voltage winding and the lead.

5►Coil oil circuit

The coil oil channel mainly has spoke oil channel and axial oil channel. Layer type coil has only axial oil passage, no spoke oil passage. The pancake type coil has both kinds of oil passages.

Forced oil circulation coil insulation structure, the coil to increase the oil retaining ring, the end end ring also often take measures to extend the oil flow path, the upper part is basically the same as the conventional structure, the outer regulating pressure is generally taken to self-cooling mode.

The design should pay attention to ensure the oil flow smoothly, and should pay attention to prevent the oil flow with point, the flow rate of the lower oil guide hole should be controlled within 0.5m/s, so a reasonable oil guide hole area should be given. And pay attention to the head at the enclosure screen openings need to be sealed tightly to prevent oil leakage.