Reverse Feed of Step Down Transformer

Winding ratio and sizing is the only difference. The laminated iron core can't tell the difference between which winding set is the source and which is the load.

Thank you so much for your reply. This is the difference what we know as Electrical Engineer. But certainly for a transformer design engineer, there must be some difference that's why step down transformer with same rating cost 70% as the cost of step up transformer.

Secondly, I make an analysis of both type of transformer under same condition for different circumstances and I found a big difference in response.

You do have to use the transformer at the same spec, same voltage and amperage in as was out, and same voltage and amperage out as was in...

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-9/step-up-and-step-down-transformers/

So, refer to the sizing protocols embraced by the design company.

There is no point in wasting time by waiting for a prejudicially-desired response from this forum!

" there must be some difference that's why step down transformer with same rating cost 70% as the cost of step up transformer."

Too often the reasoning is simply, 'Because they can.' and I have dealt with that mentality on parts suppliers all my life.

What has cost got to do with anything?

Cost is certainly nothing to do...

but one of our customer has selected Step down transformer with same rating for step up application. Because the cost is really low.. now we are facing a terrible problem regarding the core saturation during load rejection.

That is a sales opportunity.

Then there is the rest of the question: will core saturation limit the power input when using a step-down transformer as a step-up transformer?

The answer is an unqualified yes.

The correct response would be to tell them not to ever do this again, unless they want the family stones to be lost in the river.

A properly designed GSU will have more iron in the core to accommodate a higher level of overexcitation, as well as higher than normal terminal voltage (V/Hz) excursions. That's just one of the reasons why a properly designed, engineered, and specified GSU costs more.

I barely can remember the formula I used in computing for the core area..... I know the power transformed value, as the key factor in the computation for the size of the core square area..

The flux density characteristics of the silicon steel material to be used is sure is a factor as well....

Bad news. If it is bolted direct to a generator, a generator transformer must be designed to withstand a considerable overvoltage on load rejection, around 140% comes to mind. You have obviously become well acquainted with the saturation problems got if you use a standard transfo.

I assume you trip the generator excitation field suppression contactor and prime mover at the same time as the breaker, it might make the transfo last until fastest possible delivery/installation of proper transfo (which will be bigger).

Depending on the scenario one certainly can use a step down transformer to step up voltage. At the same time there are scenarios where this will produce catastrophic results.

The first unstated critical detail is the anticipated nominal voltages that will appear on both coil windings when used "backwards". A 10:1 step down from 120 VAC to 10 VAC cannot be safely used to step up 120 VAC to 1.2K VAC. But this transformer can certainly be used to boost a 1.2 VAC signal to 12 VAC, as long as the wattage is correct. Then there are a myriad of plausible fault conditions a transformer may find. A few of these will be load shedding power surges, lightning direct and indirect (coupling) effects.

Vsar is correct.

On large distribution transformers, the winding location with respect to the core and its exact construction changes for primary and secondary. This is mostly to survive fault conditions such as short circuits or high voltage transients usually seen at the primary.

Another factor not mentioned above are the protection relays that may not operate properly in reversed power.

But for a small transformer (200kVA or less), in low voltage applications, as long as the respective rated coil voltage and current are respected, you can use them as step up or step down for laboratory work. Do not use them this way in a commercial application as the UL or CSA approbations will not stand.

P.S. While this is an interesting question, our friend OP seems to have us do most of his EE work. This is not the first post of this kind.

OP, be careful when using info from this site. It is all on your head. You seem to be involved in subjects that are far beyond your training. Be very careful as using our help, you may look more knowledgeable than you really are but some day you will have to face the music. Make sure a competent EE is involved in the implementation of all these comments...

Actually you're describing the constructional differences between equally rated distribution/transmission transformers and those specifically designed for GSU (Generator Step Up) duties.

A GSU usually has to withstand the highest short circuit duties of any other transformer on the system; it not only has to withstand the SCC (Short Circuit Contribution) from the the system it is feeding, it also has to withstand the SCC from the generator, both of which are additive if the fault occurs anywhere near the transformer and the generator terminals. That's one of the reasons why Isolated Phase Buss contruction is used between the two.

Another reason why a GSU is different is the possible requirement for a lower impedance than other transformers. The desire for lower voltage drop/regulation has to be balanced with the increased available SCC which is a result of the lower impedance. A further consequence of the desire for lower impedance may result in larger conductor size for both lower real power losses and greater mechanical strength, all of which results in more copper and iron than a "regular" transformer, with a subsequent increase in size and cost.

There are other considerations as well; now it is time for OP to seek the advice of a power engineer well versed in these matters long before an inadequately sized (but cheaper) unit is purchased. If on the hand the transformer is destined to be located remote from any generation/concentrated load or between two systems, then a less robust unit may do.

A specific application that requires stepping-up (increasing) the voltage from the available level to the voltage required by the application.

Thank you for sharing good information.