75
Power Transformers
To check the winding insulation completely, it is necessary to measure the
capacitance and the DF of all insulation gaps (in this case six capacitors).
In IEEE Std. 62-1995 [3.4] the test procedure is described for transformers with
two and three windings. Figure 6-4 shows the six measurements.
Figure 6-4 Three-winding transformer test according to IEEE 62-1995
T Tertiary winding C T Cap. T to Ground C H-T Cap. H to
T
Caution: All phases and the neutral terminal of one winding (H, L and T) have
to be short-circuited. Due to the inductance of the windings resonant effects
may occur and influence the measurement.
Test Mode Energize Ground Guard UST Measure
GST HIGH – LOW, TERT. – C
H
GST LOW –
TERT., HIGH
–C
L
GST TERT. – HIGH, LOW – C
T
Supplementary test for interwinding insulations
TERT. LOW HIGH
C
HT
C
LT
C
HL
C
T
C
L
C
H
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