Circuit design for reproducible on-site measurements of transfer function on large power transformers using the SFRA method

By the analysis of transfer function mechanical deformations or displacements of transformer windings can be detected. For on site measurements on large power transformers essential attention has to be put on accurate cabling and handling of grounding conductors. Of further concern is the selection of a meaningful upper frequency limit, from where beyond measurements of frequency responses are not practicable on site. A set-up concept for reproducible measurements that has been developed by the authors shows satisfying results. Investigations on the transfer function of power transformers identical in construction have been carried out: Depending on the type, size and winding construction the transfer function of a winding system of identical transformers is well comparable to frequencies above 1 MHz. A mathematical approach to evaluate the quality of different comparison methods using the coherence function is presented.