Siemens siprotec 7SA522 - Page 275

2.15 Undervoltage and overvoltage protection (optional)
275
7SA522 Manual
C53000-G1176-C155-3
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
Sequence System
3U
0
Figure 2-115 depicts the logic diagram of the zero sequence voltage stage. The fun-
damental frequency is numerically filtered from the measuring voltage so that the har-
monics or transient voltage peaks remain largely harmless.
The triple zero sequence voltage 3U
0
is fed to the two threshold stages 8! and
8!!. Combined with the associated time delays 78! and 78!! these stages
form a two-stage overvoltage protection for the zero sequence system. Here too, the
drop-off to pick-up ratio can be set (8!!5(6(7). Furthermore, a stabilization
delay can be configured which is implemented by repeated measuring (approx. 2 pe-
riods).
The overvoltage protection for the zero sequence system can also be blocked via a
binary input ´!8!!%/.µ. 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 2.19.1, margin heading “Fuse Failure
Monitor (Non-symmetrical Voltages)” or when the trip of the mcb for voltage transform-
ers has been signalled via the binary input ´!)$,/)HHGHU97µ (internal indication
“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 pick-up 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 2-115 the device calculates the voltage to be monitored:
3·U
0
= U
L1
+ U
L2
+ U
L3
.
This applies if no suitable voltage is connected to the fourth measuring input U
4
.
However, if the displacement voltage U
en
of the voltage transformer set is directly con-
nected to the fourth measuring input U
4
of the device and this information was entered
during configuration, the device will automatically use this voltage and calculate the
triple zero sequence voltage.
3·U
0
= 8SK8GHOWD ·U
4
Since the voltage transformation ratio of the voltage transformer set is usually
the factor is set to 8SK8GHOWD = 3/√3
= √3 = 1.73. For more details, refer to 3RZHU
6\VWHP'DWD in Section 2.1.5.1 at margin heading “Voltage Connections” via
address .