ABB RET650 Applications Manual page 122

Section 7
Current protection
116
Example for time coordination
Assume two substations A and B directly connected to each other via one line, as
shown in the Figure 47. 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 47.
A1
I>
t=0 t=t
1
The fault B1 and A1 B1 trips and
occurs start
IEC05000205 V2 EN-US
Figure 47: Sequence of events during fault
where:
t=0 is when the fault occurs
t=t is when protection IED B1 and protection IED A1 start
1
t=t is when the trip signal from the overcurrent protection at IED B1 is sent to the circuit breaker.
2
t=t is when the circuit breaker at IED B1 opens. The circuit breaker opening time is t
3
t=t is when the overcurrent protection at IED A1 resets. The protection resetting time is t
4
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 25.
t=t t=t
2 3
Breaker at
A1 starts B1 opens
1MRK 504 169-UEN A
B1
Fault
I>
t=t
4
Protection
A1 resets
=IEC05000205=2=en=Original.vsd
3
Transformer protection RET650 2.2 IEC
Application manual
Feeder
Time axis
- t
2
- t .
4 3
.
3