a) for LVI-testing of phases «А-С» of LV winding;
b) for LVI-testing of phases «А-B» of LV winding;
c) for LVI-testing of phases «B-C» of LV winding.
Figure 7. LVI-oscillograms of LV winding of 250 MVA/220 kV transformer after short-circuit shot with 85 % value of transient (aperiodic) current in the phase «A», illustrating the appearance of significant amplitude-frequency changes with value to 1,5 Volts, short-circuit impedance ΔZ s-c = +1 % (radial deformations).
The changes in the spectra of windings, which occurred as a result of radial deformations, bear in essence amplitude nature, while changes in the frequency are less significant. One of the fundamental resonance frequencies (Figure 8) appears frequently in the period of 3 microseconds, i.e., 330 kHz.
Figure 8. Calculated FRA-spectrum of LV winding of 250 MVA/220 kV transformer after short-circuit shot with 85 % value of transient (aperiodic) current in the phase «A», built on the basis of LVI-oscillograms.
It is confirmed by the calculations of spectra that the frequency of 320 kHz is one of the fundamental resonance frequencies. Resonances are also the frequencies of 110 kHz and 510 divided by 550 kHz. It is possible to establish an increase in the amplitudes after the appearance of the deformations of at frequencies 320 kHz and 550 kHz (Figure 8).
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