Method for on-line monitoring of an electric arc furnace and method of control

Abstract: A method for on-line monitoring and/or control of an electric arc furnace utilizing a method of data transfer between a programmable logic controller and a microcomputer comprising monitoring data from the furnace over a fixed time cycle, assigning data values to the monitored data during such time cycle, dividing the fixed time cycle into a multiple of N time subintervals, arranging the data values into data subsets, assigning each of the data subsets to a designated time subinterval, transmitting data to the microcomputer in each time subinterval to execute program code corresponding to the data subset in such time subinterval and calculating control factors from the transmitted data for providing control information.

Description: FIELD OF THE INVENTION This invention relates to electric arc furnaces and, more particularly, to an improved process to monitor and/or control electric arc furnaces. BACKGROUND OF THE INVENTION For many years, electric arc furnaces have been operated by manually controlled relay panels. As a result of competitive pressures and a desire to improve the control of arc furnaces, programmable logic controllers (PLC's) have slowly been replacing the relay panel controllers. A PLC enables continuous on-line monitoring of various furnace conditions and also serves as an input processor for higher level main frame computers, which perform the heavy-duty data processing for on-line control. PLC's are generally configured to withstand the rigors of the furnace area environment, but are limited in their data processing and memory capabilities. To date, the PLC and the higher level main frame computers have performed discrete hierarchial tasks. The data processing main frame computer capabilities and the front end data collection capabilities of the PLC are integrated in a system designed to be located on the factory level in the industrial environment of a steel mill for operation, in accordance with the present invention, to monitor and/or control the operation of the electric arc furnace. A method for controlling an arc furnace is described in US-A-4,607,374 wherein an arc furnace controller receives detected values of momentary currents from current transformers provided on the primary and secondary sides, respectively, of the furnace transformer. However, this method of obtaining electrical data is inaccurate and misleading due to generated electrical noise, harmonic distortion of the signals and magnetic field effect. US-A-4,578,795 describes a process for controlling the position of a consumable electrode in a consumable electrode furnace by monitoring the gap voltage between the consumable electrode and the molten surface of the ingot in order to control the electrode gap. However, this method is not applicable to a three electrode type arc furnace system described in the present invention where no mechanical electrode movement is performed. In Proceedings IECON'85, November 18 to 22, 1995, San Francisco CA, USA, Vol. 1, p. 413-418 an integrated system utilizing a programmable controller, a personal computer, photoelectric sensors and shaft encoders is described to manage material flow and in-process storage facilities in an automated material handling and job order management system of a boxboard manufacturing plant. Also this method cannot be applied to the arc furnace system of the present invention. The present invention employs an arc furnace monitoring system including one or more conventional PLC's and one or more conventional data processing microcomputers. The PLC's function is to perform high speed data collection of electrical, mechanical and physical parameters of the furnace, such as, but not limited to, pulse rates from watt/var, current and potential transducers, transformer tap positions, arc length settings, hydraulic variables, positions of mechanical furnace components, and scrap charge weights and present the collected data to the data processing microcomputer in a usable format. The present invention utilizes conventional microcomputers to receive, through a high speed interface, the data transmitted by the PLC in a cost effective manner and provide the operator with control information corresponding to the above identified variables by means of reports, displays and/or commands. The key to this invention consists of the integration of the PLC(s) and microcomputer(s) and the method of information handling to provide on-line data collection and data communication between the programmable logic controller and the data processing microcomputer in a stand-alone unit for control of the furnace. Accordingly, it is an object of this invention to provide an improved process to monitor and control an electric arc furnace in the manufacture of steel. It is another object of this invention to provide a stand-alone, on-line arc furnace monitor and control system which employs commercially available PCL's and microcomputers and eliminates the need for higher level computers. It is still a further object of this invention to provide the operation of the furnace with process and control information and/or process commands to enable him to control the chemistry and slag characteristics of the furnace operation and more accurately predict when to charge additional scrap steel into the furance. This, in turn, results in a more efficient utilization of the electric arc furnace. SUMMARY OF THE INVENTION The method of the present invention for monitoring and controlling an arc furance employs a system which includes one or more small PLC's and one or more small data processing microcomputers. The microcomputers include all operator interfaces. The processing system monitors the operating conditions of an electric arc