Siemens siprotec 7SD5 User Manual

2.17 Undervoltage and Overvoltage Protection (optional)
333
7SD5 Manual
C53000-G1176-C169-1
connected phase provided that the voltage transformers are located on the outgoing
side.
Undervoltage Posi-
tive Sequence
System U
1
The device calculates the positive sequence system according to its defining equation
U
1
=
1
/
3
·(U
L1
+ a·U
L2
+ a
2
·U
L3
)
where a
= e
j120°
.
The resulting single–phase AC voltage is fed to the two threshold stages U1< and
U1<< (see Figure 2-138). 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 un-
dervoltage 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 automat-
ically blocked if voltage failure is detected („Fuse–Failure–Monitor“, also see Section
2.22.1) or, if the trip of the mcb for the voltage transformer is indicated via the binary
input „>FAIL:Feeder VT“ (internal blocking).
Figure 2-138 Logic diagram of the undervoltage protection for positive sequence voltage system
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 positive sequence system so that they do not respond to the reduced voltage
caused by the disconnected phase in case the voltage transformers are located on the
outgoing side.
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Siemens siprotec 7SD5 Specifications

General

Brand	Siemens
Model	siprotec 7SD5
Category	Relays
Language	English