ABB REL 501-C1 2.5, Technical Reference Manual

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Automatic switch onto fault logic (SOTF) Chapter 4
Line distance
2 Automatic switch onto fault logic (SOTF)
2.1 Application
The main purpose of the SOTFswitch-on-to-fault function is to provide high-speed tripping
when energizing a power line on to a short-circuit fault on the line.
Automatic initiating of the SOTF function using dead line detection can only be used when the
potential transformer is situated on the line-side of the circuit breaker. Initiation using dead line
detection is highly recommended for busbar configurations where more than one circuit breaker
at one line end can energize the protected line.
Generally, directional or non-directional overreaching distance or overcurrent protection func-
tions are used as the protection functions to be released for direct tripping during the activated
time. When line-side potential transformers are used, the use of non-directional protection zones
secures switch-on-to-fault tripping for fault situations where directional information can not be
established, for example, due to lack of polarizing voltage. Use of non-directional protection
also gives fast fault clearance when energizing a bus from the line with a short-circuit fault on
the bus.
2.2 Functionality
The SOTF function is a logical function built-up from logical elements. It is a complementary
function to the distance or overcurrent protection functions.
It is enabled for operation either by the close command to the circuit breaker, by a normally
closed auxiliary contact of the circuit breaker, or automatically by the dead line detection. Once
enabled, this remains active until one second after the enabling signal has reset. The protection
function(s) released for tripping during the activated time can be freely selected from the func-
tions included within the terminal. Pickup of any one of the selected protection functions during
the enabled condition will result in an immediate trip output from the SOTF function.
2.3 Function block
xx00000188.vsd
SOTF-
SOTF
BLOCK
NDACC
DLCND
BC TRIP