NCEES 506 transformer ratio problem

I'm sure it's because I'm tired, but I can't follow the solution of "because the connection, the high side is the phase voltage"......


 

The only thing that i can think of, is it says "single phase transformers" so if you assume single phase, then it's the phase of delta to the phase of wye, and delta is VLL but Wye phase is VLN. This would trip me up too.

Ampera18 said:

The only thing that i can think of, is it says "single phase transformers" so if you assume single phase, then it's the phase of delta to the phase of wye, and delta is VLL but Wye phase is VLN. This would trip me up too.

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Yeah I seen single phase transformers, so I took the root3 of both. 

It helps to draw it all out. You must divide the Secondary line voltage by root3 because it is wye connected. The three phase wye phase voltage is applied across the secondary phase windings, not the line voltage. For the Delta side, the three phase delta line voltage is applied across the windings, thus we do not need to divide by root3. 

Terrible drawing and description but hopefully it helps. The area outside the dashed line is the 3 phase rating given by the problem. The area within the dashed line are the individual transformer connections and what the solution is asking for. 

Another way I like to think about 3-phase transformer banks is this:

For each individual 1-phase transformer of a 3-phase transformer bank, the individual turns ratio a 1-ph will depend on the PHASE voltages of the transformer windings. The PHASE voltages in turn depend on whether the windings are delta-connected or Y-connected.

For a delta-connected winding, the phase voltage equals the line-to-line voltage of the 3-phase system.

For a Y-connected winding, the phase voltage equals the line-to-neutral of the 3-phase system.

The individual turns ratio a 1-ph is just the ratio of the primary PHASE voltage and the secondary PHASE voltage of the 1-phase transformer.

zgsharon is right in that when you first do such 3-phase transformer bank problems you should draw it out. After I drew it out similar to how he did, from there on I just use the fact that Y-winding phase voltages are line-to-neutral voltages and delta-winding phase voltages are line-to-line voltages.

Attached is how I quickly do such 3-phase transformer bank problems with individual 1-phase transformers. I simply identify the primary and secondary phase voltages of each 1-phase transformer based on how the windings are connected (delta or Y), and then use a 1-ph = V1 ph / V2 ph.

zgsharon said:

It helps to draw it all out. You must divide the Secondary line voltage by root3 because it is wye connected. The three phase wye phase voltage is applied across the secondary phase windings, not the line voltage. For the Delta side, the three phase delta line voltage is applied across the windings, thus we do not need to divide by root3. 

Terrible drawing and description but hopefully it helps. The area outside the dashed line is the 3 phase rating given by the problem. The area within the dashed line are the individual transformer connections and what the solution is asking for. 

View attachment 19688

Click to expand...

Thank you, yes I understand the transformer connection of the delta-why I guess I just wasn't expecting them to call each side the "single phase transformers" and it retain the entire line voltage of the 3 phase system. Just something I forgot could happen I guess. 

That or my mind was just fried from hammering out problems.