Siemens siprotec 7SA6 User Manual page 398

Functions
Undervoltage
Phase–Phase
Positive Sequence
System
Undervoltage U
1
6-216
case of severe voltage drops the 8SKH stage (address ) with a time delay
7 8SKH (address ) is active.
The setting of voltage and time values depends on the intended use, that is why
general setting recommendations cannot be given. With regard to load shedding, for
example, the values mostly depend on a priority grading schedule. If stability problems
occur, admissible undervoltages and their duration must be considered. With
induction 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
cases are or remain picked up, the current criterion &855683 8SKH (address
) is switcheG 2Q. With busbar side voltage transformers it can be switched 2II.
However, with a dead busbar the undervoltage protection picks up and expires, if it is
not blocked by other criteria or binary inputs.
Basically, the same considerations apply as for the phase undervoltage stages. These
stages may replace the phase voltage stages or be used additionally. Address 
8SKSK is set to 2Q, 2II or $ODUP 2QO\.
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 7 8SKSK (address ) und 7 8SKSK
(address ).
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 &8556838SKSK (address ) is
switcheG 2Q. With busbar side voltage transformers it can be switched 2II. However,
with a dead busbar the undervoltage protection picks up and expires, if it is not blocked
by other criteria via binary inputs.
The positive sequence undervoltage stages can be used instead of or in addition to
previously mentioned undervoltage stages. The address  8 is set to 2Q,
2II or $ODUP 2QO\, accordingly.
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 in most applications.
To achieve the two-stage condition, the 8 –stage (address ) is combined with
a greater time delay 7 8 (address ). The 8 –stage (address ) with a
shorter time delay 7 8 (address ).
Note that the positive sequence system is established according to its defining
1 ⋅|U
equation U = / + a⋅U
1 3 L1
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 &8556838 (address )
is switcheG 2Q. With busbar side voltage transformers it can be switched 2II.
However, with a dead busbar the undervoltage protection picks up and expires, if it is
not blocked by other criteria via binary inputs.
2 ⋅U
+ a |. For symmetrical voltages this is equivalent
L2 L3
7SA6 Manual
C53000-G1176-C133-1