Abstract
The paper deals with thermal modelling of transformers with automatic control of the cooling system. The case study was done on 725 MVA transformer with OFAF cooling, having eight compact coolers, each of them with their own pump and hysteresis control: four coolers, when top oil temperature drops below 30 °C, and six coolers, when top oil temperature rises above 45 °C. The consequence of operation with different number of coolers is change of thermal characteristics of the transformer. Additional phenomenon causing change of thermal characteristics is fouling of the coolers. These variable thermal characteristics are included in mathematical models used for the calculation of top oil and hot-spot temperatures in all applications of such calculation algorithms: protection, monitoring, estimation of overload capability, calculation of thermal ageing, etc. Copyright © 2012 John Wiley & Sons, Ltd.
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