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Phase–Earth Loops
7SA6 Manual
C53000-G1176-C156-2
U
Lx
U
Ly
I
Lx
I
Ly
1202 Iph>
>
I
Lx
I
>
from state
Ly
recognition
Figure 6-23
Logic of the phase-phase measuring system
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
Z
U
I
–
I
=
L3
L
E
E
Z
is replaced by (Z
/Z
E
E
Z
E
⋅
⋅
⋅
------ -
I
Z
–
I
Z
L3
L
E
L
Z
L
From this the line impedance can be extracted
U
L3–E
Z
----------------------------------------- -
=
⁄
⋅
L
Z
Z
I
–
I
L3
E
L
I
L3
U
L3–E
I
E
Figure 6-24
Short circuit of a phase-earth loop
The factor Z
/Z
only depends on the line parameters and no longer on the fault dis-
E
L
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.
Measuring
System
L
–L
x
y
&
L3–E
)·Z
and the result is:
L
L
=
U
L3–E
E
Z
L
Z
E
Functions
R
; X
x–y
x–y
L1
L2
L3
E
6-43
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