furnace in an effort to increase the efficiency of producing steel by: providing better control of the electrical energy required, providing better control of the consumable materials (i.e. electrodes, oxygen, etc.) providing better control of good steel making practices in operating the furnace (i.e. deep foamy slag, minimum furnace to caster delays, etc.) and providing quality control records of the entire process and the process parameters. The processing system employs a method of data transfer, which enables it to perform on-line monitoring of the arc furance using a programmable logic controller and a microcomputer as a stand alone unit. The method comprises: (a) monitoring data pulses from the arc furnace representative of the electrical energy consumption of the furnace over a fixed time cycle; (b) converting said pulses into a data value representing the furnace's electrical energy consumption during such time cycle; (c) monitoring all analog data signals other than electrical pulses, such as hydraulic flow rates and all digital signals corresponding to furnace status and/or conditions and assigning a data value for each such signal during such time cycle; (d) dividing said fixed time cycle into a multiple of N time subintervals; (e) arranging said data values into data subsets; (f) assigning designated data subsets to each of said time subintervals; (g) transmitting data to said microcomputer in each time subinterval in accordance with a predetermined sequence for executing program code corresponding to the subset of variable data in such time subinterval; and (h) calculating control factors from the transmitted data for providing control information to optimize the furnace operation. A method is also provided for on-line control of an electric arc furnace in the manufacture of steel using a data processing system for providing information to control the electric arc furnace comprising the steps of: (a) monitoring electrical data from the arc furnace representative of the electrical conditions and electrical power delivered to the furnace; (b) calculating a Stability Factor (SF) from said electrical data in accordance with the following algorithm: Stability Factor = 200 - Operating Reactance Short Circuit Reactance * 100 Where: Operating Reactance = measured Megavars 3* Electrode Current calculated 2 Electrode Current (calculated = MVA 3 1/2 * Vprimary *N Short Circuit Reactance = Constant which is derived by submerging the electrodes into the liquid steel bath and performing the following calculation: Reactance (at short circuit) = Megavars 3*Current 2 * N 2 Where: Megavars = measured primary reactive power; Current = measured average primary current N = transformer voltage ratio (c) comparing said calculated Stability Factor with a pre-established Stability Factor for said furnace operation corresponding to a given period of furnace operation; and (d) introducing raw material into the furnace when the calculated Stability Factor is below the pre-established level for said period of furnace operation to raise the Stability Factor to said pre-established level. DESCRIPTION OF THE DRAWINGS Fig. 1 is a functional block diagram of an arc furnace monitor and control system in accordance with the invention. Fig. 2 is a chart which indicates various arc furance parameters that are monitored and transmitted to the control processor. Fig. 3 is a high level block diagram showing the interconnection between a microcomputer and a display. Figs. 4a - 4g illustrate a high level flow diagram for the system of Fig. 1. Fig. 5 illustrates a Heat Log report in accordance with the invention. Fig. 6 is a Heat Electrical Summary report. Fig. 7 is a Daily Production Summary report. Fig. 8 is a Daily Electrical Summary report. Fig. 9 is a Daily Delay Summary report. DETAILED DESCRIPTION OF THE INVENTION Referring now to Fig. 1, Arc Furnace System 10 is of the conventional three electrode type and is provided with the standard complement of controls, sensors and indicators (not shown). The major categories of sensed phenomena are shown as inputs to I/O Interface 12. Those inputs include, but are not limited to, electrode regulator pressures and flows, various scale weights, furnace conditions and furnace electrical information. A plurality of outputs are provided for various furnace control functions. I/O Interface 12 conditions all received signals and provides those signals for storage into PLC 14 on a continuing basis. PLC 14 contains a Data File 16, which has an allocated storage area for each indication received from I/O Interface 12. The operation of Data File 16 is controlled by Ladder Logic 18. The operation of PLC 14 is essentially that of a sophisticated input/output buffer which receives all indications from I/O Interface 12 on a continuing basis; samples and stores each; and periodically and selectively transfers information to Microcomputer 22. Microcomputer 24 receives melt temperatures and chemistry inputs and provides calculating capability to derive control signals, which are correspondingly transferred to data file 16 for the arc furnace. Microcomputer 22 also calculates performance factors from accumulated data and provides control signals through PLC 14 to associated furnace controls. Microcomputer 22 further integrates Display 26, Keyboard 27 and Printer 28. Display 26 provides the supervisor with a continuing indication of critical measured values monitored during the heat and is accompanied by Keyboard 27, which provides the supervisor with an input capability to the system. Printer 28 provides various heat, shift and daily summary reports. Microcomputer 22 may also be connected to a local area network so that its accumulated data can be transferred to a higher level computer for data base updating and further report