Siemens siprotec 7SA6 User Manual page 216

Functions
6-34
When evaluating phase–phase loops, the sensitivity towards phase–phase faults is
particularly high. In extensive compensated networks this selection is advantageous
because it excludes pick-up as a result of single earth faults on principle. With two-
and three-phase faults it automatically adapts to the prevailing infeed conditions, i.e.
in the weak-infeed operation mode it becomes more current-sensitive, with strong
infeed and high load currents the pick-up threshold will be higher. If only phase–phase
faults are evaluated, the measuring loop is independent of the earth-fault detection,
therefore this procedure is not suitable for earthed networks (see Table 6-4).
Table 6-4 Loop and phase indication for single–phase
Phase–phase voltage program (address )
Pick-up Measured Measured
module current
L1 L1
L2 L2
L3 L3
If the option has been chosen whereby voltage loop selection is dependent on earth-
fault detection, then high sensitivity applies to phase-earth faults and to phase–phase
faults. On principle, this option is independent of the treatment of the network neutral
point, however, it requires that the earth–fault criteria according to Section 6.2.1 are
met for all earth faults or double earth faults (see Table 6-5).
Table 6-5 Loop and phase indication for single–phase
Phase–phase voltage program in the event of earth faults, phase–phase
voltages without earth faults (address )
Pick-up Measured Measured
module current
L1 L1
L2 L2
L3 L3
L1 L1
L2 L2
L3 L3
Finally, it is also possible to only evaluate phase-earth voltage loops if an earth fault
has been detected. For phase–phase faults only the overcurrent ,SK!! will then pick
up. This is advantageous in networks with neutral points that have been earthed low–
resistance, i.e. using earth-fault current limiting means (so-called "semi–solid"
earthing). In these cases only earth faults shall be detected by the U/I pick-up. In this
network it is often undesirable that phase–phase short-circuits shall result in a U/I pick-
up.
The measuring loop is independent of the setting SK )$8/76
6-6 shows the assignment of phase currents, loop voltages and measuring results.
Earth fault
voltage detection
L1–L2
L2–L3 any
L3–L1
Earth–fault
voltage detection
L1–L2 no
L2–L3 no
L3–L1 no
L1–E yes
L2–E yes
L3–E yes
U/I pick-up
;
Parameter Valid
3K )$8/76
loop
L1–L2
any L2–L3
L3–L1
U/I pick-up
;
Parameter Valid
3K $15
loop
L1–L2
any L2–L3
L3–L1
L1–E
any L2–E
L3–E
(address $)
C53000-G1176-C133-1
Signalled
Phase(s)
L1, L2
L2, L3
L1, L3
Signalled
Phase(s)
L1, L2
L2, L3
L1, L3
L1, E
L2, E
L3, E
. Table
7SA6 Manual