furnace transformers and reactors design and features - Tamini

Founded in 1916 for the production
of electric transformers, Tamini
has grown to become the leading Italian
manufacturer of industrial transformers
and of the largest power transformers
for HV and EHV (up to 420 kV).
The Tamini Group is fully controlled
by the Tamini family and operates four
manufacturing plants in Italy. These are
located at Legnano, Melegnano and Novara
in the Milan area and the fourth one is
located near Vicenza. Their production is fully
integrated, with each factory specializing
in a selected range of transformers.
The Headquarters are in Melegnano, where
the Engineering, Administration, Procurement
and Commercial offices are located.
In North America our operation office and
service unit is in Oak Brook, Illinois, USA.
1

TAMINI GROUP FOR IRON AND STEEL
AND ELECTRO-METALLURGICAL INDUSTRIES
The whole Tamini Group
operates according with the
ISO 9001-2000 Standard for
Quality Assurance and the
following Tamini Q. A. Manual.
The Quality Assurance
Certificate number 9101
has been issued by the Italian
Institute CISQ/CSQ,
the qualified member
of the European Association for
Quality Assurance EQNET.
The Group’s target has always been to produce
power and industrial transformers of high quality and
reliability, designed to satisfy the most varied and
sophisticated technical requirements. A large share
of its resources are devoted to research and
development of industrial and special transformers
and reactors for any special application, such as the
iron and steel and electrometallurgical industries.
Thanks to this, Tamini has reached a prominent
position worldwide for the supply of arc furnace
transformers of any type and size, reactors for AC
arc furnace, step-down transformers and special
transformers for metallurgical plants.
Almost the fifty percent of the Tamini Group’s
production is directed to such industries, the majority
of which is exported all over the world, including the
highly industrialized markets of Europe, North
America and other continents. In the last fifteen
years Tamini has manufactured almost 1000
transformers among them more than 400 units are
industrial transformers.
(For detailed information see relevant reference list)
1. A 140 MVA AC-EAF transformer 69/1.7 - 1.07 kV with OLTC and a 81.6 MVAR series reactor with OLTC for USA.
2. 190 MVA AC-EAF transformer 34.5/1.5-0.850 kV and 59.4 MVAR series reactor for USA
3

INTRODUCTION AND OPTIONS
ELECTRIC ARC FURNACE TRANSFORMERS AC TECHNOLOGY
Aiming to improve the efficiency and quality of the
melting process the iron and steel and
electrometallurgical engineers and the arc furnace
manufacturers have become more and more
attentive to any improvement directed to satisfy their
demand for:
maximum stability of the arc during the different
stages of the whole melting process
reduction of electric disturbances (flicker) on the
power supply network during melting process
productivity increase
reduction of electrode consumption
optimization of the cost of electric arc furnace
equipment and of its operating costs.
Conscious that proper design and adequate
production technology of the furnace transformers are
fundamental for a high efficiency of the plant
operation, Tamini is continuously devoting a
prominent attention to the developments in the arc
furnace conception and to the updating of the
melting process requirements.
Tamini has consequently always been in close contact
with the arc furnace manufacturers and operators, in
order to adapt the design and the characteristics of
the furnace transformers to the most advanced
technologies. In particular, the operation with long
arcs has demanded:
much higher secondary voltage (up to 1500 V
and over, at highest tap)
installation of series reactors, in order to increase
the total reactance of the system
universal adoption of on-load tap-changer for the
secondary voltage regulation
adoption of on-load tap-changers for the
reactors.
The result of these technologies , together with a
greater use of chemical energy (burners, lances) and
the installation of ladle furnaces, has been a dramatic
reduction of electrode and energy consumption.
The increase of the productivity is very high.
Recent years have also seen the development of DC
electric arc furnaces which have proved to be in
some cases an interesting alternative to the more
popular AC furnaces.
Tamini technology for AC and DC furnace
transformers is illustrated in the relevant paragraphs.
3. Two 93.5 MVA 30/0.9-0.5 kV AC-EAF transformers for Indonesia
TRANSFORMERS AND SERIES
REACTORS FOR AC FURNACES
The typical electric diagram of a modern AC arc
furnace is shown here aside.
This diagram does not show the complete auxiliary,
control and protection equipment normally
associated with the arc furnace plant and which is
selected taking into account the main parameters of
the installation (characteristics of the feeding
4. A 123 MVA AC-EAF transformer for France coupled with the saturable reactor see fig. 7
network, requirements of the supply utility, type of
instrumentation and automation etc.)
