Power System Design for High-power Electric Smelting and Melting Furnaces

8/20/2019 Power System Design for High-power Electric Smelting and Melting Furnaces

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POWER SYSTEM DESIGN FORHIGH-POWER ELECTRIC SMELTING AND MELTING FURNACES

T. Ma, G.J. Bendzsak and M. PerkinsHatch Associates Ltd., 2800 Speakman Drive,

Mississauga, Ontano, Canada L5K 2R 7

ABSTRACT

High power electric smelting furnaces operate typically at power levels in the 30 to 60 MW range,and frequently, the furnace dynamic load swings can have a significant impact on the generationequipment, transient stability of the power system and the power quality to other interconnectedloads.

Power systems for these furnaces are designed with the objectives of increasing the average -furnace power levels while meeting utility load restrictions, disturbance limits and equipmentperformance limitations.

System design considerations include generation frequency swings, bus voltage fluctuations,harmonic filtering, furnace and power system controls. A systematic design approach consists ofestimation of furnace load fluctuations, dynamic numerical simulations of furnace and powersystem equipment , followed by simulation and analysis of process controls.

Reprinted from:The Proceedings of the International Symposiumon Non-Ferrous Pyrometallurgy:Trace Metals, Furnace Practices and Energy EfficiencyEdmonton, Alberta, Canada

August 23-27, 199231st Conference of Metallurgists

of the Metallurgical Society of CIM

INTRODUCTION

High power electric smelting furnaces have to satisfy operating restrictions dictated by the utility.Specialized power system compensation equipment such as tuned harmonic filters, primaryreactors or controlled reactive power compensation may be required. Electrical separation of thefurnace bus from other loads may be necessary. These increasingly stringent measures arisewhen the furnace load becomes a large portion of the generation capacity on the line. In thesituation where furnace load is supplied from captive (dedicated) generation, the furnace load canapproach 80 % of the generation capacity. These circumstances underline the importance ofdesigning and operating the power system and furnace load as a Combined System.

Furnace load instabilities originating from both arc behaviour and from furnace operating upsetsare discussed from process and electrical perspectives. The interactions of these disturbanceswith furnace power supply equipment and generation are presented.

This paper presents a method of analyzing the operation of large electric furnace loads on a powersystem. The paper characterizes process specific furnace loads according to power conversionmechanisms and heat transfer. Simulation techniques for evaluating performance of the furnace



arc, power generation, control systems, as well as corrective measures are described. Applicationof simulation results to power system design is discussed.

CHARACTERIZATION OF SMELTING FURNACE LOADS

Most electric smelting furnaces contain a molten bath of conductive metal or matte on the hearth,

underlying a relatively resistive slag layer onto which unmelted charge mix is added. There arefour distinctive types of electric smelting operations, characterized primarily by the mechanisms ofpower conversion to heat and transfer of the liberated heat to the furnace charge:

➤ Immersed Electrode

➤ Open Arc

➤ Shielded Arc

➤ Submerged Arc

In practice, the four methods are distinguished by the operational positions of the electrode tipsrelative to the molten bath and the presence and depth of unmelted charge cover surrounding theelectrodes. These features, along with the associated secondary circuit electrical parameterstypical for a 40-50 MW furnace, are summarized for each furnace type in Table I.



Table I – Types of Smelting Furnaces

ImmersedElectrode

Open Arc Shielded Arc SubmergedArc

Figure 2 Figure 3 Figure 4

Electrode Tip PositionRelative to Slag Bath Immersed inbath Above Bath Above Bath AboveBath

Charge Cover at Electrodes Bath Coveredor Open Around

Electrodes

Open AroundElectrode Tips

Charge Cover Around/OverElectrode Tips

Deep ChargeCover Over

Electrode Tips

Furnace Resistance Slag BathResistance

Only

Arc + Slagresistance

Arc + SlagResistance

Mainly Arc

Resistance Distribution

Bath

Arc

R

R≅ 0

Bath

Arc

R

R> 3

Bath

Arc

R

R= 3 to 5

Bath

Arc

R

R> 5

Power Liberation 2 I

Bath R 2

I ( Bath

R + Arc

R ) 2 I (

Bath R +

Arc R ) 2

I ( Bath

R + Arc

R )

