Siemens 7SC80 Manual page 53

Optimizing the Configured Capacitance Values
In many cases, the exact values for the bushing capacitance and for the line and stray capacitance will be
unknown. Besides that, the capacitance of the voltage inputs has a tolerance of ±20 %.
These uncertainties can cause amplitude and phase errors of the measured voltage.
If the primary voltage is known, the value for the bushing capacitance (C
mized later. This is based on the fact that an incorrect configuration of the bushing capacitance usually affects
more the secondary voltage value and less the phase angle. We can gain information on the amplitude error
by comparing the primary phase-to-ground voltages in the operational measured values with their setpoint
values. The value of the configured bushing capacitance should be increased by the percentage that the
displayed phase-to-ground voltage is too large or it should be reduced by the percentage that the displayed
phase-to-ground voltage is too small.
If the phase angle of the primary voltage to the primary current is known, the setting value for the sum of line
capacitance and stray capacitance (C
fact that these capacitances usually affect the phase angle of the secondary voltage and not so much the
amplitude. We can gain information on the phase errors by comparing the phase angles (φ A, φ B and φ C) in
the operational measured values with their setpoint values. The configured value must be corrected by 4 %
per degree angle error (actual angle minus desired angle). If the angle error is positive, the configured value
has to be reduced accordingly; if the angle error is negative it has to be increased accordingly. The prerequisite
for the phase angle between phase-to-ground voltage and phase current to be displayed is that the current
amounts to at least 10 % of the nominal value.
The optimization steps for the capacitances to be configured for each voltage channel may have to be
repeated until the desired accuracy has been achieved.
Impact of the Capacitive Voltage Measurement
Capacitive voltage measurement can cause increased tolerances for all voltage-dependent protection func-
tions due to an unfavorable capacitance ratio to the rated voltage.
Nominal Values of Current Transformers (CTs)
At addresses 204 CT PRIMARY and 205 CT SECONDARY information is entered regarding the primary and
secondary ampere ratings of the current transformers. It is important to ensure that the rated secondary
current of the current transformer matches the rated current of the device, otherwise the device will calculate
incorrect primary data. At addresses 217 Ignd-CT PRIM and 218 Ignd-CT SEC, information is entered
regarding the primary and secondary ampere rating of the current transformers. In case of a normal connec-
tion (starpoint current connected to Ι
to the same value.
In US device versions (MLFB position 10 = C), parameter 205 and parameter 218 are preset to 5 A.
Amplitude Correction of the Current Transformers
You can enter the phase-specific correction factor for the current transformer amplitudes at address 340 CT A
AMPL COR, address 341 CT B AMPL COR and address 342 CT C AMPL COR. At address 343 IGND AMPL
COR, enter the correction factor for the ground current.
This allows system-related deviations of current measurement to be compensated for.
Correction of the Phase Sequence for the Current Transformers
You can adapt the relation between the current measurands and the physical measuring inputs to the actual
conditions at address 337 CT Ph. Connect..
Nominal Values of Voltage Transformers (VTs)
At addresses 202 Vnom PRIMARY and 203 Vnom SECONDARY, information is entered regarding the primary
nominal voltage and secondary nominal voltage (phase-to-phase) of the connected voltage transformers.
At addresses 333 Vnom prim Vx and 325 Vnom sec Vx, you can enter the primary and secondary rated
voltage for the 4th voltage input Vx.
SIPROTEC Compact, 7SC80, Manual
E50417-G1140-C486-A8, Edition 07.2017
) and input capacitance can still be optimized later. This is based on the
S,Lx
transformer), 217 Ignd-CT PRIM and 204 CT PRIMARY must be set
N
) to be configured can still be opti-
D,Lx
Functions
2.1 General
53