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of the minimum fault current.
For grading of the current settings, the terminal furthest from the power source is set to the lowest
value and the terminals closer to the power source are set to a higher value.
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.
Figure 2.1.17
Time setting
Time setting is performed to provide selectivity in relation to the relays on adjacent feeders.
Consider the minimum source impedance when the current flowing through the relay reaches a
maximum. In Figure 2.1.17, in the event of a fault at F2, the operating time is set so that terminal A
may operate by time grading Tc behind terminal B. The current flowing in the relays may
sometimes be greater when the remote end of the adjacent line is open. At this time, time
coordination must also be kept.
The reason why the operating time is set when the fault current reaches a maximum is that if time
coordination is obtained for a large fault current, then time coordination can also be obtained for
the small fault current as long as relays with the same operating characteristic are used for each
terminal.
The grading margin Tc of terminal A and terminal B is given by the following expression for a
fault at point F2 in Figure 2.1.17.
T
= T
+ T
+ T
c
1
2
m
where,
T
: circuit breaker clearance time at B
1
T
: relay reset time at A
2
T
: time margin
m
(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
A
F1
Current Settings in Radial Feeder
⎯
⎯
26
6 F 2 S 0 8 4 2
B
F2
F3
C
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