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Oliveira, L. M. R. and Cardoso, A. J. M.: "A coupled electromagnetic transformer model for the analysis of winding inter-turn short-circuits", Record of the IEEE International Symposium on Diagnostics for Electrical Machines, Power Electronics and Drives (IEEE SDEMPED'01), pp. 367372, Grado, Italy, September 1-3, 2001.

A Coupled Electromagnetic Transformer Model for the Analysis of WindingInter-Turn Short-Circuits
Luís M. R. Oliveira (1) (2)

A. J. Marques Cardoso (1)

Universidade de Coimbra, Departamento de Engenharia Electrotécnica Pólo II - Pinhal de Marrocos, P – 3030-290 Coimbra, Portugal

Escola Superior de Tecnologia da Universidade do Algarve Campus da Penha, P – 8000-117 Faro, Portugal generalised perspective of this phenomenon, it becomes necessary todevelop a digital model for internal fault studies in three-phase power and distribution transformers, which is the scope of this paper. For this type of studies, an open-structure transformer model is necessary, i.e., a model in which it would be possible to manipulate the windings arrangement. The coupled electromagnetic model [3-6], allowing for the modelling and simulation of the transformer inits natural technology, so that the cause-and-effect relationships can be closely investigated [4], becomes the natural choice for the analysis of transformer internal faults.

Abstract — This paper presents the development and implementation of a coupled electromagnetic transformer model for the simulation of three-phase power and distribution transformers affected by the presence of windinginter-turn short-circuits. Generically, the coupled electromagnetic transformer model is based on the simultaneous consideration of both magnetic and electric equivalent circuits. To incorporate the internal faults in this model a suitable equivalent circuit of the faulty winding is proposed. Both simulation and laboratory tests results are presented in the paper, which demonstrate the adequacy of theproposed model. Index Terms — Transformers; modelling and simulation; coupled electromagnetic model; winding inter-turn short-circuit faults; Park's Vector Approach.

I. INTRODUCTION The sweeping changes that have overtaken the electricity supply industry during the closing decade of the 20th century have created a powerful new commercial climate. Power utilities and supply companies must nowcompete in increasingly open, de-regulated markets, on an increasingly international scale. To ensure commercial survival in this aggressively competitive environment, it is essential that key items of power generation plants and power transmission and distribution networks not only operate efficiently, but with maximum availability and with the lowest possible cost of ownership over – andfrequently beyond – their design life. Power and distribution transformers have formed an essential part of electricity supply networks since the alternating current system was adopted more than a century ago [1]. Therefore it is quite obvious the need for the development of on-line diagnostic techniques, that would aid in transformers maintenance. A survey of the most important methods, actually in use,for condition monitoring and diagnostics of power and distribution transformers, presented in [2], stresses the need for the development of new diagnostic techniques, which can be applied without taking transformers out of service, and which can also provide a fault severity criteria, in particular for determining transformers winding faults. Preliminary experimental results, presented in [2],concerning the use of the Park's Vector Approach, have demonstrated the effectiveness of this non-invasive technique for diagnosing the occurrence of inter-turn short-circuits in the windings of operating three-phase transformers. In order to obtain a deeper knowledge in the study of inter-turn short-circuits occurrence, and also to acquire a

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