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.
