Reasons and solutions for common short-circuit damage in dry transformer

1.In recent years, dry-type transformer accidents have occurred from time to time. From the analysis of dry-type transformer accidents, short-circuit damage has become the main cause of power dry-type transformer accidents, causing great harm to the power grid and seriously affecting the safe operation of the power grid. The damage accidents caused by external short-circuit of power dry-type transformers in Shanghai Electric Power Company in the past ten years are classified and analyzed, and then the problems existing in the selection of electromagnetic wires and the measures to reduce such accidents are put forward.

2. Dry-type transformer short-circuit accident From January 2010 to May 2022, there were 17 dry-type transformer short-circuit damage accidents in Shanghai Power Grid, accounting for 77.3% of the total damage accidents, which was the main reason for the damage, with a total capacity of 2,750 MW. Among them, 2 sets of 500 kV, 13 sets of 220 kV, and 2 sets of 110 kV. One set of 220 kV and one set of 110 kV had to be replaced due to serious deformation of the low-voltage coil. During the transformation of the dry-type transformer, it was found that four sets of 220kV low-voltage windings were deformed, and two sets of 500kV windings showed signs of deformation during operation. Especially since 2011, the dry-type transformer damage accident has been on the rise, and the scope of the accident has been expanding. The main forms of accidents are:
1) After many external short-circuit impacts, the coil deformation is gradually serious, and the insulation breakdown damage is more frequent;
2) The exterior is frequently damaged by short-circuit impact in a short period of time;
3) Long-term short-circuit impact damage;
4) Short-term short-circuit impact damage.

3. The main forms of short-circuit damage to dry-type transformers:
3.1 Axial instability: This kind of damage is mainly caused by the axial electromagnetic force generated by radial magnetic flux leakage, which causes the axial deformation of the dry-type transformer winding. This type of accident accounts for 52.9% of the total damage accident.
3.1.1 The upper and lower bending deformation of the winding coil: This damage is due to the permanent deformation of the coil between the two axial spacers due to the excessive bending moment under the action of the axial electromagnetic force. Usually, the deformation between the two winding coils is symmetrical .
3.1.2 Winding or coil collapse. This damage is due to the wire squeezing or colliding with each other under the action of axial force, resulting in oblique deformation. If the wire is slightly inclined, the axial force will increase the inclination, and it will collapse in severe cases; the larger the aspect ratio of the wire, the easier it is to cause collapse. In addition to the axial component, the leakage magnetic field at the end also has a radial component. The combined electromagnetic force generated by the flux leakage in both directions causes the inner winding wire to turn inward and the outer winding to turn outward.
3.1.3 When the winding stress rises, the pressure plate will spread out. This damage is usually caused by excessive axial forces, insufficient strength and stiffness of the end supports, or assembly defects.

3.2 Radial instability: This type of damage is mainly caused by the radial electromagnetic force generated by the axial magnetic flux leakage resulting in radial deformation of the dry-type transformer winding, accounting for 41.2% of the total number of damage accidents.
3.2.1 Insulation damage due to elongation of the outer winding. Radial electromagnetic forces try to make the diameter of the outer winding larger. Permanent deformation occurs when the tensile stress acting on the wire is too high. This deformation often results in shorts between turns due to damaged wire insulation. In severe cases, the coil will be out of order, collapsed or even broken.

However, due to the compression deformation of the iron core and the different support methods of the struts, the stress along the circumference of the winding is not uniform, resulting in local instability of the coil and buckling deformation.

3.3 Unstable wire fixation: This damage is mainly caused by the electromagnetic force between the wires, resulting in wire vibration and short circuit between wires. Such accidents are rare.

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4. Common parts of dry-type transformer short-circuit damage:
4.1 The reason for the deformation of the corresponding part under the yoke is:
(1) The magnetic field generated by the short-circuit current is enclosed by the oil and the tank wall or iron core. Because the magnetic resistance of the magnetic yoke is relatively small, it is mostly closed between the oil circuit and the magnetic yoke, the magnetic field is relatively concentrated, and the electromagnetic force acting on the coil is relatively large;
(2) The gap of the inner winding sleeve is too large or the iron core is not bundled tightly enough, which causes the two sides of the iron core to shrink and deform, resulting in warping and deformation of the winding on the iron yoke side;
(3) Structurally, the axial compression of the yoke corresponding to the winding part is less reliable, and the wire cake of this part is often difficult to achieve the pre-tightening force, so the wire cake of this part is easily deformed.
4.2 The voltage regulating tap area and the parts corresponding to other windings are in this area, due to:
(1) The number of ampere turns is unbalanced, the leakage flux distribution is unbalanced, and additional axial external forces are generated in the coil due to the additional leakage magnetic field generated by its amplitude, and the direction of these forces always increases the asymmetry of these forces. The axial external force is the same as the axial internal force generated by the normal amplitude flux leakage, which bends the wire cake in the vertical direction and compresses the spacer of the wire cake. In addition, some or all of these forces are transmitted to the iron yoke, trying to make it away from the stem, the wire cake is deformed or flipped into the middle of the winding;
(2) In order to strive for the ampere-turn balance or the appropriate insulation distance in the tap interval, more spacers are often added. The thicker the spacer, the slower the force transmission, and the greater the impact on the winding coil;
(3) After the winding is set, the central reactance cannot be highly aligned, which further aggravates the unbalance of the ampere-turns;
(4) After running for a period of time, the thick pad naturally shrinks greatly, which aggravates the ampere-turn imbalance on the one hand, and also aggravates the beating when it is subjected to short-circuit force;
(5) In the design, in order to strive for the balance of ampere-turns, the magnet wire in the tap area is selected with a narrower or smaller size, resulting in a reduction in short-circuit resistance.

4.3 Transposition part: The deformation of this part is common in the transposition of the transposition wire and the standard transposition of the single helix. The transposition of the transposed wire is steeper than that of the ordinary wire, so that the transposition of different turning radii produces opposite tangential forces. This pair of equal and opposite tangential forces make the transposition diameter of the inner winding smaller and the direction deformed, while the transposition of the outer winding strives for the same turning radius, so that the transposition is straight, the inner transposition is deformed at the center, and the outer transposition is deformed. The position is deformed outward, and the thicker the thickness of the transposed wire, the steeper the climbing, and the more serious the deformation. In addition, there is also an axial short-circuit current component at the transposition, and the additional force generated will cause the deformation of the winding coil to be aggravated. The standard transposition of a single helix occupies one turn in space, resulting in an unbalanced ampere-turn at this part. At the same time, it has the characteristics of transposition deformation of the transposed wire, so the wire cake in this part is more easily deformed.
4.4 The lead wire of the winding is commonly used in the winding of the oblique spiral structure. The winding of this structure, due to the unbalanced ampere-turn of the two spiral openings, has a large axial force and an axial current at the same time, which makes the corner of the lead wire produce a transverse direction. Twisting deformation occurs due to force. In addition, during the winding process of the spiral winding, there is residual stress, which will make the winding try to return to its original state. Therefore, the winding of the spiral structure is more likely to be twisted and deformed under the impact of short-circuit current.

4.5 Leads are commonly found between low-voltage leads. The low-voltage leads have a large current due to low voltage, and the phase is 120 degrees, which makes the leads attract each other. If the leads are not fixed properly, phase-to-phase short-circuits will occur. Cause Analysis of Short-Circuit Damage in Dry-Type Transformer Operation

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