Experimental investigation on terminal connection and system function pair during SFRA testing on three phase transformers

It is highly desirable to maximize the ability of detecting even a slightest movement in the winding-core arrangement of a power transformer while employing Sweep Frequency Response Analysis (SFRA) measurements. The terminal connection employed and system function considered significantly influences the detection sensitivity that can be achieved. Being an off-line, low-voltage method, SFRA tests permit any possible terminal connection and system function amongst the numerous possibilities. Currently available literatures and the IEEE guide on SFRA measurements (IEEE Std C57.149™-2012) list out most of these possibilities for both single phase and three phase transformers. The terminal connection and system function listed in the IEEE guide on SFRA measurements is an outcome of past experience and knowledge. The IEEE guide provides much attention in detecting outer phase symmetry, mid phase asymmetry, changes in the inductance and other minute mechanical deviations. Despite all, an expert opinion is inevitable while making a meaningful interpretation from the measured SFRA data. In particular, more expertise is required while ascertaining the minor winding deviations which usually manifest in the mid frequency range. A possible way to simplify this task is by obtaining more information (distinguishable resonant peaks) in the measured amplitude frequency response function. It simply means enabling the SFRA method to reveal all the natural frequencies manifested by the winding-core assembly. The author has already addressed this issue for single phase, two-winding transformers. However, three phase multi-winding transformers are quite different in construction. So, the applicability of the same configuration (single phase, two-winding) on three phase transformers remains to be ascertained. Also, there exist several possibilities of alternate terminal connection and system function that might provide more insight into the status of the winding-core assembly. In conclusion, it is believed that the proposed alternate terminal connection and system function pair can also be included during routine SFRA tests on three phase transformers after adequate verification.