Heat Transfer to Charge Convection byHot CirculatingSlag Currents

Arc Radiation &Convection viaHot FurnaceFreeboard

Arc Radiation &Convection

Direct to ChargeBanks

Arc Radiation &Convection

Direct to Charge/ Coke Bed

Typical Electrical Parameters (sec’dry)

Resistance per Electrode (mohms) 5-10 5-15 10-40 1-2

Reactance per Electrode (mohms) 2-2.5 3-5 3-5 ~1-2

Sec. Volts/Electrode 100-300V 100-500V 500-700V ~75V

Sec. Current kAmps 20-50 20-50 10-30 ~100Power Factor 0.97-0.98 0.8-0.9 0.95 ~0.7

Current /Power Swings Small Swings Large Swings Large Swings ModerateSwings

About Avg. Levels Infrequent Frequent Frequent Moderate

Electrode Speed Slow Very Fast Fast Very Slow

Regulation Required <50 cm/min >150cm/min 100-150 cm/min < 50 cm/min

Immersed Electrode -Operation

To assist an understanding of the specific electrical features of each furnace type, it is useful toexamine the behaviour of electrode resistance with electrode tip position, as illustrated in Figure 1.In the IMMERSED ELECTRODE mode, the electrode tips are immersed into the slag bath, whichforms the only significant resistance in the circuit, and power is liberated solely by Joule heating:

E P =

2 I

Bath R

The liberated energy superheats the slag locally establishing circulating flows that distribute theheat to the charge banks (Figure 2). Electrical conversion to heat energy is very stable. Bath



resistance fluctuations and the associated power swings are very small. Low speed electroderegulation is sufficient for power set point regulation. The power factor is high, typically above 0.95.The merits of immersed electrode smelting are found in these important features. This smeltingmode suffers from limited hearth power densities. Large furnace sizes are required to controlerosion of the sidewall refractories by the hot circulating slag currents.

Figure 1 – Load Resistance versus Electrode Tip Position

Figure 2 – Immersed Electrode Operation and Figure 3 – Open Arc Operation



The slag bath resistance is dependent primarily on the slag resistivity, which is dependent on slagcomposition and temperature, electrode size and the immersed depth of the tips into the slag. Fora given operation, the resistance increases as the electrode tip is moved upward from the slag-metal interface to the slag surface, as shown in Figure 1. This simple resistance to immersiondepth relationship provides the basis for regulating the furnace power. The transformer secondaryvoltage tap is set at the desired value and the electrodes are raised or lowered to maintain the set

point resistance or impedance. Control of load resistance essentially controls furnace powerthrough the relationship:

E P = (

Bath R

PF xV 2

)

where E

P = Electrode Power

V = Electrode Voltage to BathPF = Power Factor

Bath R = Bath resistance per Electrode

Arcing Operation

Referring to Figure 1, it is noted that the resistance begins to rise more rapidly as the electrodetips approach the slag surface, mainly due to the reduced contact area of the pointed tips with theslag bath. Micro-arcing between the electrode and the slag commences before the electrode tip isfree from the slag surface. This condition is commonly referred to as "Brush Arcing". Steadyoperation in this zone is generally undesirable since it loses the inherent stability of theIMMERSED mode, does not provide the desired resistance boost needed for high voltage levelsand suffers from the instabilities derived from variable electrode tip geometry and the wave motionof the slag surface.

As the electrodes are raised above the brush arc zone, the rate of resistance increase with

electrode position assumes a much steeper slope, as shown in Figure 1, reflecting the onset ofstable arcing conditions. Energy liberated in the high voltage arcs is transferred either directly tothe charge in the case of Shielded-Arc operation (Figure 4) or a combination of direct transfer andfreeboard re-radiation in the Open Arc mode (Figure 3). Since transfer of the arc energy to thecharge does not rely on superheated slag recirculation, the main cause of sidewall refractoryattack, much higher power densities can be used than are possible in Immersed Electrodesmelting. Since Shielded-Arc smelting results in more moderate freeboard and off-gas conditionsthan are achieved in the Open-Arc mode, it should be adopted, wherever conditions such as gasevolution rate, charge mix size, porosity and sintering temperature of the banks permit.

