2 Functions
100
7SD5 Manual
C53000-G1176-C169-1
I
C
= 3.63 · 10
–6
· U
N
· f
N
· C
B
' · s
with
I
C
Charging current to be calculated in A primary
U
N
Nominal voltage of the network in kV primary
f
N
Nominal frequency of the network in Hz
C
B
'
Per unit line length service capacitance of the line in nF/km or nF/mile
s
Length of the line in km or miles
For lines with multiple ends, the total sum of all line sections is taken as the length.
Considering the variations of voltage and frequency, the value set should be at least
2 to 3 times higher than the calculated charging current. Moreover, the pickup value
should not be less than 15% of the operating nominal current. The nominal operating
current either derives from the nominal apparent power of a transformer in the protect-
ed area (as described in 2.1.4.1 under margin heading „Topological Data for Trans-
formers (optional)“, or from the addresses
1104
FullScaleCurr.
according to
Section 2.1.4.1 under margin heading „Nominal Values of Protected Lines“. It must be
equal at all ends of the protected object.
If no voltages are measured, the compensated charging current is taken into account
in address
1224
IcSTAB/IcN
. This ratio is normally 2.5, so that the current set by it
is 2 to 3 times, or
IcSTAB/IcN
·
I-DIFF>
, of the determined charging current.
If setting is performed from a personal computer using DIGSI
®
, the parameters can be
set either as primary or as secondary quantities. If secondary quantities are set, all
currents must be converted to the secondary side of the current transformers.
Calculation Example:
110 kV single-conductor oil-filled cable 240 mm
2
in 50-Hz network with the following
data:
s (length) = 16 km
C
B
' = 310 nF/km
Current transformers 600 A/5 A
From that the steady-state charging current is calculated:
I
C
= 3.63 · 10
–6
· U
N
· f
N
· C
B
' · s = 3.63 · 10
–6
· 110 · 50 · 310 · 16 = 99 A
For the setting with primary values at least the double value is to be set, i.e.:
Setting value
I-DIFF>
= 200
Setting value with charging current compensation
I-DIFF>
= 100 A
For the setting with secondary values this value has to be converted to secondary
quantity:
If a power transformer with voltage regulation is installed within the protected zone
consider that a differential current may be present even during normal operation, de-
pendent on the position of the tap changer. Therefore it is essential to calculate the
maximum differential current at the end positions of the tap changer and to add this
current to the pickup value setting for
I-DIFF>
(referred to the nominal transformer
current).
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