Siemens siprotec 7SA6 - Page 233

Functions
6-437SA6 Manual
C53000-G1176-C156-2
Figure 6-23 Logic of the phase-phase measuring system
Phase–EarthLoops For the calculation of the phase-earth loop, for example during a L3–E short-circuit
(Figure 6-24) it must be noted that the impedance of the earth return path does not
correspond to the impedance of the phase. In the loop equation
Z
E
is replaced by (Z
E
/Z
L
)·Z
L
and the result is:
From this the line impedance can be extracted
Figure 6-24 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 6.22) provides the corresponding block signal. A logic block diagram of
the phase-earth measuring system is shown in Figure 6-25.
I
Lx
I
Ly
I
Lx
>
I
Ly
>
&
Measuring
System
L
x
–L
y
from state
recognition
R
x–y
;X
x–y
U
Lx
U
Ly
1202 Iph>
I
L3
Z
L
I
E
Z
E
U
L3–E
=⋅–⋅
I
L3
Z
L
I
E
Z
L
Z
E
Z
L
-------
U
L3–E
=⋅⋅–⋅
Z
L
U
L3–E
I
L3
Z
E
Z
L
⁄ I
E
⋅–
------------------------------------------
=
I
L3
I
E
Z
L
Z
E
L1
L2
L3
E
U
L3–E