The arc resistance increases with arc length, but not in an invariant manner. In practice, thevoltage drop across an arc increases virtually proportionally to the arc length over a fairly wide

range of current levels. Typically, the arc voltage gradient is about 15 volts per cm of arc length, inboth the open and shielded-arc modes. Since the rate of resistance change with electrode positionis much steeper in the arcing mode than with immersed electrode - operation, the furnaceelectrical parameters are much more sensitive to arcing mode upsets in electrode position relativeto the bath surface.

This paper excludes discussion of low-resistance submerged arc smelting operations (ferroalloy,pig iron, phosphorous) where the electrodes are deeply buried in a conductive charge mix, withmicro-arcing from the tips to a floating coke bed.



Figure 4 – Shielded Arc Operation

Figure 5 – Ideal Arc Voltage and Current Waveforms



Figure 7 – Furnace Power Supply Equipment 6 Electrode Furnace

Figure 9 – Frequency Versus time Captive Generation 44MW Furnace Tip

Arc Instabilities

A major electrical instability introduced by arcing arises from the requirement to re-ignite the arceach half cycle, e.g. 120 times per second for 60 Hz power. If the arc fails to ignite, current flow isinterrupted in the electrode, causing a 33 % to 50 % reduction in furnace power and excessivelevels of power unbalance and negative sequence current. Re-ignition difficulties and thecorresponding electrical instabilities are more severe in the open arc mode since hot plasma isswept from under the tips into the freeboard, thereby leaving a much colder, less conductiveenvironment for arc re-ignition. Arc re-ignition produces reactive power (MVAR) swings on the

power system which cause supply voltage fluctuations. These disturbances, referred to as voltageflicker [2], range in frequency from 1 to 20 hz, resulting in objectionable light flicker. By contrast, inthe shielded arc mode, the environment in the arc crater under the electrode is contained andprotected from the furnace atmosphere. Re-ignition is much easier and hence power instabilitiesare considerably reduced. This benefit of shielded arcing is evident from the reduced incidence ofresistance and power fluctuations seen in Figure 4 compared to the open arcing data shown inFigure 3. The improved stability of shielded arcs allows the use of higher voltage levels.



The ignition voltage requirement of the arc introduces a phase delay angle between the arc currentand supply voltage, denoted by e in Figure 5. This phase shift is referred to as an equivalent flarereactance" and is manifested as a reduction in furnace power factor. The arc reactance follows therelationship:

arc X =

1 K -

arc R

where1

K is empirically measured and is process specific.

The non-sinusoidal arc current of Figure 5 contains higher order frequencies, known as harmoniccomponents. Both utility grids and captive generation are sensitive to current harmonics for thefollowing reasons:

➤ Distortion of the supply voltage waveform to other customers.

➤ Amplification of supply voltage distortions due to resonance at the power factor correctioncapacitors of other customers [1].

➤ Interference with communication circuits and sensitive electronic equipment.

➤ Derating of power system equipment due to increased harmonic current losses.

Operation Load Disturbances

In addition to the power system disturbances resulting from arcing, instabilities also result fromnormal operating upsets themselves such as power ramping and unbalances. Start up and shutdown power ramp rates are normally governed by the rate at which the generation can beregulated to meet the load.

Furnace unbalanced operation due to electrode breakage and rebaking, or tapping temperatureadjustments is governed by the power supply and motive load negative sequence capability.

Lastly, furnace full load trips result in an immediate step mismatch between load and generation.This step mismatch results in frequency oscillations as the generator controls adjust to the loss ofload.

THE FURNACE POWER SUPPLY

Smelting Furnace Electrode Configurations

The majority of large electric furnaces are either three electrode round furnaces or six-electrode-in-line rectangular furnaces. Furnace operation and dynamic control is influenced by the number ofelectrodes. A six electrode furnace has an additional degree of flexibility by virtue of individualfurnace transformer tap changer control as well as individual regulation on each of the sixelectrode columns. This provides for a wide range of power control flexibility in furnace operations.

