Transformer with High Voltage Ratio

Short circuit current is not a more serious problem for high ratio transformers than for low ratio transformers. You can design a high ratio transformer to have any secondary short circuit current desired.

The problem is the same insulation clearance is require for a 1 kVA high voltage transformer as for a 5000 kVA high transformer. So small kVA high voltage transformer cost is the same for insulation and bushings as a large kVA high voltage transformer.
This means that the cost/kVA for small high voltage transformers are (usually much) higher than the cost/kVA for large high voltage transformers.

For DC VSD drives I personally prefer Drive Type isolation transformers with a shield between windings. However, primary voltages are usually 2300 or 4600 volt.

Your ratio seems a bit extreme, but I suppose in the correct environment it could be reasoable - I just can't think of the situation.

Voltages above 33 kv are used in transmission of high power because of lower losses. 400 volts are at the lowest level of distribution setup. The application are totally different. In theory while it is possible to build transformers with such voltage ratios (overcoming big technology challenges), the sheer cost involved will make it a non starter.

The primary restriction would be economic, as transformers rated for that primary voltage are very expensive compared to a typical distribution-grade unit. Metering potential transformers for transmission circuits can have very high ratios, but they have very little capacity (usually 2 kVA or less).

However, high ratio power transformers are sometimes used to provide auxiliary power at remote transmission switching stations. There are places where you don't have a medium voltage circuit handy and getting one to the site would cost more than a special-design transformer.

No it is not recommended, in your example you are trying to take a very high voltage transmission level supply and feed it directly into a low voltage distribution supply (you don't mention the transformer size, so I will keep the examples generic). The power grid is not designed and set up for this, that's one reason we have substations all over the place and smaller 11kV to 400V (for example) distribution transformers to actually step down the voltage for the consumer.

For a near-by dedicated very large factory load, perhaps, but even then large loads are likely to be operated on high or medium voltage (standard specific) rather than 400V low voltage so a single high voltage ratio transformer doesn't make sense.

With regards to high voltage distribution transformers outside of a substation, how do you even connect up to the 275kV line way up on those tall transmission pylons or buried under the ground, their are no easy branch access points to allow connection except at each end when they connect to either a power station or a substation.

Is this homework, because it sounds like it is?

_{Jack - In the middle of designing a real substation.}

It is not homework, it is rather hearing from electrical engineers, especially who are in the middle of designing real substation

Just about anything is potentially possible but customer requirements, protection equipment, economics, industry standards, safety and common sense will limit the options.

Have you looked at different transformer manufacturer websites to get a bit of an idea what is commonly used?

Selecting such a large voltage ratio significantly reduces the options for system expansion and alteration in the future, and might be described in some quarters as "myopic".

I think that you should re-visit that voltage ratio because it appears to be very extreme. Typically, 11kv, 6kv are transmitted from the sub-station to private industries which will break it down to 415v/230v for workshop, office buildings etc. Typical transformer ratings are: 100kVA, 4kv/415v or 100kVA, 4kV/230v to domestic residence. May I suggest that you should check out the rating of the transformer and get the full load primary current and full load secondary current as follows: 100kVA/4kv = Ip and 100kvVA/415 = Is and then see if the load fits into this for whatever you want to do. What you have there is too extreme.

Apart from technical issues cost would be prohibitive. Nobody wants to connect their tv and microwave directly to the transmission network.