engleski

Determining the equivalent Young’smodulus of elasticity of foil windings

Distribution transformers are usually designed with non-circular windings due to economic advantages. The non-circular shape of the winding (such as oval or rectangular) in comparison to round shape gives lower weight and dimensions of the transformer for the same transformer ratings (voltage, current, impedance voltage, temperature rise, no-load losses, load losses). On the other hand, non-circular windings are more susceptible to radial forces and stresses during a short circuit. In distribution transformers, low voltage winding is usually made with foil conductor. To increase the stiffness of the windings, diamond dotted presspaper (shorter DPP or DDP) is used between the foil layers. DPP is epoxy coated paper and therefore needs to be properly technologically treated throughout the process of transformer production. For the two adjacent layers of the epoxy coated paper to adhere to each other or to foil conductor and thus increase the stiffness of the winding, it is necessary to achieve a curing temperature within a maximum of 4 hours (minimum 105 °C, maximum 120 °C) and maintain this temperature for a specified time (3 hours at 105 ° C ; 1.5 hours at 120 ° C). As mentioned before, these types of windings are more susceptible to radial forces and stresses in the short circuit. The most critical is lower voltage foil winding (or more precisely the outermost part of the foil winding divided by cooling ducts) which could buckle when loaded with higher stress than critical. In order to evaluate the critical stress, Young’s modulus of elasticity of such composite structure needs to be known. Critical stress is calculated by considering the whole thickness of the foil winding (or its outer part when cooling duct exists). The Young’s modulus of elasticity is a material property which describes its stiffness and is, therefore, one of the most important properties of solid materials. In other words, it describes how easily it is bent or stretched. Materials deform differently when loads are applied, and the relationship between stress and strain is typically defined by Young’s modulus. In this paper Young’s modulus of elasticity of foil winding which is a composite of copper or aluminium foil and diamond dotted presspaper is determined. Testing samples were made in a way to represent the real low voltage foil windings (or its outer part when cooling duct exists). The 24 testing samples were made, 12 with copper foil conductor and 12 with the aluminium foil conductor. Different foil conductor thicknesses were combined with two layers of DPP of a total thickness of 0.15 mm. Testing samples were heated and cured in an oven at the temperatures similar to those in real transformers during the heating and curing process (usually in low-frequency heating plant – LFH). An equivalent Young’s modulus of elasticity of foil winding was determined by applying the theory valid for the simply supported beam loaded with a bending force concentrated in the middle of the beam. The Young’s modulus was obtained by measuring the deflection of the sample model loaded with bending force on a tensile test machine (weight of the sample model was neglected).

non-circular, foil winding, radial forces, stress, buckling, equivalent modulus of elasticity

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