Effect of Radial and Axial Movement of Winding on Coherence Function in a 220/132 kV, 100 MVA, Auto Transformer

The short circuit force generated in Powers Transformers due to system fault is known to cause movement of the windings. Several diagnostic methods have been employed to identify the movement of internal components of transformer e.g. Core, winding, coil bulging, coil twisting, Inter turn fault etc. The methods include – Sweep Frequency Response Analysis, Current reflection time, wavelet transform and Coherence Function (CF). The present work reports a detailed analysis of Coherence Function due to radial and axial movement of coil by theoretically simulating the HV winding of 220/132 kV, 100 MVA autotransformer. The results show a variation in magnitude of CF for dominant frequencies and that even a small change in radial or axial distances cause significant change in magnitude of CF. It is observed that sensitivity of detection of winding movement by CF for minor faults is moderately better than Frequency Response, since CF is related to the amount of linearity between input and output. It is observed that CF and FRA are complimentary to each other.