Oil as coolant
In most transformers, a dielectric fluid (mineral oil, synthetic, ...) is used in the cooling circuit for dissipating heat from the magnetic circuit and windings.
The oil is moving in a transformer in operation, by a convection natural or forced * (using pumps or fans). It is cooled by ambient air through radiators or cooling towers in which fans force air circulation.
* Convection: it is for example the warm air rising and the hottest water that goes to the surface
Cooling is characterized on transformers by the following letters:
the type of fluid: A = Air, OR = Oil, and Water =
the type of traffic: N = Natural, Forced F = D = Headed
Exemples:
ONAN (Oil Natural Air Natural):
natural circulation of the oil in the transformer and air on its exterior surfac ONAN (Natural OIl Air Force):
natural circulation of the oil in the transformer and air ventilated on its radiators.
ODAF, OFAF, ODAF (with water!)
The iron and copper dissipate energy loss as heat. This results in system temperature increase. A balance will be reached wherein the heat generated is dissipated to the refrigerant.
In the great majortié transformers, the temperature limit is set by the degradation of paper which to work in good conditions must be below 100 ° C. Effective cooling is essential.
The heat capacity, or specific heat and thermal conductivity of the oil have an important influence on the rate of heat dissipation in the transformer
The oil as insulation
In many electrical devices the oil allows the isolation between the elements to different electrical potentials that must be isolated from each other.
The transformers are often defined to support above their nominal operating voltages for brief periods. Which involves holding the electrical stresses due to transients, switching or lightning strikes.
The oil is needed, and with a substantial contribution to the efficiency of solid insulating penetrating into those below and filling the spaces between layers of paper in particular.
Since the first transformers immersed in oil, the dielectric strength test was the only indicator of the electrical quality of the oil. Even today, while other more sophisticated tests exist, dielectric strength test is still used as a simpler and more practical to be done on the ground.
The importance of oil for the maintenance and diagnostics
The oil properties of a transformer used to define a maintenance plan tailored to the health status of the unit. Indeed, the chemical composition of the oil changes with time and aging of the particular unit of its active part.
A small amount of oil taken and analyzed in the laboratory allows for cheaply a good vision:
The insulating properties of the oil
The presence of moisture in the paper
The aging of the active part
The presence of any electrical or thermal fault inside the transformer
For this, the analysis laboratories carry out physicochemical measurements of the properties of the oil, the main trial or measured magnitude are:
dissolved gas analysis
dielectric strength or breakdown voltage
Water content
Acidity
furan derivatives
Tangent delta or dissipation factor
Particle count
Analysis of metals in the presence
PCB
Additive content
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