Nanoengineering of Temperature Sensitive Magnetic Fluids For Cooling Applications In Power Transformers

Abstract

Transformers generate heat through energy losses and get heated due to iron core and copper losses that results into elevated resistance within the windings, increased hysteresis loss and decreased saturation magnetization of the core and degradation of the transformer s insulation. Ultimately these will lead to significant and permanent efficiency reduction. Therefore, it is necessary to dissipate the heat generated by the core and windings so that the temperature of the transformer can be kept below a threshold at which the insulation begins to deteriorate. To inhibit temperature rise, transformers are cooled with mineral oils. The final, steady-state temperature of the transformer reflects equilibrium between power losses and heat dissipation properties of the coolant. As the oil is heats up, it experiences decrease in the density. Oil in contact with the transformer coils absorbs highest amount of heat and as a result it becomes least dense and starts rising relative to the surrounding oil. The rising oil makes contact with the walls of the housing, transfers heat to the walls and ultimately to the exterior environment. The cooled oil moves downward and replace the heated oil, which is rising from the windings. This natural convection caused by the interplay between the gravity and heat induced density variation represents the cooling mechanism most commonly utilized in commercial high power transformers. Gravitational forces that circulate oil in transformers are relatively weak. Temperature gradients across the oil reservoirs are often observed to be quite large, which results into poor heat transfer. Transformer windings frequently develop hot spots that can cause insulation break-down. These factors can lead to significant and permanent efficiency reduction. Development of coolants with enhanced thermal conductivity and better dielectric properties is critical for uninterrupted operation of high power transformers. Magnetic fluids can provide thermal and dielectric benefits over transformer oils.