generation. Microcomputers 22 and 24 are conventional microcomputers. The Ladder Logic 18 continuously samples the various inputs from I/O Interface 12 and updates the values stored in the corresponding memory areas of Data File 16. In other words, each allocated memory position in Data File 16 is periodically overwritten with new data, so that there is always an indication of the most recent measurement in each memory position. The primary function of PLC 14 is to process all data received through the I/O Interface 12 during a time cycle. The data received and processed by PLC 14 includes, but is not limited to: pulse strings, representing the furnace's electrical energy consumption; analog and digital signals, representing furnace status and/or condition; and timing values, representing the duration or absence of an event or delay. The processing of all received data is accomplished by monitoring the incoming data over a fixed time cycle and converting the data into data sets or values representing appropriate engineering units. Examples of such data processing would be: the metering of incoming electrical pulses and conversion of such rate into kilowatt hours; analog signals into data values corresponding to hydraulic flow rates and pressures; and digital signals into data values, representing discrete furnace conditions (i.e. roof open), or scale readings. In addition to data collection and processing, the PLC 14 must further transmit the processed data to Microcomputer 22. To limit the number of data words transferred from PLC 14 to Microcomputer 22, and thus reduce the total transmission time, a multiplexing algorithm is utilized. The algorithm, which is evoked by PLC 14, provides the functions of dividing a fixed time cycle into subintervals of time and allocating transfer locations in the data file 16 for the data values to be transferred during the subintervals of time within the time cycle. The algorithm further arranges the data values into data subsets, preferably of about the same size, corresponding to the given number of time subintervals within the time cycle. Datum from each subset is then selectively assigned to a transfer location during an appropriate time subinterval. A portion of the processed data is continuously assigned to selective transfer locations and transferred throughout the entire time cycle. This is to say that certain transfer locations always contain the same data variable; however, the data content of these data variables may change in each time subinterval. Other processed data is intermittently assigned and transferred so that transfer locations change data variables as well as content during each time subinterval. No data is transferred until the entire subset has been assigned a location for transfer. The subsets of data, the time subinterval in which they are transferred and a partial list of the data each contains is shown in Fig. 2. Upon receipt of valid data, the Microcomputer 22 executes program code, which performs data accumulation as well as calculation. The data accumulation represents a current history of furnace conditions and may be further utilized to calculate certain furnace control factors. Two such furnace control factors, Minutes to Back Charge and Stability Factor, are detailed on pages 14 and 15, respectively. Both of these factors provide the operator with timely information, which may require operator action or furnace adjustment to control the operation of the furnace and/or to optimize furnace operation. To expedite the execution of the program code in Microcomputer 12, the software selectively executes only the portion of program code which pertains to the subset of variable data transferred during the present time subinterval. Each execution of the subintervals program code contains a portion of common code which is, therefore, executed once during every time subinterval. The execution of each subinterval's unique program code, as well as the common code, is completed before the expiration of the present time subinterval in which it is executing. The described data transfer protocol is controlled by software in PLC 14. PLC 14 provides instructions to Microcomputer 22, which enable the microcomputer 22 to receive and process the various categories of data indicated in Fig. 2. Multiplex control 20 controls the output data to Microcomputer 22. Referring now to Fig. 3, a high level block diagram is shown of Microcomputer 22 and its interconnection with Display 26. Microcomputer 22, in its commercially available configuration, has a serial RS232 data port 30 for communicating with refresh Buffer 31 in Display 26. The data transfer capabilities of Microcomputer 22 do not enable, within a single refresh cycle, the transfer of sufficient data to refresh Buffer 31 to enable complete update of a screen viewed on CRT 32. To overcome this problem, the screen data update from Microcomputer 22 is segmented, in much the same way, as the input data to Microcomputer 22 from PLC 14. More specifically, Microcomputer 22 only transmits during one time subinterval a portion of the refresh data for Display 26. It then transmits the remaining update data during a subsequent time subinterval so that over two time subintervals the entire screen is updated, reference Fig. 2. All data required for screen refresh is calculated during the last time subinterval of the previous time cycle. Referring now to Figs. 4A-4G, a high level flow diagram illustrates the operation of software within Microcomputer 22 and its control of PLC 14, Display 26 and Microcomputer 22. As indicated in Fig. 4A, Microcomputer 22 obtains data from PLC 14, for instance, during the second time subinterval. It then stores the received data into allocated areas within its memory (Box 4). The program then determines whether the third subinterval