Siemens siprotec 7SA522, User Manual

Page: 183 / 628

Functions
6-35
7SA522 Manual
C53000-G1176-C119-2
Figure 6-21 Short circuit of a phase-earth loop
The factor Z
E
/Z
L
only depends on the line parameters and no longer on the fault dis-
tance.
The evaluation of the phase-earth loop does not take place as long as the affected
phase is switched off (during single-pole dead time), to avoid an incorrect measure-
ment with the undefined measured values existing in this state. A state recognition (re-
fer to section 6.17) provides the corresponding block signal. A logic block diagram of
the phase-earth measuring system is shown in Figure 6-22.
Figure 6-22 Logic of the phase-earth measuring system
Unfaulted Loops The above considerations apply to the relevant short-circuited loop. However, as all
six loops can be equated, the impedances of the unfaulted loops are also influenced
by the short-circuit currents and voltages in the short-circuited phases. During a L1–E
fault for example, the short-circuit current in phase L1 also appears in the measuring
loops L1-L2 and L3-L1. The earth current is also measured in the loops L2–E and L3–
E. Combined with load currents which may flow, the unfaulted loops produce the so-
called “apparent impedances“, which have nothing to do with the actual fault distance.
These “apparent impedances” in the unfaulted loops are usually larger than the short-
circuit impedance of the faulted loop because the unfaulted loop only carries a part of
the fault current and always has a larger voltage than the faulted loop. For the selec-
tivity of the zones, the “apparent impedances” are therefore of no consequence.
Apart from the
zone selectivity
, the
phase selectivity
is also important to achieve cor-
rect identification of the faulted phases, required to alarm the faulted phase and espe-
cially to enable single-pole automatic reclosure. Depending on the infeed conditions,
I
L3
I
E
Z
L
Z
E
L1
L2
L3
E
U
L3–E
I
Lx
I
E
I
Lx
>
&
measur-
ing syst.
L
x
–E
from state recogni-
tion
R
x–E
; X
x–E
U
Lx
I
EP
earth fault recogni-
tion
,SK!
(parallel line)