During the design of the electric system Tamini gives
all necessary assistance to the system engineer, in
order to co-ordinate the various parameters of the
auxiliary equipment. While designing the furnace
transformers and series reactors, the possibility of
abnormal loads and overvoltages foreseen by the
system engineer is carefully considered.
AC EAF Basic Diagram
High Current Connection
A.C. Furnace
Step-down Transformer
Reactor
Furnace Transformer
5

5. Three 12.5 MVA 33/0.4-0.2 kV AC-EAF transformers for South Africa
The necessity of stabilizing the arc during the
melting down phase, carried at very high secondary
voltages, with long arcs, requires an increased total
reactance of the system: this is normally achieved
by means of the installation of a reactance of
suitable value, in series with the transformer.
It is interesting to note that series reactors were
already used many years ago in the small furnace
installations ( up to 10 MVA ), in order to stabilize
the arc, specially at the beginning of the melting
process, due to the very low reactance of such
small furnaces. In recent years Tamini has become
one of the leading manufacturers of oil immersed
series reactors for arc furnaces. New solutions have
been developed and now practically all series
reactors are equipped with tap-changers (either
off-circuit or on-load).
SECONDARY VOLTAGE REGULATION
The operation of the arc furnaces demands that the
transformer is equipped with a tap-changer, for the
selection of the most suitable voltage tap for each
phase of the process.
Many solutions are possible and the transformer
manufacturer selects, for each specific transformer,
the most convenient configuration.
It should be noticed that any of the following
Voltage Regulation Diagrams can be completed with
the addition of a reactor with on-load or off-circuit
tap-changer which can be installed in the same
transformer tank or in a separate tank.
TAP-CHANGER ON PRIMARY SIDE
DIAGRAM A
The regulation of the secondary voltage through tap
changing on the primary side can be an advantage,
because it is the only solution which allows the use
of a single magnetic core with a reduction in total
weight and losses. This solution is however not
convenient for very high primary voltages or very
high primary currents, because of the difficulty in
finding suitable tap-changers. Transformers with tapchanger
on the primary side can be equipped with
an additional off-circuit star-delta switch which gives
the possibility of a wider secondary voltage range.
AUTOTRANSFORMER
DIAGRAM B
With this diagram it is easy to obtain a system with
equal steps, also of very small value. For this reason
this solution (like the booster transformer) is
sometimes used for the submerged arc furnaces.
Current transformers in the intermediate circuit give
a signal proportional to the secondary current,
independently from the position of the tap-changer.
Also in this case, a possible limit of utilization of the
autotransformer diagram can be the availability of a
suitable tap-changer.
BOOSTER TRANSFORMER
DIAGRAM C
With this diagram the on-load tap-changer is
installed on the tertiary winding.
The voltage and current values of the tertiary winding
are selected by the transformer designer with a view
of using the most convenient type of the on-load tap-
A. Tap-changer on Primary Side Diagram
- Δ
H.V.
B. Autotransformer Diagram
AC TECHNOLOGY
L.V.
H.V. L.V.
C. Booster Transformar Diagram
H.V.
L.V.
Main Transformer
Booster Transformer
7

D. Series Reactor with On-load Tap-changer on Booster Transformer
H.V.
6. A 150 MVA EAF transformer with built-in reactor OLTC’s for voltage and reactance control. (Schematic diagram as fig. D)
L.V.
Main Transformer
Booster Transformer
Reactor
changer, bearing in mind also the cost factor. With a
proper sizing, also the maintenance requirements for
the on-load tap-changer are reduced.
As with the autotransformer, also in the booster
transformer diagram the current transformers
installed in the tertiary winding can give a signal
proportional to the electrode current, independently
from the tap-changer position.
Moreover, this solution permits multiple feeding
voltages, for instance 10 kV or 20 kV, through a
simple change of connection on the primary side.
SERIES REACTOR
WITH ON-LOAD TAP-CHANGER
DIAGRAM D
The possibility of regulating on-load both the
secondary voltage and the system total impedance,
is considered as very interesting and useful by the
furnace operators.
The possibility of optimizing, at every moment of the
process, both the parameters, has brought, in many
steel-works, considerable advantages in tap-to-tap
time as well as in the energy consumption.
