Siemens siprotec 7SD5 User Manual page 116

2 Functions
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,
however, it requires that the earth–fault criteria according to Section Earth Fault De-
tection are met for all earth faults or double earth faults (see Table 2-7).
Table 2-7
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 Iph>> 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 measures (so-called semi–solid earth-
ing). In these cases only earth faults must be detected by the U/I pickup. In such net-
works it is usually even undesirable that phase-phase faults lead to a U/I pickup.
The measuring loop is independent of the setting 1ph FAULTS. Table 2-8 shows the
assignment of phase currents, loop voltages and measuring results.
Table 2-8
The pickup signals of the loops are converted into phase signals so that the faulted
phase(s) can be indicated. If an earth fault has been detected, it will also be alarmed.
Voltage and Angle- Phase-angle controlled U/I pickup can be applied when the U/I characteristic criteria
dependent Current can no longer distinguish reliably between load and short-circuit conditions. This is the
Pickup U/I/ϕ case with small source impedances together with long lines or a sequence of lines and
intermediate infeed. Then the local measured voltage will only drop to a small extent
in the event of a short-circuit at the line end or in the back-up range of the distance
protection so that the phase angle between current and voltage is required as an ad-
ditional criterion for fault detection.
The U/I/ϕ pickup is a per phase and per loop pickup mode. It is crucial for the phase
currents to exceed the pickup threshold, with the pickup value being dependent on the
size of the loop voltages and the phase angle between current and voltage.
116
Loops and phase indications for single-phase overcurrent pickup U I; Phase-to-
earth-voltages program for earth fault, phase-to-phase voltages without earth
fault
Pickup Measur- Measur-
Module ing ing
Current Voltage
L1 L1 L1-L2
L2 L2 L2-L3
L3 L3 L3-L1
L1 L1 L1-E
L2 L2 L2-E
L3 L3 L3-E
Loops and phase indications for single-phase overcurrent pickup U I; Phase-to-
earth-voltages program for earth fault, I>> without earth fault
Pickup Measur- Measur-
Module ing ing
Current Voltage
L1 L1 L1-E
L2 L2 L2-E
L3 L3 L3-E
L1 L1 L1-E
L2 L2 L2-E
L3 L3 L3-E
Earth Fault Parameter
Detection 1ph FAULTS
No
No any
No
Yes
Yes any
Yes
Earth Fault Parameter
Detection 1ph FAULTS
Yes
Yes any
Yes
No
No any
No
Valid Alarmed
Loop Phase(s)
L1-L2 L1, L2
L2-L3 L2, L3
L3-L1 L1, L3
L1-E L1, E
L2-E L2, E
L3-E L3, E
Valid Alarmed
Loop Phase(s)
L1-E L1, E
L2-E L2, E
L3-E L3, E
no pickup
no alarm
via U </I>
Ph-E
7SD5 Manual
C53000-G1176-C169-1