Resonance Behavior and Sensitivity to Detect Mechanical Change in Transformer Winding: Shunt Current Versus Neutral Current

Frequency-response measurement offers by far the most sensitive known technique to detect any mechanical deformation in transformer windings. By default, during this measurement, the neutral or line current is considered as the winding response. The overall sensitivity to detect a mechanical change depends not only on the response quantity and the terminal connections chosen, but, also on the arrangement of poles and zeros of the system function. Some pole-zero arrangements are inherently better suited to reveal mechanical changes. Because of this, it is imperative to examine whether neutral current is indeed the best response quantity to consider or if there exists any other quantity, say, for example, total shunt current of the winding (also referred to as the tank current), to accomplish this task better. With this motivation, this paper compares the resonance behavior of neutral and tank current. Based on analytical derivations and actual experiments, it emerges that for detecting simulated mechanical changes, the tank current seems to be a more competent alternative compared to neutral current. It offers enhanced detection sensitivity, especially for interleaved windings. The possible reasons to explain why it is so are presented. The author believes that this finding needs reinforcement by field trials to ascertain its viability.