Siemens 7SC80 Manual page 51

In addition to the bushing capacitances, the line and stray capacitances also affect the measured voltage fed
to the protection device. These capacitances are primarily determined by the type and length of the connec-
tion line.
The voltage inputs of the device have a resistive component of 1.2 MΩ.
Two capacitance values must be configured for each of the three voltage inputs when using capacitive voltage
measurement.
•
The first value to be configured is the bushing capacitance (C
•
The second value to be configured is the sum of the line and stray capacitance (C
The capacitances are configured as follows:
Phase A 241 Volt.trans.A:C1
242 Volt.trans.A:C2
Phase B 243 Volt.trans.B:C1
244 Volt.trans.B:C2
Phase C 245 Volt.trans.C:C1
246 Volt.trans.C:C2
Phase Vx 247 Volt.tran.Vx:C1
248 Volt.tran.Vx:C2
Boundary Conditions for the Capacitive Voltage Measurement
The voltages at the inputs of the protection devices are the result of the primary rated voltage, the capaci-
tances in the power system and the impedances of the voltage inputs which are taken into account. These
voltages can assume different values for 3 voltage inputs. The voltage V
mined using the following formula:
[7sc80-kapazitive-spgmess-24012013, 1, en_US]
with
V
prim, Lx
V
sec, Lx
C
D,Lx
C
S,Lx
f
The following figure represents the above equation graphically. The frequency is 50 Hz. With a frequency of
60 Hz, the ratio of secondary voltage to primary voltage is about 20 % higher than the values in this example.
The x-axis shows the value of the bushing capacitance. The y-axis shows the resulting ratio of secondary
voltage to primary voltage. As an additional parameter, the value C
and stray capacitance, is varied in the range from 2000 pF to 10 000 pF in increments of 500 pF.
SIPROTEC Compact, 7SC80, Manual
E50417-G1140-C486-A8, Edition 07.2017
= C
D,A
= C
S,A
= C
D,B
= C
S,B
= C
D,C
= C
S,C
= C
D,Vx
= C
S,Vx
Primary voltage of phase x
Voltage at the voltage input of the protection device
Value of the bushing capacitance for phase x
Value of the bushing capacitance for phase x
System frequency (50 Hz or 60 Hz)
).
D,Lx
).
S,Lx
for phase Lx can be deter-
secondary, Lx
, which is the sum of line capacitance
S,LX
Functions
2.1 General
51