Siemens Siprotec 7VK61 Manual page 104

Functions
2.5 Under and over-voltage protection (optional)
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 the stages of the undervoltage protection are automati-
cally blocked in the disconnected phase so that it does 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.
This automatic blocking function requires the condition of the system to be detected without doubt. This means
that it is only active if all three current and voltage transformers are connected.
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< and U1<< (see Figure 2-26).
Combined with the associated time delays T U1< and T U1<< these stages form a two-stage undervoltage
protection for the positive sequence system.
Current can be used as an additional criterion for the undervoltage protection of the positive sequence system
(current supervision CURR.SUP.U1<). 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 2.7.1) or, if the trip of the mcb for the voltage transformer is indicated via the
binary input „>FAIL:Feeder VT" (internal blocking).
104
1
2
+ a·U + a ·U )
L2 L3
.
SIPROTEC, 7VK61, Manual
C53000-G1176-C159-3, Release date 05.2009