Siemens siprotec 7SD5 User Manual page 329

Even during single-pole dead time (with internal automatic reclosure function) the
stages of the negative sequence overvoltage protection are automatically blocked
since arising negative sequence values are only influenced by the asymmetrical
power flow, not by the fault in the system. If the device cooperates 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 negative se-
quence system must be blocked via a binary input during single-pole tripping.
Overvoltage Zero Figure 2-136 depicts the logic diagram of the zero sequence voltage stage. The fun-
Sequence System damental frequency is numerically filtered from the measuring voltage so that the har-
3U monics or transient voltage peaks remain largely harmless.
0
The triple zero sequence voltage 3·U
3U0>>. Combined with the associated time delays T 3U0> and T 3U0>> these stages
form a two-stage overvoltage protection for the 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 asymmetrical voltage failure was detected
(„Fuse–Failure–Monitor", also see Section 2.22.1, margin heading „Fuse Failure
Monitor (Non-symmetrical Voltages))" or when the trip of the mcb for voltage trans-
formers has been signalled via the binary input „>FAIL:Feeder VT" (internal indi-
cation „internal blocking").
The stages of the zero sequence voltage protection are automatically blocked (with
the internal automatic reclosure function) during single-pole automatic reclose dead
time to avoid pickup with the asymmetrical power flow arising during this state. If the
device operates with an external automatic reclosure function or if single-pole tripping
can be triggered by a different protection system (operating in parallel), the overvolt-
age protection for the zero sequence system must be blocked via a binary input during
single-pole tripping.
According to Figure 2-136 the device calculates the voltage to be monitored:
This applies if no suitable voltage is connected to the fourth measuring input U
However, if the displacement voltage U
connected to the fourth measuring input U
entered during configuration, the device will automatically use this voltage and calcu-
late the triple zero sequence voltage.
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
General Power System Data (Power System Data 1) in Section 2.1.4.1
at margin heading „Voltage Connections" via address 211.
7SD5 Manual
C53000-G1176-C169-1
3·U = U + U + U .
0 L1 L2 L3
= Uph / Udelta ·U
3·U
0
2.17 Undervoltage and Overvoltage Protection (optional)
is fed to the two threshold stages 3U0> and
0
of the voltage transformer set is directly
Delta
of the device and this information was
4
4
.
4
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