Transformers and current limiting

[dm9289]

I was recently doing some transformer calculation refresher material, and it was mentioned that by nature transformers are current limiting. Also it was mentioned that this could possibly make OVCP slightly less important.

I seen transformer secondary short circuit formulas and to me it seemed like the current could be 50 times more than the transformers rating. I guess it is technically limiting but it can still have extremely high currents.

For example if I put a 200amp load on a transformer secondary rated at 100amps with no protection would the current somehow be limited by transformer impedance to 100amps?

I had little luck on web searches

 

[Speedskater]

It depends on the transformer:
a] in some transformers, the core will saturate with over-current (effectively limiting current)
b] in dome transformers, an internal thermal fuse will blow with continued over-current.
c] some transformers are very conservatively speced. They will proved way more current than their rating.

 

[electrofelon]

A distribution or power transformer can supply around 20-70 times the rated current. This is determined by taking the transformer full load current divided by the impedance. Impedance values vary from about 1.5-5.5%. this is assuming a perfectly stiff supply system, but in reality the supply will have impedance which will probably lower these figures.

 

[dm9289]

It depends on the transformer:
a] in some transformers, the core will saturate with over-current (effectively limiting current)
b] in dome transformers, an internal thermal fuse will blow with continued over-current.
c] some transformers are very conservatively speced. They will proved way more current than their rating.

Thank you. I did see online there are some specific current limiting transformers.
I did see one transformer that had an integrated breaker in practice.

What i see in the industrial process I work in is many transformers used for isolation dry type 480-480 no fuses or breakers.
Would the rated secondary i used as an example 100amp perhaps being overbuilt supply 140amps. If it had improper on no protection after that level what would happen? Would it just burn open? Or would it be a large event?

Thank you for your responce

 

[Elect117]

The transformer is just converting the energy from one current/voltage to another. If you overload it for a long period of time it usually ends up with burnt/failed insulation in the windings, small arcing events until it fails catastrophically. More often than not, the conductor is the limiting factor. The conductor insulation or connections might melt and fault before the transformer does.

If you are talking about faults on the secondary side then it can depend on different variables like the type of connections, the type of fault, etc.

 

[ron]

A transformer primary/secondary power path has significant impedance as compared to just conductors. It is a good method to reduce fault current.

 

[jim dungar]

A transformer does not really limit 'full load' current. Its impedance limits fault current.
But its limiting effect can be 'hidden' by change in voltage. It might be easier if you look at the transformer as limiting short circuit kVA instead.