has commenced (decision Box 5). Assuming that it has not, the program continues to determine whether an end of heat has occurred or an end of shift. If the answer is yes in either case, reports are generated for the aforestated period of operation. If not, the program (decision Box 16, Fig. 4B) determines whether a screen change has been required by the user on Display 26. In other words, in lieu of viewing a screen indicative of furnace status, the user may request a furnace "setup" status screen or a heat log screen, etc. In such case, a new screen is generated and displayed (Box 17). If no new screen has been requested, the program determines whether the first time subinterval has commenced (decision Box 19). In such case, monitored data is accumulated (Box 18), and data is transferred to Display 26 to update a first portion of the screen then being displayed (Boxes 22, 24, 26, 27). In addition, the screen is updated to indicate any user entered keyboard data (Boxes 83-95). It will be hereinafter noted that Display 26 is updated during every time subinterval to indicate keyboard entered data. This enables the user to rapidly see on the screen the results of keyboard entries without having to wait for a plurality of time subintervals for keyboard updates. Returning to decision Box 19, if it is determined that the program is in the second time subinterval. The procedure is essentially the same as for the other branch of the program; the data transferred during the second time subinterval is accumulated; a portion of Display 26 is updated; and keyboard entered data is indicated on the screen of display (Boxes 44-51, 83-95). Returning to decision Box 5, if it is determined that the third time subinterval has commenced, monitored data is accumulated (Boxes 28-43, 52-65) and all system control calculations are performed (Box 82). The program has now determined whether an "event" has occurred. An event is a major action within arc furnace system 10, e.g. the addition of scrap steel to the furnace; the commencement of a melt cycle; an addition to one of the electrodes; a change of furnace state (from melt to refine), etc. Delays, such as power demand, maintenance and production problems, are also considered events and are handled in the same manner. If an event has occurred, a line is added to the Heat Log indicating that fact. If no event has occurred, the Heat Log is merely updated, as is Display 26, to indicate keyboard entries. Referring now to Fig. 5-9, exemplary reports are shown that are generated by Microcomputer 22 to enable monitor and control of Arc Furnace System 10. In Fig. 5, a Heat Log report is indicated. The system inserts a line into the log each time an "event" or "delay" occurs. An event may, as aforestated, be a change of state of the furnace, a delay created by a furnace problem, etc. As shown by the report of Fig. 5, at clock time 8:40 the first charge added to the furnace is completed. The charge which ended 2.9 minutes into the heat had a weight of 42,100 pounds. The minutes to the next charge at the tap change is calculated to be 7.8 minutes. The equation used to calculate the prediction of minutes to next charge is shown below: minutes to next charge prediction = [(Total Charge Weight * KWH/Charge Ton Factor)] Avg Megawatts - [(MWHR Run Total * Scaling Factor)] Avg Megawatts The furnace's total charge weight is multiplied by the number of kilowatt hours required per charge-ton factor. A further multiplier is the number of total megawatt hours expended so far during the run and the final multiplier is a scaling factor. The product of the numerator is then divided by the average megawatts, thus giving the predicted number of minutes to the next charge. The displayed value for minutes to charge continuously decreases when there is power to the furnace reflecting the increasing amount of power consumed by the furnace. Returning to Fig. 5, it can be seen that at clock time 8:46 the bore-in portion of the heat ends while at time 8:56 the first recharge to the heat is completed. The process continues until the cycle is completed and the steel is removed from the furnace, at which point, a Heat Log report and a Heat Electrical Summary report are generated showing all of the events during the heat, as shown in Figs. 5 and 6. Production, Electrical and Delay summary reports are then produced at the end of each day, as outlined in Figs. 7-9, which enable a supervisor to assess the efficiency of operation of the system. If the application of the invention is so configured, all Summary Reports could also be printed at the end of each shift. The screens and reports are designed to provide furnace operators with concise, non-technical productivity information and electrical data, while documenting critical steel furnace operational information. Another factor, which has been developed in accordance with the present invention for controlling an arc furnace's operation, is termed the Stability Factor. That factor is calculated from the following expression: Stability Factor = 200 - Operating Reactance Short Circuit Reactance *100 Where: Operating Reactance = measured Megavars 3* [Electrode Current (calculated)] 2 Electrode Current (calculated) = MVA 2 1/2 * Vprimary *N Short circuit reactance = Measured Furnace Constant The short circuit reactance in the Stability Factor equation is a constant quantity based on the design factors for each furnace and is derived by submerging the electrodes in the liquid steel and then recording the various electrical parameters used in the following equation: Reactance (short circuit) = Megavars 3 * Current 2 * N 2 Where: Megavars = measured primary reactive power; Current = measured average primary current N = transformer voltage ratio During the operation of an arc furnace, after an initial