Toshiba GRL100-701B Instruction Manual page 402

<Accuracy limit factor of CTs is given>
Td [ms]
35
50
75
100
150
Td [ms]
35
50
75
100
150
R
=
n ' VA
R
n' : Equivalent accuracy limit factor defined by the equation above.
I : Secondary rated current
n
R : Rated secondary burden [VA]
VA
R : Secondary CT resistance [ohms]
ct
R : Actual secondary burden [ohms]
2
n : Rated accuracy limit factor
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
Note : The values in the table are based on the following assumption.
- 100% DC component is superimposed.
- Only one CT is saturated.
- No remnance flux is assumed.
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 GRL100
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
I < 65 X I
fmaxth n
where,
I : Secondary maximum through fault current
fmaxth
I :Rated secondary current
n
Table K-2 CT Requirement defined by n'
Requirement 1
n' I ≧ 3.75 × I
n fmax
n' I ≧ 3.75 × I
n fmax
n' I 5 × I
≧
n fmax
n' I 5 × I
≧
n fmax
n' I ≧ 10 × I
n fmax
Requirement 3
n' I ≧ 2.5 × I
n f_rev_max
n' I ≧ 3.75 × I
n f_rev_max
n' I ≧ 7.5 × I
n f_rev_max
n' I ≧ 7.5 × I
n f_rev_max
n' I ≧ 7.5 × I
n f_rev_max
+
2
/ I R
⋅
n ct n
+
R
2 ct
 401 
Requirement 2
n' I ≧ 7.5 × I
n f_z1_max
n' I ≧ 8.75 × I
n f_z1_max
n' I ≧ 10 × I
n f_z1_max
n' I ≧ 10 × I
n f_z1_max
n' I ≧ 10 × I
n f_z1_max
Requirement 4
n' I ≧ I
×18
n Lmax
n' I ≧ I
×25
n Lmax
n' I ≧ I
×36
n Lmax
n' I ≧ I
×46
n Lmax
n' I ≧ I
×63
n Lmax
(K-1)
fmaxth
6 F 2 S 0 8 5 0