Undervoltage And Overvoltage Protection (Optional); Overvoltage Protection - Siemens SIPROTEC Manual

2.19

Undervoltage and Overvoltage Protection (optional)

Voltage protection has the function of protecting electrical equipment against undervoltage and overvoltage.
Both operational states are unfavourable as overvoltage may cause, for example, insulation problems or und-
ervoltage may cause stability problems.
The overvoltage protection in the 7SD5 detects the phase voltages U
voltages U
L1-L2
any other voltage that is connected to the fourth voltage input U
the device calculates the positive sequence system voltage and the negative sequence system voltage so that
the symmetrical components are also monitored. Here compounding is also possible which calculates the
voltage at the remote line end.
The undervoltage protection can also use the phase voltages U
ages U , U
L1-L2
These voltage protection functions can be combined according to the user's requirements. 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 dual-stage, i.e. it is provided with two threshold settings each
with the appropriate times 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 with the generator discon-
nected 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-energised 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 impending stability problems. With induction machines under-
voltages have an effect on the stability and permissible torque thresholds.
2.19.1

Overvoltage protection

Overvoltage Phase-to-Earth
Figure 2-172 depicts the logic diagram of the phase voltage stages. The fundamental component is numerically
filtered from each of the three measuring voltages so that harmonics or transient voltage peaks are largely elim-
inated. Two threshold stages Uph-e> (address 3702) and Uph-e>> (address 3704) are compared with the
voltages. If a phase voltage exceeds these thresholds, it is indicated in a phase-segregated way. Furthermore,
a general pickup indication „Uph-e> Pickup" and „Uph-e>> Pickup" is given. The drop-out to pickup
ratio can be set (Uph-e>(>) RESET (address 3709)).
Every stage starts a time delay which is common to all phases. Expiry of the respective time delay T Uph-e>
(address 3703) or T Uph-e>> (address 3705) is signalled and results in the trip command „Uph-e>(>)
TRIP".
The overvoltage protection phase-to-earth can be blocked via a binary input „>Uph-e>(>) BLK".
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
, U and U , as well as the displacement voltage 3U
L2-L3 L3-L1
and U , as well as the positive sequence components.
L2-L3 L3-L1
2.19 Undervoltage and Overvoltage Protection (optional)
, U and U
L1-E L2-E
. Instead of the displacement voltage
0
of the device can be detected. Furthermore,
4
, U and U , the phase-to-phase volt-
L1-E L2-E L3-E
Functions
, the phase-to-phase
L3-E
361