CP TD1 Reference Manual V 1.44
174
12.9.14 DF Measurements on Transformer Windings
A transformer contains a complicated insulation system. High- and low-voltage
windings have to be insulated to tank and core (ground) and against each other.
All these insulation gaps should be checked regularly.
Figure 12-31 2-winding transformer
Normally in a two-winding power transformer, C-Tan-Delta measurements are
made for all insulation gaps: HV to LV, HV to ground, LV to ground (Figure
12-31). A three-winding transformer is much more complicated and more tests
are necessary to measure all gaps.
The dissipation factor is an indicator of the oil-paper insulation quality of the
single gaps. Degradation of oil, water content and contamination with carbon
and other particles can increase the DF. New HV-power transformers and
transformers of good quality have values for DF below 0.5%. Figure 12-32
shows a DF measurement HV-winding to LV-winding.
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