Functions
2.1 General
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
42
Optimizing the Configured Capacitance Values
In many cases the exact values for the bushing capacitance and for the line and stray capacitance will be un-
known. 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
D, x
) to be configured can still be opti-
mized later. This is based on the fact that an incorrect configuration of the bushing capacitance usually affects
the secondary voltage value and not so much the phase angle. We can gain insight into 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
S,x
) and input capacitance can still be optimized later. This is based on
the fact that these capacitances usually affect the phase angle of the secondary voltage and not so much the
amplitude. We can gain insight into 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 less 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.
Influence of the Capacitive Voltage Measurement
The following table shows how the capacitive voltage measurement affects the voltage-dependent device func-
tions.
Table 2-1 Influence of the capacitive voltage measurement
ATEX100 (Power System)
Parameter 235 ATEX100 enables meeting the requirements for protecting explosion-protected motors for
thermal replicas. Set this parameter to YES to save all thermal replicas of the 7SK80 devices in the event of a
power supply failure. After the supply voltage is restored, the thermal replicas will resume operation using the
stored values. Set the parameter to NO, to reset the calculated overtemperature values of all thermal replicas
to zero if the power supply fails.
Function Effect
Directional time overcurrent protection 67,
67N Ground
operational
Voltage protection 27, 59 operational
Please observe the increased tolerances of the measured voltage.
Ground fault detection 64, 50Ns, 67Ns The voltage elements are not available.
The current elements always operate non-directional.
Frequency protection 81 O/U operational
Flexible protection functions Operating modes that use the power are not available.
Fuse failure monitor not operational
Operational measured values Power and energy not available
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