A new solution, shown in the diagram, specially
interesting for high power furnaces, has been
developed by Tamini for a 80 MVA transformer
installed in a US steelplant in 1993 and since then
applied in several plants. In this case the series
reactor, with on-load tap-changer, is installed on the
tertiary winding of the booster transformer.
The picture 15 shows the 80 MVA transformer unit.
In that specific case the secondary voltage range on
the transformer is divided in 26 steps; the
corresponding series reactor has a reactance divided
in 12 steps.
In another US steelplant, Tamini has adopted the
same solution, for a 190 MVA transformer with
series reactor with on-load tap-changer, for a
furnace which is considered to be one of the most
powerful in the world (see picture 2).
SATURABLE REACTOR
DIAGRAM E
Developed many years ago, this solution has been
recently revived and applied to large arc furnaces,
to be connected to weak electric network.
Tamini has contributed to this application, developing
an innovative and reliable saturable reactor which
can definitely contribute to reduce the disturbance
(flicker effect) of the furnace on the electric HV
network. The diagram shows a saturable reactor.
Its calculation and project require a proper innovative
design capability, specially in the core design and
E. Saturable Reactor, 1-Phase Diagram
A.C. Power Supply A.C. Windings
D.C. Windings
D.C. Control
AC TECHNOLOGY
Smoothing Reactor
A.C. Power Outlet to Eaf
9

7. A 146 MVAR three-phase saturable reactor for France.
dimensioning. It has been developed by Tamini, in
strict compliance with the technical specification of a
well known furnace manufacturer, with the aim to
limit the current peaks during the melting process,
and consequently to reduce the flicker effect.
The unit is normally composed of a set of six
saturable reactors.
The basic principle is that magnetic transducers can
be used to keep the load current practically constant
even if the load impedance varies.
Each of the six saturable reactors has an AC load
winding and a DC control winding.
When the instantaneous magneto-motive-force
(m.m.f. ) of the AC winding are lower than those of
the DC winding, then the reactor is saturated; in that
condition the effect of the reactor is almost
negligible.
As soon as the load current is higher than the DC
current, the reactor desaturates (it should be proper
to call them “desaturable reactor”) and reacts with a
flux variation and voltage impulse to any current
variation. The result is that the current wave is cut,
depending on the DC imposed m.m.f.
The picture 7 shows a saturable reactor unit rated
146 MVAR with a reactive power of 6x24.3 MVAR in
operation in a steelworks in France.
OTHER POSSIBLE SOLUTIONS FOR
SECONDARY VOLTAGE REGULATION
Among the various possible solutions for secondary
voltage regulation, the following are of particular
interest, in order to solve peculiar lay-out or
operational problems.
a) Regulating transformer plus fixed ratio furnace
transformer.
The regulating transformer can operate in this case
also as a step-down transformer; it is installed in the
steelworks substation, connected directly to the HV
incoming line (up to 400 kV).
The furnace transformer has then a fixed voltage
ratio. The connection between the two transformers
is normally made by cable.
Auxiliary services as well as equipment for power
factor correction, can be connected to a tertiary
winding of the regulating step-down transformer.
With this alternative the maintenance of the on-load
tap-changer is considerably easier, as the step down
transformers are installed outdoors, and not in a
F. A schematic diagram of 90 MVA EAF transformer for South Africa
vault of limited dimensions, as normally foreseen for
furnace transformers.
In the Chapter “Step-down Transformers for Iron
Steel and Electrometallurgical Works Substations”
this solution is explained with more details.
b) For certain installations, it is sometimes requested
that the three phases can operate with unbalanced
voltages.
Tamini has designed and supplied several three-phase
transformers equipped, on the HV side, with three
independent single-phase on-load tap-changers,
which enable operation of the furnace with
unbalanced secondary voltages. With this solution
there is no circulating current and no zero-sequence
flux in the core.
The same result can be obtained, in submerged arc
furnaces of large size, with the installation of three
2W
2V
2U
2V 2W
2U
OL.TC.
1W
1W
OL.TC.
1V
1V
OL.TC.
1U
1U
single-phase transformers, each one equipped with
on-load tap-changer. The diagram F. with a star
connected primary and a delta secondary, refers to
two 90 MVA furnace transformers supplied to a
South African steelworks (see picture 8).
CT3 5 CT2 CT1
6
3
4
1
2
8. Two 90 MVA 33/0.99-0.45 kV AC-EAF transformers for South Africa
AC TECHNOLOGY
11

G. DC EAF Basic Diagram
H.V.
