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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 type GRL100-700 is a current differential with back-up distance protection. CT requirement
for GRL100-700 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 GRL100
must be considered as part of the combined requirements for both protections.
<Step 1> Check the maximum through fault current I
I
< 65 X I
fmaxth
where,
I
: Secondary maximum through fault current
fmaxth
I
:Rated secondary current
n
<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>
Td [ms]
35
50
75
100
150
Td [ms]
35
50
75
100
150
V
: Knee point voltage [V]
k
R
: Secondary CT resistance [ohms]
ct
R
: Actual secondary burden [ohms]
2
I
: Maximum secondary fault current
fmax
I
: Maximum secondary fault current at the zone 1 reach point
f_z1_max
I
: Maximum secondary fault current for a close-up reverse fault
f_rev_max
I
: Maximum secondary load current
Lmax
n
Table K-1 CT Requirement defined by V
Requirement 1
V
≧ I
(R
+ R
)×3
k
fmax
ct
2
V
≧ I
(R
+ R
)×3
k
fmax
ct
2
V
≧ I
(R
+ R
)×4
k
fmax
ct
2
V
≧ I
(R
+ R
)×4
k
fmax
ct
2
V
≧ I
(R
+ R
)×8
k
fmax
ct
2
Requirement 3
V
≧ I
(Rct+ R
k
f_rev_max
V
≧ I
(Rct+ R
k
f_rev_max
V
≧ I
(Rct+ R
k
f_rev_max
V
≧ I
(Rct+ R
k
f_rev_max
V
≧ I
(Rct+ R
k
f_rev_max
400
fmaxth
k
Requirement 2
V
≧ I
k
f_z1_max
V
≧ I
k
f_z1_max
V
≧ I
k
f_z1_max
V
≧ I
k
f_z1_max
V
≧ I
k
f_z1_max
Requirement 4
)×2
V
≧ I
(Rct+ R
2
k
Lmax
)×3
V
≧ I
(Rct+ R
2
k
Lmax
)×6
V
≧ I
(Rct+ R
2
k
Lmax
)×6
V
≧ I
(Rct+ R
2
k
Lmax
)×6
V
≧ I
(Rct+ R
2
k
Lmax
6 F 2 S 0 8 5 0
(R
+ R
)×6
ct
2
(R
+ R
)×7
ct
2
(R
+ R
)×8
ct
2
(R
+ R
)×8
ct
2
(R
+ R
)×8
ct
2
)×14.4
2
)×20
2
)×28.8
2
)×36.8
2
)×50.4
2
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