Power and distribution transformers are all basically the same design and vary only in the method of cooling them, oil or air.  By far the vast majority of transformers have been oil cooled using mineral oil.

Recent environmental and fire hazard considerations however have resulted in greater numbers of air cooled transformers being manufactured without necessarily taking into consideration the alternatives or implications of the decision.  Below is a summary of the various alternatives.

Mineral oil filled transformers.

This is the traditional distribution transformer as has been in operation for many years.


  1. Lowest cost
  2. Largest installed base of units for interchangeability
  3. Good ability to withstand overload
  4. Operates at relatively low temperature (max 105⁰C but normally 60-80⁰C)
  5. Physically compact
  6. Robust construction
  7. Fully enclosed
  8. Components including oil readily available


  1. Prone to leakage
  2. Mineral oil is an environmental hazard in the event of spillage
  3. Mineral oil has a low flash point in the event of fire
  4. PCB levels need to be controlled.

Dry type or Air cooled transformers

There are two main methods of manufacture of dry type transformers, namely Vacuum Pressure Inpregnated insulated windings or Cast Resin windings.


  1. No fluid which can become an environmental or fire hazard
  2. Cast Resin windings are fully encapsulated preventing the ingress of foreign particles


  1. Higher cost.  An oil cooled transformer will operate at around 3 – 3.5 A/mm2whereas a dry type transformer will operate at around 1.5 – 2 A/mm2 .  There is thus approx double the amount of copper in a dry type transformer when compared to an oil cooled one.
  2. The no-load losses in a dry type transformer are normally higher than those on a similar sized oil cooled transformer
  3. The efficiency of air as a cooling medium is far less than that of oil.  As a result air cooled transformers normally have an IP23 enclosure rating, although IP55 is possible but the enclosure is massive in order to accommodate the cooling required.
  4. The operating temperature of dry type transformers is normally in excess of 120⁰C.  Although the windings and insulation are rated for this temperature other objects which may come to rest or be placed on the transformer are normally not rated for such high temperatures.  This presents a fire hazard.
  5. The ability to withstand overload is significantly lower than oil cooled transformers.

Seed or silicon oil filled transformers

These are standard oil cooled transformers which are filled with either a seed or silicon oil.  Silicon oil has the advantage of higher temperature ratings but has the same environmental disadvantages as mineral oil and is therefore not normally considered an option.

Seed oil which can be manufactured from various different seeds or vegetables not only has a significantly higher temperature rating but is fully bio-degradable.


  1. Environmentally acceptable.
  2. High temperature rating
  3. Existing mineral oil cooled transformers can be converted to seed oil
  4. Will still operate at the same relatively low temperature as mineral oil cooled transformers.
  5. Good ability to withstand overload.
  6. Physically compact
  7. Robust construction
  8. Fully enclosed
  9. Lower cost than dry type transformers


  1. Higher cost than mineral oil cooled transformers
  2. Leakage will remain a problem, though the oil leaked is bio-degradable.
  3. The oil is not as readily available as mineral oil
  4. There is a risk of theft of the oil.

Attached please find some literature on seed oil which details the advantages.

Cost comparison

The costs of the various alternatives will vary depending on the specific enquiry however a rough rule of thumb would be as follows, assuming a standard mineral oil cooled transformer as being 100

Mineral oil cooled transformer                                    100

Dry type transformer (IP23)                                       160

Dry type transformer (IP55)                                       200

Seed oil cooled transformer                                       130


Where environmental and fire hazards are a major consideration then seed oil cooled transformers are a viable economic alternative.

Peter Flint

Managing Director

Armcoil Afrika (Pty) Ltd