Instantaneous High-Current Switch-Onto-Fault Protection (Sotf); Functional Description - Siemens SIPROTEC Manual

Functions

2.14 Instantaneous High-Current Switch-onto-Fault Protection (SOTF)

2.14 Instantaneous High-Current Switch-onto-Fault Protection (SOTF)
2.14.1

Functional Description

General
The instantaneous high-current switch-onto-fault protection function is provided to disconnect feeders that are
switched onto a high-current fault immediately and without any time delay. It serves, e.g., as a rapid protection
for connecting a feeder with closed earthing isolator.
A second stage works fast and without delay, regardless of the circuit breaker position.
I>>> stage
The pickup of the I>>> stage measures each phase current and compares it to the setting value I>>>(address
2404). The currents are numerically filtered to eliminate the DC component. DC current components in the fault
current as well as in the CT secondary circuit after switching off large currents practically have no influence on
this high-current pickup operation. If the setting value is exceeded by more than twice its value, the stage will
automatically use the peak value of the unfiltered measured quantity. In this way, extremely short command
times are possible.
The I>>> stage works in 2 modes which can be selected using the parameter CBaux for I>>> (address
2406). If the parameter CBaux for I>>> is set to local only for the first mode, only the position of the
local circuit breaker is considered. If the circuit breaker is open (at least for the time set in parameter SI Time
all Cl. (address 1132)) and an energization takes place, the I>>> stage is activated for the time set in pa-
rameter SI Time all Cl.. The I>>> stage trips unselectively in case that the threshold is exceeded.
If the parameter CBaux for I>>> is set to local and rem. for the second mode, the switch positions of
all circuit breakers of the devices involved in the constellation are taken into consideration. The I>>> stage is
only active if all circuit breakers of the constellation are open and the local device is the first device that ener-
gizes the line. For this, the circuit breaker auxiliary contacts of all devices must be connected. If the circuit
breaker of the second device of the constellation are closed, the I>>> stage is not activated because the line
(the object to be protected) is already energized and a huge current flow can only be caused by an external
fault.
The high-current instantaneous tripping gets the position of the circuit breaker via the central function control
(see also Section 2.25.1).
Figure 2-147 shows the function logic of the High Current Switch-onto-Fault Protection. The I>>> stage at the
bottom of the diagram operates separately for each phase via „SOTF-O/C Rel. Lx". Single-pole tripping with
this stage is possible, but only if the device provides single-pole tripping.
Figure 2-146 shows the generation of the enable signals for the I>>> stage.
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SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011