ABB RELION 670 Series Applications Manual page 442

Section 8
Current protection
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
EQUATION1430 V1 EN-US
where:
t
cbopen
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 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 265: Time sequence
t2MPh : Time delay of the back-up 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 back-up trip delay for multi-phase faults. Typical
setting is 90 – 150 ms.
t3 : Additional time delay to t2 for a second back-up trip TRBU2. In some applications there
might be a requirement to have separated back-up trip functions, tripping different back-up
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
436
+ + +
t t t
cbopen BFP reset _ margin
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
cbopen
Minimum back-up trip delay t2
Critical fault clearance time for stability
Trip and Start
CCRBRF
after re-trip
t
BFPreset
1MRK 504 152-UEN B
Margin
Time
IEC05000479_2_en.vsd
Application manual
(Equation 461)