This paper introduces new criteria based on moving average of apparent power for sizing transformers in railway systems. Usually, transformers are designed to fulfill thermal criteria which are evaluated along several load cycles reported in the standards and literature. Nevertheless, real railway load cycles are quite variable and difficult to predict, therefore simulation techniques are necessary to estimate them. Detailed thermal studies require electric and thermal simulations for very particular operation, maintenance and climatic conditions. Hence these studies are focused on that special conditions and imply high computational costs too. To face this problem new sizing design criteria are proposed. These are based on the calculation of the moving average of the demand power, considering a time window which depends on the thermal time constant of the transformer. Finally, the results obtained with the proposed design criteria are discussed and compared with those obtained with the conventional hot-spot temperature based criterion.

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