Siemens siprotec 7SA6 User Manual page 331

Definite Time
Overcurrent Stages
Iph>, 3I0>
7SA6 Manual
C53000-G1176-C133-1
s (length) = 60 km
= 0,19 Ω/km
R /s
1
= 0,42 Ω/km
X /s
1
Short circuit power at the beginning of the line:
S ' = 2,5 GVA
k
current transformers600 A/5 A
The line impedance Z and source impedance Z
L
follows:
/s = √0.19 2
Z + 0.42
1
= 0.46 Ω/km · 60 km
Z
L
2 2
110 kV
Z = ----------------------------- - =
S
2500 MVA
The three phase short circuit current at the end of the line is I
⋅
1.1 U
=
I ------------------------------------- -
F end
⋅ (
3 Z
+
S
With a safety margin of 10 % the resultant primary setting value is:
Set value I>> = 1.1 · 2150 A = 2365 A
or the secondary setting value:
Setting value I>> 1.1
=
i.e. if the fault current is greater than 2365 A (primary) or 19.7 A (secondary) the fault
is definitely on the protected feeder. This fault may be cleared immediately by the
overcurrent protection.
Comment : The calculation was carried out with scalar quantities which is sufficient for
overhead lines. If there is a large difference in the angle of the source and line
impedance, the calculation must be done with complex values.
An analogous calculation can be done for earth faults, whereby the maximum earth
fault current that flows during an earth fault at the end of the line is decisive.
The set time delays are pure additional delays, which do not include the operating time
(measuring time).
The parameter ,!! 7HOHS%, (address ) determines whether the delay times
7 ,SK!! (address ) and 7 ,!! (address ) may be bypassed via the
binary input "!2& ,QVW75,3" (F.No. ) or via the automatic reclose ready state.
The binary input (if assigned) is common to all stages of the overcurrent protection.
With the parameter ,!! 7HOHS%, = <HV it is determined that the I>>–stages trip
without delay following pick up if there is an operating signal present at the binary
input; if the setting is ,!! 7HOHS%, = 1R the set delay times always come into effect.
If the I>>-stage is to trip when switching the line on to a fault with or without a short
delay, 627) 7LPH '(/$< (address , see above and refer to Sub-section
"General"), the parameter ,!! 627) (address ) must be set to Yes . For this fast
switch on to a fault protection any other stage may also be selected.
For the setting of the current pick-up threshold ,SK! (address ), the maximum
operating current that can occur is decisive. Pick-up due to overload must be excluded
as the device operates as short-circuit protection with correspondingly short tripping
times and not as overload protection. The setting is therefore: on overhead lines
2 Ω/km = 0.46 Ω/km
= 27.66 Ω
4.84 Ω
⋅
1.1 110 kV
N
=
--------------------------------------------------------------- -
) ⋅ ( 4.84 Ω
Z 3
+
L
2150 A
⋅ ⋅
------------------ - 5 A 19.7 A
=
600 A
are calculated with these values as
S
:
F end
= 2150 A
27.66 Ω )
Functions
6-149