Causes and hazards of transformer winding deformation.

1. What is winding deformation?

2. Causes of winding deformation

The main causes of winding deformation are:

2.1 Short-circuit fault current impulse

During the operation of the power transformer, it is inevitable to suffer from the impact of various short-circuit currents, especially the transformer outlet or near

Zone short-circuit fault, huge short-circuit impulse current will make the transformer winding subject to a large amount of electrical power (tens to hundreds of times that of normal operation), and make the winding heat up sharply. At higher temperatures, the mechanical strength of the wire becomes smaller, and the electrodynamic force is more likely to break or deform the windings.

Short-circuit fault current impulse is the most important external cause of transformer winding deformation.

As we all know, power transformer coils are made up of copper or aluminum wire segments separated by insulating pads. The dynamics of such a system are in

It is variable in the event of a sudden short circuit. Because the elasticity of insulating heat is related to the degree of compression, i.e. to the force. Electrodynamics

It is not constant in itself, but changes according to complex laws. Although the electricity acting on the transformer coil when there is a short circuit

The study of dynamics began in the forties, but so far it has not been possible to calculate theoretically due to the complexity of the analysis of dynamic processes

The results correctly reflect the transformer's ability to impulse with sudden short-circuit currents.

a) Expansion radial force b) Compression radial force

Theoretical analysis shows that the electrodynamic force acting on the transformer can be divided into two types: axial (longitudinal) and radial (transverse) forces. radial

The direction of action of the force depends on the position of the coils against each other and the direction of their current, for the two-coil transformer the square, the radial force stretches outside

coils, running through the inner coils, in order to increase the stiffness of the internal coils against radial forces. Usually the coil is wound in an insulating cylinder

brace on the strip. At this time, the coil will not only act with the compressive force, but also be affected by the bending force generated by the brace at the same time.

If the resultant stress exceeds the yield point of the coil stiffness, it will inevitably lead to permanent deformation of the coil, which is often seen

Deformation of plum blossom or bulging winding.

The axial force on the transformer coil can bend the line segments and turns in the vertical direction, compressing the pad display between the line segments and merging them

The earth is passed to the iron yoke, and it is strived to leave the pillar of the heart. In general, the greatest bending force is generated in the segment located at the end of the coil:

The greatest compressive force occurs on the pad, which is located in the center of the coil height. When the coils are not of equal height (mainly due to high-voltage taps

or uneven magnetic potential, the axial force is more likely to cause a transformer accident than the radial force.

It can be seen that when the transformer suffers from a sudden short-circuit fault current shock during operation, each coil will be macroscopic

to the combined effect of strong radial and axial forces. Most of the manifestations of transformer winding fault are the deformation of the inner winding (especially for the self-converting transformer), the irrecoverable deformation phenomenon such as bulging, twisting, and displacement, and the typical form of its development is insulation failure, followed by intercake breakdown, turn short circuit, main insulation discharge or complete breakdown.

2.2. In the process of transportation, installation or hanging cover, it may be subject to accidental collision, bump and vibration, etc., resulting in deformation of the winding.

2.3 The protection system has a dead zone and the action fails.

The existence of dead zones or malfunctions in the protection system will lead to a long time for the transformer to withstand the action of stable short-circuit current, which is also one of the reasons for the deformation fault of the transformer winding. According to rough statistics, about 30% of the transformers are damaged due to failure to trip in time when they suffer from external short circuits.

2.4 The ability of the winding to withstand short circuit decreases

When there is a short circuit in the transformer winding, it will be deformed because it cannot withstand the impact of the short-circuit current. In recent years, on the whole country

The statistical analysis of the 110Kv power transformer accident shows that it has become a power transformer accident due to the insufficient ability of the winding to withstand short circuit

The primary internal cause seriously affects the safe and reliable operation of power transformers.

3. The hazard of winding deformation

Winding deformation is a major hidden danger in the safe operation of power transformers. Practical test experience of multiple transformers shows that the windings are deformed

After that, the insulation test and the oil test are difficult to find, and it is manifested as a latent fault.

According to the assessment of the 12th International Conference on Large Power Grids, many insulation failures of transformer windings are due to the most insulation

Initial mechanical damage. The transformer is subjected to a short-circuit fault current shock and the windings are locally deformed, even if not immediately

Damage, which can also leave a serious risk of failure, such as:

a) The insulation distance changes, the solid insulation is damaged, resulting in partial discharge, when encountering lightning overvoltage, there is a possibility of inter-turn and cake breakdown, resulting in sudden insulation accidents, and even under normal operating voltage, insulation breakdown accidents occur due to the long-term effect of partial discharge.

6) The mechanical properties of the winding are reduced, and when the short circuit accident is suffered again, it will not be able to withstand the huge electrodynamic action and cause damage.

Since the deformation of the transformer winding is inevitable, how to detect whether the transformer winding has been deformed, what is the degree of deformation, can it continue to operate, if there is serious deformation, the location of the deformation, and how to deal with it?

Therefore, actively carry out the diagnosis of transformer winding deformation, find those transformers with winding deformation in time, and plan it

Carrying out inspection and maintenance of the staff cover can not only save a lot of manpower and material resources, but also prevent the occurrence of transformer accidents

to the role.

At present, all countries in the world are actively carrying out the diagnosis of transformer winding deformation, and some countries even put this family in the transformer pre-diagnosis

The first place in the anti-performance test project.

In the State Power Corporation issued No. I20001589 document "25 Key Requirements for Preventing Major Accidents in Power Production",

It has been clearly included in the winding deformation test as a mandatory test item after the transformer leaves the factory, is handed over and after the occurrence of a short circuit accident. Some of the relevant terms

Here's an excerpt:

(2) Clause 15.6: After the transformer suffers a sudden short circuit in the near area, it shall do a low voltage short circuit impedance or test the winding with the frequency response method

deformation, and compare with the original record, judge that the transformer is fault-free, and then put into operation.

(3) Article 20.2.9: When ordering a transformer, the manufacturer should be required to provide the transformer winding frequency response characteristic curve, the test report of the transformer that has done the sudden short circuit test and the dynamic calculation report of the protest short circuit capacity; the frequency response characteristic test should be added to the installation and commissioning; the frequency response test should be carried out after the transformer outlet short circuit fault occurs during operation, and the test results of the winding deformation shall be used as one of the criteria for whether the transformer can continue to operate.

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