2.17 Undervoltage and Overvoltage Protection (optional)
337
7SD5 Manual
C53000-G1176-C169-1
The dropout to pickup ratio can be set in address . This parameter can only be altered
with DIGSI
®
under
Additional Settings
.
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
Phase–Phase
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 .
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
®
under
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 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, set address
3771
U1<(<)
to
ON
,
OFF
,
Alarm Only
or .
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
1
=
1
/
3
·|U
L1
+ a·U
L2
+ a
2
·U
L3
|. For symmetrical voltages this is equivalent to a
phase-earth voltage.
The dropout to pickup ratio can be set in address . This parameter can only be altered
with DIGSI
®
under
Additional Settings
.
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