On line assessment of winding deformation in a power transformer can be performed via a bushing tap excitation with optimized high frequency excitation. A benchmark layer winding is used to validate the use of the specific high frequency excitation. Axial displacement, compression and buckling of the winding are simulated and the changes identified. Measurements are performed on a voltage transformer to verify the suitability of the technique. The procedure will result in a considerably shorter time for measurement. © 2010 IEEE.