One characteristic of all transformers, regardless of size, style, or construction, is that when energized, they create losses in the circuit.
Some of these losses are from energizing the core and creating a magnet field, and some losses are resistive losses (I²R) from load currents flowing in the conductors of windings.
Both types manifest themselves in the form of heat, and heat is the number one enemy of insulation material.
The task for transformer designers is thus to allow transformers to dissipate excess heat and thereby ensure longer insulation life.
For air cooled transformers this is accomplished by providing adequate ventilation and cooling ducts in the coils. Where there is not enough air flow, fans are added to increase heat transfer away from magnetic elements and vulnerable dialectic insulating components.
For liquid filled transformers the approach is similar. Cooling ducts in the coils must be in sufficient number and size to allow dielectric fluid to flow through the coils to remove heat. This fluid can move by simple convection, or it can be “force cooled” by pumping fluid. Additionally, the tank surface must be large enough to transfer heat away from the fluid by a combination of conduction, convection, and radiation. As transformers get larger, tank surface area becomes a constraint, and external radiators are added to increase the surface area for heat transfer. To maximize this process, cooling fans can be added to expedite the heat removal through radiators.
How do transformer manufacturers indicate information on transformer rating plates?
For dry type transformers which are air cooled, ANSI/IEEE Standard C57.12.01 provide the following designations:
1. Ventilated self-cooled class: Class AA
2. Ventilated forced-air-cooled class: Class AFA
3. Ventilated self-cooled / forced-air-cooled class: Class AA/FA
4. Non-Ventilated self-cooled class: Class ANV
5. Sealed -self-cooled class: Class GA
Liquid filled transformers offer a few more options for cooling. ANSI/IEE Standard C57.12.00 defines a 4 digit code to describe the cooling attributes of the transformer.
The first letter designates the internal cooling medium in contact with the windings.
* O= mineral oil or synthetic insulation fluid with a fire point ≤ 300°C
* K = insulating fluid with a fire point > 300°C
* L = insulating liquid with no me3asurable fire point.
The second letter designates the circulation mechanism for internal cooling medium
* N = Natural convection flow through cooling equipment and in windings
* F = Forced circulation through cooling equipment and natural convention flow in the windings (also called “directed flow”)
* D = Forced circulation through cooling equipment, directed from the cooling equipment into at least the main windings
The third letter designates external cooling medium
* A = Air
* W = Water
The fourth letter designates the circulation mechanism for the external cooling medium.
* N = Natural convection
* F = Forced circulation (Fans (air cooling), pumps (water cooling))
For example: ONAN designates an oil filled unit that has natural convection flow in the tank and utilizes natural air convection cooling externally.
If this transformer has fans added for forced air externally, the designation would be ONAF.
A transformer that has natural convection cooling as a base rating and an elevated rating when fans were added later, would be designated as ONAN/ONAF.
High fire point fluids use the designation of “K” for fluid type. Thus a naturally cooled high fire point fluid would be KNAN and the same unit with fans would be KNAF.