Question

A three phase transformer having a line voltage ratio of 400/33000 v is connected in the star-delta. The cts on the 400v side have a ct ratio of 1000/5. What must be the ratio of cts on the 33000 side?

Answer 1

The ratio of CTs in the 33000 sides is 1.4

Let in the primary side 1000 A current is flowing,

As the power levels remain same at the two sides of transformer,

√3 x 400 x 1000 = √3 x 33000 x I₂

I₂= 400/33

Current through the secondary of CT on the primary side = 5A

Current through the pilot wire = 5√3 A

So CTs on the secondary side being star connected will have 5√3 A.

CT ratio on 33000 V side = (400/33) ÷5√3  = 1.4