Thermal analysis and derating of a power transformer with harmonic loads

Abstract

Thermal conditions of power transformers in abnormal operating conditions such as harmonic loads can beconsidered as a critical case. In this study, thermal analysis of transformer and additional imposed heat on it, caused by nonlinear loads, are addressed. For this purpose, an appropriate thermal equivalent circuit is used in which temperature of differentparts of oil-immersed power transformers are obtained for harmonic loads. The losses of various components of transformer asheat-generating sources are estimated. At this end, an appropriate three-dimensional finite element method is applied toestimate the losses of different parts of transformer. To keep the transformer temperature rise around the permissible value, thetransformer is derated. Transformer derating based on the IEEE Std C57.110-2008 recommendation considerably reduces thetransformer loading capability. Therefore, this study introduces a new technique for derating of transformer for harmonic loads.This enhances the transformer thermal limits and transformer loading capacity.