ABB REL650 Applications Manual page 204

Section 7
Current protection
198
have instantaneous function. The overcurrent protection of IED A1 must have a delayed
function. The sequence of events during the fault can be described using a time axis, see
figure 95.
A1
51
t=0 t=t
1
The fault Protection
occurs B1 trips
ANSI10000279 V1 EN
Figure 95: Sequence of events during fault
where:
• t=0 is when the fault occurs.
• t=t1 is when the trip signal from the overcurrent protection at IED B1 is sent to the
circuit breaker. The operation time of this protection is
• t1, t=t2 is when the circuit breaker at IED B1 opens. The circuit breaker opening time
is t2 - t1.
• t=t3 is when the overcurrent protection at IED A1 resets. The protection resetting
time is t3 - t2.
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
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 110.
D ³ + +
t 40 ms 100 ms
EQUATION1266 V1 EN
where it is considered that:
B1
51
t=t t=t
2
Breaker at Protection
B1 opens A1 resets
+ =
40 ms 40 ms 220 ms
1MRK 506 334-UUS A
Feeder
Time axis
3
ANSI10000279-1-en.vsd
Application manual
. There
3
(Equation 115)