Influence of Transformer Structures on the Frequency Response Analysis: A Laboratory Case Study

Frequency response analysis (FRA) is used in the electrical industry for condition assessment of power transformers. The method is sensitive to even slight variations occurring in the active parts of transformers. Over the years, FRA has demonstrated its good capacity for detection of mechanical and electrical failure modes This paper explores different measurements taken on a laboratory transformer model where the influence of cylindrical grounded structures (simulating a tank and a core) is investigated. The cylindrical structures were added in order to study their impacts on the frequency response with a specific focus on the inductance and capacitance changes. The core and tank influences on the frequency response measurements are explained by shunt capacitances variation in both cases causing similar changes to the traces. Besides, radially induced currents in the tank made of magnetic steel affected the main inductance of the winding causing a shift of the first anti-resonance frequency. Such radially induced currents were prevented by the design of the simulated core made of aluminum strips. The results indicated that the transformer structure has a significant influence on the frequency response, related to changes in the main inductance and capacitances of the equivalent transformer circuit model. These academic experiments help contributing to a better understanding and further support of transformer FRA trace interpretations.