I'm trying to solve some problems and I'm confusing one thing . We have , let's say a 30MVA, 24.5/345 KV 3-phase transformer connected in wye-delta. I try to use one-phase analysis and want to find the voltage ratio and make use of the following equation: $$S=\sqrt 3VI$$.

The values V and I are phase or line voltage/current? And when I find the voltage ratio which of the following is right? $$a=\frac{24.5}{345}$$ $$a=\frac{\frac{24.5}{\sqrt3}}{345}$$

I divide by the root of 3 because of the Y connection.


The nominal voltages given to describe a 3-phase transformer are always the line-to-line voltages, regardless of the connection. The voltage used for the single-phase equivalent circuit are always line-to neutral voltages. The transformer ratio is 24.5/345 but the single phase transformer voltages are those voltages each divided by root 3.

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  • \$\begingroup\$ Thank you for answering to so many of my electrical machine questions sir. I'd like to ask one more thing which I forgot to mention. If in this case I'm given the voltage across the load and the impedence of the transformer can I use the voltage ratio and from the given load voltage find the primary voltage or do I need to take impedence into account? In fewer words , does the ratio include the voltage drop because of the impedence or not? \$\endgroup\$ – John Katsantas Jun 8 '17 at 14:30
  • \$\begingroup\$ If you are given the impedance of the transformer you should probably assume that the voltages given are the open-circuit voltages and you must take the impedance into account. \$\endgroup\$ – Charles Cowie Jun 8 '17 at 15:03

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