Question

A 3-phase transformer rated for 33/6.6 kV is connected to a star/delta and the current transfer on the LV side has a ratio of 500 : 5. Determine the ratio of the current transformer on the HV side.
1. 100 : 5
2. \(110:\frac{5}{{\sqrt 3 }}\)
3. \(100:\frac{5}{{\sqrt 3 }}\)
4. 110 : 5

Answer 1

Correct Answer - Option 3 : \(100:\frac{5}{{\sqrt 3 }}\)

The line current on the star side of the power transformer will be

\(500 \times \frac{{6.6}}{{33}} = 100\ amps\)

The CTs on the star side are delta connected and the current required on the relay side of the CT is 5 amps. Therefore, the current in the CT secondary is 5/√3. The CT ratio of the HV side will be

\(\begin{array}{l} 100:5/\sqrt 3 \\ = 20\sqrt 3 :1 \end{array}\)