Rockwell Automation 865 Differential Protection Relay - Manual (Page 55)

Protection Functions 3-11
865-UM001A-EN-P – July 2009
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 3.9 shows a functional block diagram of the I> overcurrent
stage with definite time and inverse time operation time. Figure 3.10
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 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.
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.
Overcurrent Protection
I> (50/51)