Settings For Inverse Time Overcurrent Protection; Current Setting; Time Setting; Current Settings In Radial Feeder - Toshiba GRE110 Instruction Manual

Settings for Inverse Time Overcurrent protection

Current setting

In Figure 2.1.7, the current setting at terminal A is set lower than the minimum fault current in the
event of a fault at remote end F1. Furthermore, when also considering backup protection for a fault
on the next feeder section, it is set lower than the minimum fault current in the event of a fault at
remote end F3.
To calculate the minimum fault current, phase-to-phase faults are assumed for the phase
overcurrent element, and phase to earth faults for residual overcurrent element, assuming the
probable maximum source impedance. When considering the fault at F3, the remote end of the next
section is assumed to be open.
The higher the current setting, the more effective the inverse characteristic. On the other hand, the
lower the setting, the more dependable the operation. The setting is normally 1 to 1.5 times or less
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 2.1.7 Current Settings in Radial Feeder

Time setting

Time setting is performed to provide selectivity in relation to the relays on adjacent feeders.
Consider a minimum source impedance when the current flowing through the relay reaches a
maximum. In Figure 2.1.7, 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 2.1.7.
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
A
F1
 15 
6 F 2 T 0 1 7 2
B
F2 F3
C