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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?

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? Correct Answer 7/5

As the power levels remain same at the two sides of transformer, √3*400*1000 = √3*33000*IL2 IL2= 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 33000V side = 400/(33*5√3 ) = 7/5.

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