Functions
2.8 Undervoltage and Overvoltage Protection 27/59 (Optional)
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
123
Undervoltage Phase-to-Phase
Basically, the phase-to-phase undervoltage protection operates like the phase-to-ground protection except that
it detects phase-to-phase voltages. Accordingly, both phases are indicated during pickup of an undervoltage
element if one of the stage thresholds
27-1-Vpp PICKUP
(address
3762
) or
27-2-Vpp PICKUP
(address
3764
) was undershot. Apart from that, Figure 2-50 also applies generally.
It is sufficient for the current criterion that current flow is detected in one of the involved phases.
The phase–phase undervoltage protection can also be blocked via a binary input
„>27-Vphph BLOCK“
.
There is an automatic blocking if the measuring voltage failure was detected or voltage mcb tripping was indi-
cated (internal blocking of the phases affected by the voltage failure).
Undervoltage Positive Sequence System V
1
The device calculates the positive sequence system according to its defining equation
V
1
=
1
/
3
·(V
A
+ a·V
B
+ a
2
·V
C
)
where a = e
j120°
.
The resulting positive sequence voltage is fed to the two threshold elements
27-1-V1 PICKUP
(address
3772
) and
27-2-V1 PICKUP
(address
3774
) (see Figure 2-51). Combined with the associated time delays
27-1-V1 DELAY
(address
3773
) and
27-2-V1 DELAY
(address
3775
) these elements form a two-element
overvoltage protection for the positive sequence system.
Current can be used as an additional criterion for the undervoltage protection of the positive sequence system
(current criterion
CURR.SUP. 27-V1
, address
3778
). An undervoltage is only detected if the current flow is
detected in at least one phase together with the undervoltage criterion.
The undervoltage protection for the positive sequence system can be blocked via the binary input
„>27-V1
BLOCK“
. The stages of the undervoltage protection are automatically blocked if voltage failure is detected
(„fuse failure monitor“, also see Section 2.14.1) or, if the trip of the mcb for the voltage transformer is indicated
via the binary input
„>FAIL:Feeder VT“
(internal blocking).