ABB ETL600 - Blocking Schemes

ABB Switzerland Ltd 1KHW001489-EN ETL600
or human error will cause unwanted, usually three-phase tripping of
the line and will block the operation of any auto-reclosure relay.
A genuine transfer tripping signal, on the other hand, must on no
account be lost whether there is interference on the communications
channel or not, because then a line fault would not be isolated with
correspondingly serious consequences.
Extremely high security and high dependability are therefore more
important than transmission time for direct transfer tripping.
In order to fulfil these requirements, the NSD600 uses dual tone
tripping signals. Dual tone signals provide the necessary additional
security against incorrect interpretation of speech, audio signals
coupled into the system and incorrect manipulations on the equipment.
The following configuration and settings are recommended for breaker
failure protection, compensator protection and power transformer
protection (but not for steady-state signals):
Command application direct
Command output type: solid state or relay
Command prolongation: 100 ms min.
Command outputs: set to the quiescent state in case of a
SNR or level alarm
With the above settings, a signal-to-noise ratio of about 1 dB (6 dB
required to fulfill IEC 60834-1, Figure 21) is necessary for successful
transmission of direct tripping commands (dependability Pmc of less
than 10
-4
) at a transmission time of less or equal than 40 ms. The rate
of unwanted commands for noise bursts of 200 ms duration and the
worst possible signal-to-noise ratio is theoretically always less than
10
-9
(security Puc 10
-6
required to fulfill IEC 60834-1, Figure 21). Since
this extremely high level of security is achieved without blocking the
unit, the NSD600 is capable of tripping without intentional delay at any
time as soon as a signal-to-noise ratio of about 6 dB is regained.
The following configuration and settings are recommended for line
protection with direct transfer tripping:
Command application direct
Command output type: solid state
Command prolongation: 20 ms
The corresponding configuration, setup and further programming
instructions are to be found in section 5.3.3.
3.6.5.3. Blocking Schemes
In a blocking scheme no tripping signals are transmitted along the
faulted line. Instead the blocking schemes of all the surrounding
healthy lines transmit signals to their remote ends to prevent tripping of
the overreaching relays there.
The protection equipment of a blocking scheme usually consists of
distance relays with overreaching first zones measuring into the line
3-60 November 2005 Structure and Function