3 Mounting and Commissioning
418
7SA522 Manual
C53000-G1176-C155-3
3.3.10 Polarity Check for the Current Input I
4
If the standard connection of the device is used whereby current input I
4
is connected
in the starpoint of the set of current transformers (refer also to the connection circuit
diagram in the Appendix A.3), then the correct polarity of the earth current path in
general automatically results.
If, however, the current I
4
is derived from a separate summation CT or from a different
measuring point, e.g. transformer starpoint current or earth current of a parallel line,
an additional polarity check with this current becomes necessary.
If the device is provided with the sensitive
current input I
4
and it is connected to an iso-
lated or resonant-earthed system, the polarity check for I
4
was already carried out with
the earth fault check according to the previous section. Then this section can be ig-
nored.
Otherwise the test is done with a disconnected trip circuit and primary load current. It
must be noted that during all simulations that do not exactly correspond with situations
that may occur in practice, the non-symmetry of measured values may cause the mea-
sured value monitoring to pick up. This must therefore be ignored during such tests.
DANGER!
Hazardous voltages during interruptions in secondary circuits of current trans-
formers
Non-observance of the following measure will result in death, severe personal injury
or substantial property damage.
Short-circuit the current transformer secondary circuits before current connections to
the device are opened.
I
4
from Own Line To generate a displacement voltage, the e–n winding of one phase in the voltage
transformer set (e.g. L1) is bypassed (refer to Figure 3-25). If no connection on the
e–n windings of the voltage transformer is available, the corresponding phase is open
circuited on the secondary side. Via the current path only the current from the current
transformer in the phase from which the voltage in the voltage path is missing, is con-
nected; the other CTs are short-circuited. If the line carries resistive-inductive load, the
protection is in principle subjected to the same conditions that exist during an earth
fault in the direction of the line.
At least one stage of the earth fault protection must be set to be directional (address
31x0 of the earth fault protection). The pick-up threshold of this stage must be below
the load current flowing on the line; if necessary the pick-up threshold must be re-
duced. The parameters that have been changed, must be noted.
After switching the line on and off again, the direction indication must be checked: In
the fault log the messages ´()3LFNXSµ and ´()IRUZDUGµ must at least be
present. If the directional pickup is not present, either the earth current connection or
the displacement voltage connection is incorrect. If the wrong direction is indicated,
either the direction of load flow is from the line toward the busbar or the earth current
path has a swapped polarity. In the latter case, the connection must be rectified after
the line has been isolated and the current transformers short-circuited.
In the event that the pick-up alarms were not even generated, the measured earth (re-
sidual) current may be too small.
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