Siemens siprotec 7SA522 - Page 182

Functions
6-34 7SA522 Manual
C53000-G1176-C119-2
recognition (refer to Section 6.17) provides the corresponding block signal. A logic
block diagram of the phase-phase measuring system is shown in Figure 6-20.
Figure 6-20 logic of the phase-phase measuring system
Phase–Earth Loops For the calculation of the phase-earth loop, for example during a L3–E short-circuit
(Figure 6-21) 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
I
Lx
I
Ly
I
Lx
>
I
Ly
>
&
Measur-
ing syst.
L
x
–L
y
from state recogni-
tion
R
x–y
; X
x–y
U
Lx
U
Ly
 ,SK!
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
⋅–
------------------------------------------
=