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1MRK 505 370-UEN D
where:
t
CB_open
t
BFP_reset
t
margin
It is often required that the total fault clearance time shall be less than a given critical time.
This time is often dependent of the ability of the power system to maintain transient stability
in case of a fault close to a power plant.
Protection
operate time
The fault
occurs
IEC05000479 V2 EN-US
Figure 102: Time sequence
t2MPh : Time delay of the backup trip at multi-phase start. The critical fault clearance time is
often shorter in case of multi-phase faults, compared to single phase-to-earth faults.
Therefore there is a possibility to reduce the backup trip delay for multi-phase faults. Typical
setting is 90 – 150 ms.
Note that for a protected object which are always tripped three-phase (e.g. transformers,
generators, reactors, cables, etc.) this timer shall always be set to the same value as
t3 : Additional time delay to t2 for a second backup trip TRBU2. In some applications there
might be a requirement to have separated backup trip functions, tripping different backup
circuit breakers.
tCBAlarm : Time delay for alarm in case of indication of faulty circuit breaker. There is a binary
input CBFLT from the circuit breaker. This signal is activated when internal supervision in the
circuit breaker detect that the circuit breaker is unable to clear fault. This could be the case
when gas pressure is low in a SF6 circuit breaker, of others. After the set time an alarm is
given, so that actions can be done to repair the circuit breaker. Note that the time delay for
backup trip
tPulse : Trip pulse duration. This setting must be larger than the opening time of circuit
breakers to be tripped from the breaker failure protection. Typical setting is 200 ms.
Busbar protection REB670
Application manual
is the maximum opening time for the circuit breaker
is the maximum time for breaker failure protection to detect correct breaker function
(the current criteria reset)
is a safety margin
Normal t
cbopen
Retrip delay t1
t
after re-trip
cbopen
Minimum back-up trip delay t2
Critical fault clearance time for stability
Trip and Start
CCRBRF
t2 is bypassed when the CBFLT is active. Typical setting is 2.0 seconds..
t
BFPreset
Margin
IEC05000479_2_en.vsd
Section 7
Current protection
Time
t2 timer.
179
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