Siemens siprotec 7SA6 User Manual page 224

Functions
Phase–Earth Loops
6-42
U
L1–E
U
L2–E
Figure 6-22 Short circuit of a phase-phase loop
The calculation of the phase-phase loop does not take place as long as one of the
concerned phases is switched off (during single-pole dead time), to avoid an incorrect
measurement with the undefined measured values existing during this state. A state
recognition (refer to Section 6.20) provides the corresponding block signal. A logic
block diagram of the phase-phase measuring system is shown in Figure 6-23.
U
Lx
U
Ly
I
Lx
I
Ly
!! Dƒu3
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
⋅ ⋅
I Z – I Z = U
L3 L E E
Z is replaced by (Z /Z
E E
Z
E
⋅ ⋅ ⋅
Z Z ------ -
I – I
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
Z
I
L1 L
Z
I
L
L2
Measuring
System
L –L
x y
&
L3–E
)·Z and the result is:
L L
U
=
L3–E
E
L1
L2
L3
E
R ; X
x–y x–y
7SA6 Manual
C53000-G1176-C133-1