Functional Description
19
measurements. This involves a lot of work and time, especially when on-site measurements are being performed
on large power transformers.
Using the different Test Modes, the test object only has to be connected once for measurement and all relevant
capacitances can be measured by switching the connections as required. The selected Test Mode connects the
DUT current path(s) to the internal current measuring shunt R
X
and the other (not measured) connected leads to
the V-potential (reference point) of the system. All capacitances connected to this reference point are bypassed
and are not influencing the selected measurement.
Measuring setup on a single phase transformer with two low voltage windings.
Note: The connection between HV GND on the measuring instrument and the earth point of the test object is a
normal measuring channel as well. A good clean contact is essential.
The table below gives an overview which test mode measures which capacitances.
Test Mode
Explanation
INPUT A
connecte
d to (S1)
INPUT B
connected
to (S2)
HV GND
connected
to (S3)
Actual
measured
C
X
UST A
Ungrounded Specimen Test, A used as
measuring channel, B and HV GND connected
to V- potential point (bypassed)
R
X
V V C
HL
UST B
Ungrounded Specimen Test, B used as
measuring channel, A and HV GND connected
to V- potential point (bypassed)
V R
X
V C
HT
UST A+B
Ungrounded Specimen Test, A and B used as
measuring channels, HV GND connected to V-
potential point (bypassed)
R
X
R
X
V
C
HL
+
C
LT
GST A+B
Grounded Specimen Test, A and B and HV
GND used as measuring channels.
R
X
R
X
R
X
C
HL
+
C
HT
+
C
HG
GSTgA
Grounded Specimen Test with guarding (V-
potential) connected to A (bypassed). HV
GND and B are used as measuring channels.
V R
X
R
X
C
HT
+
C
HG
GSTgB
Grounded Specimen Test with guarding (V-
potential) connected to B (bypassed). HV
GND and A are used as measuring channels.
R
X
V R
X
C
HL
+
C
HG
GST gA+B
Grounded Specimen Test with guarding (V-
potential) connected to A and B (bypassed).
Only HV GND is used as measuring channel.
V V R
X
C
HG
Note: For testing the insulation secondary winding – tank, the HV cable and measuring cables shall be exchanged.
The HV shall be connected to the secondary winding(s) and the measuring cable to the primary winding. The
measured capacitances in the table will change accordingly.
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