ABSTRACT
Since the introduction of the first overhead line in the end of the nineteenth century, the development of overhead power transmission lines has been a continuous process. Although both approaches of ac and dc power transmission were considered at the start, the former has rapidly gained its predominance. Favored by the flexibility of power transformers in modulating the power supply to the local needs and the favorable conditions for the development of current interrupting apparatus, ac transmission system has reached the level of 200-300 kV and extended over distances of several hundred of kilometers during the first part of the twentieth century.
