- 1699 -
CT REQUIREMENT
Ideally it would be preferable to employ current transformers that did not saturate; this is
particularly desirable if operation of the protection is to be avoided during external faults.
However, there are circumstances due to accommodation requirements and occasionally on the
basis of cost where this is not always possible.
The GR relay is a current differential with back-up distance protection. CT requirement for
GR relay is determined by the requirement for current differential protection and for distance
protection.
CASE 1: The case with the distance protection applied as a main protection in addition to the
current differential protection when the distance protection is applied as a main
protection the CT requirement for GR relay must be considered as part of the
combined requirements for both protections.
<Step 1> Check the maximum through fault current I
fmaxth
I
fmaxth
< 65 X I
n
where,
I
fmaxth
: Secondary maximum through fault current
I
n
: Rated secondary current
<Step 2> Check the CT satisfies the condition given by table K-1 or K-2 depending on CT types.
Each table has 4 requirements. Every requirement must be satisfied.
<Knee point voltage of CTs is given>
Table K-1 CT Requirement defined by V
k
V
k
≧ I
f_z1_max
(R
ct
+ R
2
)×6
V
k
≧ I
f_z1_max
(R
ct
+ R
2
)×7
V
k
≧ I
f_z1_max
(R
ct
+ R
2
)×8
V
k
≧ I
f_z1_max
(R
ct
+ R
2
)×8
V
k
≧ I
f_z1_max
(R
ct
+ R
2
)×8
V
k
≧ I
f_rev_max
(Rct+ R
2
)×2
V
k
≧ I
f_rev_max
(Rct+ R
2
)×3
V
k
≧ I
f_rev_max
(Rct+ R
2
)×6
V
k
≧ I
f_rev_max
(Rct+ R
2
)×6
V
k
≧ I
f_rev_max
(Rct+ R
2
)×6
V
k
: Knee point voltage [V]
R
ct
: Secondary CT resistance [ohms]
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