Voltage Transients in Arc-Furnace Circuits

1. Voltage transients encountered in primary circuits of furnace transformers have characteristics similar to those of ordinary power-system voltage transients, except that of frequency of occurrence. Primary-circuit transient overvoltages in arc-furnace installations occur during the melting-down operation and at the time of opening or closing the furnace breaker. The highest magnitudes appear to be associated with breaker opening operations. Accordingly, there are more transients in furnace transformer circuits over a given period of time, because furnace breakers operate 40 to 50 times per day compared to a few times a year for the general use of circuit breakers. 2. Insulation levels recommended for general use in power installations also appear to be satisfactory for most furnace installatiors. For some devices, however, there are wide variations of the insulation level for the same voltage rating, and in those cases where the operating voltage approaches the ``voltage class,´´ it may be desirable to use apparatus of a higher actual insulation level as compared to the rating or to apply suitable protective equipment such as capacitors and lightning arresters. 3. There does not appear to be any functional relationship between either furnace size or primary voltage and the number and magnitude of transients on a times normal basis. 4. The magnitude of secondary transients suggests that they originate from current discontinuities in the secondary circuit. 5. This report does not cover internal transformer voltages. 6.