Unblocking - ABB ETL600 Instruction Manual

ETL600
and reverse-looking directional units. A through-fault is seen by the
directional unit which sends a signal to block the distance relay on the
healthy line behind it. For a fault on the line, the reverse-looking
directional units at the two ends do not send blocking signals and the
overreaching first zones trip their respective circuit-breakers.
A teleprotection equipment for a blocking scheme has to fulfil only
modest requirements. An incorrect signal (fault on the protected line)
cannot prevent tripping, but only delay it. The loss or appreciable delay
of the blocking signal for a fault in the overreach section of an adjacent
line will, however, cause false tripping of the protected line.
Since the overreaching first zones of the relays have to be delayed
sufficiently long to allow time for a blocking signal to be received, a
short transmission time for the blocking signal is essential.
It follows from these considerations that a short transmission time and
good dependability are more important than security.
The following configuration and settings are recommended for a
blocking line protection scheme:
Command application
Command output type:
Command prolongation:
The corresponding configuration, setup and further programming
instructions are to be found in section 5.3.3.
With the above settings, a signal-to-noise ratio of about 4 dB (6 dB
required to fulfill IEC 60834-1, Figure 21) is necessary for successful
transmission of blocking commands (dependability Pmc of less than
-3
10 ) at a transmission time of less or equal than 15 ms. The rate of
unwanted commands for noise bursts of 200 ms duration and the
worst possible signal-to-noise ratio is always less than 10
Puc 10
3.6.5.4.

Unblocking

The standard NSD600 includes an unblocking function.
An 'unblocking' command used in directional comparison or in
permissive tripping schemes with overreaching first zone can be
allocated to one or more outputs on the relay interface module. Note
that „unblocking' commands are not transmitted from the remote end,
but automatically produced by certain types of line faults.
Should PLC communication be almost completely lost, e.g. in the
seldom case of a phase-to-phase fault involving the phases used for
PLC coupling in the immediate vicinity of the station, or in the case of
single-phase PLC coupling, the transmission of transfer tripping
signals cannot be absolutely guaranteed. Without a tripping signal a
distance relay will only trip in its second time step. The purpose of the
unblocking function is to avoid this delay. It does so by closing the
unblocking contacts of the NSD600 for 200 ms in the event of a PLC
channel failure, i.e. when the NSD600 is receiving neither guard nor
Structure and Function
1KHW001489-EN
-3
required to fulfill IEC 60834-1, Figure 21).
blocking
solid state
0 ms
November 2005
ABB Switzerland Ltd
-4
(security
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