Siemens siprotec 7SA6 User Manual page 342

Functions
Sensitive Earth
Fault Directional
Determination
6-160
The direction of the earth fault can be determined from the direction of the earth fault
current in relation to the displacement voltage. The only restriction is that the active or
reactive current components must be available with sufficient magnitude at the point
of measurement.
In networks with isolated starpoint, the earth fault current flows as capacitive current
from the healthy lines via the measuring point to the point of fault. This capacitive
current determines the direction.
In networks with arc suppression coils, the Petersen coil superimposes a
corresponding inductive current on the capacitive earth fault current when an earth
fault occurs, so that the capacitive current at the point of fault is compensated.
Dependent upon the point of measurement in the network the resultant measured
current can however be inductive or capacitive and the reactive current is therefore
not suitable for the determination of direction. In this case, only the ohmic (active)
residual current which results from the losses of the Petersen coil can be used for
directional determination. This earth fault ohmic current is only a few percent of the
capacitive earth fault current.
In 7SA6 the earth fault direction is determined from a highly accurate calculation of
active and reactive power using the definitions:
t T
+
1
∫
⋅ ⋅
-- -
P = u t ( ) i
E E E
T
t
where T equals period of integration.
The use of an efficient calculation algorithm and simultaneous numerical filtering
allows the directional determination to be achieved with high accuracy and sharply
defined threshold limits (see Figure 6-89) and insensitivity to harmonic influences —
particularly the third and fifth harmonics which are often large in earth fault currents.
The directional decision results from the signs of active and reactive power.
Reverse
I 90° 60°
Er
Figure 6-89 Directional earth fault measurement characteristic in a resonant-earthed system
⋅
t ( ) t d and
U
I
Ea,
300 mA
Forward
10 mA
3 mA
30° 0°
t T
+
π
1
∫
⋅
-- - -- -
Q = u t –
(
E E
T 2
t
E
Forward
–30° –60° –90°
C53000-G1176-C133-1
⋅ ⋅
) i t ( ) t d
E
Reverse
7SA6 Manual