Smoothing Reactor
H.V.
High Current Connection
Step Down Transformer
Rectifier Transformer
RECTIFIER
High Current Connection
D.C. Furnace
H. 2x3 Phase Bridge Connection With Wye-Delta Secondary Windings
L.V.
RECTIFIER TRANSFORMERS FOR DC
FURNACES
The typical DC furnace electric diagram, with its
main components, including step-down transformer,
furnace transformer, rectifier system and arc furnace,
is shown in the simplified diagram here aside.
The electric system feeding a DC furnace is
substantially different from that foreseen for feeding
an AC furnace as in this case, the furnace is not
directly fed by the furnace transformer but through a
rectifier high current DC connections and a
smoothing reactor.
The DC technology offers satisfactory performances
in terms of electrode consumption and reduction of
network disturbances but with higher investment and
operative costs and with the disadvantage of relying
on delicate DC and associated electronic equipment
which may be a drawback, particularly in a heavy
industrial process. This has not to be
underestimated.
The diagram G. is the most commonly used diagram
for DC furnace transformers: a double six-phasebridge
for a 12-pulse system with two 30° shifted
secondary windings.
For high power, some additional phase shift windings
have to be provided to obtain systems of 18 pulses
or more. This means transformers with 2, 3 or 4
primary windings and 2,3 or 4 secondary windings.
Moreover in a DC furnace transformer the
secondaries have usually to be magnetically
uncoupled in order to reduce the electromagnetic
interference between the different rectifier units and
to reduce problems in thyristors control.
9. A 100 MVA 33/0.77-0.45 kV DC-EAF transformer for China.
The DC furnace transformers can be quite simple for
what concerns the regulating windings as the
voltage change can be made by an off-circuit motordriven
tap-changer.
Sometimes a fixed ratio transformer is used without
any tap-changer: in this case the voltage is regulated
by the thyristor control only. The two systems can
also be used together.
It has to be underlined that the control by thyristors
causes substantial increase of the eddy losses in the
windings and stray losses in the external structures,
due to high harmonic content of the currents.
This is a basic aspect to be considered in DC EAF
transformer design.
An additional important aspect to be considered,
when applicable, is the possible unbalanced
operation of secondary windings, should one or
more bottom electrodes not conducting.
10. Two 70 MVA 30/0.82-0.67 kV DC-EAF transformers for Germany
DC EAF TECHNOLOGY
13

REDUCTION OF ELECTRIC DISTURBANCES (Flicker)
Tamini does not design or manufacture equipment
for VAR compensation on the power networks,
nevertheless a short mention to this specific matter
is worthwhile, as it represents an element of
substantial importance for any iron and steelworks,
conscious of the problems consequent to the flicker
effect on the surrounding areas, and respectful of
the Standards on Electromagnetic Compatibility.
In this specific case Tamini is asked to co-operate
with the suppliers of compensation systems.
In order to compensate the flicker caused by the arc
furnace operation, conventional static VAR
compensators, with thyristor-switched capacitors or
thyristor-controlled reactors, are used.
Such conventional compensation is generally
capable of a 2:1 reduction of flicker.
11. A 2x35 MVA 15/6x9.3 kV transformer for a compensator of an EAF steel plant in USA
In order to overcome any problem related with the
utility supplying electricity to its works, a primary
steel company in US has adopted an innovative
system, to replace its old conventional VAR system.
Without entering here in the description of such
innovative application (specific documentation can
be requested to the steel Company), Tamini has
been required to supply a very special three-phase
coupling transformer, rated 70 MVA, designed to
properly withstand the severe harmonic content
during the arc furnace operation.
This transformer, designed and manufactured by
Tamini in strict technical co-operation with the steel
Company and with the system supplier, has been
the first of this type and for this specific application.
The picture 11 shows a special tansformer for a
compensator of an EAF steel plant.
STEP-DOWN TRANSFORMERS FOR IRON AND STEEL
AND ELECTRO-METALLURGICAL WORKS
Steel and iron works generally receive electricity
from the utilities at a voltage value between 110
and 400 kV which is then reduced to 20 to 60 kV,
more suitable for the furnace transformer, utilizing a
step-down transformer installed in the steel works
substation.
The step-down transformers for iron and steel and
electrometallurgical works are usually three-phase
units (or composed by three single-phase units, in
case of very large ratings) and foreseen for outdoor
installation. They do not differ substantially from the
power transformers installed in the network
substations, but they must be specially designed to
handle continuous heavy loading with instantaneous
overloading of up to 100%, frequent on/off
switching, high peak currents and sometimes poor
surge protection.
