Tettex MIDAS 2881, Operating Instructions Manual

Page: 132 / 164

122 Applications Guide
Autotransformers (3 phase and single-phase)
Contrary to the two-winding transformer the windings of an autotransformer can not be separated.
The winding of an autotransformer is a combination of the high- and low-voltage windings (HV and
LV, see figure below).
For testing the insulation of an autotransformer all seven bushings (three bushings for a single-phase
unit) have to be connected together (HV1+HV2+HV3+LV1+LV2+LV3+0).
For a conventional autotransformer without a tertiary winding only an overall test to ground can be
performed (C
HG
).
If an autotransformer is equipped with a tertiary winding which is accessible, the test procedure is
exactly the same as described in section “Two Winding Transformer”.
Measurement connections of an autotransformer with tertiary winding for measurement of C
HG
& C
HT
Test Connections
Seq.line DUT INPUT A
to INPUT
HV GND
to Test Mode High Voltage
to
1 C
HT
T Tank GND UST A HV+LV+0
2 C
HG
T Tank GND GST gA+B HV+LV+0
3 C
TG
HV+LV+0 Tank GND UST A T
4 C
TG
+ C
HG
- Tank GND GST gA+B HV+LV+0+T
Note: Test line #4 can be used to inter-check the measurement results. (#4 = #2 + #3). Additional
measurements in other test modes can be executed to inter-check the measurements results.