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 39.
A1 B1
Feeder
Time axis
t=0 t=t
1
t=t
2
t=t
3
The fault
occurs
Protection
B1 trips
Breaker at
B1 opens
Protection
A1 resets
en05000205_ansi.vsd
51
51
ANSI05000205 V1 EN
Figure 39: Sequence of events during fault
where:
t=0 is when the fault occurs
t=t
1
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 t
1
t=t
2
is when the circuit breaker at IED B1 opens. The circuit breaker opening time is t
2
- t
1
t=t
3
is when the overcurrent protection at IED A1 resets. The protection resetting time is t
3
- t
2
.
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
3
. 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 33.
1MRK 511 286-UUS A Section 6
Current protection
101
Application manual
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