7.10.7 Operation principle
7.10.7.1 Measured quantities
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The voltage-restrained time overcurrent protection VRPVOC function is always
connected to three-phase current and three-phase voltage input in the configuration
tool (ACT), but it will always measure the maximum of the three-phase currents
and the minimum of the three phase-to-phase voltages. If frequency tracking mode
for preprocessing blocks is used, then the function operates properly in wide
frequency range (e.g. 10-90 Hz).
7.10.7.2 Base quantities
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GlobalBaseSel defines the particular Global Base Values Group where the base
quantities of the function are set. In that Global Base Values Group:
IBase shall be entered as rated phase current of the protected object in primary
amperes.
UBase shall be entered as rated phase-to-phase voltage of the protected object in
primary kV.
7.10.7.3 Overcurrent protection
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The overcurrent step simply compares the magnitude of the measured current
quantity with the set start level. The overcurrent step starts if the magnitude of the
measured current quantity is higher than the set level.
Voltage restraint/control feature
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The overcurrent protection operation is made dependent of a measured voltage
quantity. This means that the start level of the overcurrent step is not constant but
decreases with the decrease in magnitude of the measured voltage quantity. This
feature affects the start current value of both definite time and inverse time IDMT
overcurrent protection; in particular the overcurrent with IDMT curve operates
faster during low voltage conditions. Two different types of dependencies are
available:
• Voltage restrained overcurrent (when setting parameter VDepMode = Slope);
the start level of the overcurrent stage changes according to the
Figure 122.
The voltage restrained characteristic is defined by the two points:
(0.25*UBase ; VDepFact *StartCurr/100*IBase) and (UHighLimit/
100*UBase; StartCurr/100*IBase). In the first point the factor 0.25 that
multiply UBase cannot be changed.
Section 7 1MRK 506 382-UEN A
Current protection
254 Line distance protection REL650 2.2 IEC
Technical manual
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