Siemens siprotec 7SA6 User Manual page 345

Voltage Settings
Directional
Determination
7SA6 Manual
C53000-G1176-C133-1
The displacement voltage is the pickup threshold of the earth fault detection and is set
in address  8!.
If the displacement voltage U
the fourth voltage measuring input U
the configuration, the device will use this voltage, multiplied by the factor 8SK 
8GHOWD (address ). For the usual transformation of the voltage transformer with
e–n–winding
⁄ ⁄
U U U
Nprim Nsec
----------------- - --------------- - --------------- -
3 3
the factor is set to 1,73 (√3) (see also Subsection 6.1.1, margin heading "Voltage
Transformer Connection"). In case of a complete displacement of a healthy voltage
triangle the displacement voltage has a value that is √3 times the phase-to-phase
voltage.
If no displacement voltage is connected to the device, the device calculates the
monitored voltage from the total of the voltages:
3⋅U = |U + U + U
0 L1 L2
In case of a complete displacement of a healthy voltage triangle the displacement
voltage also has a value that is √3 times the phase-to-phase voltage.
Since, in case of earth faults in isolated or resonant-earthed systems, the complete
displacement voltage emerges, the setting value is uncritical; it should approx. be
between 40 % to 50 % of the displacement voltage: for U
70 V to 90 V.
The earth fault is only recognized and announced if the displacement voltage has
been "present" for the time set in 7 6HQV() (address ). This stabilizing period
is also enabled if earth fault conditions change (e.g. change of direction).
If tripping is also required for earth faults (address , 6HQV (DUWK )OW = 21),
a delay time can be set in address  7 8!
For phase determination 8SKH PLQ (address ) is the criterion for the earth-
faulted phase, when simultaneously the other two phase voltages have exceeded
8SKH PD[ (address ). Accordingly, 8SKH PLQ must be set lower than the
minimum operational phase-earth voltage. This setting is, however, also not critical,
40 V (factory setting) should always be adequate. 8SKH PD[ must lie above the
maximum operational phase-earth voltage, but below the minimum operational
phase-phase voltage, therefore, for example, 75 V at U
the faulty phase is a further precondition for annunciation of an earth fault.
For determination of the direction of the earth fault, in principle, the threshold current
,! (address ) should be set as high as possible to prevent faulty operation due
to asymmetrical currents in the network and the current transformers (particularly in
Holmgreen connection). Dependent upon the treatment of the network star point, the
magnitude of the capacitive earth fault current (for isolated networks) or the wattmetric
residual current (for compensated networks) is decisive.
In isolated networks an earth fault in a cable will allow the total capacitive earth fault
currents of the entire electrically connected network, with the exception of the faulted
cable itself, to flow through the measuring point. It is normal to use half the value of
this earth fault current as the threshold value.
Example: A 25 kV bus-bar feeds seven cable circuits. Each circuit has a current
transformer set 300 A/1 A. The earth fault current is 2.5 A/km. The cables might be as
follows:
of the voltage transformer set is directly connected to
en
of the device and if this was predefined during
4
Nsec
3
|.
L3
Functions
= 100 V therefore between
N
= 100 V. The identification of
N
6-163