charge of scrap steel, erratic arcing occurs between the graphite electrodes and the steel. As the scrap steel liquifies, the arcs become more continuous and a slag layer forms on top of the liquid steel. The slag layer prevents energy. from being radiated to the sidewalls of the furnace and confines the arc ionization in tunnels in the slag layer. The reactive component (MVAR) of the electrical current tends to vary erratically during the initial period, and then stabilizes as the arc's become steadier and the slag depth increases. The direct measurement of electrode current in electric arc furnaces has been found to be somewhat inaccurate and misleading due to generated electrical noise, harmonic distortion of the signals and magnetic field effects. Therefore, accurate results of Operating Reactance and Stability Factor computations cannot be derived from the measured electrode current throughout the cycle. However, it has been discovered that by calculating the current from the following measured quantities on the utility side of the furnace transformer, Megavars, Megawatts, and voltages, results in accurate numbers, which agree with theoretical computations under sinusoidial or non-distorted electrical signal conditions. This is due to the effect that the furnace transformer acts as a filter of electrical and magnetic noise. Therefore, the measured electrical signals on the primary or utility side of the furnace transformer have less distortion and represent sinusoidial conditions more accurately. Also, the metering on the primary side of the furnace transformer is of much higher accuracy than the secondary or electrode side of the furnace. The following equation is used in calculating electrode current: Electrode current (calculated) = MVA 1.732 * Vprimary * N where: MVA = (Megawatts 2 + Megavars 2 ) 1/2 Vprimary = Average measured primary phase-to phase voltage N = Furnace Transformer Voltage Ratio The Stability Factor may be used to control the furnace for both optimization of the electrical energy usage and maximizing the life of the furnace components. It is an indication of arc steadiness, which is directly effected by the depth of the slag on the liquid steel. The steadier the arc, the higher the average power level and the greater the electrical efficiency. This is due to the arcs being submerged in the slag and the energy being directed into the steel bath rather than being radiated to the furnace walls. This also results in extending the life of the furnace components, such as, refractory materials and furnace wall panels. The optimum slag depth can be controlled by monitoring the Stability Factor. There is a balance point whereby increasing the depth of the slag improves the furnace operating efficiency until a plateau is reached. At this point, the Stability Factor (S.F.) percentage reaches a peak value. This value is a function of the furnace design, type of steel, and type of slag produced and changes for each point of the operating cycle. During the melting period, the S.F. is at a relatively low percentage, below 50% for example. During the refining period after the steel scrap has melted and is liquid, the S.F. increases to approximately 75%. A foamy slag period follows as the slag depth is increased by the addition of oxygen and coke, and the S.F. reaches an optimum point. The S.F. may peak at 85%, for example. At this time, the oxygen and coke injection systems are shut off to conserve on raw materials and stabilize the slag. If by chance the electrodes are submerged into the liquid steel, which short circuits the arcs, the S.F. would indicate 100%. The submersion of the electrodes is not a recommended nor desirable practice, since it will alter the steel chemistry and should rarely occur. Therefore, the stability factor is an excellent control parameter for maintaining maximum furnace performance while lowering operating costs. The optimum level for the Stability Factor can be pre-established by earlier melts to provide a historical basis for controlling the operation at a given optimum level.

Claims: A method for on-line control of an electric arc furnace in the manufacture of steel using a data processing system for providing control information to control the electric arc furnace wherein: a) electrical data from the arc furnace representative of the electrical conditions and electrical power delivered to the furnace are monitored, characterized in that said electrical data are measured on the primary side of the furnace transformer; and b) a Stability Factor (SF) from said electrical data is calculated in accordance with the following algorithm: Stability Factor = 200 - Operating Reactance Short Circuit Reactance * 100 where Operating Reactance = measured Megavars 3* Electrode Current ( calculated ) 2 Electrode Current ( calculated ) = MVA 3 1/2 * Vprimary * N Short Circuit Reactance = Constant which is derived by submerging the electrodes into the liquid steel bath and performing the following calculation: Reactance ( as short circuit ) = Megavars 3* Current 2 * N 2 Where: Megavars = measured primary reactive power; Current = measured average primary currant N= transformer voltage ratio (c) said calculated Stability Factor is compared with a pre-established Stability Factor for said furnace operation corresponding to a given period of furnace operation; and (d) raw material is introduced into the furnace when the calculated Stability Factor is below the pre-established level for said period of furnace operation to raise the Stability Factor to said pre-established level.

Inventors: GULDEN GEORGE JR, PENKAL KENNETH WALTER, GULDEN, GEORGE, JR., PENKAL, KENNETH WALTER
Applicants: UCAR CARBON TECH, UCAR CARBON TECHNOLOGY CORPORATION
Application year: 1990
Main classification: G05B19
IPC codes: H05B7, G05B19