A review of transformer FRA measurement and diagnosis techniques
FRA is a technique that has been successfully applied to power transformers to detect mechanical damage to windings since its development over 40 years ago
byY. Yang et al.
Frequency Response Analysis (FRA) is a technique that has been successfully applied to power transformers to detect mechanical damage to windings since its development over 40 years ago [1]. One example of such damage is shown in Figure 1.1. FRA grew from low voltage impulse test techniques, which record, for example, the neutral current when an impulse is applied to a terminal. These techniques had been successful in both detecting winding damage and understanding the impulse response for system transient studies. The raw impulse response, however, varies with cabling and impulse generator parameters, and the frequency response (the ratio of output voltage to input voltage across a range of frequencies) eliminated some of the uncertainties caused by measurements, has better reproducibility and was easier to interpret.
Frequency response can be obtained using a fast Fourier transform algorithm (FFT) to convert impulse data (or any time series data with a suitable frequency content) into the frequency domain, but it is generally considered easier to obtain good signal-to-noise ratios, good frequency definition and rejection of interference required for reliable site measurements using a network analyser. A network analyser uses a swept frequency sine wave as the input signal whilst measuring the input and output signals through a narrow band filter tracking the frequency. This is the technique (sometimes called SFRA) most commonly used today.
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