2.17 Undervoltage and Overvoltage Protection (optional)
325
7SD5 Manual
C53000-G1176-C169-1
2.17 Undervoltage and Overvoltage Protection (optional)
Voltage protection has the function of protecting electrical equipment against under-
voltage and overvoltage. Both operational states are unfavourable as overvoltage may
cause, for example, insulation problems or undervoltage may cause stability prob-
lems.
The overvoltage protection in the 7SD5 detects the phase voltages U
L1-E
, U
L2-E
and
U
L3-E
, the phase-to-phase voltages U
L1-L2
, U
L2-L3
and U
L3-L1
, as well as the displace-
ment voltage 3U
0
. Instead of the displacement voltage any other voltage that is con-
nected to the fourth voltage input U
4
of the device can be detected. Furthermore, the
device calculates the positive sequence system voltage and the negative sequence
system voltage so that the symmetrical components are also monitored. Here com-
pounding is also possible which calculates the voltage at the remote line end.
The undervoltage protection can also use the phase voltages U
L1-E
, U
L2-E
and U
L3-E
,
the phase-to-phase voltages U
L1-L2
, U
L2-L3
and U
L3-L1
, as well as the positive sequence
system.
These voltage protection functions can be combined according to the user's require-
ments. They can be switched on or off separately, or used for alarm purposes only. In
the latter case the respective trip commands do not appear. Each voltage protection
function is two-stage, i.e. it is provided with two threshold setting stages, each one with
its respective time delay.
Abnormally high voltages often occur e.g. in low loaded, long distance transmission
lines, in islanded systems when generator voltage regulation fails, or after full load
shutdown of a generator from the system. Even if compensation reactors are used to
avoid line overvoltages by compensation of the line capacitance and thus reduction of
the overvoltage, the overvoltage will endanger the insulation if the reactors fail (e.g.
due to fault clearance). The line must be deenergized within very short time.
The undervoltage protection can be applied, for example, for disconnection or load
shedding tasks in a system. Furthermore, this protection scheme can detect menacing
stability problems. With induction machines undervoltages have an effect on the sta-
bility and permissible torque thresholds.
2.17.1 Overvoltage Protection
Overvoltage
Phase–Earth
Figure 2-132 depicts the logic diagram of the phase voltage stages. The fundamental
frequency is numerically filtered from each of the three measuring voltages so that har-
monics or transient voltage peaks are largely eliminated. Two threshold stages
Uph-
e>
and
Uph-e>>
are compared with the voltages. If a phase voltage exceeds these
thresholds it is indicated phase-segregated. Furthermore, a general pickup indication
„Uph-e> Pickup“
„Uph-e>> Pickup“
is given. The drop-out to pickup ratio can
be set (
Uph-e>(>) RESET
).
Every stage starts a time delay which is common to all phases. Expiry of the respective
time delay
T Uph-e>
or
T Uph-e>>
is signalled and usually results in the trip
command
„Uph-e>(>) TRIP“
.
The overvoltage protection phase–earth can be blocked via a binary input
„>Uph-
e>(>) BLK“
.
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