Figure 1.2 Direction Of The Displacement Voltage And The Earth Current (Earth-Fault Isolates At T1) - Siemens 7SN71 Instruction Manual

1
Application
Transient earth-fault relay 7SN71
At measuring point A, as a result of the transformer
summation circuit, the earth current of the faulted line
is not included in the measurement, as this current
portion flows through the summation transformer of
the relevant Holmgreen circuit and back, thereby can
celling itself out. It is the total of the capacitative
earth currents from the non-faulted system which
has an effect. In the diagram they are summated on
the upper line. The capacitative currents of the non-
faulted lines 1, 3 and 2, 4 accumulate vectorially,
which is why only three arrows instead of four are
shown at measuring point A.
With a transient earth-fault relay, the equalizing cur
rent forming with a damped oscillation of 100 to more
than 1 000 Hz decays after only a few periods.
Earth fault
beginning
end
V
E
I
0
t
1
Figure 1.2 Direction of the displacement voltage and the earth current (earth-fault isolates at t
Examples of typical applications:
If the system is of radial configuration (figure 1.3 a),
the red lamp immediately indicates the faulted line.
If one of the lines consists of several sections (figure
1.3 b), the fault is upstream of the last red lamp.
The transient earth-fault relay can also be used wi
thout restrictions in any type of meshed network
1 - 2
Instruction Manual Order No.: E50410-A0007-U501-A1-7691
The displacement voltage V
to zero. In earthed systems this takes place after a
number of periods (decay of the Petersen coil - earth
capacitance oscillation circuit); in non-earthed sy
stems this occurs after a very short time (figure 1.2).
In the case of a continuous earth fault, the equalizing
current in the non-earthed system changes into the
mostly capacitive continuous earth current or, in com
pensated systems, into the relatively low residual ac
tive current.
(figures 1.3 c and 1.3 d). The directional indications
clearly identify the fault location.
Transient earth-fault relays distributed at suitable
points throughout the system allow detection of the
earth-fault location from the directional indications
(see examples in figure 1.3).
thereupon also returns
E
t
) in isolated systems
1
Siemens AG March 1997