89
Power Transformers
For oil paper insulation, the range of the DF values for new and aged
transformers are published in some standards like [3.4] and in other literature
[3.1], [3.2]. In IEEE Std. 62-1995 [3.1] the following limits for DF values are
given:
It is always better to measure the values regularly and save them for comparison
to tests in the past and in the future. In this way, trends can be observed and the
evaluation of results is of much higher quality.
6.4.2 Capacitance Measurement
The capacitance of the insulation gaps between the windings to each other and
to ground depends mainly on the geometry of the winding. Windings may be
deformed after transport of the transformer or nearby through faults with high
currents. Changes in capacitance serve as an excellent indicator of winding
movement and structural problems (displaced wedging, buckling etc.). If a
winding damage is suspected then the capacitance measurement should be
supplemented by a leakage reactance measurement. A separate test can be
done for each phase with this measurement technique. Therefore this method is
more sensitive to small changes in one phase.
6.5 References
Table 6-4 DF values for oil paper insulation
Condition of insulation
Transformer Good May be accept-
able
Should be investi-
gated
New DF < 0.5% - -
Service-aged DF < 0.5% 0.5% < DF < 1% DF > 1%
All values measured at 20 °C
[3.1] US Bureau of Reclamation: "Maintenance of liquid insulation
mineral oils and Askarels", Facility instructions, standards and
techniques - Vol. 3-5, 1992
[3.2] Möllmann, A., Lütge, H.: IEC / VDE Standards für flüssige
Isolierstoffe zur Diagnostik von Transformatoren und Wandlern,
ETG-Fachbericht "Diagnostik elektrischer Betriebsmittel", VDE-
Verlag GmbH Berlin 2002, S. 205-210
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