ONAN or ONAF_ What is the Difference-

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ONAN or ONAF_ What is the Difference- Powered By Docstoc
					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 (I 虏 R) 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 鈥渇 orce cooled
鈥?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
 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 鈥搒 elf-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 掳 C
 * K = insulating fluid with a fire point > 300 掳 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
* 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
 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
 To know more about ONAN,ONAF and liquid filled distribution transformers. Visit
Pacific Crest Transformers website

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