1. Departamento de Energia; Universidad Autonoma Metropolitana-Azcapotzalco; 02200 D.F. Mexico
2. On sabbatical leave at the Department of Electrical and Computer Engineering; University of Alberta; Edmonton Alberta T6G 2V4 Canada
3. On sabatical leave from Programa de Graduados e Investigacion en Ingenieria Electrica; Instituto Tecnologico de Morelia; Michoacan 58120 Mexico
4. Engineering Department; Pennsylvania Transformer Technology; Inc. Canonsburg 15317 PA, USA
5. School of Electrical and Computer Engineering; National Technical University of Athens; GR 15780 Athens Greece