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Overloading of power transformers can become necessary in open electricity markets due to economic reasons or simply to ensure continuous energy supply. During an overload circle accelerated ageing and damages have to be strictly avoided. In order to control overload cycles intelligent on-line monitoring systems are needed. In this contribution the on-line calculation of the overload capacity and the integration in a monitoring system are described. By measurement of environmental and loading conditions it delivers continuously information on the maximum continuous and short time overload considering the actual preload of the transformer according to IEC 603.54. The presentation of practical experiences shows the considerable possibilities regarding optimization of transformer operation during, normal cyclic loading and emergency cases. Also a load dependent control of the cooling unit by the monitoring system allows to increase the overload capacity and optimize the hot-spot temperature.

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