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IEEE power & energy magazine 63T THIS ARTICLE DESCRIBES THE first "polyphase" (more than one phase) system developed for the distribution of alternating current (ac) power. This two-phase system was subsequently rendered obsolete, however, by the superior three-phase system that is now universally used throughout the world.

Recent investigations of systems involving vacuum switching have forced a reappraisal of the traditional protectionmethods necessitating major changes inproteetion philosophy. The new philosophy has been developed on industrial systems supplying arc furnace transformers where the exceedingly large number of switchings makes such systems particularly sensitive. This paper discusses a new transient voltage protection system which has been developed in response to these recent investigations.In support ofthe development ofthe new protection system, transient voltage measurements have been made on 14 arc furnace systems at various locations within the United States and abroad.This paper also documents the instrumentationused to monitor transients on these systems and describes its application. Typicaloscillographic records are presented as indications of the performance of the system with and without the new transient voltage protection system.In 1972, several arc furhace transformers suffered high-voltage insulation failures that could not, at first, be explained. These failures occurred at onlytwo plant locations, under conditions that suggested a possible interaction among the power distribution system, the arc furnace transformer, and its vacuum switch.As had beenthe practice for severalyears, in each case, a suitable station-type lightning arrester and a surge capacitor, sized to limit no-load switching chopping voltages, were connected from each high-voltage bushing to ground. These were mounted on the transformer tankin order that connections would be as short as possible. Figure 1 shows the elements of a typicalpower distribution system for an electric arc furnace system.The furnace transformer uses a delta secondary while the primary may be connected in delta or wye via an internal switch. Committee and the IEEE Switchgear Committee of the IEEE Power Engineering Paper T 74 405-7, recommended and approved by the IEEE Transformers Society for presentation at the

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