Understanding Eddy Current Loss and Cooling in Power Transformers

Eddy current loss is a critical factor in the design and efficiency of power transformers. This phenomenon, while inversely related to temperature, contributes only a small portion to the total winding loss. Interestingly, as the temperature increases, the overall winding loss also rises. This means that in a transformer winding, the cables located at the top generate more losses than those at the bottom, despite both sets carrying the same current. The implications of this behavior are significant for transformer design, particularly in managing heat and ensuring optimal performance.

Cooling is another vital consideration in transformer operation. When a transformer is energized, oil circulation is initiated as the cold oil from the radiator is heated by the winding conductors and then rises to the top. The hot oil at the top subsequently flows to the radiator, where it dissipates heat to the air. To ensure sufficient oil flow through the windings, the vertical oil ducts must be adequately sized to minimize resistance. However, this requirement must be balanced with the need for dielectric strength, leading to a typical duct thickness of between 6 mm and 12 mm.

Research indicates the presence of a boundary oil layer adjacent to the winding surface, which is about 6.5 mm thick. This layer is crucial as it facilitates 90% of the oil flow, with maximum velocity occurring near the winding surface. If the duct size is smaller than this boundary layer, it can significantly impede oil flow, leading to increased temperature differentials between the incoming and outgoing oil. While a minimum thickness of 6.5 mm appears reasonable, further studies are essential to fully understand the thermal behavior of smaller ducts.

Moreover, the geometry of the winding plays a crucial role in cooling efficiency. A larger cooling surface area for the winding enhances heat dissipation, lowering conductor temperatures. However, the design must also account for the number and width of radial spacers, which are essential for maintaining short-circuit strength. The configuration of horizontal ducts within the winding is particularly important, as oil flow in these ducts can improve heat transfer through convection, as opposed to conduction, which is less effective.

Finally, transformer sizes significantly influence the design of these cooling mechanisms. Larger transformers tend to have greater winding resistance due to larger cable sizes and limited space for horizontal ducts. Consequently, ensuring effective oil flow through small horizontal ducts becomes even more critical. Innovative solutions, such as the implementation of oil flow guide washers, can enhance the flow conditions in these ducts, ensuring better cooling and overall performance of the transformer. Understanding these dynamics is essential for engineers and designers aiming to optimize transformer efficiency and reliability.