VAMP 265 - 2.5. Overcurrent Protection I> (50;51)

2.5 Overcurrent protection I>
(50/51)
2 Protection functions
Technical description
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VM265.EN011
2.5. Overcurrent protection I> (50/51)
Overcurrent protection is used against short circuit faults and
heavy overloads.
The overcurrent function measures the fundamental frequency
component of the phase currents. The protection is sensitive for
the highest of the three phase currents. Whenever this value
exceeds the user's pick-up setting of a particular stage, this
stage picks up and a start signal is issued. If the fault
situation remains on longer than the user's operation delay
setting, a trip signal is issued.
Two independent stages
There are two separately adjustable overcurrent stages: I>, I>>
I‟>and I‟>>. The first stage I> can be configured for definite
time (DT) or inverse time operation characteristic (IDMT). The
stage I>> has definite time operation characteristic. By using
the definite delay type and setting the delay to its minimum, an
instantaneous (ANSI 50) operation is obtained.
Figure 2.5-1 shows a functional block diagram of the I>
overcurrent stage with definite time and inverse time operation
time. Figure 2.5-2 shows a functional block diagram of the I>>
overcurrent stage with definite time operation delay.
Inverse operation time
Inverse delay means that the operation time depends on the
amount the measured current exceeds the pick-up setting. The
bigger the fault current is the faster will be the operation.
Accomplished inverse delays are available for the I> stage. The
inverse delay types are described in chapter 2.13. The relay will
show the currently used inverse delay curve graph on the local
panel display.
Inverse time limitation
The maximum measured secondary current is 50xI
N
. This
limits the scope of inverse curves with high pick-up settings.
See chapter 2.13 for more information.
Setting groups
There are two settings groups available for each stage.
Switching between setting groups can be controlled by digital
inputs, virtual inputs (mimic display, communication, logic)
and manually.