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2.1.1.3 Instantaneous Overcurrent Protection
In conjunction with inverse time overcurrent protection, additional overcurrent elements provide
instantaneous or definite time overcurrent protection.
OC1 to OC4 and EF1 to EF4 are phase fault and earth fault protection elements, respectively. Each
element is programmable for instantaneous or definite time delayed operation. (In case of
instantaneous operation, the delayed pick-up timer is set to 0.00.) The phase fault elements operate
on a phase segregated basis, although tripping is for three phase only.
Selective Instantaneous Overcurrent Protection
When they are applied to radial networks with several feeder sections where ZL (impedance of the
protected line) is large enough compared with ZS (the impedance between the relay and the power
source), and the magnitude of the fault current for a local end fault is much greater (3 times or
more, or (ZL+ZS)/ZS≧3, for example) than that for a remote end fault under the condition that ZS
is maximum, the pick-up current can be set sufficiently high so that the operating zone of the
elements do not reach the remote end of the feeder, and thus instantaneous and selective protection
can be applied.
This high-set overcurrent protection is applicable and effective particularly for feeders near the
power source where the setting is feasible, but the longest tripping times would otherwise have to
be accepted.
As long as the associated inverse time overcurrent protection is correctly coordinated, the
instantaneous protection does not require setting coordination with the downstream section.
Figure 2.1.6 shows operating times for instantaneous overcurrent protection in conjunction with
inverse time overcurrent protection. The shaded area shows the reduction in operating time by
applying the instantaneous overcurrent protection. The instantaneous protection zone decreases as
ZS increases.
Operate time
Figure 2.1.6 Conjunction of Inverse and Instantaneous Overcurrent Protection
The current setting is set 1.3 to 1.5 times higher than the probable maximum fault current in the
event of a fault at the remote end. The maximum fault current for elements OC1 to OC4 is
obtained in case of three-phase faults, while the maximum fault current for elements EF1 to EF4 is
obtained in the event of single phase earth faults.
2.1.1.4 Staged Definite Time Overcurrent Protection
When applying inverse time overcurrent protection for a feeder system as shown in Figure 2.1.7,
well coordinated protection with the fuses in branch circuit faults and high-speed protection for
the feeder faults can be provided by adding staged definite time overcurrent protection with
time-graded OC2 and OC3 or EF2 and EF3 elements.
A
B
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15
T
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6 F 2 S 0 7 5 8
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