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2.14 Intermittent transient earth fault
protection I0T> (67NT)
2 Protection functions
Technical description
74
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VM50.EN004
Figure 2.14-1 Typical phase to earth voltages, residual current of the faulty
feeder and the zero sequence voltage U
0
during two transient earth faults in
phase L1. In this case the network is compensated.
Direction algorithm
The function is sensitive to the instantaneous sampled values
of the residual current and zero sequence voltage. The selected
voltage measurement mode has to include a direct
U
0
measurement.
I
0
pick-up sensitivity
The sampling time interval of the relay is 625
s at 50 Hz (32
samples/cycle). The I
0
current spikes can be quite short
compared to this sampling interval. Fortunately the current
spikes in cable networks are high and while the anti-alias filter
of the relay is attenuates the amplitude, the filter also makes
the pulses wider. Thus, when the current pulses are high
enough, it is possible to detect pulses, which have duration of
less than twenty per cent of the sampling interval. Although
the measured amplitude can be only a fraction of the actual
peak amplitude it doesn't disturb the direction detection,
because the algorithm is more sensitive to the sign and timing
of the I
0
transient than sensitive to the absolute amplitude of
the transient. Thus a fixed value is used as a pick up level for
the I
0
.
Co-ordination with U
0
> back up protection
Especially in a fully compensated situation, the zero sequence
voltage back up protection stage U
0
> for the bus may not
release between consecutive faults and the U
0
> might finally do
an unselective trip if the intermittent transient stage I
0T
>
doesn't operate fast enough. The actual operation time of the
I
0T
> stage is very dependent on the behaviour of the fault and
the intermittent time setting. To make the co-ordination
between U
0
> and I
0T
> more simple, the start signal of the