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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
from Own Line
To generate a displacement voltage, the e–n winding of one phase in the voltage
4
transformer set (e.g. L1) is bypassed (refer to Figure 3-30). 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
31x 0 of the earth fault protection). The pickup threshold of this stage must be below
the load current flowing on the line; if necessary the pickup threshold must be reduced.
The parameters that are changed, must be noted.
After switching the line on and off again, the direction indication must be checked: in
the fault log the messages „EF Pickup" and „EF forward" 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 pickup alarms were not even generated, the measured earth (re-
sidual) current may be too small.
7SD5 Manual
C53000-G1176-C169-1
3.3 Commissioning
501
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