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Functions
2.8 Undervoltage and Overvoltage Protection (optional) 27/59
2.8
Undervoltage and Overvoltage Protection (optional) 27/59
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 over-
voltage may cause insulation problems.
The overvoltage protection in 7SD80 measures the phase voltages V
voltages V
, V
A-B
B-C
sequence 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
C-A
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
commands 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
undervoltages have an effect on the stability and permissible torque thresholds.
Overvoltage Protection (ANSI 59)
2.8.1
Overvoltage Phase–Ground
Figure 2-46
depicts the logic diagram of the phase voltage elements. The fundamental frequency is numeri-
cally filtered from each of the three measuring voltages so that harmonics or transient voltage peaks are
largely eliminated. 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
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
108
. Furthermore, the device calculates the positive-sequence system and the negative-
, as well as the positive sequence system.
, 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
59-1-Vpg Pickup and 59-2-Vpg
>59-Vphg BLOCK .
SIPROTEC 4, 7SD80, Manual
E50417-G1100-C474-A2, Edition 02.2018
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