The three electrode furnace differs in that the three furnace transformer secondary windings areelectrically connected to each other by secondary bus connections made either in the transformervault or near the electrode clamps. This close electrical phase coupling of the three electrodefurnace causes a current fluctuation in one electrode to be reflected into the other two electrodes.

Following the loss of arc conditions in a six electrode furnace, the total power is reduced by 33%.For the same conditions, the total power reduction of a three electrode furnace is 50%. A sixelectrode furnace therefore presents reduced MW fluctuations, and power unbalance to the powersupply system during loss of arc.



The Furnace Transformer and Power Supply

The operating ranges of furnace electrical parameters are established by the transformerspecifications. The transformer voltage tap range, currents and furnace load range are graphicallydisplayed using a Power/Voltage/Current (PVI) diagram. A typical PVI diagram for a 3 electrodefurnace is shown in Figure 6. An operating point of 50 MW, 32 kA, 1042 volts, 18 mohm is

indicated for a 72 MVA furnace transformer. This diagram also displays both the immersedelectrode operating region as well as high voltage arcing operation.

A typical power supply arrangement for a 6 electrode furnace powered by 3 single phase furnacetransformers is shown in Figure 7. The key equipment items with power control capability are thefurnace transformer tap changer, and the electrode regulation. The transformer voltage tapchanger enables a range of discrete secondary voltage levels as shown in the PVI diagram. Theelectrode regulator controls the load resistance, and hence the power level, by raising or loweringthe electrodes.

GENERATION, TRANSMISSION AND DISTRIBUTION OF FURNACE LOAD

Generation

The electrical transmission system connecting generation to the furnace load is shown in Figure 8.Furnace load can be supplied from either Captive Generation or a Utility Grid. A utility grid consistsof multiple generation sites, each site consisting of two or more synchronous generators.

A summary of high power electric furnace loads supplied from both Utility grids and captivegeneration is shown in Table II.

Table II – Power Supply to Furnace Load

Load Type Project Fce Generation PFCorrection

ElectrodeResistance

FaultLevel

Fur. Ty.

Pri.

Fault LevelUtility

Supply

MW Type MW MVAR mohm MVA MVA

Captive Power System

Open Arc Iscott 2x45 GasSteam

3x88+100MWGrid

SVC 2x65SWC30

5 600 1200

Open Arc Steel Plant 1x17 Diesel 4x12 SVC+45/-10

5 157 240

Shielded Arc Falcondo 1x80 Steam 3x70 None 135/2 660 800

Shielded Arc PT Inco 3x55 Hydro 3x60 None 15 700 950

UTILITY GRID 45

Cerro Matoso 1x48 Grid None 5 600

Ipsco 2x40 Grid SynchronousCondenser s 120MVAR

5 334 1340

Sysco 1x60 Grid SVC 1x40 5 900 2100

Immersedelectrode

Falconbridge 2x21 Grid None 35/2 300 1500

Immersedelectrode

Cyprus 1x40 Grid None 12/2 370 1180

Immersedelectrode

Impala 1x30 Grid None 15/2 700 2000



Figure 6 – Power/Voltage/Current (PVI) Curves 3 Electrode Furnace

Figure 8 – Power System Block Diagram

The traditional approach has been to obtain smelter power from the closest utility grid provided ithas the required capacity and acceptable reliability. In new greenfield projects, an adequate utilitysupply is often not available. Consequently, installation of a local generation plant is required. Thechoice of generation type depends mainly on the relative supply costs of competing energysources. The potential for using waste heat from the smelter to augment the energy supply as wellas waste heat from the generation plant in the smelter should also be examined.



A brief discussion of the main types of captive generation follows.

Hydroelectric Generation

The stored water energy is directed through penstocks to turbines which supply mechanicalenergy to the synchronous generators. The turbine wicket gates control the mechanical energy to

the turbine according to the speed control set by the governor. Fast furnace load sheds result inwater flow diversion from the turbine to a bypass valve. Frequency swings of up to 15% arepossible. Start Up/Shut Down power ramps are controlled by the turbine wicket gates. The wicketgate and bypass valve also provide protection from the furnace load dynamics. Hydroelectricgeneration operates at low speeds and high inertia, and is well suited for furnace load dynamics.

