Functions
2.16 Backup Time Overcurrent Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
310
Calculation Example:
110 kV overhead line 150 mm
2
:
s (length) = 60 km
R
1
/s = 0.19 Ω/km
X
1
/s = 0.42 Ω/km
Short-circuit power at the beginning of the line:
S
k
' = 2.5 GVA
Current Transformer 600 A / 5 A
From that the line impedance Z
L
and the source impedance Z
S
are calculated:
Z
1
/s = √0.19
2
+ 0.42
2
Ω/km = 0.46 Ω/km
Z
L
= 0.46 Ω/km · 60 km = 27.66 Ω
The three-phase fault current at the line end is I
sc end
:
With a safety factor of 10 %, the following primary setting value is calculated:
Set value I>> = 1.1 · 2150 A = 2365 A
or the secondary setting value:
i.e. in case of fault currents exceeding 2365 A (primary) or 19.7A (secondary) you can be sure that a short-
circuit has occurred on the protected line. This fault can immediately be cleared by the time overcurrent pro-
tection.
Note
: the calculation was carried out with absolute values, which is sufficiently precise for overhead lines. If the
angles of the source impedance and the line impedance vary considerably, a complex calculation must be
carried out.
A similar calculation must be carried out for earth faults, with the maximum earth current occurring at the line
end during a short-circuit being decisive.
The set time delays are pure additional delays, which do not include the operating time (measuring time).
The parameter I>> Telep/BI (address 2614) defines whether the time delays T Iph>> (address 2611) and
T 3I0>> (address 2613) can be bypassed via the binary input „>O/C InstTRIP“ (No. 7110) or by the op-
erational automatic reclosure function. The binary input (if allocated) is applied to all stages of the time-over-
current protection. With I>> Telep/BI = YES, you define that the I>> stages trip without delay after pickup if
the binary input was activated. For I>> Telep/BI = NO, the set delays are always active.
Instantaneous tripping by the operational auto-reclosure function should only be chosen if the overcurrent pro-
tection is set to emergency function
. Since the fast main protection function - differential protection and/or dis-
tance protection - guarantees a fast and selective tripping with or without auto-reclosure, the overcurrent pro-
tection as a backup protection
may not perform a non-selective trip, even before auto-reclosure.
If the I>> stage, when switching the line onto a fault, is to re-trip without delay or with a short delay, SOTF Time
DELAY (address 2602, see above under margin heading „General“), the parameter I>> SOTF (address 2615)
is set to YES. Any other stage can be selected as well for this instantaneous tripping.
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