Siemens siprotec 7SD5 User Manual page 337

The dropout to pickup ratio can be set in address . This parameter can only be altered
with DIGSI
The settings of the voltages and times depend on the intended use; therefore no
general recommendations for the settings can be given. For load shedding, for exam-
ple, the values are often determined by a priority grading coordination chart. In case
of stability problems, the permissible levels and durations of overvoltages must be ob-
served. With induction machines undervoltages have an effect on the permissible
torque thresholds.
If the voltage transformers are located on the line side, the measuring voltages will be
missing when the line is disconnected. To avoid that the undervoltage levels in these
cases are or remain picked up, the current criterion CURR.SUP. Uphe< (address
3758) is switched ON. With busbar side voltage transformers it can be switched OFF.
However, if the busbar is dead, the undervoltage protection will pick up and expire and
then remain in a picked-up state. It must therefore be ensured in such cases that the
protection is blocked by a binary input.
Undervoltage Basically, the same considerations apply as for the phase voltage stages. These
Phase–Phase stages may replace the phase voltage stages or be used additionally. Depending on
your choice, set address 3761 Uph-ph<(<) to ON, OFF, Alarm Only or .
As phase–to–phase voltages are monitored, the phase–to–phase values are used for
the settings Uph-ph< (address 3762) and Uph-ph<< (address 3764).
The corresponding times delay are T Uph-ph< (address 3763) and T Uphph<< (ad-
dress 3765).
The dropout to pickup ratio can be set in address . This parameter can only be altered
with DIGSI
If the voltage transformers are located on the line side, the measuring voltages will be
missing when the line is disconnected. To avoid that the undervoltage levels in these
cases are or remain picked up, the current criterion CURR.SUP.Uphph< (address
3768) is switched ON. With busbar side voltage transformers it can be switched OFF.
However, if the busbar is dead, the undervoltage protection will pick up and expire and
then remain in a picked-up state. It must therefore be ensured in such cases that the
protection is blocked by a binary input.
Undervoltage Posi- The positive sequence undervoltage stages can be used instead of or in addition to
tive Sequence previously mentioned undervoltage stages. Depending on your choice, set address
3771 U1<(<) to ON, OFF, Alarm Only or .
System U
1
Basically, the same considerations apply as for the other undervoltage stages. Espe-
cially in case of stability problems, the positive sequence system is advantageous,
since the positive sequence system is relevant for the limit of the stable energy trans-
mission.
To achieve the two-stage condition, the U1< stage (address 3772) is combined with a
greater time delay T U1< (address 3773), and the U1<< stage (address 3774) with a
shorter time delay T U1<< (address 3775).
Note that the positive sequence system is established according to its defining equa-
tion U
phase-earth voltage.
The dropout to pickup ratio can be set in address . This parameter can only be altered
with DIGSI
7SD5 Manual
C53000-G1176-C169-1
2.17 Undervoltage and Overvoltage Protection (optional)
®
under Additional Settings.
®
under Additional Settings.
1 2
= / ·|U + a·U + a ·U
1 3 L1 L2
®
under Additional Settings.
|. For symmetrical voltages this is equivalent to a
L3
337