The knowledge of the steel melting process, peculiar
for a manufacturer like Tamini, represents a basic
guarantee also for the life of the step-down
transformers installed in the iron and steel and
electrometallurgical works.
The picture 12 shows an interesting application of
this type.
Additionally it has to be mentioned that sometimes
there is a requirement for the step-down transformer
to be able to operate also as a regulating
transformer. This solution is briefly presented under
the point a) of the paragraph “Other possible
solutions for the secondary voltage regulation”.
Tamini has manufactured and supplied several such
units with connection diagrams specially designed in
accordance with the different operational
requirements of the system engineers.
Different solutions are available in order to satisfy
such specific requirements. As an example, a special
diagram is diagram I: it has proved to have a
satisfactory problem-free impact on the system
operation, even after years of duty.
The diagram I. refers to a step-down transformer
supplied to a steelworks in France. The incoming
line voltage is 225 kV and the furnace transformer
has a fixed ratio. In order to assure the LV
12. One of three single-phase step-down transformers for a three-phase bank 120/63/27 MVA -
400/20/20 kV for Spain.
15

egulation on the furnace, Tamini has designed and
supplied a 100 MVA, 225/35-12 kV regulating
transformer, with an independent regulation on each
secondary phase, with the aim to balance the
reactance of the furnace.
In this case, the high voltage primary windings and
the regulating secondary windings are star-and-delta
connected respectively. The transformer also has a
I. Diagram of the 100 MVA EAF regulating transformer for France
1U
1N
1W 1V
V
Delta tertiary winding, which can feed a power
factor compensation and auxiliary circuits.
When the tap-changer on one phase of the LV side
is in a different position compared with those of the
other two phases, then a circulating current is
created in the delta-connected secondary and tertiary
windings and of course it is under control, this situation
occurs either during on-load or no-load operation.
50
T 14 58
59
TI 1 51
TI 9
37
71 72 TI 2
TI 11
55
38
5453
52
68
TI 8 69
2V
U
2U
k
– +
2W
13. A 100 MVA AC-EAF regulating transformer, 220 kV with three
independent tap changers.
k
– +
TI 10
56
TI 13
57
72 73
– +
k
W
3U
3W
67
66
T 17
65
64
T 16
T 15
63 62 6160
3V
FURNACE TRANSFORMERS
The content of this section refers specially to electric
arc furnace transformers for iron and steel works
which are subject to exceptional mechanical and
electrical stresses during melting process.
The design and features described below are
basically the same used for furnace transformers for
electrometallurgical works.
Mechanical Stresses on Winding
During furnace operation, the transformers undergo
thermal and mechanical stresses due either to
frequent short circuits on arc or to continuous
energizing and deenergizing operations during the
daily steel melting process. Continuous stresses and
vibrations may loose the windings if they are not
properly treated and robustly clamped. To avoid any
inconvenience due to such heavy and frequent
stresses, Tamini has adopted improved procedures
during manufacture for winding pressure and
thermal treatment operation; furthermore Tamini has
since long time developed a special windings
clamping system which guarantees an exceptional
resistance against any electrodynamical stresses
even under the most arduous operating conditions.
The system has been used successfully for many
years but it is subject to continuous review for any
possible further improvements.
Electrical Stresses
These are mainly due to the variation in electric arc
overvoltages, which involves either LV bushing, bar
insulation, or the LV winding itself. Electrical stresses
are also caused by a transient state resulting from
FURNACE TRANSFORMERS
AND REACTORS DESIGN AND FEATURES
a sudden disconnectin of HV circuit breakers
expecially if vacuum-type circuit breakers are
installed when the low currents must be cut off (as
for the transformer’s no load current). To overcome
these dangerous overvoltages, RC devices and
surge arresters are frequently used and installed by
the electrical contractor. Nevertheless it is
important that the transformer design itself is
improved. Tamini pays very particular attention to
this problem adopting a very specific manufacturing
procedure suitable to guarantee a stronger
transformer insulating structure and safer operation.
