Discussion
A three-phase transformer bank consisting of three single-phase transformers is used to step down the voltage of a three-phase, 6600 V transmission line. If the primary line current is 10 A, calculate the secondary line voltage for the star/delta connection. The turns ratio is 12.
Answer: C
The primary is star connected on a 6600 V line, so the primary phase voltage is
\[ V_{1ph} = \frac{6600}{\sqrt{3}} = 3810.5\ \text{V} \]
The turns ratio of each single phase transformer is 12, so the secondary phase voltage is
\[ V_{2ph} = \frac{3810.5}{12} = 317.5\ \text{V} \]
In a delta secondary the line voltage equals the phase voltage, so the secondary line voltage is 317.5 V, which is the 317.55 V of the options. The primary current of 10 A is not needed for the voltage. The other values are 10, 100 and 1000 times off. The answer is option C.
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