Tank Current Measurement of Three-Phase Transformer: Its Resonance Behavior and Sensitivity to Detect Mechanical Faults

In this paper, a novel technique to improve the fault detection sensitivity in a three-phase transformer using the tank current method is reported. The tank current method was first published by the first author in recent past for single isolated transformer winding. A fault in single isolated transformer winding was uniquely identified employing the typical resonance behavior of tank current, but for three-phase transformer, such implementation is yet to be explored. The present scope of this paper is only for uniformly wound three-phase transformer windings. Implementing the tank current method for such a three-phase transformer may not be very simple, since the neighboring windings, unlike in the case of isolated winding, strongly influence the measurement of the tank current. Therefore, this paper appropriately considers the effect of neighboring windings in a three-phase transformer and investigates the resonance behavior of its tank current to propose a promising innovative extension approach of the method. Experimental results on actual transformers are also presented for practical validation of the proposed method. Results imply that the tank current method is indeed very sensitive to identify the fault in an actual transformer compared to the conventional neutral current method. The authors also believe that the method needs to be reinforced by field measurements on several transformers to ascertain its viability.

I. Introduction and Background

Ever since the frequency response analysis (FRA or SFRA) was introduced for transformer diagnostics in late ’70s [1]–[3], several responses like line current, neutral current, transferred surge, neutral-end voltages have been explored and analyzed to examine their sensitivity to detect the mechanical changes in transformer winding [4]–[8]. Even to increase the sensitivity of FRA measurement several investigations are reported in the literature to identify the best terminal configuration for the tested and non-tested windings [9]–[15], but surprisingly, the tank current response was never explored for FRA diagnostics. Therefore, the author recently started an investigation to know whether the tank current measurement could be of any use for fault diagnostics in transformer winding [16]. Investigation was initially limited to explore the measurement sensitivity of tank current to identify a fault in single isolated transformer winding. Tank current response actually happens to be the current to the tank through ground capacitances of the winding and its contributions are distributed throughout the length of the winding. Therefore, it intuitively seems that it may contain identical winding related information similar to any other measured response of the winding. Intuition was also proven correct by the investigation in [16]. Once a fault was introduced in isolated winding, tank current measurement exhibited new resonance peaks in subsequently measured frequency response and also identified the change/shift in magnitude and position of the existing peaks and troughs. The second aspect is the most common for all FRA measurements and is also employed for measuring the fault detection sensitivity in conventional method. But, the first-one i.e. the occurrence of new resonance peaks is found very unique for tank current magnitude response. This feature enables the tank current measurement to identify a fault in isolated transformer winding simply by observing new resonance peaks in subsequently measured frequency response as compared to the peaks in healthy response. To the best of the authors’ knowledge, similar study is never reported in published literature to assess the sensitivity of tank current measurement for fault identification in three-phase transformer. Therefore, in such a scenario, it is imperative to investigate the resonance behaviour of tank current for three-phase transformer and to examine its potential for fault diagnostics.