Siemens Siprotec 7VK61 Manual page 100

Functions
2.5 Under and over-voltage protection (optional)
zero sequence system. Here too, the drop-off to pickup ratio can be set (3U0>(>) RESET). Furthermore, a
restraint delay can be configured which is implemented by repeated measuring (approx. 3 periods).
The overvoltage protection for the zero sequence system can also be blocked via a binary input „>3U0>(>)
BLK". The stages of the zero sequence voltage protection are automatically blocked as soon as an asymmet-
rical voltage failure was detected („Fuse–Failure–Monitor", also see Section 2.7.1, margin heading „Fuse
Failure Monitor (Non-symmetrical Voltages))" or when the trip of the mcb for voltage transformers has been
signalled via the binary input „>FAIL:Feeder VT" (internal indication „internal blocking").
The stages of the zero sequence voltage protection are automatically blocked during single-pole automatic
reclose dead time to avoid pickup with the asymmetrical power flow arising during this state. If the device co-
operates with an external automatic reclosure function, or if a single-pole tripping can be triggered by a different
protection system (working in parallel), the overvoltage protection for the zero sequence system must be
blocked via a binary input during single-pole tripping.
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.
According to Figure 2-24 the device calculates the voltage to be monitored:
3·U = U + U
0 L1
This applies if no suitable voltage is connected to the fourth measuring input U
However, if the displacement voltage U
measuring input U
matically use this voltage and calculate the triple zero sequence voltage.
= Uph / Udelta ·U
3·U
0
Since the voltage transformation ratio of the voltage transformer set is usually
the factor is set to Uph / Udelta = 3/√3 = √3 = 1.73. For more details, refer to Power System Data 1 in
Section 2.1.5.1 at margin heading „Voltage Connections" via address 211.
100
+ U .
L2 L3
delta
of the device and this information was entered during configuration, the device will auto-
4
4
of the voltage transformer set is directly connected to the fourth
C53000-G1176-C159-3, Release date 05.2009
.
4
SIPROTEC, 7VK61, Manual