Transformer Loadability Calculations
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The manufacturer shall submit a loadability study of transformers or autotransformers based on load flows, in order to define the limits of overload by adopting the recommendations and calculation methodologies expressed in the standard IEC 60076-7 “Power Transformers – Part 7: Loading guide for oil-immersed power transformers. The study presented should be performed for the following cases of preload (0.5, 0.6, 0.7, 0.8, 0.9 and 1) and report the overload limits for 30 minutes and 60 minutes without the presence of the effects indicated in the following paragraph, considering the Maximum ambient temperature of the installation site.
The Manufacturer shall ensure that the implications of overcharging a transformer are evaluated and that in no case will the following effects be incurred by overloading the transformer to the limits reported by the Manufacturer:
a. Reduction of the dielectric withstanding of the insulation system by the possible presence of gas bubbles in zones of high dielectric stress due to the formation of steam by the boiling of the water contained in the solid insulation adjacent to the hottest points of the winding.
b. Generation of gas by thermal decomposition of the insulating oil or the paper that is in contact with areas that reach high temperatures in the windings or external to it, which can cause gas saturation of the insulating oil and formation of gas bubbles, reducing the dielectric withstand.
c. Decrease in bearing resistance to short-circuit stresses by reducing mechanical properties and increasing the aging rate of insulating materials at high temperatures.
d. Increased internal bushing pressure due to oil expansion, which can result in oil leakage failure.
e. Risk of failure in the switching of high currents in the OLTC.
f. Deterioration of autotransformer gasket materials by temperature,