ABB REB670 Applications Manual page 210

Section 7
Current protection
204
t1: Time delay of the re-trip. The setting can be given within the range 0 – 60s in steps of
0.001 s. Typical setting is 0 – 50ms.
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 – 200ms
(also dependent of re-trip timer).
The minimum time delay for the re-trip can be estimated as:
³ + +
t 2 t 1 t t
cbopen BFP reset _
EQUATION1430 V1 EN
where:
t is the maximum opening time for the circuit breaker
cbopen
t is the maximum time for breaker failure protection to detect correct breaker function (the
BFP_reset
current criteria reset)
t is a safety margin
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.
+
t
margin
1MRK 505 337-UUS -
(Equation 42)
Application manual