Diesel

The generators are driven by diesel engines coupled to large flywheels, which provide additionalinertia for dynamic load changes. Fast diesel throttle control and a specially designed fuelturbocharger provide a proper response for full load rejections. Diesel generation, operating at lowspeeds of 400 rpm is a solution for the supply of furnace power.

Steam Generation

Steam is produced in a coal, oil or gas fired boiler, and is admitted to the turbine through a mainstop valve and downstream throttle valves, freely expanding through the turbine and exhausting aslow pressure steam to a condenser. In the event of a furnace load shed, over speed protection isfurnished by the main stop valve, limiting over speed to 6%. Load regulation is provided by thesteam control valves and the speed regulator. Steam turbines operate at speeds of 3000 rpm andare mechanically more sensitive to furnace dynamic load changes.

Gas Turbines

Gas powered turbines have been used for the supply of open arc furnace load. In this instance,the system frequency excursions were limited to 0.4% or 0.24 hertz, requiring a thyristor-switched

ballast resistor (static WATT control-SWC) for power system stabilization [3].

Generation Controls

The generating plant supplies two power components to the furnace; the MW or real powercomponent that is used for smelting and the MVAR or inductive component, which is the storedenergy in the magnetic fields of the power transformers and secondary bus. The MW and MVARfurnace power requirements are both met by the synchronous generators. For a steady furnaceoperation, balances for both the MW and MVAR loads exist between the generation and thefurnaces. A furnace load disturbance, however, upsets the balances.

A transient power system MW mismatch is first drawn from the rotating inertia of the generationmachines, resulting in a speed change, and corresponding frequency swing. Generation speed -governor controls then react to restore system frequency to the set point value by regulating theprime mover mechanical power. The speed control loop is typically slow, in the order of 5 to 30seconds. Significant frequency excursions may result, as seen in the case of hydroelectric captivegeneration. Where tighter frequency limitations must be met, additional MW compensation isrequired.

Reactive power MVAR mismatches resulting from arc instabilities cause a system voltage change.The generation voltage regulator senses the change and adjusts the internal field of the generator



to re-establish set point voltage. In doing so, the MVAR output of the generation is matched to thecorresponding new furnace MVAR requirements. This voltage regulation control loop has aresponse of 0.5 to I second and is generally too slow for open arc flicker control. In this situation,an additional source of controlled MVAR compensation may be required. Static VARcompensation (SVC), consisting of fixed harmonic filters and a thyristor-controlled reactor (TCR)may be appropriate. Other solutions involve raising the utility fault level with additional generation

and transmission. The fault level is a measure of the short circuit current at the point of interestand is expressed as the short circuit volt-amperes supplied by the generating sources. Table IIprovides typical fault levels for furnace installations. In general, increasing fault level represents a"stiffer" power supply which reduces the impact of arc-generated voltage flicker and harmonicvoltage distortion to other customers. System fault levels normally increase with increasing supplyvoltage. Consequently, many smelter operations are supplied at voltages of 69 kV and higher.

The next section is devoted to the analysis and correction of furnace load disturbances on thepower system.

ANALYSIS, CORRECTION AND CONTROL OF POWER SYSTEM DISTURBANCES

Proper selection and specification of the power equipment for a furnace load requires the detailedanalysis of furnace load profiles operating on selected generation and transmission systemconfigurations. The analysis involves the examination of furnace steady state and dynamic loads,solution of the furnace supply circuit equations, simulation of appropriate generator and regulatorcontrol systems, and simulation of corrective compensation equipment.

Presently available software packages, running on engineering work stations, are sufficientlypowerful for power system simulations [4]. Application of furnace resistance and arc models,equipment control systems and reactive power compensator schemes [5] can be incorporatedwithin the software packages.

