Siemens siprotec 7SD5 User Manual page 332

2 Functions
Figure 2-137 Logic diagram of the undervoltage protection for phase voltages
Phase-phase Und- Basically, the phase-phase undervoltage protection operates like the phase-earth pro-
ervoltage tection except that it detects phase-to-phase voltages. Accordingly, both phases are
indicated during pickup of an undervoltage stage if one of the stage thresholds Uph-
ph< or Uph-ph<< was undershot. Beyond this, Figure 2-137 applies in principle.
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
„>Uphph<(<) BLK". There is an automatic blocking if the measuring voltage failure
was detected or voltage mcb tripping was indicated (internal blocking of the phases
affected by the voltage failure).
During single-pole dead time for automatic reclosure (using the internal automatic re-
closure function) the stages of the undervoltage protection are automatically blocked
in the disconnected phase so that it does not respond to the undervoltage of the dis-
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7SD5 Manual
C53000-G1176-C169-1