GATE EE 2016 Set 2 — Question 61
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Electrical Machines → Three-Phase Transformers → Connections & Vector Groups
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Question
Three single-phase transformers are connected to form a delta-star three-phase transformer of 110 kV/ 11 kV. The transformer supplies at 11 kV a load of 8 MW at 0.8 p.f. lagging to a nearby plant. Neglect the transformer losses. The ratio of phase currents in delta side to star side is
Correct answer
(A) 1: 10√(3)
Solution
For a three-phase transformer, the voltage rating (110 kV/ 11 kV) refers to the line-to-line voltages ().
Therefore, the ratio of phase currents in the delta side to the star side is .
1.Primary Side (Delta connection):
Phase voltage kV.2.Secondary Side (Star connection):
Phase voltage kV.3.Phase Current Ratio:
For an ideal transformer, the ratio of phase currents is inversely proportional to the ratio of phase voltages:Therefore, the ratio of phase currents in the delta side to the star side is .
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