Siemens siprotec 7SA6 User Manual page 396

Functions
Overvoltage
Phase–Earth
Overvoltage
Phase–Phase
Positive Sequence
System
Overvoltage U
1
Negative Sequence
System
Overvoltage U
2
6-214
voltages; the symmetrical negative sequence system can also be used for
overvoltage. Any combination is possible. Detection procedures that are not required
are switched 2II.
The phase voltage protection stages can be switched 2Q or 2II in address 
8SKH! ! . In addition to that you can also set $ODUP 2QO\; i.e. these stages
operate and transmit signals. Without generating a trip command.
The settings of the voltage and time values depend on what they are used for. If
steady-state overvoltages are to be detected on long unloaded lines, the 8SKH!
stage (address ) is set to at least 5 % above the maximum stationary phase–to–
earth voltage that is to be expected in operation. Additionally, a high drop-off to pick-
up ratio is required (address  8SKH! ! 5(6(7 =   = presetting). This
setting can only be changed with DIGSI
time 7 8SKH! (address ) should be a few seconds so that overvoltages with
short duration may not result in tripping.
The 8SKH!! stage (address ) is provided for high overvoltages with short
duration. Here, an adequately high pick-up value is set, e.g. the 1.5–fold of the nominal
phase–earth voltage. 0.1 s to 0.2 s are sufficient for the time delay 7 8SKH!!
(address ).
Basically, the same considerations apply as for the phase voltage stages. These
stages may be used instead of the phase voltage stages or be used addionally. The
address  8SKSK! ! is set to 2Q, 2II or $ODUP 2QO\.
As phase–to–phase voltages will be detected, phase–to–phase values are used for
the settings 8SKSK! (address ) and 8SKSK!! (address ).
The same aspects as mentioned above apply to the time delays 7 8SKSK! (address
) and 7 8SKSK!! (address ) as well as to the drop-off to pick-up ratios
(address  8SKSK! ! 5(6(7). This setting can only be changed with DIGSI
4 under "Additional Settings".
The positive sequence voltage stages can be used instead of or in addition to
previously mentioned overvoltage stages. The address  8! ! is set to 2Q,
2II or $ODUP 2QO\, accordingly.
These stages are particularly suited to the detection of steady-state overvoltages on
long, weak-loaded transmission lines (Ferranti effect). Here too, the 8! stage
(address ) with a longer delay time is used for the detection of steady-state
overvoltages, the 8!! stage (address ) with the short delay time 7 8!!
(address ) is used for high overvoltages that may jeopardize insulation.
Note that the positive sequence system is established according to its defining
⋅|U
1
equation U = / + a⋅U
1 3 L1
to a phase–to–earth voltage.
The drop-off to pick-up ratio (address  8! ! 5(6(7) is set as high as possible
with regard to the detection of even small steady-state overvoltages. This setting can
only be changed with DIGSI
The negative sequence system voltage stages detect asymmetrical voltages. If such
voltages shall cause tripping, set the address  8! ! to 2Q. If these states shall
be signalled only, set the address 8! ! to $ODUP 2QO\, in any other cases to 2II.
This protective function also has in two stages, one being 8! (address ) with a
greater time delay 7 8! (address ) for stationary asymmetrical voltages and
®
4 under "Additional Settings". The delay
⋅U
2
+ a |. For symmetrical voltages this is equivalent
L2 L3
®
4 under "Additional Settings".
®
7SA6 Manual
C53000-G1176-C133-1