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Functions
2.5 Under and over-voltage protection (optional)
2.5
Under and over-voltage 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 of 7VK61 detects the phase voltages U
voltages U
L1-L2
other voltage that is connected to the fourth voltage input U
device calculates the positive sequence system voltage and the negative sequence system voltage so that the
symmetrical components are also monitored.
If there is only one voltage transformer in the system, the parameter 106 VT CONNECTION must be set ac-
cordingly during configuration. Also, it is indispensable in this case that the secondary winding of the voltage
transformer is connected to the input U
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 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 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 menacing stability problems. With induction machines under-
voltages have an effect on the stability and permissible torque thresholds.
2.5.1
Overvoltage Protection
Overvoltage Phase–Earth
Figure 2-21 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 elim-
inated. 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".
96
, U
and U
, and the displacement voltage 3 U
L2-L3
L3-L1
L1
and U
, as well as the positive sequence system.
L2-L3
L3-L1
L1-E
0
of the device can be detected. Furthermore, the
4
of the 7VK61.
, U
L1-E
, U
and U
, the phase-to-phase
L2-E
L3-E
. Instead of the displacement voltage any
and U
, the phase-to-phase volt-
L2-E
L3-E
SIPROTEC, 7VK61, Manual
C53000-G1176-C159-3, Release date 05.2009
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