Siemens siprotec 7SA6, Manual

Page: 228 / 876

Functions
6-38
7SA6 Manual
C53000-G1176-C156-2
and for phase–to–phase measuring loops separately. Address
1601 PROGAM U/I
states which loop voltages shall apply to phase–to–earth (
Ph-E:
) and which ones to
phase–to–phase (
Ph-Ph:
).
In networks with earthed star point, a selection using U
Ph–E
with earth faults and
U
Ph–Ph
with non-earthed faults is often preferred (address
1601 PROGAM U/I
=
Ph-E: Uphe/ Ph-Ph: Uphph
). 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
Ph-E
for all fault
types may be useful (address
1601 PROGAM U/I
=
Ph-E: Uphe/Ph-Ph: Uphe
),
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 overcur-
rent pick-up. Therefore, the address
1601 PROGAM U/I
=
Ph-E: Uphe/Ph-Ph: I>>
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 tran-
sient, this can be delayed using a parameter
T3I0 1PHAS
. If the earth fault pick-up
can also be exceeded during steady-state conditions,
T3I0 1PHAS
(address
1206
)
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
1601 PROGAM U/I
=
Ph-E: Uphph/Ph-Ph: Uphph
). 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.
General
Settings
for Pick-Up The maximum operational load current that can occur is crucial for the setting of over-
current pick-up. Pick-up due to overload must be ruled out! Therefore the pick-up val-
ue 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:
Iph>>
= I
L max
⋅
1.2 = 680 A
⋅
1.2 = 816 A
This value is sufficiently below the minimum short-circuit current of 1200 A. When con-
figuring via PC and DIGSI
®
4 this value can be entered directly. Conversion to secon-
dary quantities is
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.
If U/I pick-up is required because the minimum short-circuit current is below the max-
imum load current (incl. a safety factor of 1.2), the condition for maximum load current
in respect to
Iph>>
still has to be observed. Then, the minimum current limit
Iph>
Iph>>
816 A
5 A
600 A
----------------
⋅
=
= 6.8 A