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Technical description
2 Protection functions
2.7 Directional overcurrent protection
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VM259.EN007
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2.7.
Directional overcurrent protection
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Directional overcurrent protection can be used for directional
short circuit protection. Typical applications are
Short circuit protection of two parallel cables or overhead
lines in a radial network.
Short circuit protection of a looped network with single
feeding point.
Short circuit protection of a two-way feeder, which usually
supplies loads but is used in special cases as an incoming
feeder.
Directional overcurrent protection in low impedance earthed
networks. Please note that in this case the device has to
connected to line-to-neutral voltages instead of line-to-line
voltages. In other words the voltage measurement mode has
to be "3LN" (See chapter 4.7).
The stages are sensitive to the amplitude of the highest
fundamental frequency current of the three measured phase
currents. The phase angle is based on the phase angle of the
three-phase power phasor. For details of power direction see
chapter 4.9. A typical characteristic is shown in Figure 2.7-1.
The base angle setting is –30°. The stage will pick up, if the tip
of the three phase current phasor gets into the grey area.
NOTE! If the maximum possible earth fault current is greater than the used most
sensitive directional over current setting, the device has to be connected
to the line-to-neutral voltages instead of line-to-line voltages in order to
get the right direction for earth faults, too. (For networks having the
maximum possible earth fault current less than the over current setting,
use 67N, the directional earth fault stages.)
Figure 2.7-1 Example of protection area of the directional overcurrent
function.