7SR18 Applications Guide
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4.3 CT Formulae P Class CTs
Example. For 5P20 15 VA CT: -
Knee point Vk (1 A) = VA x A.L.F + (R
CT
x A.L.F. x I
n
) = 15 x 20 + (0.05 x 20 x 1) = 301 V
In 1
Knee point Vk (5 A) = VA x A.L.F + (R
CT
x A.L.F. x I
n
) = 15 x 20 + (0.01 x 20 x 5) = 61 V
In 5
If the R
CT
is not known the second term i.e. (R
CT
x A.L.F. x I
n
) can be ignored as this term would be a low value.
CT ratio selection would typically be: -
e.g. I
fmax
= 50000 = 2500:1
ALF 20
A ALF of 20 means the CT is accurate to 20 x the primary CT current i.e. 20 x 2500 = 50000
e.g. I
fmax
= 25000 = 1250:1
ALF 20
Select a ratio of 1500:1 from Standard CT Series.
A ALF of 20 means the CT is accurate to 20 x the primary CT current i.e. 20 x 1500 = 30000
Alternately for 5P20 15 VA System CT: -
Knee point Vk (1 A) = A.L.F x I
sec
x (R
Relay
+ VA ) = 20 x 1 x (0.05 + 15 ) = 301 V
(I
sec
)
2
1
Knee point Vk (5 A) = A.L.F x I
sec
x (R
Relay
+ VA ) = 20 x 5 x (0.01 + 15 ) = 61 V
(I
sec
)
2
25
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