Hi guys so I recently just stumbled and realized I’m not sure on this topic. I was asked by an apprentice about the construction of a transformer specifically for the wye configuration. He asked why the X0 connection doesn’t short circuit the transformer if it’s connected to all 3 phases? How exactly is the X0 connected internally to the transformer I tried looking this up but nothing helps.
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This is a good question and I remember wondering about this myself early on.
In the basic Wye config, one end of each winding is connected to a common point (the neutral bus). So yes, in theory the windings are all shorted together.
The reason why the transformer does not blow up is because of the phase offset. Each phase voltage is entering the transformer at a different point and therefore there is a potential difference between them.
Each phase is landing on the neutral in a different sequence, they share the same space at different times. This concept may be hard to visualize, so I will try and dig up some diagrams to help explain but this is the basic why in a nutshell.
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Whether you are talking about a Single phase, Delta, Wye, or Zigzag transformer, the phases and ground/neutral are connected via metal (copper, aluminum wire/bus).
There are a few theoretical laws at play, but 2 main theories. First is Lenz’s law. How is it that a transformer is different from just shorting the phases together with a piece of bus? Both ohm out in milliohms if checked with a fluke. The answer is the coil (winding), as current passes along a conductive surface, a magnetic field is generated, and as the wire is turned onto itself, the field of the magnetic flux counteracts itself causing reactive resistance(inductive impedance). This phenomenon can also be seen by the large inrush of current that stabilizes over a quick period upon energization. A bus shorted together would continue the rush of current into an Arc Blast since there is no reactive resistance.
Second theory at play is what SecondGen was talking about, and that is symmetrical components. When 3-phase Wye is perfectly balanced, there is no voltage/current on the neutral. Depending on the imbalance, you start to discuss the symmetrical components (Positive, Negative and Zero Sequence) and that topic is much more intense for a simple blog post.
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I feel as though both responses were too indepth for a simple but important question. The impedance of the windings prevents the short circuit situation the apprentice speaks of. An electrical short is a scenario where no impedance exists between the Line-line or line-neutral. If there is no impedance you will get infinite current (or as much as can possibly be supplied). The same question could be asked about any electrical device, why doesnt a light bulb cause a short as the L-N are connected to each other but due to the resistance of the bulb current cannot flow freely to the neutral but us restrained by the resistance of the bulb.
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I see what you are saying but the original question referenced the X0 in a wye transformer so I assumed they were talking specifically about three phase.
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For sure, I was just trying to simplify for the sake of understand sometimes its easiest to bring thing to single phase especially for a level 1 apprentice I assume.
As far as the theory I do no feel it is a correct way of thinking that due to the phases being 120deg apart no phase will be “high” if you will at the neutral at different times. Current flow through the windings will basically cancel at the neutral as vectoral math states of you add 100A angle0 + 100A angle-120 + 100Aangle120 will equal 0 perfectly balance. If there is some unbalance current it will either return through the neutral (NGR) or flow through the windings to the adjacent winding. What are your thoughts on this?
Its such a small question but it is very fun to discuss just based on how different folks have been taught in school across the land.
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Thank you guys for the feedback I really appreciate it! Everyone I’ve asked has had a different way of explaining it. So it’s very informative to see different perspectives of it. I’m currently studying to take my level 3 and the fact that I couldn’t answer this was a bit embarrassing haha! 
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Trying to teach is when you really start to understand;)
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Your response is in reference to symmetrical components. But the fault in your logic would be explaining it for Open Delta configuration or High-Leg Delta. These can still be visually and mathematically calculated using symmetrical components.
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