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Polarity check for current input I
If the standard connection of the device is used with current input I
current transformers (also refer to the connection circuit diagram in Appendix A.3), the polarity of the ground
current path is in general automatically correct.
But if the current I
necessary.
If the device features the sensitive current input for I
the polarity check for I
Then this section can be ignored.
Apart from that the test is carried out with a disconnected trip circuit and primary load current. It must be noted
that during all simulations not exactly corresponding with cases that occur in practice, the asymmetry of mea-
sured values may cause the measured value monitoring to pick up. They must therefore be ignored during such
tests.
DANGER!
Hazardous voltages during interruptions in secondary circuits of current transformers
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
4
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 3-33). If no connection to 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 resistive-inductive load, the protection is in principle subject 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 pickup threshold of this stage must be below the load current flowing on the line; if necessary
the pickup threshold must be reduced. Note down the parameters that you have changed.
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 po-
larity. In the latter case, the connection must be rectified after the line has been isolated and the current trans-
formers short-circuited.
The voltages can be read on the display at the front, or called up in the PC via the operator or service interface,
and compared with the actual measured quantities as primary or secondary values. The absolute values as
well as the phase differences of the voltages are indicated so that the correct phase sequence and polarity of
individual transformers can also be seen. The voltages can also be read out with the Web-Monitor.
In the event that the pickup alarms were not even generated, the measured earth (residual) current may be too
small.
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
4
is supplied by a separate summation CT, an additional direction check on this current is
4
was already carried out with the earth fault check according to the previous section.
4
connected at the starpoint of the set of
4
and if it is used in an isolated or resonant-earthed system,
4
Mounting and Commissioning
3.3 Commissioning
553
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