Mounting and Commissioning
3.3 Commissioning
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
457
3.3.12 Polarity Check for Current Input I
N
General
If the standard connection of the device is used with current input I
N
connected in the starpoint of the set of
current transformers (see also connection circuit diagram in the Appendix A.3), then the correct polarity of the
ground current path usually occurs automatically.
If, however, current I
N
is derived from a separate summation CT (see e.g. a connection circuit diagram in the
Appendix A.3), an additional direction check with this current is necessary.
If the device features the sensitive
current input for I
N
and if it is used in an isolated or resonant-grounded
system, the polarity check for I
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.
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 3-37). 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 3-38). 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 resis-
tive-inductive load, the protection is subject to the same conditions as exist during a ground fault in line direc-
tion.
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 checked via operational measured values.
Important! If parameters were changed for this test, they must be returned to their original state after comple-
tion of the test !
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