Chapter 2. Diagnostics and Transformer Testing Experience by LVI/FRA Methods
Abstract
LVI-testing, FRA and short-circuit inductive reactance measurements are sensitive to detecting such typical transformers winding faults as buckling, axial shift and other. It is given an algorithm of the frequency spectral analysis. The examples of diagnostics by LVI-testing, frequency spectral analysis of capacity range from 20 to 400 MVA and voltage range from 35 to 220 kV transformers and autotransformers are showed in service and during short-circuit testing. The studies are carried out for dependency of LVI oscillograms and signal spectra of power transformer winding of one type manufacturing.
Keywords: Short-circuit, Testing Laboratory, Winding fault, Electrodynamic deformation, Low voltage impulse method
2.1. Introduction
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.
2.2. LVI-Testing and FRA Method for 250 MVA/220 kV Transformer Diagnostic
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.
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).