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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 example, 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
observed. 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.
Phase-to-phase undervoltage
Basically, the same considerations apply as for the phase voltage stages. These 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 U<Alarm U<<Trip.
As phase-to-phase voltages are monitored, the phase-to-phase values are used for the settings Uph-ph< (ad-
dress 3762) and Uph-ph<< (address 3764).
The corresponding time delays are T Uph-ph< (address 3763) and T Uphph<< (address 3765).
The dropout to pickup ratio Uphph<(<) RESET can be set in address 3769. This parameter can only be
altered in DIGSI at Display Additional Settings.
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 positive sequence system U
The positive sequence undervoltage stages can be used instead of or in addition to previously mentioned un-
dervoltage stages. Depending on your choice, set address 3771 U1<(<) to ON, OFF, Alarm Only or
U<Alarm U<<Trip.
Basically, the same considerations apply as for the other undervoltage stages. Especially 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 transmission.
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 calculated according to its defining equation
U
=
·|U
+ a·U
1
/
1
L1
3
The dropout to pickup ratio U1<(<) RESET can be set in address 3779. This parameter can only be altered
in DIGSI at Display Additional Settings.
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.U1< (address 3778) 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.
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
1
2
+ a
·U
|. For symmetrical voltages this is equivalent to a phase-to-earth voltage.
L2
L3
2.19 Undervoltage and Overvoltage Protection (optional)
Functions
375
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