A summary of furnace load disturbances, power supply standards and corrective measures ispresented in Table III. Technical selection of corrective measures is assisted by power system

studies. The results of representative case studies for furnace power supply analyses coveringvoltage flicker, frequency fluctuations, arc harmonic filtering and furnace unbalance are presentedbelow.



Table III - Correction of Furnace Load Disturbances

Standard Spec Corrective Measures

Arc Based Distrubances

Furnace MVAR swings causeexcessive voltage flicker which

disrupts other sensitive loads on thesystem.

UIE 2 0.28 % at 7hz

(sinusoidal)

➤ Separate the furnace andmotive bus

➤ Increase Power Supply FaultLevel

➤ MVAR CompensationEquipment

➤ Series Buffer Reactors➤ Restrict, Change Operations

Furnace MW swings causeexcessive frequency swings whichdamage motive load drives

IndividualUtilityRequirements

➤ Separate Motive Load Bus➤ Electrode Regulation Control➤ Generation Controls➤ Dynamic MW Compensator

Excessive Arc Harmonics IEEE Std. 519 ➤ Harmonic Filters

Operations Based Disturbances

Start up and shut down load can bedelayed by scheduling of water orsteam

Utility 2-10MW/Minute

➤ Computer Assisted Control

Sustained unbalanced furnace loadoccurring due to electrode breakageor other sustained furnace operationwith unbalanced furnace power.

2% VoltageUnbalance.DerateMotors to95% ofNameplate

➤ Increase unbalance ratings ofrotating machines

➤ Automated control of furnacephase power set points.

CASE 1 VOLTAGE FLICKEROpen arc furnace operation on a Utility grid was measured and calculated with and withoutoperation of MVAR compensation by a thyristor-controlled reactor (TCR) and fixed capacitorscheme [6].

The furnace, power system, and arc parameters, denoted in Figure 7 are:

Generation GRIDFault Level at Incoming Bus 2100 MVA

Furnace Operating Power 60 MW

Power Factor 0.9

Total Fixed Capacitors 110 MVARTCR Rating 140 MVAR

Equation (1) kl 0.20

MEASURED DATA WITH TCR

Flicker Voltage Magnitude at 7 hertz 0.3 % - 0.4 %Flicker Voltage Magnitude at 20 hertz 0.4 % - 0.8 %

Initial GRID MW and MVAR power flows were calculated using a Gauss Seidel load flow pro-ram[7]. An empirical arc model was developed which includes the described arc instabilities,



measured arc reactance, and measured harmonic current source generation for open arcs.Numerical calculations were carried out using a commercial software package [4]. Simulationcalculations verifying SVC correction of low frequency flicker and amplification of high frequencyflicker are in progress.

CASE 2 FREQUENCY DYNAMICS

Furnace load trips on a captive power system were causing excessive frequency fluctuations of +15 %. Calculations were required to determine the magnitude and ramp rate of the MWcompensation needed to reduce the frequency fluctuations to 6% for a 50 MW load shed.


Generation 190 MW Captive

Fault Level at Incoming Bus 850 MVA

Furnace Operating Power 52 MW Shielded Arc

Power Factor 0.92

Equation (1) kl 0.217

The calculated frequency swings for both uncompensated and compensated furnace load trip areshown in Figure 9.

A calculated ramp rate of 10 MW/sec, immediately following a furnace trip reduces the frequencyswing to 6%. Process control and equipment modifications are presently in progress to implementthe above strategy. This design parameter was obtained from numerical simulation of thegenerator electromechanical and control loop differential equations representing the machineequations, voltage regulator and speed governor control, a shielded arc model and electroderegulator equations. The simulation software was written using commercial software [8].

CASE 3 REVIEW OF HARMONIC FILTER DESIGN

A performance review of harmonic filters for open arc load and thyristor-controlled reactoroperation was carried out.


Generation 45 MW CAPTIVE

Fault Level at Incoming Bus 240 MVAFurnace Operating Power 27 MW Open Arc

Power Factor 0.7

2nd Harmonic Filter 18 MVAR

3rd Harmonic Filter 12 MVAR

5th Harmonic Filter 15 MVAR

TCR Rating 55 MVAREquation (1) k1 0.20

Harmonic filters are designed to provide a current sink for arc and TCR generated harmonics,thereby reducing the magnitude of the net harmonic injection into the utility grid. The filters alsominimize parallel resonance of the power factor correction capacitors, and limit the harmonicvoltage distortion of the utility bus [9].



