2.13 Instantaneous High-Current Switch-onto-Fault Protection (SOTF)
265
7SD5 Manual
C53000-G1176-C169-1
2.13 Instantaneous High-Current Switch-onto-Fault Protection (SOTF)
2.13.1 Functional Description
General
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 serves, e.g. as a rapid protection for connecting a feeder with closed grounding
disconnector. In order to function properly, the devices at all ends of the protected
object must know the circuit breaker positions (breaker auxiliary contacts).
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>>>
. The currents are numerically filtered so that only the fundamental
frequency is evaluated. DC current components in the fault current and in the CT sec-
ondary circuit following the switching off of 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 mea-
sured quantity so that extremely short command times are possible.
This stage is only enabled when the local circuit breaker is closed while all remaining
line ends of the protected object are open. The devices exchange the status of their
respective circuit breakers continuously via the communication link. If the protected
object is already live (from a different end) the stage is not effective. An indispensable
precondition for the functioning of the
I
-STUB stage is that the auxiliary contacts of the
circuit breakers are connected at all ends of the protected object and allocated to the
relevant binary inputs. If this is not the case, this stage is not effective. The central
function control communicates the information of the circuit breaker position to the
high-current instantaneous tripping (see also Section 2.23.1).
Figure 2-116 shows the logic diagram. The
I
-STUB stage at the bottom of the diagram
operates separately for each phase. During the manual closing of the circuit breaker
all three phases are enabled via the internal signal „SOTF enab. L123“ which is issued
by the central functional control of the protection, provided that the manual closing can
be recognized there (see Section 2.23.1).
Tripping can also be enabled separately for each phase by the signals „SOTF enab.
Lx“. This applies also, for example, to automatic reclosure after single-pole tripping.
Then, single-pole tripping with this stage is possible, but only if the device is designed
for single-pole tripping.
I
>>>> Stage
The
I
>>>> stage trips regardless of the position of the circuit breakers. Here, the cur-
rents are also numerically filtered and the peak value of the currents is measured from
the double setting value onwards. Figure 2-116 shows the logic diagram in the upper
part.
Therefore, this stage is used when current grading is possible. This is possible with a
small source impedance and at the same time a high impedance of the protected
object (an example can be found in the advice on setting notes, Section 2.13.2).
The
I
>>>> stage is enabled automatically by the current-step monitoring
dI
/
dt
of the
device for a duration of 50 ms. This stage operates separately for each phase.
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