[lo_Seal-in_Rel2, 1, en_US]
Figure 6-31 Logic Diagram of the Definite-Time Overcurrent Protection, Voltage-Released Undervoltage
Seal-in, Part 2
Voltage Release
In addition to the current criterion with undervoltage seal-in, a voltage-released logic must be present to issue
the indication
Pickup
. The voltage-released logic monitors the negative-sequence voltage and phase-to-
phase voltages respectively for detecting unsymmetrical faults and symmetrical faults. With the voltage-
released logic, the setting value of the parameter Threshold can be reduced in a certain range and the relia-
bility and sensibility of this function can be improved correspondingly.
Undervoltage Seal-In
In generators where the excitation voltage is derived from the machine terminals, the short-circuit current
decreases quickly in the event of close-in faults (for example, in a generator or a generator-transformer
range). The current decreases to a value below the current threshold within a few seconds because of the
absence of the excitation voltage. To avoid a dropout of the relay, the positive-sequence voltages are used as
an additional criterion for detecting a short circuit.
The pickup signal is maintained for a settable seal-in time Duration of V-seal-in time, if the positive-
sequence voltage falls below a settable threshold V-seal-in threshold after an overcurrent pickup, even
if the current falls below the threshold again. If the voltage recovers before the seal-in time has elapsed, or if
the undervoltage seal-in is blocked via a binary input
>Block V-seal-in
, the signal
Pickup I>+V-seal-
in
drops out immediately.
You can switch off the undervoltage seal-in via the parameter State of V-seal-in.
Blocking of the Undervoltage Seal-in with Measuring-Voltage Failure
The Undervoltage seal-in can be blocked if a measuring-voltage failure occurs. In case of a blocking, the
pickup signal
Pickup I>+V-seal-in
drops out immediately. The following blocking options are available
for the Undervoltage seal-in:
•
From an internal source upon pickup of the Measuring-voltage failure detection function.
•
From an external source via the binary input signal
>Open
of the function block VT miniature CB, which
links in the tripping of the voltage-transformer circuit breaker.
The parameter Blk. by meas.-volt. failure can be used to control the measuring-voltage failure
detection. The Undervoltage seal-in remains unaffected if the parameter Blk. by meas.-volt.
failure is switched off.
Application and Setting Notes
Parameter: Operate & flt.rec. blocked
•
Default setting (_:2) Operate & flt.rec. blocked = no
With the parameter Operate & flt.rec. blocked, you can block the operate indication, the fault
recording, and the fault log.
6.4.6.2
Protection and Automation Functions
6.4 Voltage-Dependent Overcurrent Protection, Phases
374 SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017
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