Parallel transformers benefits

[MercedesLopez]

Could you please explain what are the benefits of parallel transformers?

[MathieuGuillot · Accepted solution]

Hello Mercedes

Thanks for your interesting question. Could you be more explicit on your application?  Anyway, I will try to bring answer for MV/LV transformer application in buildings.

Paralleling two or more transformers can be done for two main reasons: To increase the power delivered to LV system or to Increase availability of the supply of LV system (redundancy)

For a given load of a low voltage installation supplied by MV grid there are several possible architectures

- Using several smaller separate MV/LV transformers supplying several sub-systems in LV

- Using one MV/LV transformer covering all needs supplying one LV system

- Using two (or more) smaller MV/LV transformers in parallel supplying one LV system

- Using two (or more) transformers able to supply all the LV system each

I will leave aside first case. But why should we prefer one big transformer or two smaller in parallel? For example, if a building has a maximum power demand estimated around 1450kVA is it better to have one 1600kVA MV/LV transformer or two 800kVA in Parallel?

Benefits of two transformers solution:

• Small is beautiful … Smaller transformer will be lighter and easier to handle especially in basement or electrical room difficult to reach. But be careful, weight and size are not proportional to rated power. Volume and weight per kVA is lower for bigger rating. (1600kVA oil transformer is about 3000kg when 800kVA is around 1800kg
• Smaller transformer can fall in more standard range, in the example before 800kVA MV/LV transformer is more common than 1600kVA so price and/or delivery time can be more attractive.
• For business with a significant number of transformer (multi site business …) it may make sense to “standardized” one rating like 630 800 or 1000kVA and play with the number of transformers according to the need of each site. It allows to have a standard “brick” and a shared spare. (Transformer are not failing often but they are not so easy to get in a short delay…)
•  
• In addition having two transformers of half the power will allow a “partial” redundancy in case of failure of one transformer, the site may continue to operate with a reduce load.

• Full redundancy is possible but in that case each transformer shall have the full power capability and in that case we lose the previous benefits. And it will rise some other questions (Active or passive redundancy, selectivity …)

  Paralleling transformers requires some attention on the transformers : same winding arrangement and vector group, same transformation ratio same impedance drop. Rated power of each can be different. for detail see Schneider Electric - Square D technical note on paraleling transformers

Mathieu Guillot

Power System Expert - Schneider Electric