Complex Imaginary Test 2 Question 45

[pbo064]

On CI test 2 question 45 the student is given a wye-wye 230 kV 3-phase transmission line being stepped down to 200 kV using three autotransformers connected between the neutral and and the line. The problem asks the student to find what the voltages across the series coil is for each autotransformer.

The solution begins by finding dividing each voltage by sqrt(3) to the phase voltage. It then subtracts low side from high side to get series voltage.

I don't understand why the answer wants a phase-to-phase voltage since both sides of the transformer are in line-to-neutral and the autotransfomers are stated to be connected between the neutral and the line. Are series coil voltaes always expressed in phase to phase voltage, or am I missing something else?

 

[Wael]

1- Since you are dealing with 3 Auto transformers, then each one will be phase to neutral connected. Therefore, first thing you do is to get the Line to neutral voltage.

2- In Auto transformer, Sires coil is the top one (which is indicate the primary), the common coil is the bottom one (secondary).

I hope this helps.

All the best