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Suppose I have an industrial plant, where a switchboard is fed through a transformer. The switchboard have only "passive" loads, i.e. they don't contribute with short circuit currents.

If there is a short circuit on the switchboard feeding that transformer (on the grid side), my intuition tells me that the transformer would be demagnetized (disregarding remanent flux), thus energy must go from the transformer to either the secondary or primary side. Since current can't immediately change in an inductor, I think this energy should go to the secondary side.

My simulations show this behavior. As the image below show, there is current/power flowing for about 2 ms (the results on the primary side is identical, so I haven't included it). However, I'm not sure if this is the correct physical representation or a result of inaccurate mathematics. The minimum integration step I can select is 1 ms, so it might be a result of that (I think).

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The reason why I doubt the results is because the exact same current flows from the shorted bus and in to the transformer on the primary side. Investigating the results on both sides of the transformer, I see that the exact same amount of energy goes in and out after the primary side is shorted. This tells me the transformer is not demagnetized.

Now, as we all learned in kinder garden, current in must equal current out, so how could I expect something different? Well, it's not really true in all situations (you can for instance charge a capacitor). I know the energy store in a transformer is magnetic energy, not electrical.

Still, there is something here that doesn't seem right to me. I appreciate any input!

Thanks!

Update

It is possible that my explanation has not been good enough. The short circuit I'm talking about is a fault that occurs after a certain time, 3-phase, line-line, line-line-ground or line-ground.

Here's a part of the single line diagram. I don't have any "justification" for why the delta-star connection is relevant for the analysis (I don't think it is, it's simply how the system is designed). There can be many reasons, elimination of zero sequence currents for one, but this shouldn't be relevant for this quesiton.

The diagram below is how the transformer is represented in my simulation tool. I can't "show" how the circuit is shorted. The way it is done is I add an "event": "Create a (3 phase / Line-Line / ...) fault after 3 seconds. Then I can see the system response as a result of this event.

I would think the general behavior of the transformer is independent of the topology, vector group etc.

enter image description here

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  • \$\begingroup\$ why don't you draw a simplified schematic. We don't need to know about grids and switchboards just show the minimum to explain your thoughts. \$\endgroup\$ – Andy aka Feb 28 '14 at 9:55
  • \$\begingroup\$ That doesn't help - I think you need to show a transformer connected to a load with the primary connected to an ac source with a switch that shorts the primary out. I'm guessing here because you sound like you are asking a simple question but you're complicating stuff with all the star/delta symbols - think minimum OR justify why all the star/delta stuff is needed for the analysis. \$\endgroup\$ – Andy aka Feb 28 '14 at 10:11
  • \$\begingroup\$ @Andyaka, please see the new edit. I agree with you, and believe too that I'm asking a simple question and that details regarding vector groups, how it is connected etc. is irrelevant. If the question is still not clear, I appreciate your advice on how I can improve it. It was not my intention to include too much information in the question, but I thought it would be good to include my thoughts, so that you (and other users) might see where me logic (or simulation tool) fails. \$\endgroup\$ – Stewie Griffin Feb 28 '14 at 11:01
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Think of a single phase transformer feeding an open circuit on the secondary. The AC is a normal AC signal then suddenly the AC signal instantly becomes 0 V (this represents the short on the primary feed). There will be some amount of magnetic flux (aka energy) in the transformer core that will push a current back out of the primary windings into the 0 V AC source.

If the transformer is perfect in all respects (other than it has a magnetizing inductance), the current will flow into the "0 V" indefinitely BUT there are of course resistive losses so this current will be an exponential decay to zero. If there is no leakage inductance on the primary winding, the 0 V (aka short) will ensure that there is nothing seen on the secondary other than the cessation of the AC waveform when it instantly falls to zero.

If there is leakage inductance on the primary (normal of course) then there will be a small kick-back voltage seen on the secondary due to the magnetizing inductance of the primary not being perfectly shorted to 0 V.

With or without a load it won't make a difference - there will be a small kick-back voltage seen on the secondary as the magnetic flux (and current) exponentially decay to zero.

It should also be noted that if the AC falls to zero volts at the very peak of its waveform, the current in the magnetizing inductance of the primary is zero and no effect will be seen. This is because, at that instant in time, the flux in the core will also be zero. If, on the other hand, the AC voltage halts at a zero-cross (and stays at zero), the flux in the primary will be at a maximum and the effect described above will take place. At all other points the effect will be proportionally less.

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