Hitachi Relion 670 Series Applications Manual page 124

Section 8
Current protection
possible time difference, the operation time of the protection, the breaker opening time and the protection
resetting time must be known. These time delays can vary significantly between different protective
equipment. The following time delays can be estimated:
Protection operation time: 15-60 ms
Protection resetting time:
Breaker opening time:
Example for time coordination
Assume two substations A and B directly connected to each other via one line, as shown in the Figure
57. 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 57.
A1
t=0
The fault
occurs
ANSI05000205 V1 EN-US
Figure 57:
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
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 40.
118
15-60 ms
20-120 ms
51
t=t
1
Protection Breaker at
B1 trips B1 opens
Sequence of events during fault
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B1
51
t=t t=t
2 3
Protection
A1 resets
3
Feeder
Time axis
en05000205_ansi.vsd
- t
3 2
- t .
4 3
. There are uncertainties in the values of
Phasor measurement unit RES670
1MRK511407-UUS Rev. N
Application manual