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Functions
2.8 Undervoltage and Overvoltage Protection 27/59 (Optional)
2.8
Undervoltage and Overvoltage Protection 27/59 (Optional)
Voltage protection has the function to protect electrical equipment against undervoltage and overvoltage. Both
operational states are unfavorable as for example undervoltage may cause stability problems or overvoltage
may cause insulation problems.
The overvoltage protection in 7SD80 measures the phase voltages V
voltages V
, V
A-B
quence system of the voltages so that the symmetrical components can be monitored, too.
The undervoltage protection can also use the phase voltages V
V
, V
and V
A-B
B-C
These voltage protection functions can be combined according to the user's requirements. They can be
switched on or off separately, or used for indication purposes only. In the latter case, the respective trip com-
mands do not appear. Each voltage protection function comprises two elements, i.e. it is provided with two
threshold settings each with the appropriate time delays.
Abnormally high voltages often occur e.g. on long transmission lines under low load conditions, 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 de-energized within a 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 pending stability problems. With induction machines undervolt-
ages have an effect on the stability and permissible torque thresholds.
2.8.1
Overvoltage Protection (ANSI 59)
Overvoltage Phase–Ground
Figure 2-46 depicts the logic diagram of the phase voltage elements. The fundamental frequency is numerically
filtered from each of the three measuring voltages so that harmonics or transient voltage peaks are largely elim-
inated. Two threshold elements 59-1-Vph PICKUP (address 3702) and 59-2-Vph PICKUP (address 3704)
are compared with the voltages. If a phase voltage exceeds these thresholds it is indicated separately for each
phase. Furthermore, a general pickup indication „59-1-Vpg Pickup" „59-2-Vpg Pickup" exists for each
element. The dropout ratio can be set (59-Vph RESET (address 3709)).
Each element starts a time delay which is common to all phases. Expiry of the respective time delay 59-1-
Vph DELAY (address 3703) or 59-2-Vph DELAY (address 3705) is signaled and usually results in the trip
command „59-Vpg TRIP".
The overvoltage protection phase–ground can be blocked via a binary input „>59-Vphg BLOCK".
116
. Furthermore, the device calculates the positive-sequence system and the negative-se-
B-C
, as well as the positive sequence system.
C-A
, V
and V
and the phase-to-phase
A-N
B-N
C-N
, V
and V
, the phase-to-phase voltages
A-N
B-N
C-N
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
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