Functions
6-191
7SA522 Manual
C53000-G1176-C119-2
6.13
Overvoltage Protection
General
The overvoltage protection is primarily intended for the recognition of steady-state ov-
ervoltage conditions on very long weakly loaded EHV transmission lines. These over-
voltages are caused by the shunt capacitance of the line and are most severe if the
line is unloaded (Ferranti–effect).
To avoid these overvoltage conditions, compensating reactors are often installed to
compensate the capacitance of the line and to thereby reduce the overvoltages. If
these reactors fail (e.g. due to short circuit protection operation) the insulation of the
line is at severe risk due to the overvoltages: the line must be tripped in a very short
time.
The overvoltage protection in the 7SA522 measures the voltages U
L1-E
, U
L2-E
and
U
L3-E
, as well as the displacement voltage 3U
0
. The phase voltage stages on the one
hand and the displacement voltage stage on the other hand can be switched on and
off independently or be switched to only output an alarm.
Instead of the displacement voltage it is possible to measure any other voltage that is
connected to the fourth voltage input U
4
of the device.
6.13.1 Method of Operation
Phase Voltages
In Figure 6-94 the logic diagram of the phase voltage stage is shown. The fundamental
frequency is filtered out numerically in each of the three measured voltages, so that
higher harmonics or transient voltage peaks are largely suppressed. If a phase voltage
threshold
is exceeded a phase segregated alarm is issued. Furthermore, a
general pick-up annunciation “
8SKH3,&.83
” is available. After expiry of a set com-
mon time delay
the trip command is issued.
As the cause of the overvoltage condition can not always be removed by tripping the
local line end, it is possible to transfer the trip command to the other line end. The im-
pulse output “
8SKH6(1'
”, the duration of which can also be set with the parameter
, is used for this purpose. This impulse can be transmitted to the
other line end (e.g. via optical fibre, PLC or microwave radio).
Figure 6-94 Logic diagram of the phase voltage overvoltage protection
U
L1-E
U
ph
8SKH!
U
L2-E
U
L3-E
U
ph
U
ph
≥
1
T
0
T
78SKH
76HQ,PS8SKH
Uphe PICKUP L1
4342
Uphe PICKUP L2
4343
Uphe PICKUP L3
4344
Uphe PICKUP
4341
Uphe TRIP
4361
Uphe SEND
4345
Summary of Contents for siprotec 7SA522
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