Reconstruction of the transformer response from on-line FRA measurements performed via bushing taps

On-line measurement of the FRA spectrum of large power transformers requires a signal injection and extraction methodology that can isolates the measurement system from the high voltages present in an operational transformer. It has been shown by numerous researchers, that the bushing test tap of capacitivly-graded or condenser bushings can be a used as a convenient point for capacitivly injecting and extracting signals from a transformer. These signals can that then be used for condition monitoring purposes such as FRA and PD. The presence of this capacitive injection and extraction path in the on-line FRA measurement chain will result in a modification of the measured transformer spectrum when compared to the windings on their own. This modification of the on-line FRA spectrum can be accounted for if an accurate electrical model of the on-line FRA measurement chain is developed that includes an accurate model of the bushings used. An equivalent “Pi” model of an epoxy cast bushing with an integral CVT voltage tap is developed through measurement and simulation. The work presented in this paper, further looks at the development of a cascade two-port transmission matrix model for the on-line FRA measurement chain that includes the developed bushing model. Using matrix manipulation of this transmission matrix model, it is possible to extract the transformers winding response spectrum from the measured on-line FRA spectrum. Experimental results are performed to validate the developed mathematical models