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Installation and Commissioning
7.2.2
Check of the complete protection system with operating currents
7.2.2.1
Directional check of the input currents
7-8
When checking the connection circuits, one prerequisite for a correct comparison of
the currents of all feeders was a uniform (normalized) transformation ratio between the
primary currents from the main current transformers and the secondary currents from
the summation current transformers or matching transformers, which were the basis
for the measurement.
Once this prerequisite is fulfilled, the currents must be fed into the protection circuit in
the same direction, so that in fault-free load operation of the station the differential cur-
rent paths of the measuring systems do not carry any currents with wrong polarity
which might lead to overfunctioning. Therefore, it must be possible to reverse the po-
larity of the secondary currents from the summation current and input transformers to
match it to the downstream measurement setup.
The directional check of the feeder currents should be performed with operational load
currents from the station, with the tripping lines of the busbar protection interrupted or
the TRIP command blocked.
Reliable measurements are performed with 20 % of the normalized rated current in
each feeder. This can usually be achieved with load current if the power system is
switched accordingly.
Where this is not possible, it is recommended to feed into the busbar from any feeder
a testing current that is sufficiently high for a reliable measurement. In that case a
three-pole short-circuit is generated successively into each feeder outside the differ-
ential protection zone during the test.
The subsequent measuring procedure comprises the measuring system of one bus-
bar only. If the system has several busbar sections with one measuring system allo-
cated to each section, any of these will be selected for starting.
The differential and restraint currents can be output on the display for evaluation. In all
tests, the differential currents may differ from zero only by a few percent (approx. 3 to
5 %).
7SS60 Manual
E50417-G1176-C132-A1
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