2 Functions
224
7SA522 Manual
C53000-G1176-C155-3
2.12 Instantaneous high-current switch-on-to-fault protection (SOTF)
The instantaneous high-current switch-onto-fault protection function is provided to dis-
connect immediately and without delay feeders that are switched onto a high-current
fault. It is primarily used as fast protection in the event of energizing the feeder while
the earth switch is closed, but can also be used every time the feeder is energized —
in other words also following automatic reclosure — (selectable).
The energization of the feeder is reported to the protection by the circuit breaker state
recognition function. This function is described in detail in Section 2.20.1.
2.12.1 Method of Operation
Pickup The high-current pickup function measures each phase current and compares it with
the set value ,!!! (address ). The currents are numerically filtered so that only
the fundamental frequency is evaluated. If the measured current is more than twice
the set value, the protection automatically reverts to the unfiltered measured values,
thereby allowing extremely fast tripping. DC current components in the fault current
and in the CT secondary circuit following the switching off of large currents virtually
have no influence on the high-current pickup operation.
The high-current switch-on-to-fault function can be phase segregated or three-phase.
In case of manual closure of the CB, it operates always in all three phases via the in-
ternal release signal “SOTF O/C Release 3ph” supplied by the central function control
of the protective relay, provided that the manual closure can be detected there (see
Subsection 2.20.1).
If further criteria were determined during configuration of the line energization detec-
tion (address /LQH&ORVXUH, refer to Section 2.1.5.1) the release signal
“SOTF-O/C Release. Lx” can be initiated selectively for each phase. This only applies
to devices that can trip single-pole, and is important in conjunction with single-pole au-
tomatic reclosure.
Tripping is always three-pole. The phase selectivity only applies to the pick-up due to
the coupling of the high current criterion with the circuit breaker pole which is closed.
In order to generate as quickly as possible a trip command after an energization, the
fast switch-on-to-fault protection is released selectively for each phase already when
the line is open.
The following figure shows the logic diagram.
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