Installation and Commissioning
8-54 7SA6 Manual
C53000-G1176-C133-1
I
4
Measured on the
Protected 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 8-25). 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
connected; the other CTs are short-circuited. If the line carries load in the first
quadrant, 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
[ 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
reduced. 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 messages (refer also to Sub-section 7.1.1.3) the messages “()3LFNXS” and
“()IRUZDUG” must at least be present. If the directional pick up 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
(residual) current may be too small.
DANGER!
Working on measurement transformers requires the highest precautions!
Short-circuit the secondary side of the current transformers before any current
connections to the device are opened!
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