Functions
2.3 Differential Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
103
2.3.2 Setting Notes
General
The differential protection can be 1201 STATE OF DIFF. ON or OFF. If a single device is switched off at any
end of the protected object, a calculation of measured values becomes impossible. The entire differential pro-
tection of all ends is then blocked. If the distance protection is available and configured, it performs the main
protection function.
Pickup value of differential current
The current sensitivity is set with address 1210 I-DIFF>. It is determined by the entire current flowing into a
protected zone in case of a short-circuit. This is the total fault current regardless of how it is distributed between
the ends of the protected object.
If the charging current compensation in address 1221 is switched Ic-comp. = ON, the pickup value I-DIFF>
can be set to 1 · I
cN
. Thus the residual error of the charging current compensation is considered.
Without charging current compensation (address 1221 Ic-comp. = OFF), this pickup value must be set to a
value that is higher than the total steady-state shunt current of the protected object. For cables and long over-
head lines, the charging current is to be considered in particular.
The charging current is calculated from the operational capacitance:
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
Nom
Nominal voltage of the network in kV primary
f
N
Nominal frequency of the network in Hz
C
B
' Per unit line length 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 set value 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 rated opera-
tional current. The rated operational current either derives from the nominal apparent power of a transformer
in the protected zone (as described in Section 2.1.4.1 under margin heading „Topological Data for Transformers
(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 identical 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.
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