2.7 Earth fault overcurrent protection in earthed systems (optional)
167
7SA522 Manual
C53000-G1176-C155-3
Figure 2-72 Curve parameters in the logarithmic–inverse characteristic
Zero Sequence
Voltage Stage with
Inverse
Characteristic
If you have configured the zero sequence voltage controlled stage (address
(DUWK)DXOW2& = 8LQYHUVH) the operating mode is initially set: address
2SPRGH,S. The stage can be set to operate )RUZDUG (usually towards line),
5HYHUVH (usually towards busbar) or 1RQ'LUHFWLRQDO (in both directions). If the
stage is not required, set its mode to ,QDFWLYH.
Address ,S3,&.83 indicates the minimum current value above which this
stage is required to operate. The value must be exceeded by the minimum earth fault
current value.
The voltage-controlled characteristic is based on the following formula:
U
0
is the actual zero sequence voltage. U
0 min
is the setting value 8LQYPLQLPXP
(address ). Please take into consideration that the equation is based on the zero
sequence voltage U
0
, not on 3U
0
. The function is illustrated in the Technical Data.
Figure 2-73 shows the most important parameters. 8LQYPLQLPXP displaces the
voltage-controlled characteristic in direction of 3U
0
. The set value is the asymptote for
this characteristic (t → ∞). In Figure 2-73 a' shows an asymptote that belongs to the
characteristic a.
The minimum voltage 8!8LQY (address ) is the lower voltage threshold.
It corresponds to the line c in Figure 2-73. In characteristic b (asymptote not drawn)
the curve is cut by the minimum voltage 8!8LQY (line c).
An additional time can be set for directional-controlled tripping at address 7
IRUZ8LQY that is added to the voltage-controlled characteristic.
With the non-directional time 7UHY8LQY (address ) a non-directional
back-up stage can be generated.
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