When must the 30deg phase shift be accounted for in calculations?

I've found that sometimes the 30 degree XFMR phase shift is used, and sometimes it's seemingly irrelevant or disregarded.  This is even the case when we aren't necessarily going through a transformer, but even when simply converting from a phase to a line voltage, for example -- this is perplexing.  I don't have any specific examples at the moment, but if someone has a general explanation of when the phase shift must be used in PE-exam-relevant calculations, I'd be very grateful.

The only time it really matters is if one side of the transformer or the other is referenced at 0 degrees and the question is concerned about the voltage phasor on the other side of the transformer.  The only other time to concern yourself with it is if you wish to do live bus transfers from one source to another.  That doesn't work too well if they're out by 30 degrees.

nukem2k5 said:

I've found that sometimes the 30 degree XFMR phase shift is used, and sometimes it's seemingly irrelevant or disregarded.  This is even the case when we aren't necessarily going through a transformer, but even when simply converting from a phase to a line voltage, for example -- this is perplexing.  I don't have any specific examples at the moment, but if someone has a general explanation of when the phase shift must be used in PE-exam-relevant calculations, I'd be very grateful.




 




 




 




 

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Good Question!

If we are only worried about magnitudes in the answer, then a lot of times the phase angles and the 30° phase shifts are ignored.   

Example:  If a question is asking to solve for apparent power in VA, but not complex power, then most of the time phase angles are ignored in calculations because they represent reactive power components.  If you are not sure, you can always include phase angles in your calculations then only use the magnitude for the answer. 

Now for the 30° phase shifts:

For a wye system, the line to line voltage will always lead the respective line to neutral voltage by 30° if the system is positive sequenced and balanced.
For a delta system, the line current will always lag the respective phase current by 30° if the system is positive sequenced and balanced.

Almost every system will be positive sequence and balanced unless the problem states otherwise. 

If the system is negative sequence (ACB or BCA) then instead of leading, they will lag, and vice versa.  That is because the vectors now rotate in opposite directions about the origin. 

If the system is not balanced then the phase shift will be something other than 30°, and could very well be different for each of the three phases (i.e. each of the three phase shifts will not be the same).

It helps to have a look at the phasor diagrams:

For a wye system:



For a Delta System:

nukem2k5 said:

I've found that sometimes the 30 degree XFMR phase shift is used, and sometimes it's seemingly irrelevant or disregarded.  This is even the case when we aren't necessarily going through a transformer, but even when simply converting from a phase to a line voltage, for example -- this is perplexing.  I don't have any specific examples at the moment, but if someone has a general explanation of when the phase shift must be used in PE-exam-relevant calculations, I'd be very grateful.




 




 




 




 




 

Click to expand...

The only other time you will see a 30° phase shift is with three phase transformer connections, either wye to delta, or delta to wye.

Of course you will also see a change in magnitude depending on the transformer ratio but we will focus on the phase shift since you asked. 

Positive 30° (lead) phase shift

Of the line to line voltages across the primary to secondary of a delta to wye transformer connection



Negative 30° (lag) phase shift

Of the line to line voltages across the primary to secondary of a wye to delta transformer connection



Hope this helps anyone looking for answers! 

It is an often confused topic. 

We go into a lot more detail about it on our website: www.electricalpereview.com