a
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~ 0 13,
. NO. 2,
1987
published by the Center for Metals Production
-
An EPRl Sponsored R&D Applications Center
Understanding
Electric Arc Furnace Operations
For Steel Production
Introduction
silicon and the burning of natural gas
The use of electric arc furnaces (EAF)
with
oxy-fuel burners. About
53% of the
for steelmaking has grown dramatiin the
cally in the last decade in the United Furnaces are often classified by power total energy leaves the furnace
States. In 1975 electric furnaces acrequirement levels. A scale indicating liquid steel, while the remainder is lost
counted for 20% of the steel produced power classification ranging from ultra- to slag, waste gas, or cooling.
Typical tap-to-tap time has decreased
high-power (UHP), withover 700 kVA
in the U.S.; by 1985 this figure had
from
over 2 hours in 1960 to 70-80
grown to 34%. Electric furnaces range per ton, downto low-power, with less
minutes today. Primarily responsible are
in capacity from a few tons to as many than 200 kVA per ton, is shown in
UHP furnaces, oxy-fuel burners, wateras 400, and a steelmaking shop can
Figure 1 along with some representacooled
side panels (which allow
for
have a single furnace or up to three tive furnaces.
higher
power
after
the
steel
is
molten),
or four. In brief, these furnaces melt
foamy
slag
practices
(which
also
permit
steel by applyingan AC current to a
EXAMPLES
KVA
higher power), and ladle metallurgy
PER
steel scrap charge by means
of graphite
TON
(which removes the refining function
electrodes. It requires about
500 kwh
IMX) from
the furnace and shifts it to a ladle
of electricity to produce a ton
of steel;
ULTRA-HIGH
900 into which the molten metal is poured).
consequently, these furnaces use a
.. .~
. .. . .
.
800 0 CHAPARRAL
tremendous quantity of electricity. Trans0 BETHLEHEM(JOHNSTOWNI
700
former loads mayreach 120 MVA.
Chemical
Ekctricol
The melting process involves the use 600 0 INLAND
HIGH
of large quantitiesof energy in a short
0 LUKENS “n”
500 0 ATUNTIC STEEL
time (1-2 hr) andin some instances the
400 process has caused disturbances in
MEDIUM
300 ARMCO (KANSAS CITY)
power grids. These disturbances have
7.00 usually been characterized as “flicker”
LOW
0 LTV(CLEVELAND)
- brief irregularitiesin voltage a frac100”
tion of the60 Hz cycle inlength, and
I
“harmonics” - irregularities that tend to
Figure 1
to the 60
occur in a pattern repetitive
EAF Power Classifications
Hz cycle.
The featuresof electric arc furnaces
It is important to consider the energy
were describedin a CMPTechEAF: The
Commentary on Electric Arc Furnaces balance for a typical modem
energy diagram shownin Figure 2 indi(Vol. 1, No. 3, 1985). The purposeof
cates that70% of the total energy is
the present Techcommentary is to
electrical, the remainder being chemical
give utilities and steel mills a better
of
understanding of electric furnaces from energy arising from the oxidation
Figure 2
elements such as carbon, iron, and
an electrical viewpoint.
Energy Patterns in an Electric
Furnace
Energy Needs
~
”
~
Typical
Steelmaking Cycle
A typical heat cycle appearsin Figure 3.
To achieve meltdown as quickly as
possible, electrodes are initially lowered
to a point above the material, the
current is initiated, and the electrodes
bore through the scrap to form a pool
of liquid metal. The scrap itself protects
the furnace lining from the highintensity arc. Subsequently, the arc
is
lengthened by increasing the voltage
to maximum power. Most modem
furnaces are equipped with water-cooled
panels in the upper halfof the sidewall,
rather than refractories, which allows
for longer arcs and higher energy input
into the furnace.In the final stage,
when there is a nearly complete metal
pool, the arcis shortened to reduce
radiation heat losses and to avoid refractory damage and hot spots.
After melt dawn, oxygen usually isinjected to oxidize the carbon
in the steel
or the charged carbon. This process
is
an important source of energy; the carbon monoxide that evolves helps minimize the absorption of nitrogen and
flushes hydrogen out of the metal. It
also foams the slag, which helps minimize heatloss.
