Abstract:
A circuit and method for regulating power supplied on a network which provides power to an electric arc furnace or other highly variable load. The circuit includes at least a first inverter connected in series with the arc furnace transformer, and in shunt with the power supply. The circuit and method provide an effective, low cost technique for controlling network power.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to power control. More particular, the resent invention relates to providing a controlled AC voltage to randomly-fluctuating loads such as electric arc furnaces. 
     Electric arc furnaces (EAFs) are widely used in a variety of applications, including the melting of scrap metal. EAFs represent erratic nonlinear loads to their associated power networks. This is because the arc is random and fluctuating in nature due to the unevenness of the material surface and the mechanical vibrations of the arc electrode assembly. EAFs can adversely affect power quality through, for example, poor power factor, harmonics, and “flicker”. 
     Flicker occurs in the power network as a result of a rapidly changing system voltage, mainly during the melting of scrap metal in the electric arc furnace. When scrap is provided to the furnace, arcs between the furnace electrodes and the scrap melt the metal. The molten metal drips to the bottom of the furnace, and the arcs shift to other pieces of scrap metal. The shifting of the arcs results in highly variable and reactive power consumption. Further, short circuits can result from pieces of scrap falling onto the electrodes, thus shortening the arcs and causing the furnace electrode control system to reignite the arc. It should be apparent from this description that an electric arc furnace consumes a widely and rapidly fluctuating amount of supply current and voltage, thus causing flicker and inefficiencies in the power network. As the power ratings of EAFs have increased, these problems have increased in intensity. 
     Accordingly, it is generally desirable to stabilize arc currents in a manner which minimizes disruption to the electric power supply grid. It is also generally desirable to provide steady-state regulation of the arc current, and to improve power efficiency. 
     One method of controlling the arc current is shown in FIG. 1, where power is supplied to an EAF  10  from a supply  12  through a main transformer  14  having no-load taps on the load side, and a tap-changer. EAF current can be controlled in this arrangement by temporarily suspending the operation of the furnace, and adjusting taps on the supply transformer  14  to alter the voltage applied to the electrodes of the arc furnace  10 . For example, a melt cycle begins on one tap, and at the end of the melt cycle the process is interrupted while the tap is moved to a more optimal location to complete the melting process. 
     FIG. 1 also includes a Static VAR Compensator (SVC)  16 , which can be used to reduce the side effects of the randomly fluctuating nature of the arc load. SVCs involve shunt compensation on the electric supply to the EAF. The SVC is separate from the EAF, and is usually in a physically remote location. Typically, SVCs are rated significantly higher than the EAF load- SVCs can be up to 5 times the rating of the load (e.g., 250 MVAr SVC may be required for a 50 MW electric arc furnace). 
     A direct current (DC) EAF is shown in FIG.  2 . The DC arrangement includes power electronics, in the form of a controlled thryistor bridge  18  rated for the full power of the load  10 . This arrangement supplies the load with a controlled direct current, which can be regulated while the load DC voltage varies based on the arc condition. This DC arrangement provides relatively limited control, and a SVC (not shown) may also be required. Further, additional converters may be necessary to reduce the fluctuations of the reactive current on the power supply. Practical installations typically use at least two converters, with separate transformer windings, to achieve 12-pulse operation. 
     Another solution is shown in FIG. 3, where a controlled series reactor  20  is provided between the electric supply and the furnace transformer. The series reactor  20  has a controlled thryistor portion on the high voltage side of the transformer. The series reactor  20  is physically relatively large, and has a rating substantially equal to the load. The thyristors regulate the effective reactance imposed in series with the load to partially mitigate the fluctuations of the load current. This arrangement has relatively limited compensation ability, due to the load fluctuation caused by changes in the real part of the impedance. The series reactor can affect only the imaginary portion of the impedance. Again, this arrangement may also require a SVC (not shown) to provide improved regulation. 
