2.15 Undervoltage and overvoltage protection (optional)
283
7SA522 Manual
C53000-G1176-C155-3
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 &8556838SKH (address
) 21 is switched on. With busbar side voltage transformers it can be switched
2)). 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
Phase–Phase
Basically, the same considerations apply as for the phase undervoltage stages. These
stages may replace the phase voltage stages or be used additionally. Depending on
your choice, you set address 8SKSK to 21, 2)) or $ODUP2QO\.
As phase–to–phase voltages are monitored, the phase–to–phase values are used for
the settings 8SKSK (address ) and 8SKSK (address ).
The corresponding times delay are 78SKSK (address ) and 78SKSK (ad-
dress ).
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 &8556838SKSK (address
) 21 is switched on. With busbar side voltage transformers it can be switched
2)). However, if the busbar is dead, the undervoltage protection will pick up and
expire and then remain in picked-up state. It must therefore be ensured in such cases
that the protection is blocked by a binary input.
Undervoltage Posi-
tive Sequence
System U
1
The positive sequence undervoltage stages can be used instead of or in addition to
previously mentioned undervoltage stages. Depending on your choice, you set
address 8 to 21, 2)) or $ODUP2QO\.
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 8 stage (address ) is combined with a
greater time delay 78 (address ), and the 8 stage (address ) with a
shorter time delay 78 (address ).
Please note that the positive sequence system is established according to its defining
equation U
1
=
1
/
3
·|U
L1
+a·U
L2
+a
2
·U
L3
|. For symmetrical voltages this is equivalent to
a phase-earth voltage.
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 &8556838 (address )
21 is switched on. With busbar side voltage transformers it can be switched 2)). How-
ever, if the busbar is dead, the undervoltage protection will pick up and expire and then
remain in picked-up state. It must therefore be ensured in such cases that the protec-
tion is blocked by a binary input.
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