Arc ignition peGiod
(start of power supply )
Conditions of
furnace
I
2. To stabilize the arc
3. To rapidly submerge the
Operating
objectives
I
electrode tip into
scrap
I
To protect the furr
from arc soot
by supplying high power
and increasing the boring
speed
2. To increase the boring
diameter
roof from arc
I
I
I
I 11 Optimum current
a OP
a
u 80
Characteristic
curve of arc power 2 60
? 40
00
60
Is Short-
40
c~rcuiting 40
current
20
20
‘
0
Criteria for
operation of
furnace
Detailed Electrical
Operation
Molten metal
period
Boring period
T
40 80
ArcCurrent
I20
160
‘ 0
40
80
120 160
Io %
To be judged by
the position
where the
electrode tip is
submerged for
about 1.5d
TO be judg A? t
consumption (kW
the boring period
1. The rate of lowering
the electrode
2. The lowered position of
the electrode
I
1
I
Courtesy of the Iron and Steel Society. Inc. - Electric Furnace Steelmaking 1985
-
Figure 3
I
SteelMeltingCycle
early meltdown, and they occur at varyin an
After an electric furnaceis charged with tinguished by a minor overshoot
electrode regulator or by physical move-ing frequencies.
scrap and the roofis in place, the
Many attempts have been madeto
operator lowers the electrodes, each ment of the scrap.As the scrap melts,
establish the human eye’s reaction to
it can often shift and fall away from
of which has its own regulator and
mechanical drive. The electrodes are an electrode- extinguishing the arc, the flickerof a lamp. That these endeavors have not exactly confirmed one
connected to the furnace transformer‘s or against theelectrode- possibly
another is shownin Figure 4 (from “Arc
breaking it.
secondary delta winding, which may
Furnace Power Delivery”), where perBecause of thephysical movement
be rated from;bout 600 to 850 volts.
ception is measured while disturbance
No current flows when the first elec- and settlingof the scrap, wide excursions can take place on a random basisvoltage and frequency are varied. Eye
trode contacts scrap, but
a line-to-line
response to disturbancesin the 5-10 Hz
path through the scrap and an arc are in the secondary circuit. The abrupt
range did seem to be greatest
in all
established when the second electrode initiation and interruption of current
the studies.
completes the circuit. The regulators flow provides a source of harmonic
currents and causes considerable distur- Generation of harmonics may result in
for each of thesetwo electrodes then
bance to high-impedance circuits.
( A b o u t further flicker problems, and equipsignal the drives to raise the elecment on the power system may also
trodes until the selected current-voltage 75% of the total impedanceis inthe
secondary circuit.) Voltage and current be damaged. If static capacitors are to
ratio for the arcis achieved. Initiation
of the third arc depends on the scrap’s waves deviate considerably from sym- be used to improve the power factor,
location, whichis unpredictable, hence metrical sinusoidal patterns, but they an analysisto ensure that resonance
does not exist at any
of the harmonic
do not attain full rectangular shape,
the duration of the unbalance
is short
according to findingsin the CMP report, frequencies should be made. Harmonbut random. While the scrap is still
“Arc Furnace Power Delivery Scoping
unmelted, the arc may easily
be exStudy.”’ Disturbances are worst during
.mation
Main melting period
Meltdown period
Meltdown-heating period
Tapping spout
..
. ...
lid baih
'bottom
1. To reduce the local
damages near hot spots
on lining
2. To rapidly melt the
remaining scrap
Since the arc is surrounded
withsolid materials, the
the therrnol conductivity Of
arc power is a maxlmum.
Therefore, rapid and uniform
melting should be planned
by supplying the maximum
power that the facilities permit
)P
I20
I4O
is
3
1. To reduce the heat radiation
onto the lining as well as
minimize hat spot domoge
2. To rapidly increase the
temperature of molten
steel to the appropriate
value for refining
1
la
!
160
jowr
lfter
1. Power consumption (kwh)
1. Variation of arc impedance
2. Melting Conditions of
2. Therate of the F c e
scrop
temperature (IPB)
3. Powerconsumption (kwh)
Borderlines of flicker per perceptibility for incandescent
light bulbs under laboratoryconditions (1-Commonwealth Edison. 12OV. 25
4 0 + 6OW; 2-Japan. lOOV,
60W; 3-Schwabe. 120V. 60W; 4-Carjell. 220V, 6OW;
5-Wasowsk~,220V, 40W; 6-Kendal1, 230'4 60W; 7-UIE,
230V, 60W; sinusoidal wave; 8-UIE, 230V, 60W.
square wave).
