Coherence Function method of detection of axial and radial movement of a coil in high voltage transformer

Power transformers are the most expensive equipment in a substation. Hence, to assess the health of transformers several diagnostic methods have been proposed. In this regard Frequency Response Analysis (FRA) is considered to be one of the important diagnostic tools for determination of winding movement and coil failure. The present work reports a detailed analysis of Frequency Response (FR) and Coherence Function (CF) estimation of the valve winding (HVDC winding) of a 500 kV HVDC converter transformer. The star connected winding is theoretically simulated by distributed inductances and capacitances and an equivalent electric current is derived. A standard impulse voltage is applied at the HV terminal and the current is calculated in the grounded neutral winding. By varying the radius of the coil and duct thickness the new parameters and associated neutral currents are calculated. The currents so obtained are subjected to Fast Fourier Transform (FFT) and CF analysis. A comparison of results obtained provides a relationship between FRA and CF for different radii and duct thicknesses simulating radial and axial movement.