The modern steel plant receives low current, high voltage power from the generators of the electrical utility company. The purpose of the steel plant primary transformer is to step-down this voltage. The transformer thus provides high current, low voltage power for the EAF furnace. Large furnaces with transformers rated at 100 MVA or higher is not uncommon. There are usually two stages to this process. The transformer first steps-down the voltage from high to medium levels.
There are different standards for a “medium-level voltage” in different countries; usually between 30 to 33 kV for Europe and Japan, but for the USA it is usually 34. 5 kV. Finally, a heavy duty furnace transformer powers the EAF. This special furnace transformer is designed to allow the electric arc to operate within the desired current and voltage range. To safeguard the EAF electrical system from the primary power supply are two types of switches: a vacuum switch and a motorized disconnect switch. These two switches are designed to isolate the EAF electrical system from the primary transformer in case of an emergency.
Other electrical systems around the EAF are the delta closure, the power cables, the current conduction arm, and the electrode holders. The power cables provide a connection between the delta closure and the current conducting arm. It is usually made of copper wires, with a rubber water jacket around the outside for water-cooling the cables. The power cable is connected with the current conducting arm, usually made of copper clad steel or aluminum alloys.
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This system weighs a lot less than the old bus-bar design that a lot of the older furnaces used. The current conducting arm conducts current directly from the power cable to the electrode holders. The electrode holders must withstand thermal cycling (not so much for our furnace, since our furnace should have a continuous feed) and severe mechanical loading and wear due to vibrations, torsion forces and such. They are usually cast from copper, and the contact area between it and the electrodes will need to be kept very clean, and avoid carbon build up and oxidation. Lastly, the electrode holder transmits the electrical power to the graphite electrode. Recent improvements in EAF design and electrode manufacturing have reduced typical electrode consumption from 12~14 pounds per ton of steel to 3.
5~4. 5 pounds per ton of steel.
