Abstract
Transformers are considered as the key components in power system network as they are employed in generation, transmission and distribution sectors. Hence electricity providers are highly concerned about proper operation of these valuable assets and specifically normal performance of the transformer active parts. One of the most reliable techniques to monitor the active part of the transformer is the Frequency Response Analysis (FRA). Given the fact that FRA technique has been successfully utilized in offline application for transformer diagnosis; it has been recently studied to apply the FRA once the transformer is in service, called online FRA. In this study, the transformer winding inter-disk and inter-turn faults are studied along with the effect of the bushing on frequency response signature in online FRA setup. Transformer winding physical model is created, simulated and numerically analyzed. It is revealed that the winding inter-disk and inter-turn faults lead to FRA anti-resonance frequency shift and response magnitude alteration in online FRA. Emulation of transformer bushing in the form of two paralleled capacitors unveils the significant influence of this component over the low frequency band of FRA spectrum. Destructive effect of the bushing reactive impedance should be taken into consideration during online FRA test measurements.