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Appendix
The large difference of approximately 20% indicates a failure with 20% of the
turns. Due to the non-linear behavior, it can be assumed that the current, which
is flowing through the low-voltage winding is partly flowing through the magnetic
core. This can happen when the forces have significantly deformed the inner
turns (see Figure 5-16 "Leakage flux and forces during a fault" ).
Figure 5-16 Leakage flux and forces during a fault
The winding is probably interrupted and parts of the winding are contacting the
core which can be proven by measuring a resistance of 10 mΩ between LV
winding and core. For intact windings, this ratio is nearly totally independent
from the frequency in the discussed frequency range.
The ratio was measured with a test voltage of 200 V on the HV side. The
excitation current of the defective phase was approximately 340 mA, whereas
the excitation current of the intact phases was approximately 10 mA.
5.2 References
[1]Weck, K.-H.: Instandhaltung von Mittelspannungs-verteilnetzen, Haefely
Symposium 2000, Stuttgart
[2]Seitz, V.: Vorbeugende Instandhaltung an Leistungstransformatoren –
Betriebsbegleitende Messungen an Stufenschaltern und Durchführungen,
OMICRON Anwendertagung 2003, Friedrichshafen
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