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Functions
2.15 Sensitive Earth Flt.(comp/ isol. starp.)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
296
Sensitive Earth Fault Directional Determination
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. For the determination of the direction the capacitive reactive power is
most relevant.
In networks with arc suppression coils, the Arc-suppression 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. Depending on the measuring point in the system the resultant measured current
may be inductive or capacitive. Therefore, the reactive current is not suitable for direction determination of the
earth current. In this case, only the ohmic (active) residual current which results from the losses of the arc-sup-
pression coil can be used for directional determination. This earth fault residual current is only about some per
cent of the capacitive earth fault current.
Following the phase determination the earth fault direction is determined from a highly accurate calculation of
active and reactive power. The following definitions are used for this purpose:
where T equals period of integration.
The use of an efficient calculation algorithm and simultaneous numerical filtering allows the directional deter-
mination to be achieved with high accuracy and sharply defined threshold limits (see Figure 2-149) and insen-
sitivity 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.
Figure 2-149 Measurement characteristic of the sensitive direction determination for earth fault in a reso-
nant-earthed system
Since the active and reactive component of the current – not the power – determine the earth fault directional
decision, these current components are calculated from the power components. Thus for determination of the