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6 F 2 T 0 1 7 2
Settings of Definite Time Overcurrent protection
Current setting
The current setting is set lower than the minimum fault current in the event of a fault at the remote
end of the protected feeder section. Furthermore, when also considering backup protection for a
fault in a next feeder section, it is set lower than the minimum fault current, in the event of a fault at
the remote end of the next feeder section.
Identical current values can be set for terminals, but graded settings are better than identical
settings, in order to provide a margin for current sensitivity. The farther from the power source the
terminal is located, the higher the sensitivity (i.e. the lower setting) that is required.
The minimum setting of the phase overcurrent element is restricted so as not to operate for the
maximum load current, and that of the residual overcurrent element is restricted so as to not operate
on false zero-sequence current caused by an unbalance in the load current, errors in the current
transformer circuits, or zero-sequence mutual coupling of parallel lines. Taking the selection of
instantaneous operation into consideration, the settings must be high enough not to operate for large
motor starting currents or transformer inrush currents.
Time setting
When setting the delayed pick-up timers, the time grading margin Tc is obtained in the same way as
explained in "Settings for Inverse Time Overcurrent Protection".
2.2 Instantaneous and Staged Definite Time Overcurrent Protection
In conjunction with inverse time overcurrent protection, definite time overcurrent elements OC2 to
OC4 and EF2 to EF4 provide instantaneous overcurrent protection. OC2 and EF2 also provide the
same inverse time protection as OC1 and EF1.
OC2 to OC4 and EF2 to EF4 are phase fault and earth fault protection elements, respectively. Each
element is programmable for instantaneous or definite time delayed operation. The phase fault
elements operate on a phase segregated basis, although tripping is for three phase only.
2.2.1 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 in the local end fault is much greater (3 times or
more, or (ZL+ZS)/ZS≧3, for example) than that in the 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 setting 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 2.2.1 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.
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