     U.S. Pat. No. 5,617,447 discloses a method of stabilizing a power supply network against reactive load fluctuations, and a reactive power compensation device for a DC arc furnace. Inductors are used to provide reactive compensation. The disclosed technique features a relatively complex arrangement of multiple control loops, and multiplication of power control element outputs. 
     U.S. Pat. No. 5,610,937 discloses a method for regulating a DC arc furnace. To reduce flicker, comparatively fast reactive power regulation is mathematically superimposed on the relatively slow current regulation of the DC arc furnace. 
     U.S. Pat. Nos. 5,463,653 and 5,677,925 disclose a power converter device for DC power supply to an electric arc furnace. The power converter device includes at least one transformer in which the secondary winding applies a three-phase current to a rectifier circuit. The rectifier circuit is a non-reversible “freewheel” circuit, and includes controlled semiconductors for each secondary winding. 
     While the techniques described above provide voltage regulation, none provide sufficiently simple, effective, and low cost solutions to the problem of voltage regulation in a power grid which supplies power to an electric arc furnace. 
     In view of the above discussion, it would be desirable to provide a relatively simple, effective, reliable, and low cost voltage control capability for an electric arc furnace or other similar load on a power system. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention overcomes the above-noted problems, and achieves additional advantages, by providing for a novel technique for regulating voltage provided to an AC electric arc furnace. According to exemplary embodiments, at least one inverter can be connected in shunt with the power supply and another at least one inverter can be connected in series with the EAF to modulate the AC voltage. The rating of the inverters can be less than the rating of the EAF, thereby achieving a significant cost advantage over DC EAF technology. 
     The inverter can be implemented by back-to-back inverters, for example by connecting a source inverter in shunt with the power supply network in addition to a series-connected inverter. The shunt-connected source inverter allows power transfer in either direction from the series-connected inverter to allow steady-state regulation of the arc voltage. The shunt-connected source inverter further can provide shunt reactive control to minimize fluctuations of the furnace and other loads. The inverters, under microprocessor control, effectively vary the impedance seen by the power supply source to regulate the erratic nature of the arc furnace load. 
     Thus, the present invention provides a simple but effective technique for providing a controlled AC voltage to an AC electric arc furnace. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention can be understood more fully by reading the following Detailed Description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a conventional arrangement for providing EAF control using a SVC; 
     FIG. 2 is a block diagram of a second conventional control technique using a DC arrangement; 
     FIG. 3 is a block diagram showing a third conventional control technique using a series reactor; and 
     FIG. 4 is a block diagram of one implementation of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 4 is a block diagram of a presently preferred embodiment of the present invention. In this embodiment, inverter  28  is connected in series between the furnace load  10  and the power supply  12  via series transformer  22  and main transformer  14 . Using this approach, the electrical rating of the components can be less than the full rating of the furnace, thus resulting in a significant cost advantage over prior systems. 
     In the embodiment of FIG. 4, the electric arc furnace  10  has an associated furnace transformer  14  which receives a current Inet from a power supply  12  through a voltage supply network Vsupply. A series transformer  22  is connected in series between the furnace transformer  14  and the load  10 . A DC link  24  is provided between a first (shunt) inverter  26  and a second (series) inverter  28 . The first inverter  26  is connected in shunt with the power supply  12  via a tertiary winding on main transformer  14 , and receives a current I shunt from the transformer  14 . Further, a microprocessor-based controller  30  is provided which receives a supply voltage, the DC voltage across the DC link  24 , and the load current Iload. Based on these inputs, the controller  30  outputs control signals to the inverters  26  and  28 . Alternatively, the control inputs can be synthesized from other measurements- for example, using preexisting voltage and current sensors on the series and shunt inverters rather than sensors on the main load or power system points. 
     In operation, the shunt inverter  26  permits real power to be transferred in either direction- that is, to or from the series connected inverter  28 . This action of the shunt inverter allows for steady-state regulation of the arc voltage, eliminating the need for tap changing. Further, the shunt inverter  26  provides shunt reactive control to minimize fluctuations due to the operation of the furnace or other loads connected to the power supply network. It will be appreciated that the series connected inverter  28  provides most of the regulation function, and that the shunt inverter  26  provides shunt compensation for the fluctuations not fully compensated by the series-connected inverter  22 . 
