Siemens SIPROTEC 7SD610 Manual page 73

Pickup Value of the 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.
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 overhead lines, the charging current is to be considered in particular. It is calculated
from the operational capacitance:
–6
I = 3.63 · 10
C
with
I
C
U
Nom
f
N
C '
B
s
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 rated
current. The operating rated current either derives from the rated apparent power of a transformer in the pro-
tected area (as described in 2.1.4.1 under margin heading „topological data for transformers is described in the
protection zone (optional)", or from the addresses 1104 FullScaleCurr. according to Section 2.1.4.1 under
margin heading „Rated Values of Protected Lines". It must be equal at all ends of the protected object.
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
Cross section = 240 mm
Rated frequency f
Length s = 16 km
Service capacitance C
Current transformer, transformer ratio 600 A/5 A
From that the steady-state charging current is calculated:
–6
= 3.63 · 10
I
C
For the setting with primary values at least double the value is to be set, i.e.:
Setting value I-DIFF> = 200
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, dependent on the position of the tap changer. Please
also refer to the notes in chapter 2.1.2.1 Setting information, Margin heading „Power Transformer with Voltage
Regulation".
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011
· U · f · C ' · s
N N B
Charging current to be calculated in A primary
Nominal voltage of the network in kV primary
Nominal frequency of the network in Hz
Per unit line length service capacitance of the line in nF/km or nF/mile
Length of the line in km or miles
2
= 50 Hz
N
' = 310 nF/km
B
· U · f · C ' · s = 3.63 · 10
N N B
–6
· 110 · 50 · 310 · 16 = 99 A
Functions
2.3 Differential Protection
73