7SG12 DAD N Applications Guide
5.2 CTs on the circuit side of the Circuit Breaker
Faults F1 and F2 should be correctly cleared as before, but F3
will only cause operation of the busbar protection because the
fault is outside the circuit protection zone. Thus the fault may
remain fed from the remote end of the circuit. Arrangements
must therefore be made to cause the CB at the remote end of
the line to be tripped under these circumstances.
This can be affected by a direct intertrip or CB fail scheme.
Another method, as shown, uses an interlocked overcurrent
relay. This is arranged to detect any power infeed at F3 after
the circuit breaker is opened.
This relay is a three pole over current type with a time setting of
about 0.3 second. Its operation is inhibited until the busbar
protection operates and so if the fault persists at F3 after the
circuit breaker opens, the busbar protection remains operated,
so permitting the interlocked overcurrent relay to function and
unstabilise the circuit unit protection or send an intertripping
signal to the remote end of the circuit.
Figure 10 – CTs on other side of CB
5.3 CTs on the Busbar side of the Circuit Breaker
The faults at F1 and F2 will be correctly cleared.
A fault at F3 will cause the circuit protection to trip the circuit
breaker, but the fault will remain fed from the busbars. The
busbar protection will not operate as F3 is outside its zone.
Again an interlocked Over current relay is used, but in this case,
since it is the circuit protection which remains operated for the
fault at F3, it is this protection which is used to initiate operation
of the interlocked overcurrent relay. If F3 persists for about 0.3
seconds, the interlocked overcurrent relay then operates the
tripping relays of the protection of the section of busbar to which
the circuit is selected.
Figure 11 – CTs on Busbar side of CB
5.4 Bus Section and Bus Couplers
Figure 12 – CTs on Bus Section and Bus Coupler
©2010 Siemens Protection Devices Limited
Chapter 5 Page 10 of 16
Summary of Contents for 7SG12 DAD-N
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