Toshiba GR200 Series - Page 1724

6F2S1914 (0.49)
GRL200 (Soft 033 & 037)
- 1701 -
CASE 2: The case with the distance protection applied as a back-up protection with time delay
when the distance protection is applied as back-up protection the CT requirement for
GR relay can be ignored. If instantaneous tripping of distance protection is allowed
when a communication failure occurs to the current differential protection, it is
recommended that the requirement for case 1 should be applied.
<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-3 or K-4 depending on CT types.
Each table has 2 requirements. Both must be satisfied.
<Knee point voltage of CTs is given>
Table K-3 CT Requirement defined by V
k
Td [ms]
Requirement 1
Requirement 2
35
V
k
≧ I
fmax
(R
ct
+ R
2
)×3
V
k
≧ I
Lmax
(Rct+ R
2
)×14.4
50
V
k
≧ I
fmax
(R
ct
+ R
2
)×3
V
k
≧ I
Lmax
(Rct+ R
2
)×20
75
V
k
≧ I
fmax
(R
ct
+ R
2
)×4
V
k
≧ I
Lmax
(Rct+ R
2
)×28.8
100
V
k
≧ I
fmax
(R
ct
+ R
2
)×4
V
k
≧ I
Lmax
(Rct+ R
2
)×36.8
150
V
k
≧ I
fmax
(R
ct
+ R
2
)×8
V
k
≧ I
Lmax
(Rct+ R
2
)×50.4
V
k
: Knee point voltage [V]
R
ct
: Secondary CT resistance [ohms]
R
2
: Actual secondary burden [ohms]
I
fmax
: Maximum secondary fault current
I
Lmax
: Maximum secondary load current
<Accuracy limit factor of CTs is given>
Table K-4 CT Requirement defined by n’
Td [ms]
Requirement 1
Requirement 2
35
n’ I
n
≧ 3.75 × I
fmax
n’ I
n
≧ I
Lmax
×18
50
n’ I
n
≧ 3.75 × I
fmax
n’ I
n
≧ I
Lmax
×25
75
n’ I
n
≧ 5 × I
fmax
n’ I
n
≧ I
Lmax
×36
100
n’ I
n
≧ 5 × I
fmax
n’ I
n
≧ I
Lmax
×46
150
n’ I
n
≧ 10 × I
fmax
n’ I
n
≧ I
Lmax
×63