دانلود Supply-voltage and current variations produced by a 60-ton 3-phase electric arc furnace

Abstract

The use of electric arc furnaces for the production of high-grade steels is becoming increasingly common. At the same time the size of the new furnaces is being increased to permit the melting of larger quantities of metal at a time. An arc furnace does, however, present a number of difficulties to the supply engineer on account of the rapid variations of load which it produces. The first 60-ton arc furnace in this country was installed at the end of 1954, and it was felt desirable to determine what effect it had on the supply network. The electric power for the furnace was obtained from a temporary connection to the 66-kV system through a 66/11-kV 15-MVA transformer, pending the provision of a permanent supply direct from a 132/66-kV supply point. During tests on the 11kV line, voltage fluctuations from about 75 to nearly 150% of normal voltage were recorded. These can be shown to be due to unstable arcing conditions in the furnace and to the impedance of the supply network. Calculations indicate that about 17% of these voltage variations were produced on the 66-kV network. During the initial period of a melt, arcing conditions are very unstable owing to the conditions in the furnace. Initially the metal is cold and forms a poor arcing electrode. Also, since the charge is composed of scrap material it is liable to collapse from time to time, short-circuiting an arc. Alternatively, owing to a previous over-current surge the electrode control-gear may withdraw the electrode too far, causing the arc to be extinguished. These initial fluctuations may take place several times a minute with an amplitude of several hundred amperes. As the charge begins to melt the power input is increased, and records show that unbalanced currents of about 1500 amp may occur on the 11-kV system. Measurements indicate that the actual voltage drop across the arcs between the electrodes and the charge may vary between about 110 and 400 volts, depending on the furnace conditions. Since the various voltages in the circuit are all magnetically coupled they are uniformly distributed through the various windings, and no special reinforcement of the transformer end-turn insulation is required, as in the case of lightning surges.