14. One of two 85 MVA 34.5/1.2-0.78 kV AC-EAF transformers for USA
17

15. One of two 140/157 MVA 34.5/1.35-0.85 kV AC-EAF transformers for USA
16. A 48 MVA 30/0.3 kV AC-EAF transformer withh reactor in the same tank for Poland
Basic Design Description
The description refers to the normal design for
furnace transformers including the on-load tapchanger;
obviously special design for any specific
application can be performed by Tamini in
accordance with the customer’s technical
specifications and requirements.
Magnetic Circuit
The cores are normally of three vertical limbs type
and composed of silicon cold rolled, grain oriented
magnetic steel sheets. The insulation between
laminations is of the inorganic type (carlite) with
high chemical resistance to hot oil. Additional
insulation (pressboard) is interposed between
packages of sheets. The joints between sheets are
usually of the interleaved type.
Windings
The windings are of electrolytic copper ECU 99,9
and assembled concentrically: HV winding,
regulating winding and LV winding. The insulating
material used for all copper straps is of pure
cellulose paper, thicker than for a standard design
in order to overcome possible stresses due to
exceptional overvoltage conditions in the operation
of the furnace. In order to cope with the heavy
mechanical stresses due to the frequent short
circuits in the operation of an arc furnace, all the
windings are exactly symmetric and of the same
height in relation to the mean horizontal plane of
the core. In detail the regulating winding has the
turns of every step distributed over the whole height
of the winding. Particular care is given to clamping
structure and to winding stabilization.
The supporting cylinders of the windings are thicker
than those normally requested for a standard design
for the same reasons previously mentioned. The
connections of the windings inside the transformer
tank are designed in accordance with the connection
diagrams used.
On–Load-Tap.Changer (OLTC)
The OLTC mostly used for AC furnace transformer
can be utilized with different connection diagrams as
better shown in relavant paragraphs.
The OLTC is composed of an off-load selector
normally immersed in the same oil of the
transformer and by a diverter switch located in an oil
filled sealed container separate from the transformer
oil. On request a barrier board inside the
transformer tank can be provided to divide up to a
certain height the oil of the transformer and the oil
in which the selector is immersed. With this solution
it is possible to check the tap changer selector by
only removing the oil from from the separate section
in which it is positioned. Moreover as a further
solution the whole OLTC can be located in a
separately associated oil filled tank and connected
through bushings.
This solution is also utilized when a vacuum type
OLTC is required.
The OLTC control can be local and remote
LV Outlets
Under previous technology the most utilized solution
for LV outlets consisted of copper bars mounted
either on the top cover of the transformer tank or on
FURNACE TRANSFORMERS
AND REACTORS DESIGN AND FEATURES
its side. The increase of the transformer ratings
required many bars in parallel very close together for
each LV outlet. Considerable care has to be taken
during maintenance and cleaning of the outlets
specially when mounted on the tank cover. The bars
have different arrangements depending on the type of
connection to the arc furnaces.
In recent years the improvement in technology for LV
connections brought an alternative solution in
particular for transformers of large capacity and high
current rating. Water-cooled tubular bushings have
been used instead of bars. The water-cooled bushings
are generally mounted on the side of the tank or
sometimes on the top cover of the transformer. This
system is safe and simple for both electrical and
cooling water connection. The design and construction
of the tubular bushings avoid any risk of water
leakage into the transformer oil. Any type of LV
connections, either bars or tubular bushings are
mounted on insulating plates through a set of special
ring gaskets designed to guarantee perfect insulation
even in presence of high secondary values.
The request for water-cooled tubolar bushings is
increasing even if the LV bars are still used for medium
size transformers or when the interchangeability with
existing transformers is required.
Many solutions can be adopted for LV connections
however the most used are:
water cooled bushings on the side of the tank
bar outlets on the tank cover
bars on the side of the tank
The arrangement of the LV outlets depends also on
the connection of the LV windings. The windings can
be connected either in delta or star outside or inside
19

17. A 140 AC-EAF TRANSFORMER 115.000/1300-650V
18. A 80 MVA 15/1.1-0.66 kV AC-EAF transformer for USA
the tank. When delta or star point connection is
requested inside the tank, the LV outlets normally
have a triangle shaped arrangement. This solution
facilitates the connection to the furnace and at the
same time guarantees a good symmetry of the three
phases and current distribution among them. To
improve current distribution the secondary winding
coils are divided into group whose number
corresponds to the LV connections.
COOLING SYSTEM
The furnace transformers are normally provided with
an OFW(F) cooling system. One or more coolers are
fitted on the transformer normally in a vertical position
along one of the short side of the transformer tank;
they are connected to the tank through shutt-off valves.
