English Original Reader for Technical Students. Power transformers: short-circuit testing, monitoring systems (Smart Grid) - страница 17

Power transformers are one of the basic parts in the circuitry of power transmission and delivery. Therefore, the interest to perfection of the power transformers' fault diagnostic methods is increased. The repairs of power transformers and other electrical equipment are carried out, using diagnostic measurement results.

LVI-testing, FRA and short-circuit inductive reactance measurements are sensitive to detecting such typical transformers winding faults as buckling, axial shift and other. The 70 units of 25–240 MVA 110–500 kV power transformers have been checked by low voltage impulse (LVI) method. A few power transformers were detected with winding deformations after short-circuit with aperiodical short-circuit current.

The idea of the application of an apparatus of frequency spectral analysis (FRA) for diagnostics of defects and damages of transformer-reactor electrical equipment belongs to R. Malewski, which proposed to use frequency spectra or transfer function of winding on the oscillograms of current and voltage for the analysis of the state of the transformer.

Significant amplitude-frequency changes with value to 1,5 Volts in the LVI- oscillograms, corresponding to radial deformations in LV winding of phase A, were occurred during the short-circuit tests of phase A of 250 MVA/220 kV transformer after short-circuit shot with 85 % value of transient (aperiodic) current. Conclusion was made about the impossibility of conducting further transformer testing (Figure 7) [by 1–4].

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.