Functions
2.3 Directional Gound Overcurrent Protection 67N
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
86
2.3.8 Determination of Direction
Basically, the direction determination is performed by determining the phase angle between the fault current
and a reference voltage.
Method of Directional Measurement
For the directional ground fault element there are two ways of direction determination:
• Direction determination with zero-sequence or ground quantities
• Direction determination with negative-sequence quantities
Direction Determination with Zero-sequence or Ground Quantities
For the directional ground fault elements, the direction can be determined from the zero-sequence system
quantities. In the current path, the I
N
current is valid when the transformer neutral current is connected to the
device. Otherwise, the device calculates the ground current from the sum of the three phase currents. In the
voltage path, the displacement voltage V
N
is used as reference voltage if connected. Otherwise, the device cal-
culates as reference voltage the zero-sequence voltage 3 · V
0
from the sum of the three phase voltages. If the
magnitude of V
0
or 3 · V
0
is not sufficient to determine direction, the direction is undefined. Then the directional
ground element will not initiate a trip signal. If measurement from the zero-sequence system is not possible,
e.g. because only two current transformers are connected, the directional ground element will work neither.
Direction Determination with Negative-sequence Quantities
Here, the negative sequence current and the negative sequence voltage as reference voltage are used to de-
termine the direction. This is advantageous if the zero sequence is influenced via a parallel line or if the zero
voltage becomes very small due to unfavorable zero sequence impedances. The negative sequence system
is calculated from the individual voltages and currents. As with using the zero sequence quantities, the direction
is only determined once the values required for direction determination have exceeded a certain minimum
threshold. Otherwise, the direction will remain undetermined.
When voltage transformers are open-delta-connected, direction determination is always based on the nega-
tive-sequence quantities.
Reference Voltages for Direction Determination
A single-phase fault (ground fault) is detected by the directional ground fault element. For the directional ground
fault elements, the connection requirements described previously must be met.
The following table shows the allocation of measured values for the determination of the fault direction for
various pickup conditions.
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