Protecting Dual-Circuit Lines - ABB ETL600 Instruction Manual

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tripping signals. This 'emergency' feature can be used in different
ways:
•
to switch the reach of the distance relay (under/overreaching first
zone)
•
as an enabling signal by connecting the unblocking output contact
in parallel with the tripping signal output of the NSD600. This is
used above all in the case of permissive overreaching blocking
schemes.
One or two of the potentially-free relay outputs located on the
teleprotection interface type G4AI are usually configured to signal the
unblocking command.
3.6.5.5.

Protecting Dual-Circuit Lines

Dual-circuit lines often form important links in power systems at the
higher system voltages. The line protection is usually supported by a
breaker failure protection scheme.
An NSD600 with two commands set for permissive tripping and with
two commands programmed for direct tripping is sufficient for the two
line protection signals and the two breaker failure direct tripping
signals. All commands can be transmitted and received with one single
teleprotection interface type G4AI.
Important dual-circuit lines (132 kV to 400 kV) are often equipped with
1st and 2nd main protections supported by breaker failure protection
on each circuit. Logically, redundant communication channels should
also be installed such that the protection of both circuits remains fully
intact should one set of equipment fail due to a defect or human error,
or be out of commission for servicing.
The NSD600 with its four transfer tripping signals is especially well
equipped to efficiently handle the tripping commands generated by
redundant main and breaker failure protection schemes for a dual-cir-
cuit line. An extremely reliable overall scheme would comprise two
sets of PLC and NSD600 equipment, the NSD600's having the need of
only one relay interface for four transfer tripping signals each. A
corresponding example of a redundant teleprotection scheme can be
seen in Fig. 3-38.
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1KHW001489-EN
November 2005
ETL600
Structure and Function