Medium-Frequency Induction Melting Furnace as a Load on the Power System

The variable-frequency operation of the coreless induction melting furnace (IMF) has been investigated in detail both theoretically and experimentally. The time variations of the operational impedance value of the IMF, owing to the time-varying resonance frequency and crucible conditions, have been derived for typical melting cycles by using a variable parallel RLC model derived from extensive field measurements. The work coil and its crucible are supplied at variable frequency from a multipulse rectifier and a current-fed load-resonant inverter cascade. Uncharacteristic current harmonics caused by cross-modulation phenomenon across the alternating current ac-dc-ac link have been derived theoretically and verified by field tests. These theoretical results and extensive field measurements on a sample IMF installation have shown the following: 1) The major power quality problem of an IMF as a load on the power system is the injection of various characteristic and uncharacteristic current harmonics to the grid; 2) line currents on the supply side are nearly balanced; 3) flicker contribution is below the limits specified in the standards; 4) reactive power demand is relatively low and the variations in the magnitude and rate of change of active power are not drastic; and 5) no voltage sag and swell phenomena arising from furnace operation occur.