Application; Earth Fault Directional Determination; Figure 1.1 Charging Process In Compensated Or Isolated Networks - Siemens 7SN71 Instruction Manual

Transient earth-fault relay 7SN71
1

Application

1.1

Earth fault directional determination

The highly sensitive tansient earth-fault relay 7SN71
determines the direction of transient and continuous
earth faults in systems with isolated neutral, in sy
stems with high-impedance resistive earthing and in
compensated systems.
The continuous earth-faults are only indicated in con
junction with an earth-fault directional discrimination
element.
In the event of an earth fault, the neutral-point vol
tage to earth can be as high as the full phase voltage.
The phase-to-earth capacitancies of the non-earth-
faulted phases are charged via the transformer induc
tance.
Figure 1.1 Charging process in compensated or isolated networks
Siemens AG March 1997
Instruction Manual Order No.: E50410-A0007-U501-A1-7691
This charging process is bound up with a strong cur
rent surge (starting oscillation).
The amplitude of this current surge depends on the
expands of the system and on contact resistance va
lues at the earth-fault location. This current flows via
the phase-to-earth capacitancies of the un-effected
lines to earth and enters the earth-faulted phase via
the earth-fault location.
The current flows via this line to the in-feed transfor
mers.
Thus the direction of the earth-fault induced current
surge is identical with that of the short-circuited cur
rent at the same location (see figure 1.1)
1
2
3
4
Ç Ç Ç Ç Ç Ç Ç Ç Ç Ç Ç Ç Ç Ç
Application
Ç Ç Ç Ç Ç Ç Ç Ç
1
1 - 1