2.14 Intermittent transient earth fault
protection I0T> (67NT)
2 Protection functions
Technical description
76
VAMP 24h support phone +358 (0)20 753 3264
VM50.EN004
When the time between successive faults is more than 40 ms,
the stage will release between the faults and the delay counting
is restarted from zero for every single fault and no trip will be
issued. For such cases the intermittent setting can be used.
Figure 2.14-2 shows an example of how the intermittent setting
works. The upper start and trip signals are a case with zero
intermittent setting. The lower signals are another case with
intermittent setting 0.12 s. The operation time setting is 0.14 s
in both cases corresponding to seven 20 ms time slots with
faults.
The time between the second and the third fault exceeds the
release time + intermittent time. Thus the operation delay
counter is cleared in both cases: with zero intermittent time
and with 0.12 s intermittent time.
The fourth and the next faults do occur after release time but
within release time + intermittent time. Thus the operation
delay counter is advanced at every fault in the case the
intermittent time setting is more than 100 ms (the lower status
lines in the figure) and finally a trip signal is issued at t=0.87 s.
When faults do occur more than 20 ms apart each other, every
single fault will increment the operation delay counter by 20
ms. In this example the actual operation time starting from the
third fault will be 617 ms although, the setting was 140 ms. In
case the intermittent setting would have been 0.2 s or more, the
two first faults had been included and a trip would have issued
at t=0.64 s.
Figure 2.14-2. Effect of the intermittent time parameter. The operation
delay setting is 0.14 s = 7x20 ms. The upper start and trip status lines are
for a case with the intermittent time set to zero. No trip will happen. The
lower start and trip status lines show another case with intermittent time
setting 0.12 s. In this case a trip signal will be issued at t=0.87 s.
