Knowledge Sharing | Transformer Coil Insulation

1►Types and classification of insulation structures

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)

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

③Mechanical strength (strong short-circuit resistance)

2► Basic requirements and principles of insulation design

a Basic principles of insulation structure design 

In order to obtain an insulation structure that is reasonable in design, cost effective, safe and reliable, the insulation structure must be analysed and optimised using various "field" analysis software, such as electric field analysis, leakage field analysis, temperature field analysis and electrodynamic analysis, and the insulation distance with the minimum safety margin and the most economical insulation the minimum safety margin and the most economical insulation layout.

The insulation structure of conventional products is fine-tuned and designed on the basis of a relatively mature structure that has been analyzed and optimized. For special products, under the condition that there is no ready-made experience for reference, various "field" theory analysis software, especially electric field analysis software, must be used to analyze and verify the insulation structure, so as to save costs and meet safety and reliability requirements.

The electrical strength of the coil, namely the level of insulation withstand voltage and the voltage difference between the coil and the other coils and heads, is the main design basis for deciding on the layout of the transformer insulation.

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

Angle rings, oil retaining rings and paper rings, enclosing the screen with each other 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 the end insulation, which must be arranged in such a way that the oil gap between the main airway and the end of the coil is reasonably divided to increase the breakdown voltage, and the corner rings are reasonably arranged to prevent creepage along the coil end.

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

b Arrangement of the main airway

The coil main airway is mainly composed of the oil gap divided by the enclosure and the spacer 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 screen cardboard is 2mm thick, the oil gap between the enclosure screen spacer strip is generally 6~10mm.

It is generally 6mm next to the coil stay. The distance of the main channel and the number of enclosure layers are related to the voltage insulation level of the coil and the voltage difference between adjacent coils. The number of outermost enclosures is related to the voltage insulation level and phase-to-phase voltage difference of the outermost coil, and the distance from the lead wire, fuel tank, and iron core side yoke related.

A typical main airway arrangement is shown below.

c Arrangement of end insulation

The surface of the coil and the surface of the electrostatic ring is the highest electric field strength, must be designed into 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.

The chamfer R of the edge of the electrostatic ring is generally 1/4 circle. Note that the electrostatic ring of the low-voltage coil and the voltage regulating coil should increase the chamfer R when facing the high-voltage side potential and higher field strength. Large oil gaps are generally not allowed around the electrostatic ring of coils with high voltage. If there is a large oil gap, the chamfer should be insulated to form a small oil gap and reduce the surrounding field strength.

When the corner rings are arranged, the R angle should be perpendicular to the direction of the electric field line as much as possible, similar to or coincident with the equipotential surface (the higher you go, the larger the R angle), and the positive and anti-angle rings should be arranged staggered as much as possible to increase the creepage distance. The number of corner rings on both sides of the coil is related to the voltage insulation level of the coil 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.

d Handling of the headers

When the voltage level of the coil head is very high, it is necessary to take measures to reduce the surface field strength of the head. Such as adding copper tubes to change the electrode shape, adding insulation, adding insulating moldings such as outgoing line angle rings or protective grooves, shielding cold-pressed welding parts, etc.

e Example of 220kV end axial head out

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.

The material of the insulating part: it is generally made of sulfate fiber paper, and the insulating cardboard is generally T4 cardboard, which has been densified to increase the mechanical strength. The insulating part is generally a shaped insulating part, which is more conducive to reducing the surface field strength.

Basic requirements for insulating parts: smoothly rounded edges, no burrs, no broken layers or cracks, densified, dry and free from moisture, no metal dust or other foreign matter.

4►Layout of coils

The arrangement of the high and low voltage windings of a transformer is determined by a number of factors.

For most transformers, the low-voltage winding stage is arranged inside the high-voltage winding. This is mainly a consideration from the insulation side.

In theory, no matter how the high-voltage winding or low-voltage winding is arranged, it can act as a voltage converter. However, as the core of the transformer is earthed, the low-voltage winding is close to the core, which is easy to do from the insulation point of view.

If the high-voltage winding is placed close to the core, it requires a lot of insulation material and a large insulation distance to achieve the insulation requirements due to the high voltage of the high-voltage winding. This not only increases the volume of the winding, but also wastes the insulation material. 

Furthermore, as 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 leads.

5►Coil oil circuit

The coil oil channels are mainly spoked and axial. Laminated coils have only axial oil passages and no spoke oil passages. The pancake coil has both types of oil channels.

The cooling oil flow mode of the coil is shown in the figure below:

Forced oil circulation coil insulation structure, oil retaining ring is added to the coil, and measures are often taken to extend the oil flow path at the end coil. The upper part is basically the same as the conventional structure, and the external pressure regulation generally adopts self-cooling method.

During the design, care should be taken to ensure smooth oil flow and prevent oil flow from being spotted. The flow rate of the lower oil guide hole should be controlled within 0.5m/s, so a reasonable area of the oil guide hole should be given. And pay attention to the opening of the screen at the head must be tightly sealed to prevent oil leakage.

Guangri Electronics Machinery is the first manufacturer of ring winding machine in China and has been awarded several patents and certificated by Rheinland. Product categories include automatic stator winding machine, side sliding winding machine, belt winding machine, gear winding machine and ring wrapping machine, manufactured with original parts.

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