Assessment of transformer winding displacement faults by frequency response analysis

Abstract

According to the statistics of transformer accidents from 1995 to 1998, nearly 50% of the accidents were caused by short circuit, and the number is still increasing. In other words, diagnostic test technique on winding displacement is very important, by which hidden troubles can be found in advance, so the transformer can be arranged for repairing by schedule. The main methods on detection of transformer winding displacement are Short Circuit Impedance Measurement (SCI), Low Voltage Impulse Method (LVI), and Frequency Response Analysis Method (FRA). The Short Circuit Impedance Measurement is not widely used on site because its sensitivity is low and the hidden trouble can not be found effectively. On the hand, the sensitivity of LVI and FRA is high, their principles are similar. However, the parameter of single impulse voltage source used in LVI can hardly be the same all the time, and therefore the FRA is the most suitable method on site. Every transformer winding has a unique signature that is sensitive to changes in the parameters of the winding, namely resistance, inductance, and capacitance. Frequency spectrum of a transformer is very sensitive to any displacement of the winding. Practical case studies are presented that demonstrates the effectiveness of this technique. Again FRA has proven itself to be a valuable diagnostic tool for the detection of winding displacement fault.