Toshiba GRL150-100 Series Instruction Manual page 27

Element Range
[OC3EN] Off / On
[OC4EN] Off / On
[OC-ICD] NA / BLK
(*) Current values shown in the parenthesis are in the case of a 1 A rating. Other current values are in
the case of a 5 A rating.
Inverse Time Protection
Current setting
In Figure 2.3.3.1, 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 considering also backup protection of a
fault within the adjacent lines, it is set lower than the minimum fault current in the event of a fault
at remote end F3. 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 is restricted so as not to operate on false zero-sequence currents caused by an
unbalance in the load current, errors in the current transformer circuits or zero-sequence mutual
coupling of parallel lines.
Time setting
Time setting is performed to provide selectivity in relation with the relays on the adjacent lines.
Suppose a minimum source impedance when the current flowing in the relay becomes the
maximum. In Figure 2.3.3.1, in the event of a fault at near end F2 of the adjacent line, 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 the maximum is that if time
coordination is obtained for large fault current, then time coordination can also be obtained for
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 2.3.3.1.
Tc = T1 + T2 + M
where, T1: circuit breaker clearance time at B
T2: relay reset time at A
M:
When single-phase autoreclose is used, the minimum time of the earth fault overcurrent protection
must be set longer than the time from fault occurrence to reclosing of the circuit breaker. This is to
prevent three-phase final tripping from being executed by the overcurrent protection during a
single-phase autoreclose cycle.
Step
A
Figure 2.3.3.1 Current Settings in Radial System
margin
⎯ 26 ⎯
Default Remarks
Off OC3 Enable
Off OC4 Enable
NA OC/EF/SEF blocked by irush current
B
F1 F2
6 F 2 S 0 8 2 8
C
F3