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Siemens AG ⋅ January 1999
6.7.3 Checking the
current transformer termination
The current transformer termination is checked with
primary measured signals. A load current of at least
25 % of the rated line current is necessary for this.
This check cannot replace the visual check of the
correct current transformer termination because
the result of the measurement with the summation
CT current does not always uniquely identify the
cause of the error. For instance, a connection with
incorrect phase rotation due to swapped phase or
with incorrect phase allocation but still maintaining
the correct phase rotation cannot be detected! Same
connection error at both line ends The checks accorĆ
ding to section 5.2.5 are therefore a precondition.
Secondary test equipment connected for the previous
tests and measurements must be removed. The sumĆ
mation transformer must be connected to the current
transformer.
Before switching on the line, make sure that a short-
circuit protection is available at both ends of the line.
The differential protection functions remain switched
to (addr. 10 DIFPRO).
D Open earth switches of the line. Connect line both
ends. It must be ensured that at least 25 % of the
rated line current flows through the line.If necesĆ
sary, changes in the system configuration must be
implemented. The direction of the current is irreleĆ
vant.
D The summation transformer secondary current can
be measured in the operational measured values
under addr. 84 OP.MEASV.. If connected correctly
this value corresponds to the current flowing
through the line, in relation to the rated current of
the device or the primary rated current in A.
If matching transformers are connected, their actual
transformation ratio must be taken into account.
Current transformer terminations may still be swapĆ
ped. Table 6.2 shows the measured current in relation
to the expected current for polarity errors in the curĆ
rent transformer connection:
D Isolate and earth line.
D Correct current transformer connections.
D Repeat test.
Table 6.2 Measured currents with incorrect
CT connection
measured
I1/I
expected
possible correction error
4,36 Transformer L2, incorrect polarity
Transformer L1 and L3, incorrect polarity
4,73 Transformer L3, incorrect polarity
Transformer L1 and L2, incorrect polarity
4.93 Transformer L1, incorrect polarity
Transformer L2 and L3, incorrect polarity
1.00 All transformers have incorrect polarity
incorrect phase rotation
incorrect phase allocation by correct
phase rotation
right connection
On the one hand the table shows that the measured
deviations only differ slightly in the case of reverse
polarity of single phases which is why all connections
must be checked again carefully in any case. On the
other hand, swapped phases are not detected at all.
This is basically irrelevant for the function of the diffeĆ
rential protection provided that only the connections
at both ends of the line are identical.
However, a uniform phase allocation is important for
the sensitivity of the protection in the case of double
earth faults in the non-earthed network.
Finally, switch off the line again.
The correct polarity of the earth current path (in stanĆ
dard connection according to Figure 5.3) is checked
when current I
L2
is missing (Figure 6.6):
Caution
Work on the current transformers requiĆ
res the greatest caution! Short circuit
current before disconnecting any CT
connections to the device!
Current transformer phase I
L2
is short-circuited at the
CT and the connection cable to the summation transĆ
former is disconnected. The previous load current
test is repeated.
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