Mounting and Commissioning
3.3 Commissioning
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
453
3.3.9
Direction Check with Load Current
≥
10 % of Load Current
The correct connection of the current and voltage transformers is tested via the protected line using the load
current. For this purpose, connect the line. The load current the line carries must be at least 0.1 ·
I
Nom
. The load
current should be in-phase or lagging the voltage (resistive or resistive-inductive load). The direction of the load
current must be known. If there is any doubt, network or ring loops should be opened. The line remains ener-
gized during the test.
The direction can be derived directly from the operational measured values. Initially the correlation of the mea-
sured load direction with the actual direction of load flow is checked. In this case the normal situation is
assumed whereby the forward direction (measuring direction) extends from the busbar towards the line
P
positive, if active power flows into the line,
P
negative, if active power flows towards the busbar,
Q
positive, if reactive power flows into the line,
Q
negative, if reactive power flows toward the busbar.
Figure 3-35
Apparent Load Power
All signs of powers may be inverted deliberately. Check whether polarity is inverted in address
1108
P,Q sign
in the
P.System Data 2
. In that case the signs for active and reactive power are inverse as well.
The power measurement provides an initial indication as to whether the measured values have the correct po-
larity. If both the active power and the reactive power have the wrong sign and
1108
P,Q sign
is set to
not
reversed
, the polarity according to address
201
CT Starpoint
must be checked and corrected.
However, power measurement itself is not able to detect all connection errors. For this reason, directional mes-
sages should be generated by means of the directional overcurrent protection. Therefore, pickup thresholds
must be reduced so that the available load current causes a continuous pickup of the element. The direction
reported in the messages, such as
„Phase A forward“
or
„Phase A reverse“
must correspond to the
actual power flow. Be careful that the „Forward“ direction of the protective element is in the direction of the line
(or object to be protected). This is not necessarily identical with the direction of the normal the power flow. For
all three phases, the directional messages to the power flow must be reported properly.
If all directions differ from each other, individual phases in current or voltage transformer connections are inter-
changed, not connected properly or phase assignment is incorrect. After isolation of the line and short-circuiting
of the current transformers the connections must be checked and corrected. The measurements must then be
repeated.
Finally, switch off the protected power line.
Important!
Make sure that pickup values that have been changed for testing are set back to the valid settings!
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