1. Definition
Winding deformation refers to the mechanical displacement, distortion, bulging, twisting, or compression of transformer windings caused by electrodynamic forces, mechanical impact, or other stresses.
This deformation changes the designed geometry of the coils, thereby affecting the insulation distance, mechanical strength, and electrical characteristics of the transformer.
During transformer operation, short-circuit faults are inevitable—particularly at or near the transformer’s outlet terminals.
A sudden short-circuit generates extremely large current impulses, causing the windings to experience electrodynamic forces tens to hundreds of times greater than during normal operation.
These forces produce:
Rapid heating, softening the conductor material (copper or aluminum).
Strong mechanical stress, exceeding the mechanical strength of the winding.
The forces acting on transformer windings can be divided into two categories:
Radial Forces (Transverse):
Act from the center outward or inward.
The outer coils experience expansion forces.
The inner coils are subject to compression forces.
If the internal bracing or insulation cylinder is insufficiently strong, the coils may bulge or bend, forming “plum-blossom” or swelling deformations.
Axial Forces (Longitudinal):
Act along the height of the winding.
They compress insulating spacers, cause bending of coil turns, and may displace windings vertically relative to the iron core.
Unequal coil heights (e.g., due to tap changers or magnetic imbalance) further amplify axial stress, often leading to serious accidents.
Conclusion:
Sudden short-circuit currents subject transformer windings to combined strong radial and axial forces.
The inner windings—especially in autotransformers—are most vulnerable, resulting in irreversible deformation, such as bulging, twisting, and displacement.
These deformations may evolve into insulation breakdowns, inter-turn short circuits, or catastrophic failures.
During transportation, lifting, or cover installation, the windings may suffer from collision, vibration, or accidental impact, leading to physical deformation.
If the protection relay system has a dead zone or fails to operate, the transformer may be exposed to sustained short-circuit current.
Prolonged electrodynamic stress can deform windings.
Statistical data indicate that about 30% of transformer damages result from failure to trip during external short-circuits.
If the transformer design or manufacturing process does not ensure adequate mechanical rigidity, its short-circuit withstand capability diminishes over time.
Recent statistics on 110 kV transformer failures show that insufficient short-circuit strength has become a major internal cause of transformer accidents.
Winding deformation is a major hidden danger to the safe operation of power transformers.
It is difficult to detect through conventional insulation resistance or oil dielectric tests, making it a latent fault.
Changes in insulation distance or damage to solid insulation can cause partial discharge (PD).
Under lightning or switching overvoltage, PD may lead to inter-turn or inter-disk breakdown.
Even under normal voltage, long-term PD can result in insulation failure.
Once deformed, the winding loses its mechanical strength.
If another short-circuit occurs, the weakened structure can no longer withstand electrodynamic stress, leading to further deformation or total failure.
Because winding deformation cannot be completely avoided, early diagnosis is critical.
Accurate detection can determine:
Whether deformation has occurred
The degree and location of deformation
Whether the transformer can continue to operate safely
Whether maintenance or disassembly is required
Proactive diagnosis not only saves manpower and material costs but also prevents major transformer accidents.
Global research has recognized the importance of winding deformation detection.
In many countries, it is considered a key predictive maintenance test.
According to the State Power Corporation Document No. I20001589 — “25 Key Requirements for Preventing Major Accidents in Power Production”, winding deformation testing is a mandatory inspection item under the following conditions:
After factory acceptance and handover
After a short-circuit fault event
Relevant Clauses:
Clause 15.6:
After a transformer suffers a short-circuit fault in the nearby area, a low-voltage short-circuit impedance test or a frequency response analysis (FRA) shall be conducted.
Results must be compared with the original factory data before the transformer is put back into service.
Clause 20.2.9:
When ordering transformers, the manufacturer must provide:
The frequency response characteristic curve of the windings
The short-circuit withstand test report
The dynamic calculation report of electromechanical forces
FRA should also be conducted:
During installation and commissioning
After any operational short-circuit fault
The test results must serve as one of the key criteria for determining continued operation eligibility.
Winding deformation is a complex mechanical-electrical phenomenon directly linked to transformer safety and reliability.
Its primary cause is short-circuit current impulse, but external mechanical impact and protection failure also contribute.
Early detection—especially through frequency response analysis—is essential to prevent hidden insulation deterioration, mechanical collapse, and major transformer accidents.
Kingrun Transformer Instrument Co.,Ltd.
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