If, however, current Ι
N
is derived from a separate summation CT (see e.g. a connection circuit diagram in the
Appendix C Connection Examples), an additional direction check with this current is necessary.
If the device features the
sensitive current input for Ι
N
and if it is used in an isolated or resonant-grounded
system, the polarity check for Ι
N
was already carried out with the ground fault check according to the previous
section. Then this section is not relevant.
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 measured value monitoring to pick up. This 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.
²
Short-circuit the current transformer secondary circuits before current connections to the device are
opened.
Directional Testing for Grounded Systems
The check can either be carried out with function “directional ground fault protection” (address 116) or with
the function “ground fault detection” (address 131), which can be operated as additional fault protection.
In the following the check is described using the “directional ground fault protection” function (address 116)
as an example.
To generate a displacement voltage, the e–n winding of one phase in the voltage transformer set (e.g. A) is
bypassed (see Figure 3-27). If no connection on the e–n windings of the voltage transformer is provided, the
corresponding phase is disconnected on the secondary side (see Figure 3-28). Only the current of the trans-
former which is not provided with voltage in its voltage path is fed into the current path. If the line carries
resistive- inductive load, the protection is subject to the same conditions as exist during a ground fault in line
direction.
The directional ground fault protection must be configured to enabled and activated (address 116 or 131). Its
pickup threshold must be below the load current of the line; if necessary the pickup 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 log the
messages
67N picked up
and
Ground forward
must at least be present. If the directional pickup is not
present, either the ground 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 ground
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.
If the pickup message is missing, the measured ground (residual) current or the displacement voltage
emerged may be too small. This can be checked via operational measured values.
Important! If parameters were changed for this test, they must be returned to their original state after
completion of the test !
Mounting and Commissioning
3.3 Commissioning
348 SIPROTEC 4, 7SJ80, Manual
E50417-G1140-C343-A8, Edition 12.2017
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