1 2 3 4 A - SIPROTEC

SIPROTEC, 7UT6x, Manual
C53000-G1176-C230-2, Release date 06.2012
Functions
2.2 Differential Protection
The type 4AM5120 is recommended for summation current transformers. These transformers have different
input windings which allow for summation of the currents with the ratio 2 : 1 : 3 as well as matching of different
primary currents of the main CTs to a certain extent. Figure 2-48 shows the winding arrangement.
The nominal input current of each summation CT must match the nominal secondary current of the connected
main CT set. The output current of the summation CT (= input current of the 7UT6x) amounts to I N =0.1A at
nominal conditions, with correct matching.
Figure 2-48 Winding arrangement of summation and matching transformers 4AM5120
Differential Current Monitoring
Whereas a high sensitivity of the differential protection is normally required for transformers, reactors, and
rotating machines in order to detect even small fault currents, high fault currents are expected in case of faults
on a busbar so that a higher pickup threshold (above rated current) is conceded here. This allows for a
continuous monitoring of the differential currents on a low level. A small differential current in the range of
operational currents indicates a fault in the secondary circuit of the current transformers.
When, during normal load conditions, a differential current is detected in the order of the load current of a
feeder, this indicates a missing secondary current, i.e. a fault in the secondary current leads (short-circuit or
open-circuit). This condition is annunciated with time delay. The differential protection is blocked at the same
time.
Feeder Current Guard
With busbars a release of the trip command can be set if a threshold is exceeded by one of the incoming
currents. The currents of each feeder are monitored for over-shooting of a set value. Trip command is allowed
only when at least one of these currents exceeds a certain (settable) threshold.
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