t1: Time delay of the re-trip. The setting can be given within the range 0 – 60 s in steps
of 0.001 s. Typical setting is 0 – 50 ms.
t2: Time delay of the back-up trip. The choice of this setting is made as short as
possible at the same time as unwanted operation must be avoided. Typical setting is 90
– 200 ms (also dependent of re-trip timer).
The minimum time delay for the re-trip can be estimated as:
_
2 1³ + + +
cbopen BFP reset margin
t t t t t
EQUATION1430 V1 EN (Equation 44)
where:
t
cbopen
is the maximum opening time for the circuit breaker
t
BFP_reset
is the maximum time for breaker failure protection to detect correct breaker
function (the current criteria reset)
t
margin
is the safety 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 to maintain transient stability in case
of a fault close to a power plant.
Time
The fault
occurs
Protection
operate time
Trip and Start
CSPRBRF
Normal t
cbopen
Margin
Retrip delay t1 t
cbopen
after re-trip
t
BFPreset
Minimum back-up trip delay t2
Critical fault clearance time for stability
IEC10000280-1-en.vsd
IEC10000280 V1 EN
Figure 67: Time Sequence
1MRK 505 291-UEN A Section 6
Current protection
139
Application manual
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