Extensive field measurements support new approach to protection of arc furnace transformers against switching transients

Abstract

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

Improved calculations for no-load transformer switching surges

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Fault detection in the secondary side of electric arc furnace transformer using the primary side data

“…However, there are several studies that discuss protection of EAF transformer against switching transients. [17,18] Due to immense currents of about 100 kA of the secondary windings of EAF transformers, saturation of common iron core Current Transformers (CTs) prevents monitoring and protection in the secondary side. The most commonly used protection solution is the regular over-current scheme on the primary side of EAF transformers.…”

Digital-Computer Simulation of Virtual-Current Chopping in Vacuum Circuit Breakers

“…Because of the circuit configuration involved in this application, difficulties 1 have occured due to multiple reignitions and interruptions of the circuit breaker, leading to the production of high overvoltages and consequent insulation failures in the transformer. This phenomenon has been termed Virtual current chopping' 2 " 4 because its effects are similar to those of current chopping in a circuit breaker, although, in this particular case, chopping of the current is not a necessary condition for the phenomenon to occur.…”