Siemens siprotec 7SD5, User Manual

Page: 276 / 698

2 Functions
276
7SD5 Manual
C53000-G1176-C169-1
When using a personal computer and DIGSI
®
to apply the settings, these can be op-
tionally entered as primary or secondary values. If secondary quantities are used, all
currents must be converted to the secondary side of the current transformers.
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
F 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 immedi-
ately be cleared by the time overcurrent protection.
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 by the binary
input
„>O/C InstTRIP“
(No 7110) or by the operational automatic reclosure func-
tion. The binary input (if allocated) is applied to all stages of the time overcurrent pro-
tection. 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 protection is set to emergency function. Since the fast main
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