Siemens SIPROTEC 7SD610 Manual page 162

Functions
2.11 Undervoltage and Overvoltage Protection (optional)
Phase-to-phase undervoltage
Basically, the phase-to-phase undervoltage protection operates like the phase-to-earth protection except that
it detects phase-to-phase voltages. Accordingly, both phases are indicated during pickup of an undervoltage
stage the value fell below one of the stage thresholds Uph-ph< (address 3762) or Uph-ph<< (address 3764).
Beyond this, Figure 2-61 applies in principle.
It is sufficient for the current criterion that current flow is detected in one of the involved phases.
The phase-to-phase undervoltage protection can also be blocked via a binary input „>Uphph<(<) BLK".
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).
During single-pole dead time for automatic reclosure the stages of the undervoltage protection are automati-
cally blocked in the disconnected phase so that they do not respond to the undervoltage of the disconnected
phase provided that the voltage transformers are located on the outgoing side. Only such stages are blocked
during the single-pole dead time that can actually initiate tripping according to their setting.
Undervoltage positive sequence system U
The device calculates the positive sequence system according to its defining equation
1
U = / ·(U
1 3 L1
j120°
where a = e
The resulting positive sequence voltage is fed to the two threshold stages U1< (address 3772) and U1<< (ad-
dress 3774 (see Figure 2-62). Combined with the associated time delays T U1< (address 3773) and T U1<<
(address 3775). these stages form a two-stage undervoltage protection for the positive sequence system.
The current can be used as an additional criterion for the undervoltage protection of the positive sequence
system (current supervision CURR.SUP.U1<, 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 „>U1<(<)
BLK". The stages of the undervoltage protection are automatically blocked if voltage failure is detected („Fuse
Failure Monitor", also see Section 2.15.1) or, if the trip of the mcb for the voltage transformer is indicated via
the binary input „>FAIL:Feeder VT".
162
1
2
+ a·U + a ·U )
L2 L3
.
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011