     As presently contemplated, the shunt inverter  26  is supplied via a tertiary connection to the main transformer  10 . Alternatively, the shunt inverter  26  can be supplied from a separate transformer (not shown), or other suitable source. 
     The microprocessor-based controller  30  is programmed to cause the series inverter  28  to regulate the active power portion of the AC current to maintain an operator setpoint for average value. The microprocessor  30  is further programmed to minimize AC current fluctuations within a predetermined bandwidth (e.g., up to approximately 20 Hz) to avoid irritation of other customers on the power supply network. This type of control can be achieved using known inverter current regulation techniques. The control logic may also incorporate features which increase the damping of mechanical vibrations in the electrode assembly by modulating the effective impedance, using modulation techniques known to those skilled in the art of control design for electromechanical systems. 
     The shunt inverter  26  can similarly be controlled to regulate the voltage across the DC link  24 , and to regulate (e.g., by injecting or absorbing) AC reactive current to minimize fluctuations of the AC supply voltage. In this way, the shunt inverter effectively compensates for variations beyond the control range of the series inverter  28 . The control of the shunt inverter  26  can be implemented in a manner similar to that used in connection with enhanced regenerative AC motor drives. 
     The series voltage Vser across the series transformer is controlled to regulate the effective impedance seen by the power supply network, and can compensate the real portion of the arc impedance variation to stabilize the effect of the arc furnace on the power supply network. Since the arrangement of FIG. 4 can operate in a buck or boost mode, the effectiveness can be twice the rating of the components (e.g., a 25% series inverter rating can provide 50% total voltage variation). 
     The arrangement of FIG. 4 provides unique advantages over known DC control techniques. For example, the arrangement of FIG. 4 achieves performance substantially equivalent to DC control techniques using lower-rated components. Further, the arrangement of FIG. 4 can compensate directly for variations in the real portion of the load impedance without substantially affecting the power supply network. In addition, the arrangement of FIG. 4 allows for a simpler main transformer  14 . 
     As stated above, one significant advantage of the arrangement of FIG. 4 is that series voltage injection via the series inverter and series transformer is achieved using components rated significantly less than the total load (e.g., approximately 0-25% of the load represented by the arc furnace). The shunt current injection is rated less than the total load (e.g., substantially the same as, or slightly higher than, the series voltage injection). It should be appreciated that while the embodiment of FIG. 4 shows the use of a series transformer  22  to couple the inverter between the furnace transformer  14  and load  10 , the series transformer can be eliminated, and the inverters can be connected directly to the neutral of the transformer windings. This alternative embodiment would further reduce cost. It is clear that in such an embodiment, the series inverter current rating would match the current rating of the series-connected equipment. 
     Another alternate embodiment would include elimination of the shunt inverter  26 . The shunt inverter  26  can be replaced by a thyristor bridge connected across the DC link  24  to regulate real power. This alternative would not provide direct compensation of the real portion of the load impedance, but would offer reduced cost. 
     It should also be appreciated that the embodiment of FIG. 4 contemplates that the shunt inverter  26  is implemented in a self-commutated manner. The shunt inverter  26  may also be implemented in a line-commutated manner to further reduce cost, allowing the series inverter  28  to compensate for the real portion of the load impedance but losing the ability to independently provide shunt reactive compensation. 
     Still another alternate embodiment would involve a very large capacity energy storage element across the DC link. Such an embodiment could be implemented using DC capacitors or batteries. 
     While the foregoing description includes many details and specificities, it is to be understood that these have been included for purposes of explanation only, and are not to be interpreted as limitations of the present invention. Many modifications to the embodiments described above can be made without departing from the spirit and scope of the invention, as is intended to be encompassed by the following claims and their legal equivalents.