Seg WI Series - Page 76

The impedance of the adapter CT plus WIC1 R
wic1
has a non-linear dependency on
the secondary current. Therefore the calculation of the minimum and of the maximum
requirements use di󰊞erent values of R
wic1
.
Example Calculation for a P/PR Class CT
Standard CT, 1 A I
pr
= 25 A
Maximum primary short-circuit
current
I
psc,max
= 4000 A
Rated resistive burden
R
b
=
2.5 VA
(1 A)
2
= 2.5 Ω
Burden of secondary wiring of
the standard CT
R
wiring
= 2.0 Ω
This example assumes a normal connection (i. e. no Holmgreen) for the standard CTs.
Calculation of the minimum requirement:
ALF ≥ 20 ⋅
R
ct
+ R
wiring
+ R
wic1
R
ct
+ R
b
= 20 ⋅
1.5 Ω + 2.0 Ω + 0.2 Ω
1.5 Ω + 2.5 Ω
= 18.5
The ALF of the standard CT must be greater than 18.5.
The thermal limit, too, needs to be considered. The maximum short-circuit current is:
I
psc
= 2500 A =
4000 A
25 A
/
I
n
= 160I
n
> 100I
n
The maximum short-circuit current is above the limit of the adapter CT. Therefore the
standard CT must ful󰊟ll a maximum requirement.
Calculation of the maximum requirement:
ALF ≤ 80 ⋅
R
ct
+ R
wiring
+ R
wic1
R
ct
+ R
b
= 80 ⋅
1.5 Ω + 2.0 Ω + 0.1 Ω
1.5 Ω + 2.5 Ω
= 72
So, considering the limit of the adapter CT, the ALF of the 1 A standard CT must be in the
range 18.5 … 72.
A 5P20 CT is therefore su󰊡cient for the given application.
76 WIC1 WIC1-1.0-EN-MAN
3 Hardware
3.3.3.1 Requirements for 1 A Standard CTs being Used with Adapter CTs