NCEES #126

[ElecPwrPEOct11]

Hi. I understand the basics of this question but don't feel 100% confident on the phase shift on the 4 types of transformer connections. My books don't seem to bother illustrating the phase shift either. I know a delta- wye has a 30 degree phase shift through it (but I don't know which side leads). I think a wye-wye or a delta-delta would each have zero phase shift.

If this NCEES problem had one delta-wye transformer and another wye-delta transformer in parallel, would it cause any problems on the load side? It seems the 30 degree shifts would negate each other on the shared secondary side. Thanks for the help!

 

[ElecPwrPEOct11]

5 votes 'no' but no replies. Can anyone explain why this configuration won't work?

 

[Flyer_PE]

I'm dissenting. They can, in fact, be paralleled. If you are creative with the primary and secondary connections, it is possible to parallel a delta-wye with a wye-delta transformer. For either transformer, you will have a 30 degree phase shift. The direction of that 30 degree shift can be altered by how you go about connecting the transformer to the primary and secondary systems.

See attached:Transformer_Connections.pdf

 

[ElecPwrPEOct11]

Thanks Flyer_PE. Wow I have never seen anything like this. I spent some time studying the chart and think I understand it. So hypothetically it's possible, but it's tricky. I had no idea there were so many phase shifts available from a delta wye xfmr. I'm taking it as a reference, but I hope the NEC won't ask anything this detailed.