Siemens siprotec 7SA522 - Page 167

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)DXOW2& = 8LQYHUVH) the operating mode is initially set: address 
2SPRGH,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  ,S3,&.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 8LQYPLQLPXP
(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. 8LQYPLQLPXP 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!8LQY (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!8LQY (line c).
An additional time  can be set for directional-controlled tripping at address 7
IRUZ8LQY that is added to the voltage-controlled characteristic.
With the non-directional time 7UHY8LQY (address ) a non-directional
back-up stage can be generated.