significantly between different protective equipment. The following time delays can be
estimated:
Protection operation
time:
15-60 ms
Protection resetting
time:
15-60 ms
Breaker opening time: 20-120 ms
Example for time coordination
Assume two substations A and B directly connected to each other via one line, as shown in the
Figure 90. Consider a fault located at another line from the station B. The fault current to the
overcurrent protection of IED B1 has a magnitude so that the overcurrent protection will start
and subsequently trip, and the overcurrent protection of IED A1 must have a delayed operation
in order to avoid maloperation. The sequence of events during the fault can be described using
a time axis shown in Figure
90.
I> I>
A1 B1
Feeder
Time axis
t=0 t=t
2
t=t
3
The fault
occurs
B1 trips and
A1 starts
Breaker at
B1 opens
=IEC05000205=2=en=Original.vsd
Fault
t=t
4
Protection
A1 resets
t=t
1
B1 and A1
start
IEC05000205 V2 EN-US
Figure 90: Sequence of events during fault
where:
t=0 is when the fault occurs
t=t
1
is when protection IED B1 and protection IED A1 start
t=t
2
is when the trip signal from the overcurrent protection at IED B1 is sent to the circuit breaker.
t=t
3
is when the circuit breaker at IED B1 opens. The circuit breaker opening time is t
3
- t
2
t=t
4
is when the overcurrent protection at IED A1 resets. The protection resetting time is t
4
- 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
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 29.
1MRK 505 370-UEN D Section 7
Current protection
Busbar protection REB670 151
Application manual
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