+
070
r
I
Power consumption (kwh)
a full heat. Thisis generally uneconomical due to oxidation losses and the
need to open the furnace for several
separate charges, which results inloss
of both time and heat. Noris the use of
large heavy scrap alone optimum.A
large piece might protrude and interfere
with roof closure or require placement
by magnet, a process which takes time.
The furnace operator therefore tries
to blend several types of available scrap
to suit his needs. It is beneficialto
arrange the heavier pieces near the
bottom of the charge. After about20
minutes of operation, dependingon
available power and other practices,
the electrodes will have opened some
voids, and cave-ins can occur. If large
pieces of scrap are on top of the
pile, they can possibly slide into and
break an electrode. It is generally believed that light, uniform scrap produces
a smoother meltdown than does large
heavy scrap. However, this is not
always the case. If heavy scrap is
charged, full power canbe applied. If
all the scrapis light, on full power the
electrodes may bore through, damaging the furnace bottom before
a sufficient pool of liquid metal has formed.
Generally, the initial period of melting
causes the most electrical disturbances.
As the scrap temperature begins
to rise,
a liquid pool forms, and disturbances
begin to diminish. This is generally about
10 minutes or so after power-on and
can vary depending on power levels
and shop practices.
Melt Down
ics contributeto wave distortion and
Heating steel scrapto approximately
to the increasein effective inductive
3000" F requires large quantitiesof
reactance. This increase is often in the energy rapidly applied. Therefore, full
10 to 15% range and has been repower is called for during meltdown.
25%. Current into
ported as high as
The arc during meltdown canbe long
the furnaceis therefore less than what because the electrode and arc are borwould be expected from calculations ing a hole down through the scrap,
based on sinusoidal wave shapes, and and the roof andsidewalls are not
losses in frequency-sensitive equipexposed to arc radiation. If the arc is
ment such as transformers are higher extiquished, the regulator will lower the
than the sinusoidal wave shape would electrode to re-establish it. This can
produce.
take several seconds if the scrap has
moved out from under the electrode.
Importance of Scrap Main Melting Period
Scrap is availablein a wide variety
0.20
S
8.8 10
FlickerFrequency,
IS
Hz
Figure 4
Borderline of Flicker Perceptibility
of sizes, densities,and chemical compo- After about 20 minutes, most electric
sitions, anda mixtureis usually used. If furnaces will have begun converting
only the lightest, least dense material scrap to liquid metal. Hence, wide
is charged, several buckets of scrap swings in disturbances will diminish
considerably. When sufficient molten
must be placedin the furnaceto make
metal exists(in some high-powered
as oils) and low-boiling-point nonferrous
furnaces only8-10 minutes is required), When the carbon electrode actsas the
metals. Preheat could come from Waste
the arc is shortened by an adjustment cathode, it is a good emitter of eleci ‘
heat or supplemental gas or oil burners.
trons
(hence,
carbon
cathodes
in
the
to the electrode regulators. The curThe recent trend toward using the
rent will rise since overall resistance is large mercury arc rectifiers of several
electric furnace asa main melting unit
reduced, and the power factor and arc years ago); steel, even molten,is not
has ledto the practice of leaving Some
power will decline. Arc length is changed nearly as good. During the half-cycle
slag and molten metalin the electric
when the scrap orthe bath is the
so that the shorter arc will deliver
a
furnace. CMP-AIS1 studies at Sidbeccathode, arc initiationis a little slower
higher portion of its heat
to the metal
Dosco indicated that this ractice reis
below the electrode than will the longer and weaker than when the electrode
duces arc furnace flicker.
cathodic (Figure6). This slight variaarc, which radiates more heat
to furtion between the opposite half-cycles
nace sidewalls. Many studies have
been conducted which confirm the ad- tends to create the even-numbered
vantages of thelong arc for meltdown harmonics -the second and fourth.
of heavy scrap andthe short arc for
operation after sufficient liquid metal has
been formed. The short arcis much
Tests havebeen run with hollow elecmore stable than the long arc, and
trodes, both dry and with argon, and
operation during the refining period
argon and lime injection. Work by
W.E.
follows sinusoidal concepts much more
1950’s
demonstrated
Schwabe
in
the
Many ways exist to reduce the effects
closely.
of the arc disturbances. These are deter-the stabilizing effect of hollow electrodes in furnace^.^ He described the
mined by the utility system
to which
development of the trumpet profile in
to be
the furnace or furnaces are
the tip which decreased load swings
connected, and theyare influenced
mainly by the size and stability of the during meltdown.
