One of the characteristics of all transformers, regardless of size, style or design, is that when excited, they create losses in the circuit.
Some of these losses are associated with the excitation of the active zone and the creation of a magnetic field, and some losses - resistive losses (I²R) from load currents flowing in the conductors of the windings.
Both types appear as heat, and heat is the enemy of the number one insulating material.
The task of transformer designers is to allow transformers to dissipate excess heat and ensure a longer insulation life.
For air-cooled transformers, this is achieved by providing adequate ventilation and cooling channels in the coils. Where there is not enough air, fans are added to increase heat transfer from magnetic elements and vulnerable dialect insulating components.
For transformers filled with liquid, the approach is similar. The cooling channels in the coils should be of sufficient quantity and size so that the dielectric fluid flows through the coils to remove heat. This fluid can move by simple convection, or it can be “forcedly cooled” by pumping fluid. In addition, the surface of the tank must be large enough to transfer heat from the liquid using a combination of construction, convection and radiation. As transformers increase, the surface area of the tank becomes limited, and external radiators are added to increase the surface area for heat transfer. To maximize this process, cooling fans can be added to speed up heat dissipation through radiators.
How do transformer manufacturers provide information on transformer tags?
For dry transformers that are air-cooled, the ANSI / IEEE C57.12.01 standard provides the following designs:
1. Ventilated self-cooled class: class AA
2. Ventilated class with forced air cooling: AFA class
3. Ventilated self-cooled class with forced air cooling: class AA / FA
4. Non-ventilated self-cooled class: ANV class
5. Class of hermetic self-cooling: class GA
Fuel oil transformers offer several more cooling options. The ANSI / IEE C57.12.00 standard defines a four-digit code for describing the attributes of transformer cooling.
The first letter indicates the internal cooling medium in contact with the windings.
* O = mineral oil or synthetic insulating liquid with a firing point of 300 300 ° C
* K = insulating fluid with a point of fire> 300 ° C
* L = insulating fluid without a mesostable fire point.
The second letter indicates the circulation mechanism for the internal cooling medium.
* N = natural convection flow through cooling equipment and in windings
* F = for forced circulation through cooling equipment and natural convection flow in the windings (also called “directional flow”)
* D = Forced circulation through cooling equipment, directed from the cooling equipment at least to the main turns
The third letter indicates external cooling medium.
* A = Air
* W = water
The fourth letter indicates the circulation mechanism for the external cooling medium.
* N = natural convection
* F = forced circulation (fans (air cooling), pumps (water cooling))
For example: ONAN stands for an oil-filled unit that has a natural convection flow in a tank and uses natural convection air cooling from the outside.
If this transformer has fans added for forced air from outside, the design will be ONAF.
A transformer with natural convection cooling as a base rating and an increased rating when fans were added later will be designated as ONAN / ONAF.
High-precision flame-retardant liquids use a “K” design for the liquid type. Thus, a naturally high liquid liquid with a high point of the flame will be KNAN, and the same unit with fans will be KNAF.
