Diagnosis of power transformers using the transfer function

The Transfer Functions is method for diagnoses in practical use. The reproduce ability for measurements is high enough to practise a time-based comparison for TF's. The suitability of construction based comparisons essentially depends on the vector group. This is why for transformers containing a zig-zag winding a comparison between the three phases can not be applied. Type-based comparison require an identical construction of the core-and-coil assemblies. Actual measurements must be compared to a reference characteristic. This reference must be determined out of the number of single test results. The estimated sensitivity of the TF to radial deformations is about a depth of 3 % of the diameter and an axial extension of 10% absolute height