Photos have been taken during the
refining’period to show the electric arc’s power grid. Some sizable shops require Trials with hollow electrodes and ar1970’s showed
no particular flicker control equipment. gon injection during the
action. High-speed photographyis
a
marked
smoothing
of
oscilloscope
needed to capture the60 arc cycles per It is quite possible that, if a furnace
traces
of
arc
voltage
and
current4.The
shop is fed from
a 220 kV or higher
second. It has been shown that the
favorable
effect
of
argon
resulted from
system
with
a
short-circuit
capacity
of
arc moves around on the tip of the
the arc-supporting effectof the media
electrode and, in some cases, consider- 6500 MVA or more,the utility will
due to the greater number of atoms
ably off the vertical. This movement is experience verylittle loaddisturbance,
present
for ionization. Injectionof powbelieved to be caused by the electro- and the steelmaker can have considerdered
lime
into the arc zone cut arc
able
flexibility
in
configuring
his
internal
magnetic forces induced by the high
resistance in half and eliminated all
current flow (Figure5). When the same
plant power system.
Most utilities require power factor high-frequency componentsof the arc
conditions were observed with hollow
voltage.’
correction. Shops with large electric
electrodes, the arc still moved around
Based on these favorable results,
furnaces
would
more
than
likely
use
considerably, but it appearedto be more
nearly vertical and better consolidated static capacitors; synchronous condens-trials ona commercial-sized electric
furnace were undertaken by the Center
ers of sufficient capacity wouldbe
than with solid electrodes.
for
Metals Production, with joint financprohibitively expensive for
a multifurnace
shop. Before such systems are installed, ing by 20 steel companies and the
A
transient analysis is required to determine:Electric Power Research Institute.2
20% reduction in flicker was found when
(a) Capacitor bank configuration argon and lime were injected down
(b) Need for harmonic tuning of the electrodes during these tests. Howsections
ever, savings in power and electrodes
(c) Switching procedure (This is
were minimal, offering little incentive
important to avoid a power
for adopting this practice for steelmelting.
Cycle I
Cycle 2
Cyde 3
factor penalty and does not
eliminate flicker.)
Figure 5
If additional regulationis needed, VAR
Arc Pattern Flow
control equipment would probably be Operation of an electric furnace with
required. However, if plans have alone direct-current electrode rather than
ready been made for power factor
the three electrodesof conventional
capacitors, including tuning reactors, alternating-current designs has proven
then the thyristors and main reactor are
to reduce electrical system disturbthe only further additions required.
ances. Improvements in load variation,
a 60% reduction in flicker, and elimination of phase unbalance has been reported with DCfurnace^.^ Major problems withDC operation are the relatively
Anode
short life of the bottom electrode and
Preheating the scrap charge helps re- contact between the scrap and the botFigure 6
tom electrode during initial meltdown.
duce
electrical disturbansesin melting
Variation in Arc Shape with Half
Except for the bottom electrode, most
of
and
removes
some
contaminants
(such
Cycles
f
Argon and Lime
Injection
Reducing Electrical
Disturbances
Arc Movement
DC Furnaces
h
Other Means of
d Reducing Flicker
monic filters, which would also help t
the equipment for a DC furnace is
avoid power factor penalties.It is also
conventional, and large DC loads are
helpful that makers of electronic apparanow routinely handledin steel mills. A
60 MW furnace load would be compara- tus - computers, TV, X ray, etc.-have
.. ble to rectifiers supplying a wide 7-stand improved their power supplies in recent years, making these systems less
hot stripmill for rolling steel.
vulnerable to power fluctuations.
Rectifiers for these main3000-5000
hp drive motors are of the 12-pulse
type. These rolling mill systems handle large impact loads which are close
to step functions and probably could
handle initial disturbances in DC melting
furnaces.
1. “Arc Furnace Power Delivery Scoping .
p
,
’
Most important, utilities have become
vitally interested in power quality.To
further this goal, the Electric Power
Research Institute has formed
a Power
to investiElectronics Applications Center
gate further the question
of utility power
quality.
Bibliography
Study,” Center for Metals Production,
Report No. 84-1,1984.
EAF Furnace Loads 2. “Arc Stability in Electric Furnace SteelCenterforMetalsProduction,
Are Not Unreasonable making,”
ReDozNo.
86-9,
1986.
