3 Installation and Commissioning
372
7UM62 Manual
C53000-G1176-C149-3
Figure 3-30
Measurement of the Zero Sequence Currents in a Delta Winding with
Artificial Starpoint
A zero sequence current of at least 2 % the rated generator current is required per
phase, i.e. the test current is at least 6 %. In the protection function, the sensitive pick-
up threshold must be set, and the zero voltage release disabled.
−
Switch on test current
−
Perform absolute value measurement with test current switched on
Read out the measured quantities on the device under:
Measured values
→
I-Diff,I-Stab
:
Both zero sequence currents 3I0-1 and 3I0-2 must be equal, and correspond to the
current fed in. The differential current I0-Diff is nearly zero. The restraint (stabilizing)
current I0-Stab is twice the flowing current. If the differential and the restraint current
are equal, the polarity of one CT must be wrong. Minor deviations are caused by CT
errors.
When checking the phase CTs of the allocated side, the measured values (device:
Measurement
→
Operational values, secondary
) per phase will be
1
/
3
each
of the applied zero sequence current. The phase angle is the same in all 3 phases due
to the zero sequence current.
If there are deviations, a connection error can be normally assumed (see side title “Pri-
mary Test with Generator”)
−
Disconnect test source and protected object
−
Check and correct connections and test arrangement
−
Repeat measurement
Checking the Zero
Voltage Release
If the zero voltage release is used, it must be checked during the test of the stator earth
fault protection (see Section 3.4.6). In the presence of an earth fault, the indication
3I0–1
Calculated zero sequence current of side 1 or side 2 (depending on
configuration)
3I0–2
Measured earth current I
EE2
I0–Diff
Calculated differential current
I0–Stab
Calculated restraint (stabilizing) current
∼
Test
source
7UM62