Tettex MIDAS 2881, Operating Instructions Manual

Page: 133 / 164

Applications Guide 123
Three Winding Transformers (3 phase and single-phase)
The test technique for a three-winding transformer is an extension of the two-winding transformer test
procedure.
In some cases a three-winding transformer is so constructed that one of the interwinding capacitances
is practically non-existent. This condition may be the result of a grounded electrostatic shield between
two windings, or of a concentric-winding arrangement which places one winding between two others.
The effect of the grounded shield of the sandwiched winding is to effectively eliminate the interwinding
capacitance except for stray capacitances between bushing leads.
3 phase, 3 winding transformer in Yn-Yn formation with tertiary winding. Measurement connections for
measurement of C
HG
, C
HT
and C
HL
Test Connections
Sequence
line DUT INPUT A
to INPUT B
to INPUT
HV GND
to Test Mode High Voltage
to
1 C
HT
T LV Tank GND UST A HV
2 C
HG
T LV Tank GND GST gA+B HV
3 C
HL
T LV Tank GND UST B HV
4 C
LG
T H Tank GND GST gA+B LV
5 C
TG
HV LV Tank GND GST gA+B T
6 C
LT
HV LV Tank GND UST B T
7 C
HG+
C
LG
C
TG
- - Tank GND GST gA+B HV+LV+T
Note: Test line #7can be used to inter-check the measurement results. (#7= #2 + #4 + #
5) additional
measurements in other test modes can be executed to inter-check the measurements results.