Siemens SIPROTEC 7SD80 Manual page 64

Functions
2.2 Phase Comparison Protection and Ground Differential Protection
Sensitive Ground Fault Direction Determination
The direction of the ground fault can be determined from the direction of the ground fault current in relation to
the displacement voltage. The only restriction is that the active or reactive current components must be avail-
able with sufficient magnitude at the point of measurement.
In networks with isolated starpoint, the ground fault current flows as capacitive current from the healthy lines
via the measuring point to the point of ground fault. For the determination of the direction the capacitive reactive
power is most relevant.
In networks with arc suppression coils, the Petersen coil superimposes a corresponding inductive current on
the capacitive ground fault current when a ground 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 in-
ductive or capacitive. Therefore, the reactive current is not suited for direction determination of the ground cur-
rent. In this case, only the ohmic (active) residual current which results from the losses of the Petersen coil can
be used for direction determination. This residual ground fault residual current is only about some per cent of
the capacitive ground fault current.
The active and reactive component of the power is decisive for the ground fault protection pickup.
A fault in forward direction must be detected at both ends of the protected object for the ground fault differential
protection to pick up.
In case of a single feeder, the residual current per watt at the opposite end of the infeed can be so weak that it
is impossible to determine the direction at that end. In this case, the amplitudes of the active currents of the two
ends are additionally compared to initiate pickup and localize the ground fault.
The amplitude of the active current (resonant-grounded system) and the reactive current (for isolated starpoint)
are included in the fault record. The local wattmetric ground current or reactive current is recorded as Iee1, the
wattmetric ground current or the reactive current of the opposite end as Iee2.
Pickup Logic
The following figure shows the pickup logic of the ground fault differential protection resonant-grounded or iso-
lated systems.
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SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011