Choosing a distribution transformer is one of the biggest decisions in a new substation. In Bangladesh you have two main options: an oil-immersed transformer or a dry type transformer. Both do the same job — stepping the 11 kV or 33 kV supply down to 415 V — but they are built very differently.
How an oil-immersed transformer works
As our ebook explains, an oil-immersed (oil-filled) transformer uses oil to cool its coils. The body is installed in a welded steel tank filled with insulating oil. When the transformer runs, the heat from the coils and iron core passes into the oil and then to the cooling medium, usually through radiator fins.
Depending on capacity, oil transformers are built with natural cooling (ONAN) or forced-air cooling (ONAF) using fans.
How a dry type transformer works
A dry type transformer uses no insulating liquid. Its windings and core are insulated with solid materials such as cast resin and cooled by air. Because there is no oil, there is no risk of oil leakage or oil fire.
Side-by-side comparison
| Oil-immersed | Dry type | |
|---|---|---|
| Cooling | Oil and radiators | Air (natural or fan) |
| Fire risk | Oil is flammable; needs a safe location | Very low; self-extinguishing resin |
| Location | Outdoor yard or separate room with oil pit | Indoors, close to the load |
| Maintenance | Regular oil testing and top-up | Cleaning and inspection |
| Overload capacity | Good | Good with fans |
| Initial cost | Lower | Higher |
Which one should you choose?
Choose an oil-immersed transformer when you have an outdoor yard or a separate substation room, when budget matters most, and for larger ratings where oil cooling is very efficient.
Choose a dry type transformer for high-rise buildings, hospitals, shopping malls and RMG factories where the substation is inside the building, close to people, and fire safety is the top priority. Dry type transformers are often preferred in Accord and Alliance compliant factories for this reason.
Sizing your transformer
Add up your connected load, apply a realistic diversity factor and leave room for future expansion. Remember that a PFI plant reduces the kVA your transformer has to carry, so a well-corrected system can use a smaller transformer. The transformer rating then decides the LT panel incomer.
Frequently asked questions
Is a dry type transformer more efficient?
Both types can be highly efficient. The bigger difference is in safety, location and maintenance, not efficiency.
Can a transformer be installed with busbar trunking instead of cables?
Yes. A busbar trunking system between the transformer and the LT panel is neat, safe and easy to extend.
Get a transformer quotation
Tell us the rating, voltage and installation location. We supply oil-immersed and dry type transformers, and we can install, test and commission the full substation. Request a quotation.
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