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3-36
Protection Functions
857-UM001A-EN-P – July 2009
Direction algorithm
The function is sensitive to the instantaneous sampled values of the
residual current and residual voltage. The selected voltage
measurement mode has to include a direct –U
I
pick-up sensitivity
0
The sampling time interval of the relay is 625 μs at 50 Hz (32
samples/cycle). The I
current spikes can be quite short compared to
0
this sampling interval. Fortunately the current spikes in cable
networks are high, and while the anti-alias filter of the relay
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
transient than sensitive to the absolute amplitude of
0
the transient. Thus a fixed value is used as a pick up level for the I
Co-ordination with U
> back up protection
0
Especially in a fully compensated situation, the zero sequence
voltage back up protection stage U
between consecutive faults and the U
unselective trip if the intermittent transient stage I
fast enough. The actual operation time of the I
dependent on the behavior of the fault and the intermittent time
setting. To make the co-ordination between U
simple, the start signal of the transient stage I
feeder can be used to block the U
Co-ordination with the normal directional earth fault protection
based on fundamental frequency signals
The intermittent transient earth fault protection stage I
always be used together with the normal directional earth fault
protection stages Iϕ>, Iϕ>>. The transient stage I
case detect the start of a steady earth fault in wrong direction, but
will not trip because the peak value of a steady state sine wave I
signal must also exceed the corresponding base frequency
component's peak value in order to make the I
The operation time and U
be higher than the settings of any Iϕ> stage to avoid any unnecessary
and possible incorrect start signals from the I
measurement.
0
> for the bus may not release
0
> might finally do an
0
> stage is very
0T
> and I
0
> in an outgoing
0T
> backup protection.
0
0T
> to trip.
0T
setting of the transient stage I
0
> stage.
0T
.
0
> doesn't operate
0T
> more
0T
> should
0T
> may in worst
0
> should
0T
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