Siemens SIPROTEC 4 7VK61 Manual page 106

Functions
2.5 Under and over-voltage protection (optional)
[logikdia-unterspgschutz-spgmitsys-wlk-310702, 1, en_GB]
Figure 2-30
During single-pole dead time for automatic reclosure, the stages of positive sequence undervoltage protection
are automatically blocked. In this way, the stages do not respond to the reduced positive sequence voltage
caused by the disconnected phase in case the voltage transformers are located on the outgoing side.
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.
Please bear in mind that the device is designed for a three-phase voltage connection. Single-phase connection
is also possible but no adjustments have been made in this respect. This must be taken into consideration
when selecting the stage and the pickup value.
Operation Without Current Transformer Connections
The 7VK61 relay can also be operated without current transformer connections. This is achieved by setting
parameter 107 CT CONNECTION = NO during configuration. For the undervoltage protection this has the
consequence that the current criterion cannot become active and associated parameters are hidden.
106
Logic diagram of the undervoltage protection for positive sequence voltage system
SIPROTEC 4, 7VK61, Manual
C53000-G1176-C159-5, Edition 05.2018