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 95.
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
ANSI10000279-1-en.vsd
51
51
ANSI10000279 V1 EN
Figure 95: Sequence of events during fault
where:
• t=0 is when the fault occurs.
• t=t1 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
• t1, t=t2 is when the circuit breaker at IED B1 opens. The circuit breaker opening time
is t2 - t1.
• t=t3 is when the overcurrent protection at IED A1 resets. The protection resetting
time is t3 - t2.
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
110.
40 100 40 40 220t ms ms ms ms msD ³ + + + =
EQUATION1266 V1 EN (Equation 115)
where it is considered that:
Section 7 1MRK 506 334-UUS A
Current protection
198
Application manual
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