Can a Step down transformer be used as a Step up transformer?

Yes you can do it but need to exercise some precaution: The LV winding that was intended by design to be the secondary winding, will serves as the primary & the value of the magnetizing inrush current actually will be greater than expected. When a transformer is reverse fed, the taps move to the output side and so their operation is reversed. Taps will control the output voltage so chances of over excitation will be there. This is not serious concern till the input voltage variation is within limit. OC voltage of the winding which is supposed to be the secondary will be higher than the nominal voltage. This is to allow for the drop in the winding so that the nominal voltage will be present when the transformer is loaded fully. The degree of change in the secondary voltage is expressed as the transformer's regulation; the lower the figure the better. Using the transformer in reverse you need to increase the applied voltage to the secondary which will act as primary when reverse fed.
First: You'll need to verify that the new input voltage is the same per coil winding as the original impressed across the coil to ensure the flux density in the core is per original spec and to ensure the insulation system still falls within design specs as far as strike, creep, turn to turn and layer insulation.

Second: Take the open circuit and short circuit original inductance specs and calculate the new inductances to ensure the new inductance values will not impair the xfmr application (line and load).

Third: Recalculate Watts/In^2 of cooling surface for the coils and cross-check with design specs. Recalculate new Eddy current and harmonics losses in the reverse current situation. Recalculate new watts loss (Iron loss and I^2R loss) based on reverse current flow in each coil. Depending on the rating of the insulation system you want watt loss per In^2 of surface area to be below .5 to .2 for oil filled.

Lastly: Take into account that the inrush current will either increase or decrease depending on the geometry and location of the original primary (line) side could. This could cause breaker tripping, flash over or other anomalous events on the application (load side).

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11/15/2016 4:32 PM
Could you explain the difference related the leakage flux of the primary winding  through the core limb compaired with the step up transformer? The step up transformer is  exciting and the step down transformer is no exciting. The former increased the no-load losses, noise level, temperature mainly when the the impedance is too much big.
11/8/2016 6:42 AM
It is possible, but several considerations should be checked first prior to installation. The application of a step-up transformer that is initially intended to step-down might result to issues or conflict specially in parallel operation with other transformers since the vector group of the transformer will be reversed as well. It might result to a shift in phase which is critical in parallel operation. If it is a standalone transformer, I see no problem except for the tap changer operation and grounding. If the transformer has a neutral ground at the secondary and a delta connection at the primary, reversing the transformer connection will result to grounding of the transformer at the primary. This will result to ungrounded secondary which might be an issue to your system. The conclusion is to check first its application prior to its connection.

Typical step down transformers have vector group of Dy1 or Dy11. If you use it as step-up transformer in parallel operation with other step-up transformers with typical vector group of Yd1 or Yd11, then you are paralleling a Dy1 or Dy11 to either Yd1 or Yd11 transformer. If it belongs to the same vector group (like Dy1 and Yd1), then you'll have no problem. But if you parallel a Dy1 to a Yd11, then you'll have a phase difference resulting to circulating current. So you should check first the vector group of the step-down transformer that is intended for step-up application if to be operated in parallel with the existing step-up transformers.