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6.16
Voltage Protection (optional)
General
6.16.1 Method of Operation
6.16.1.1 Overvoltage Protection
Overvoltage
Phase–Earth
7SA6 Manual
C53000-G1176-C156-2
The overvoltage protection avoids stress of electrical equipment by extremely high
voltages and the resultant insulation problems.
Abnormally high voltages often occur in weak-loaded, long distance transmission
lines, in islanded systems when generator voltage regulation fails, or after full load
shutdown of a generator and external generators (not connected to 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 deen-
ergized 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.
The overvoltage protection in the 7SA6 detects the phase voltages U
U
, the phase-to-phase voltages U
L3–E
placement voltage 3U
0
connected to the fourth voltage input U
the device calculates the positive sequence system voltage and the negative se-
quence system voltage so that the symmetrical components are also monitored.
The phase voltages U
L1–E
U
and U
, as well as the positive sequence system can also be used for the
L2–L3
L3–L1
undervoltage protection.
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. In the
latter case the respective trip commands do not appear. Each voltage protection func-
tion is dual-stage, i. e. it is provided with two threshold settings each with the appro-
priate times delay.
Figure 6-114 depicts the logic diagram of the phase voltage stages. The fundamental
frequency is numerically filtered from each of the three measuring voltages so that
harmonics 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 pick-up in-
dication " Uph–e> Pickup " and " Uph–e>> Pickup " is given. The drop-off to pick-up
ratio can be set ( Uph-e>(>) RESET ).
Every stage starts a time delay which is common to all phases. The expiry of the re-
spective time delay T Uph-e> or T Uph-e>> is indicated and issues the trip com-
mand
" Uph–e>(>) TRIP ".
The overvoltage protection phase–earth can be blocked via a binary input
" >Uph–e>(>) BLK ".
, U
L1–L2
L2–L3
. Instead of the displacement voltage any other voltage that is
of the device can be detected. Furthermore
4
, U
and U
, the phase-to-phase voltages U
L2–E
L3–E
, U
L1–E
and U
, as well as the dis-
L3–L1
Functions
and
L2–E
,
L1–L2
6-227
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