Siemens siprotec 7SA6 User Manual page 219

General
Settings
for Pick-Up
7SA6 Manual
C53000-G1176-C133-1
With the U/I(/ϕ) pick-up mode you have the option to determine the voltage measuring
and, if applicable, the phase-angle measuring for phase–to–earth measuring units,
and for phase–to–phase measuring loops separately. Address  352*$0 8,
states which loop voltages shall apply to phase–to–earth (3K( ) and which ones to
phase–to–phase (3K3K ).
In networks with earthed star point, a selection using U
with non-earthed faults is often preferred (address  352*$0 8, =
U
Ph–Ph
3K( 8SKH 3K3K 8SKSK). This mode has a maximum sensitivity for all fault
types, however it requires the unambiguous detection of earth faults via the earth-fault
detection function (also see Section 6.2.1). Otherwise, a mode using U
types may be useful (address  352*$0 8, = 3K( 8SKH3K3K 8SKH),
accepting lesser sensitivity for earth-free faults, as the overcurrent stage Iph>> usually
picks up here.
In networks with low–resistance earthed neutral point, the U/I/ϕ pick-up should only
come into effect on earth faults as phase–to–phase faults are detected by the
overcurrent pick-up. Therefore, the address  352*$0 8, =
3K( 8SKH3K3K ,!! is useful in this case.
As far as there is a danger of the earth fault detection picking up when a applicable
earth fault occurs in isolated or resonant-earthed networks owing to the starting
transient, this can be delayed using a parameter 7, 3+$6. If the earth fault pick-
up can also be exceeded during steady-state conditions, 7, 3+$6 (address
) should be set to ∞. As a result of this, pickup by one phase alone will not be
possible even with a large earth current. Double earth faults are however correctly
detected and measured according to the preference program (also see Section
6.2.3.1, "Double Earth-Faults in Non-earthed Systems").
In isolated or resonant-earthed networks it is possible to control the U/I/ϕ pick-up
using phase–to–phase voltages only (address  352*$0 8, =
3K( 8SKSK3K3K 8SKSK). Naturally, this excludes pick-up by single earth
faults, however, it also does not allow a correct double earth fault detection, therefore
it is suitable only for small isolated cable networks.
The maximum operational load current that can occur is crucial for the setting of
overcurrent pick-up. Pick-up due to overload must be ruled out! Therefore the pick-
up value must be set above the maximum (over-)load current that is expected (approx.
1.2 times). Then, it must be ensured that the minimum short-circuit current is above
this value. If this is not the case, U/I pick-up is required.
Arithmetic example:
Maximum operational current (incl. overload) is 680 A, for current transformers 600 A/
5 A, minimum short circuit is 1200 A. the following has to be set:
⋅ 1.2 = 680 A ⋅ 1.2 = 816 A
,SK!! = I
L max
This value is sufficiently below the minimum short-circuit current of 1200 A. When
configuring via PC and DIGSI
secondary quantities is
Iph>>
816 A
=
The condition for minimum short-circuit current also applies to earth faults (in the
earthed network) or for double earth faults as long as overcurrent pick-up is solely
used.
®
4 this value can be entered directly. Conversion to
5 A
⋅
--------------- -
= 6.8 A
600 A
Functions
with earth faults and
Ph–E
for all fault
Ph-E
6-37