It is true that the arc furnace incurs
short circuits. However, relative magnitude should be kept in mind:
If current
at maximum circuit power is considered to be 100 percent, or 1.O per unit,
then a short circuit will result in only
1.414 timei this full load current
- not
too serious a problem. When the primary breaker is closed on
a transformer,
the inrush current can be
10 times
rated load-far more than the1.4
times due to a secondary short in the
electric arc furnace.A hot strip rolling
mill cango from no load to 60 MW
faster than a60 MW furnace, and this
can occur every2 minutes. While the
rolling mill load is a balanced-phase
load (a major advantageover arc
furnaces), the mill load cycles much
more frequently.
3. W. E. Schwabe. “Experimental Results
with Hollow Electrod‘es,” Iron and Steel
Engineer, June 1957, pp. 84-92.
Utility Engineering
Progress
Most of the larger electric arc furnaces
installed over the past
20 years have
been fed from high-voltage, stiff utility
power grids. Problems of previous years
-transformers failing because of resonance on a certain tap combination or
utility capacitor banks failing far from
an offending furnace- have not been
repeated. Users and utilities have done
their homework priorto installation,
and potential problems have been identified and corrected. The user can decide what to do inside his plant regarding the potential flicker problem. For
example, he may wantto install har-
4. W. J. Maddever. “Gas Injection Process
During Electric Furnace Steelmaking and
Continuous Casting,” Ph.D. thesis, University of Toronto, 7978.
5. ”DCArc FurnacesforSteelProduction.”
Metals
Center
Production,
for Report
No.
86-8, 7 986.
6. “Techno-Economic Assessmentof Electric Steelmaking Through the Year 2000”
EPRI, 1987 (to be published).
CMP Publications of Interest
84-1 Arc Furnace Power Delivery (1984)
A detailed analysisof the technical problems relatingto large electric furnaces
and utilltypower grids.
85-1 Ladle Refining Furnaces for the Steel Industry
(1985)
A review of supplemental steel heating and refining units often used in
conjunction with electric arc furnaces.
85-2 Electric Arc Furnace Dust Disposal
(1985)
An analysis of dusts generated by electric furnaces and a review
of methods
for treatment and disposal.
Vol. 1, No. 3 Electric Arc Furnace Steelmaking (Techcommentary- 1985)
A description of the structure and function of electric arc furnaces.
(1986)
86-7 Electrode Tip Analysis
An examination of electrode wear by various photographic means.
86-8 DC Arc Furnaces for Steel Production
(1986)
30 ton
A comparison of the electrical energy consumption of a conventional
AC furnace with a30 ton DC furnace.
86-9 Arc Stabilityin Electric Furnace Steelmaking(1986)
Field testingthe effect on furnace performance of hollow electrodes, lime and
argon injection, and changes in other operating parameters.
The Electric Power Research Institute
(EPRI) conducts a technical research
and development program for the
U.S. electric utility industry.EPRl
promotes the developmentof new and
improved technologiesto help the
utility industry meet present and future
electric energy needs in environmentally and economically acceptable ways.
EPRl conducts research on all aspects of electric power productionand
use, including fuels, generation, delivery, energy management and conservation, environmental effects, and
energy analysis.
The Center for Metals Production (CMP)
is an R&D application center sponsored
by the Electric Power ResearchInstitute (EPRI) and administered through
Mellon Instituteof Camegie Mellon
University. CMPs goal is to develop
and transfer technical information that
improves the productivity and energy
efficiency of U.S. primary metals producing companies(SIC 33). Target
areas are reductiotdsmelting;refining
remelting; and surface conditioning1
protection.
LEGAL NOTICE
This report was prepared and sponsored by the Center for Metals Production (CMP). Neither members of CMP
nor any person acting on their behalf: (a)
makes any warranty expressed or
implied, with respect to the use of any
information, apparatus, method, or process disclosed in this report or that such
use may not infringe privately owned
rights; or (b) assumes any liabilities with
respect to the use of, or for damages
resulting from the use of, any information,
apparatus, method, or process disclosed in this report.
EPRl
Robert Jeffress, Project Manager
CMP
Joseph E. Goodwill, Director
Richard M. Hurd, Chairman
Steelmaking Processes
James M. Hensler, Manager of
Technical Projects
John Kollar, Manager of
Communications
William C. Flora, Consultant
David J. Westhead Company, 1nc.i
Advertising
Center for Metals Production
Mellon Institute
4400 Fifth Avenue
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412-268-3243
0 1987 Center for Metals Production
CMP-0787-002