Experimentally obtaining on-line FRA in transformers by injecting controlled pulses

Abstract

Transformer monitoring based on the operating condition (on-line test) has boomed in recent years because it allows early detection of risk situations or limitations regarding use to make appropriate decisions before a failure occurs. Frequency response analysis (FRA) is one of the most heavily researched techniques for on-line development; it is based on obtaining a transformer's frequency response (admittance, impedance or transfer function) in a given bandwidth. This paper presents an experimental method for obtaining the frequency response with the transformer in service by injecting controlled pulses of the voltage signal using an external electronic circuit. Once the voltage and current signals had been recorded, the wavelet transform was used via multi-resolution analysis (MRA) for filtering. This led to results being obtained without noise interference from the electrical system. The transfer function was obtained in terms of impedance, using the fast Fourier transform (FFT). The proposed method represented an alternative in solving some problems arising from the on-line FRA technique such as filtering and results' repeatability. The tests involved a 5-kVA 1150/345 V three-phase transformer and a 3-kVA, 7620/240 V single-phase transformer in the Universidad del Valle's High Voltage Laboratory.