The source harmonic currents and the calculated net harmonic injection into the generation areprovided below.

Harmonic Arc & TCR Harmonic Residual Utility AttenuationNumber Generation Harmonic Current Factor

I3 Iu I3 /Iu

% of Fundamental % of Fundamental2 1.7 0.22 7

3 3.3 0.2 174 1.3 0.2 6

5 3.5 0.2 17

The filters reduced the total harmonic distortion factor [10] to 4.6% on the interconnected rollingmill bus, thereby meeting the maximum distortion limit of 5.0%.

The calculations involve representing the power system with equivalent harmonic impedances andsolving for harmonic filter currents and residual utility currents at each harmonic frequency.

CASE 4 UNBALANCE COORDINATION OF FURNACE AND MOTIVE LOAD

Both immersed six electrode load and large synchronous motors are supplied from a common busconnected to a Utility grid. The maximum sustained furnace unbalance acceptable to the motiveload operation was calculated. The calculations were used to set the negative sequence relaysettings.


Generation GRID

Fault Level at Incoming Bus 11 80 MVA

Furnace Operating Power 18 MW Immersed ElectrodePower Factor 0.97

Motive Load 3x5OOO HP

The furnace transformer connection was grounded WYE with an insulated transformer neutralcable. Maximum furnace power with 4 electrode operation (two out of service) is 12 MW, resultingin a bus voltage unbalance of 1.2%.

Furnace unbalance limits are normally given' by either the negative sequence rating of thegeneration, or the NEMA bus voltage unbalance of 2% [10]. Analysis of these limits involvecalculation of furnace load negative sequence current, by multiplying the furnace three phasecurrents, in vector format by a transformation matrix.

CONCLUSIONS

The design of electrical power systems for smelters incorporating one or more high power electricfurnaces must take into account:

➤ Furnace operating characteristics, whether immersed electrode, open or shielded arc.

➤ Furnace power load relative to generation capacity.

➤ The method and source of power generation.



Arc-based disturbances can be corrected enabling high power furnace operation on weak powersupplies. Appropriate power system calculations are required for the design of the power supplysystem. Each furnace power supply application requires individual evaluation to establish theparameters at the interface between generation and furnace load.

REFERENCES

1. J. Arrillaga, D. A. Bradley, P. S. Bodger, "Power System Harmonics", Wiley, New York,1985, Chapter 4.

2. R. Seebald, J. Buch, D.J. Ward, "Flicker Limitations of Electric Utilities", IEEE Transactionson Power Apparatus and Systems, Vol. PAS 104, No. 9, Sept. 1985, 2627-2631.

3. D.J. Chee-Hing, F.M. Wheeler, "The New ISCOTT Meltshop, In Trinidad", Electric FurnaceConf. Proc., 1981, 39, 48-56.

4. V. Brandwaj n, " Electromagnetic Transients Program (EMTP) Revised Rule Book, Version2.0", EPRI EL-6421-L, Vol. 1, Research Project 2149-4.

5. Hatch Associates, "Static VAR Compensation of Voltage Flicker from Electric ArcFurnaces", CEA Project 042-T-818, in progress.

6. R. M. Mathur, Ed., "Static Compensators for Reactive Power Control", CEA-Cantext,Winnipeg, Canada, 1984.

7. Roettger Eng. Co., "LOADFLOW 2.2", Naples, Florida, May 1989.

8. The Math Works, Inc., "MATLAB", MA, USA, 1985 - 1990.

9. M.A. Pesonen, "Harmonics, Characteristic Parameters, Methods of Study, Estimates ofExisting Values in the Network", Electra, No. 77, Pg- 35-53.

10. NEMA Standard MG1-20.55.

Power System Design for High-power Electric Smelting and Melting Furnaces

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Transcript of Power System Design for High-power Electric Smelting and Melting Furnaces