ABB REQ650 Applications Manual page 109

1MRK 505 291-UEN A
Application manual
A1
I>
t=0 t=t
1
The fault Protection
occurs B1 trips
IEC05000205 V1 EN
Figure 50: Sequence of events during fault
where:
t=0 is when the fault occurs
t=t is when the trip signal from the overcurrent protection at IED B1 is sent to the circuit breaker. The
1
operation time of this protection is t
t=t is when the circuit breaker at IED B1 opens. The circuit breaker opening time is t
2
t=t is when the overcurrent protection at IED A1 resets. The protection resetting time is t
3
To ensure that the overcurrent protection at IED A1, is selective to the overcurrent
protection at IED B1, the minimum time difference must be larger than the time t
There are uncertainties in the values of protection operation time, breaker opening
time and protection resetting time. Therefore a safety margin has to be included. With
normal values the needed time difference can be calculated according to equation 19.
D ³ + +
t 40 ms 100 ms
EQUATION1266 V1 EN
where it is considered that:
the operate time of overcurrent protection B1 is 40 ms
the breaker open time
the resetting time of protection A1
the additional margin
B1
I>
t=t
2
Breaker at Protection
B1 opens A1 resets
1
+ =
40 ms 40 ms 220 ms
is 100 ms
is 40 ms and
is 40 ms
Section 6
Current protection
Feeder
t=t
3
Time axis
en05000205.vsd
- t
2 1
- t .
3 2
.
3
(Equation 19)
103