Functions
6-236 7SA6 Manual
C53000-G1176-C156-2
Undervoltage
Phase–Earth
The phase undervoltage stages can be switched On or Off in address 3751 Uph-
e<(<)
. In addition to this, you can set Alarm Only, i.e. these stages operate and
send alarms but do not generate any trip commands.
This undervoltage protection function has two stages. The
Uph-e< stage (address
3752) operates with the longer set time value T Uph-e< (address 3753) for a slight
undervoltages. However, it must not be set above the admissible undervoltage. In
case of severe voltage drops the
Uph-e<< stage (address 3754)withatimedelay
T Uph-e<< (address 3755)isactive.
The setting of voltage and time values depends on the intended use, that is why gen-
eral setting recommendations cannot be given. With regard to load shedding, for ex-
ample, the values mostly depend on a priority grading schedule. If stability problems
occur, admissible undervoltages and their duration must be considered. With induc-
tion machines the undervoltages influence the admissible torque thresholds.
If the voltage transformers are located on the line side, the measuring voltages will be
missing if the line is disconnected. To avoid that the undervoltage stages in these cas-
es 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.How-
ever, with a dead busbar the undervoltage protection picks up and expires, if it is not
blocked by other criteria or binary inputs.
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. Address
3761
Uph-ph<(<)
is set to On, Off or Alarm Only.
As phase–to–phase voltages are monitored, the phase–to–phase values are used for
the settings
Uph-ph< (address 3762)andUph-ph<< (address 3764).
The corresponding times delay are
T Uph-ph< (address 3763)undT Uphph<< (ad-
dress
3765).
If the voltage transformers are located on the line side, the measuring voltages will
missing if 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.Howev-
er, with a dead busbar the undervoltage protection picks up and expires, if it is not
blocked by other criteria via binary inputs.
Positive Sequence
System Undervolt-
age U
1
The positive sequence undervoltage stages can be used instead of or in addition to
previously mentioned undervoltage stages. The address
3771 U1<(<) is set to On,
Off or Alarm Only, accordingly.
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-
missioninmostapplications.
To achieve the two-stage condition, the
U1<–stage (address 3772)iscombinedwith
a greater time delay
T U1< (address 3773). The U1<<–stage (address 3774)witha
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.
If the voltage transformers are located or 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 3768)
To Next Page
Loading...
Need help?
General