Siemens siprotec 7SA522 - Page 66

2 Functions
66
7SA522 Manual
C53000-G1176-C155-3
Figure 2-12 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 2-13) 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
L3
· Z
L
– I
E
· Z
E
= U
L3-E
Z
E
is replaced by (Z
E
/Z
L
) · Z
L
yielding:
From this the line impedance can be extracted
Figure 2-13 Short-circuit of a phase-earth loop
The factor Z
E
/Z
L
solely depends on the line parameters and not on the fault distance.
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
provides the corresponding block signal. A logic block diagram of the phase-earth
measuring system is shown in Figure 2-14.