Functions
2.4 Backup Overcurrent
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
83
2.4.3 Directional Overcurrent Protection
Measured Quantities
The phase currents are fed to the device via the input transformers of the measuring input. The ground current
3I
0
is calculated from the phase currents.
For the directional Iph> elements, the used measuring voltage is determined by the fault type.
The current phase-to-ground voltage is used
• for 1-phase or 3-phase faults,
• if the phase-to-ground voltage is > 4 V,
• not within the first 50 ms after short-circuit inception as the present voltage is disturbed by transients during
this time.
The saved phase-to-ground voltage is used
• for 1-phase or 3-phase faults,
• up to max. 2 sec. after saving the phasors
• if there was not pickup before short-circuit occurrence.
The unfaulted phase-to-phase voltage is used
• for 1-phase faults
• for unfaulted phase-to-ground voltages
• if the voltage value is > 70% of the rated voltage.
The negative-sequence system quantities V
2
and I
2
are used
• for 1-phase or 2-phase faults
•if I
2
> 50 mA and V
2
> 5 V.
When using the negative-sequence system quantities, it is the short circuit with the higher current which deter-
mines the direction in case of two 1-phase short circuits.
If none of the above measured quantities is available, an already existing result of the direction determination
is used or the directional element is blocked for the corresponding phase.
The behavior in the even to a measuring voltage failure can be set. The elements operate directionally or non-
directionally.
The time overcurrent protection only operates directionally if all 3 phase-to-ground voltages are connected.
Address 144 must be set to connected here.
Directional Characteristic
The directional characteristic of the directional overcurrent elements is fixed. The angle difference Ï•(V
)-Ï•(I) is
calculated from the voltage phasors and current phasors using the impedance Z
= V/I and the direction is de-
termined using the depicted directional characteristic.
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