Power Transformer Rating Selection

[salem]

How to choose the rating of a power transformer 11/0.400 kV.
Is it to be chosen based on the total connected load to the transformer or their are other factors to be considered.
This transformer is used to supply loads inside a hospital.

 

[electricpete]

Other factors that might be considered include:

Impedance => impact on voltage under varying load conditions including motor starting.

Impedance => impact on short circuit current availability

future load growth? emergency overload in event of contingency (failure of alternate transformer)

 

[wbd]

Additionally, depending on where the transformer is located and the size, you may want to consider a dry type or using one of the new environmentally friendly liguids.

wbd

 

[salem]

Let me ask the question in another way,
Is the minimum transformer rating to be chosen equal to the total connected load (no diversity or maximum demand considered).

 

[dpc]

There is no simple answer because it depends on the nature of the loads, the application, economics and other factors. For industrial applications, I would size the transformer such that its maximum forced-cooled rating is slightly larger than the highest peak demand expected. Plus whatever allowance is appropriate for future growth.

For a hospital I would be more conservative and probably size based on connected load unless this was clearly unreasonable. As a minimum, I would size such that the transformer could carry the maximum expected peak load continuously at its self-cooled rating - no reliance on fans.