Calculating the transient recovery voltage associated with clearing transformer determined faults by means of frequency response analysis

The transient recovery voltage (TRV) associated with power-transformer terminal faults is essential for correctly applying circuit breakers. Because presently used methods only allow for roughly estimating an equivalent single TRV ringing frequency, more accurate but inexpensive methods are needed. The novel method presented in this paper employs frequency response analysis (FRA) for measuring the transformer´s complex short circuit impedance Z_X. From that, the frequency spectrum of the TRV is approximated by the ratio of Z_X over the frequency square. Inverse fast Fourier transformation finally leads to the time domain signal. Comparing measured TRV signals from several field tests with the calculated ones prove this simple but effective method. Even multimode oscillations that cannot be reproduced by rough estimating tools are covered. It is concluded that the presented method provides a valuable tool for extracting inherent TRV parameters, either from routinely performed FRA measurements or from lightning impulse tests without significant additional costs.