Siemens SIPROTEC 4 User Manual page 94

Functions
2.3 Directional Overcurrent Protection 67, 67N
Otherwise the device calculates as reference voltage the zero sequence voltage 3 · V
three phase voltages. If the magnitude of V
undefined. Then the directional ground element will not initiate a trip signal. If the current Ι
mined, e.g. because only two current transformers are utilized or the current transformers are connected in an
open delta configuration, then the directional ground elements will not be able to function. The latter is only
permitted in ungrounded systems.
Direction Determination with Negative Sequence System
Here, the negative sequence current and as reference voltage the negative sequence voltage are used for the
direction determination. 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 impedances. The negative sequence system is calculated
from the individual voltages and currents. As with the use of the zero sequence values, a direction determina-
tion is carried out if the values necessary for the direction determination have exceeded a minimum threshold.
Otherwise the direction is undetermined.
Cross-Polarized Reference Voltages for Direction Determination
The direction of a phase-directional element is detected by means of a cross-polarized voltage. In a phase-to-
ground fault, the cross-polarized voltage (reference voltage) is 90° out of phase with the fault voltages (see
Figure 2-28). With phase-to-phase faults, the position of the reference voltages changes, depending on the
degree of collapse of the fault voltages, up to 30°.
[kurzschlussfremde-spannungen-fuer-richtungsbestimmung-260602-kn, 1, en_US]
Figure 2-28
Measured Values for the Determination of Fault Direction
Each phase has its own phase measuring element. The fourth measuring element is used as ground measuring
element. If the current exceeds the pickup threshold of a phase or that of the ground path, direction determi-
nation is started by the associated measuring element. In case of a multiphase fault, all phase measuring
elements involved perform their own direction determination. If one of the calculated directions differs from
the set direction, the function picks up.
The following table shows the allocation of measured values for the determination of fault direction for
various causes of pickup.
Table 2-7
Pickup Measuring element
A
Current
A
Ι
A
B —
C —
94
Cross-polarized voltages for direction determination
Measured Values for the Determination of Fault Direction
B
Voltage Current
V - V —
B C
—
Ι
B
— —
or 3 · V is not sufficient to determine direction, the direction is
0 0
C
Voltage Current
— —
V - V —
C A
—
Ι
C
from the sum of the
0
cannot be deter-
0
ground
Voltage Current Voltage
— — —
— — —
V - V — —
A B
SIPROTEC 4, 7SJ62/64, Manual
C53000-G1140-C207-8, Edition 08.2016