Winding Failure Analysis
Winding failure is one of the most serious transformer failures.
Common winding faults
1. Inter-turn fault
A short circuit occurs between turns of the same winding.
Causes:
. Turn insulation ageing
· Local overheating
. Mechanical movement
. Transient overvoltage
. Manufacturing weakness
A small inter-turn fault can produce a very high circulating current in
the shorted turns, even though the external current may not initially
appear extremely high.
Failure progression:
Insulation weakness - turn-to-turn breakdown - circulatingcurrent - intense local heating - further insulation damage larger fault
2. Winding-to-earth faultA winding conductor contacts:
Core
Tank
· Clamping structure
Grounded metallic parts
This can produce a high-current internal fault.
3. Winding-to-winding fault
A fault occurs between HV and LV windings.
This is generally a severe internal fault because the insulation barrier
between the windings has failed.
4. Axial and radial winding displacement
During an external short circuit, very high current flows through the
windings.
The electromagnetic force is approximately related to:
Fx12
Therefore, a large short-circuit current can produce enormous
mechanical forces.
These forces can:
. Compress the winding
. Stretch conductors
. Move discs or coils
· Damage spacers
. Reduce insulation clearances
. Cause conductor buckling
The important point is:
An external fault may not immediately destroy the transformer, butit can mechanically damage the winding and create a futureinternal failure.
This is why Frequency Response Analysis (FRA/SFRA) is useful aftersevere short-circuit events and for checking whether the active partsuffered mechanical deformation. The Central Electricity Authoritymanual specifically identifies FRA as a method for detectingdeformation or movement of the winding/core and recommendscomparison with factory results.