# What happens to a transformer if the supply side is shorted?

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).

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.

• 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. Feb 28, 2014 at 9:55
• 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. Feb 28, 2014 at 10:11
• @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. Feb 28, 2014 at 11:01