Functions
2.5 Distance Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
126
In the faulted loop
the voltage U
L3-E
, the phase current I
L3
and the earth current I
E
are measured. The impedance to the fault lo-
cation results from:
and
with
U
L3-E
= r.m.s.value of the short-circuit voltage
I
L3
= r.m.s. value of the phase short-circuit current
I
E
= r.m.s. value of the earth short-circuit current
Ï•
U
= phase angle of the short-circuit voltage
Ï•
L
= phase angle of the phase short-circuit current
Ï•
E
= phase angle of the earth short-circuit current
The factors R
E
/R
L
and X
E
/X
L
are dependent only on the line constants, and no longer on the distance to fault.
The calculation of the phase-to-earth loops does not take place as long as the concerned phase is switched off
(during single-pole dead time) to avoid an incorrect measurement with the now undefined measured values. A
state recognition provides the corresponding blocking signal. A logic block diagram of the phase-to-earth mea-
suring system is shown in Figure 2-46.
Figure 2-46 Logic for a phase-to-earth measuring unit
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