Toshiba GR200 Series - 2.5.2 Nature of CT ratings matching; 2.5.3 Nature for fault current setting; (i) Setting DIFL-I1 characteristic

6F2S1914 (0.49)
GRL200 (Soft 033 & 037)
- 56 -
CT ratio:
2000/1 A
Busbar-G
Busbar-H
DIFL
DIFL
Sensitivity (primary)
CT ratio
800 A
2000 / 1 A
= 0.4 A
=
CT ratio:
4000/1 A
Setting [DIFL-I1] =
Sensitivity (primary)
CT ratio
800 A
4000 / 1 A
= 0.2 A
=
Setting [DIFL-I1] =
DIFL sensitivity required on line GH
= 800A on the primary current
Figure 2.5-2 Setting example when ratios being not identical
2.5.2 Nature of CT ratings matching
In the previous section (i) and (ii), we have the setting examples in terms of making matching
CT ratios, but we have another example when the CT rating at the remote-terminal is not
identical to the one at the local-terminal.
CT ratio:
2000/1 A
Busbar-G
Busbar-H
Sensitivity (primary)
CT ratio
800 A
2000 / 1 A
= 0.4 A
=
CT ratio:
2000/5 A
Setting [DIFL-I1] =
Sensitivity (primary)
CT ratio
800 A
2000 / 5 A
= 2.0 A
=
Setting [DIFL-I1] =
DIFL operation
at 1A rated current
DIFL operation
at 5A rated current
DIFL sensitivity required on line GH
= 800A on the primary current
Figure 2.5-3 Setting example when ratings being not identical
In terms of the sensitivity at the CT primary side, setting values for detecting a fault is
the same at both the terminal-G and the terminal-H; thereby the user can introduce the setting
values by dividing the sensitivity to the rated current. Figure 2.5-3 shows the example of
computation; the settings [DIFL-I2] and others are also obtained in the same means.
2.5.3 Nature for fault current setting
(i) Setting DIFL-I1 characteristic
The user should set the [DIFL-I1] by considering the minimum internal fault current to operate;
whereas, expect for a fault condition the user should also consider the maximum erroneous
differential current†. Accordingly, the user should set the [DIFL-I1] using below equation:



󰇟
DIFL-I1
󰇠




(2.5-1)
where,
K: Setting margin (1.2 to 1.5)