1. Transformer Engineering Consultant and Researcher; La Piedad Michoacan 59386 Mexico
2. Departamento de Energia; Universidad Autonoma Metropolitana-Azcapotzalco; 02200 D.F. Mexico
3. On sabbatical leave at the Department of Electrical and Computer Engineering; University of Alberta; Edmonton Alberta T6G 2V4 Canada
4. Department of Electrical Engineering; University of Vigo; Vigo Pontevedra 36310 Spain