ABB REC670 Applications Manual page 170

Section 8
Current protection
164
Example for time coordination
Assume two substations A and B directly connected to each other via one line, as
shown in the Figure 63. 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 63.
A1
51
t=0 t=t
1
The fault Protection
occurs B1 trips
ANSI05000205 V1 EN-US
Figure 63: 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 32.
B1
51
t=t t=t
2 3
Breaker at Protection
B1 opens A1 resets
1MRK 511 401-UUS A
Feeder
Time axis
en05000205_ansi.vsd
- t
3 2
- t .
4 3
.
3
Bay control REC670 2.2 ANSI
Application manual