Siemens siprotec 7SA6 - Page 397

Functions
6-2157SA6 Manual
C53000-G1176-C133-1
the other being 8!! (address ) with a short delay time 78!! (address )
for high asymmetrical voltages.
Note that the negative sequence system is established according to its defining
equation U
2
=
1
/
3
⋅|U
L1
+ a
2
⋅U
L2
+ a⋅U
L3
|. For symmetrical voltages and two
exchanged phases this is equivalent to the phase–to–earth voltage value.
The drop-off to pick-up ratio 8!!5(6(7) can be set in address . This setting
can only be changed with DIGSI
®
4 under “Additional Settings”.
Zero Sequence
System
Overvoltage
The zero sequence voltage stage can be switched 2Q or 2II in address . In ad-
dition, this stage can also be selected to $ODUP2QO\ i.e. these stages operate and
issue alarms only. This protection function can be used for any other single-phase
voltage which is connected to the fourth voltage measurement input U
4
. (Also refer to
Section 6.1.1 and see margin heading “Voltage Transformer Connection”).
The settings of the voltage threshold and the timer values depend on the type of ap-
plication. Here no general guidelines can be established. Generally, with a sensitive
setting of 8! (address ), i.e. close to operational values that are to be expect-
ed, not only the time delay 78 (address ) must be greater, but also the reset
ratio 85HVHW5DWLR (address ) must also be as large as possible. Usually
the presetting is sufficient. This setting can only be changed with DIGSI
®
4 under “Ad-
ditional Settings”.
Similar considerations apply if this voltage stage is used for a different voltage at the
measuring input U
4
.
When setting the voltage values please observe the following:
• If the U
en
-voltage of the set of voltage transformers is connected to U
4
and if this
was already set in the power system data 1 (refer also to Section 6.1.1 under
margin heading “Voltage Transformer Connection”, U4 transformer = 8GHOWD
WUDQVI), the device multiplies this voltage by the matching ratio 8SK8GHOWD
(address ), usually with 1.73. Therefore the voltage measured is √3
· U
en
=
3.U
0
. When the voltage triangle is fully displaced, the voltage will be √3-times the
phase-to-phase voltage
.
• If any other voltage is connected to U
4
, which is not used for voltage protection, and
if this was already set in the power system data 1 (refer also to Section 6.1.1,
“Voltage Transformer Connection”, e. g. 8WUDQVIRUPHU = 1RWFRQQHFWHG or
8WUDQVIRUPHU = 8V\QFWUDQVI), the device calculates the zero sequence
voltage according to its definition: 3 · U
0
= IU
L1
+ U
L2
+ U
L3
I. When the voltage
triangle is fully displaced, the voltage will be √3-times the phase-to-phase voltage
.
• If any other voltage is connected to U
4
, which is used for voltage protection, and if
this was already set in the power system data 1 (refer also to Section 6.1.1, “Voltage
Transformer Connection”, i. e. 8WUDQVIRUPHU = 8[WUDQVIRUPHU), this
voltage will be used for the voltage stages without any further factors. This “zero
sequence voltage protection” then is, in reality, a single-phase voltage protection for
any kind of voltage at U
4
.
Undervoltage
Phase–Earth
The phase undervoltage stages can be switched 2Q or
Off in address  8SK
H. In addition to this, you can set $ODUP2QO\, i.e. these stages operate and
send alarms but do not generate any trip commands.
This undervoltage protection function has two stages. The 8SKHstage (address
) operates with the longer set time value 78SKH (address ) for a slight
undervoltages. However, it must not be set above the admissible undervoltage. In