Smart predictive line controller for AC and DC electric arc furnaces

An electric arc furnace, comprising:

• a) a three phase power source;
• b) three opposite polarity pairs of thyristors connected to respective phases of said three phase power source;
• c) a furnace transformer having primary windings and secondary windings, said primary windings being connected to said three opposite polarity pairs of thyristors;
• d) three electrodes being connected to said secondary windings;
• e) a grounded container for receiving metal, said electrodes being disposed within said container and spaced apart therefrom, whereby application of power from said power source to said electrodes causes generation of respective electrical arcs between each of said electrodes and said container for melting said metal;
• and
• f) a predictive line controller for monitoring predetermined line currents and voltages on opposite sides of said furnace transformer and in response generating predetermined gating signals for firing respective ones of said thyristors at predetermined times so as to reduce flicker in said electric arc furnace,
• wherein said predictive line controller comprises:
• i) a signal processor for performing a frequency domain analysis on said line currents and said voltages;
• and
• ii) a microcontroller coupled to said signal processor for generating said gating signals from said analysis.

In an electric arc furnace having a power source for applying three phase power to three respective electrodes, said electrodes being spaced apart from a grounded container for receiving metal, and wherein the application of said power to said electrodes causes generation of electrical arcs between said electrodes and said container for melting said metal, a method of firing a plurality of switches connected intermediate respective phases of said power source and said three electrodes comprising the steps of:

• a) calculating the period of one line-to-line voltage generated by said power source;
• b) generating a plurality of sampling points from said period;
• c) for each one of said sampling points simultaneously sampling and holding predetermined line currents and voltages intermediate said switches and said electrodes;
• d) successively converting the sampled line currents and voltages to digital values;
• e) for each of said line voltages converted to digital values calculating two line-to-line digital voltage values;
• f) performing a running discrete Fourier series on each of said line current and digital values for generating a frequency model of said arc furnace;
• g) calculating a plurality of index values based on said model;
• h) for each of said index values forming a base address;
• i) accessing predetermined memory locations of a look-up table using each said base address and retrieving from said memory locations respective delay angle values;
• j) generating respective gating signals based on said delay angle values;
• and
• k) applying said gating signals to said switches for firing said switches according to said delay angles, thereby applying said power to said electrodes in such a manner as to reduce flicker in said electric arc furnace.

An electric arc furnace, comprising:

• a) a power source including three phases;
• b) a furnace transformer including three primary windings and three secondary windings;
• c) three switches, each said switch comprising a pair of thyristors arranged in opposite polarity to one another and being electrically connected to a respective one of said phases and a respective one of said primary windings and including a gating input for connecting and disconnecting a respective one phase to and from a respective one primary winding;
• d) three electrodes, each electrode being coupled to a respective one of said secondary windings;
• e) a container for receiving metal and being coupled to the power source, said electrodes being disposed within the container and spaced apart therefrom, whereby application of power from said power source to said electrodes causes generation of electrical arcs between each of said electrodes and said container;
• and
• f) a predictive line controller for monitoring alternating current signals on opposite sides of said furnace transformer and in response generating gating signals, said gating signals being delayed by predetermined times based on said alternating current signals, for firing respective ones of said thyristors at said predetermined times so as to reduce flicker in said electric arc furnace.