Different positioning of the coolers, including horizontal
installation, can be adopted to satisfy specific
requirements for easier connection to the external
water piping system.
The coolers are normally composed of a single-walled
or, on request, should the water pressure be higher
than the oil pressure, a double-walled system. The
design of the coolers is such that any possible risk of
water leakage into oil is absolutely avoided.
The coolers are fitted with an oil immersed motor
pump and with water and oil flow indicators with
alarm contact, water and oil thermometers, water and
oil drain taps.
Other special fittings are available, if required. When
the water is not available, the cooling can be of the
OFA(F) type. In this case oil-to-air coolers are installed
out of the transformer room; they are connected to
the transformer by an oil piping system.
TANK
The tank is made of welded steel sheets creating a
particularly strong and stiffened welded steel structure.
The internal walls of the tank are painted with a hot oil
resistant coating while externally the tank is painted
according to a standard procedure established by the
Tamini Quality Assurance. Specific painting requirements
can be adopted on request.
The tank is equipped with a separate oil conservator
and the piping system both for coolers and conservator
connection through shut-off valves.
The conservator is divided in two sections for the
transformer oil and for the OLTC oil switch. Suitable
manholes for internal inspection and maintenance are
provided on the tank cover. In correspondence of the LV
connections the tank has one or more non magnetic or
high resistance insulating plates bolted through suitable
gaskets to the tank either on the cover or on the tank’s
walls. According to the chosen diagram, and to the
characteristics and size of the furnace transformer, the
tank can be designed and built to incorporate other
equipment, such as the autotransformer, the booster
transformer and occasionally the reactor.
ACCESSORIES
The furnace transformers are normally equipped with
the following fittings and accessories:
one oil conservator as described
two air silicagel breathers for the two conservator
sections
two oil level indicators with electric contacts for the
two conservator sections
buchholz relay for transformer with alarm and trip
contacts
FURNACE TRANSFORMERS
AND REACTORS DESIGN AND FEATURES
water coolers as described
OLTC as described (or off-circuit TC if required)
gas pressure relay for OLTC switch with trip contacts
oil drain, filling and filtering valves
oil thermometer complete with alarm and trip
contacts and, on request, a device for remote
temperature transmission
HV porcelain bushings
LV outlets as described
current transformers as per customers
requirements
over-pressure vent
lifting lugs for core and winding lifting from the tank
lifting lugs for the complete transformer
rating plate
marshalling box for signalling and protection
auxiliary circuits
two earthing terminals
one oil sample cock
surge arresters on HV side (on request only)
RC surge suppressors on HV side (on request only)
capacitors on LV side (on request only)
Upon request additional and/or specific accessories
could be fitted on the transformers.
REACTORS
Reactors provide the furnace operation with the
following improvements:
arc stability and power regulation
optimisation of electric power and of electrode
consumption
limitation of current during short circuit conditions
in the furnace scrap collapsing
reduction of flicker on the feeding network
21

The external feature of the reactor is very similar to
that of an oil immersed transformer. Core and
windings are of the same type of the transformer
with the difference that in the columns of the
magnetic core are inserted suitable gaps designed
for specified reactance and reactive power values.
The reactance of the reactors will be in any case
constant for currents up to 2 times the rated value
(if requested up to 3 times or more).
When the linearity of reactance at higher current
value is required (for example 5 times the nominal
current or more), a core-less solution is appropriate.
The solution consists of windings without internal
magnetic core but with a suitable external magnetic
frame, in order to give the flux a confined path.
Compared with the “gapped-core” solution, core-less
design has the advantage to be more effective in
the limitation of possible fault currents, which may
19. A 65 MVAR reactor for a 190 MVA AC-EAF transformer for USA
occur immediately after the reactor, but not as
regards the short circuit on the furnace. The latter
frequently happens during the EAF operation, but its
amplitude is relatively small (2-3 times the rated
current), so the normal limitation effect obtained
using a gapped-core reactor is sufficient.
The construction of reactors immersed in oil specially
suits to requirement of using a Tap Changer for the
reactance variation. In particular, through remote
controlled TCs, the selection of the proper reactance
value at any operational set-point, so achieving a
quicker furnace regulation.
The most satisfactory technical solution is anyhow
the use of On Load Tap Changers which allows the
on-load regulation of the reactance from the highest
value to zero, so achieving the reactance regulation
without operating the furnace circuit breaker.
In order to optimise the choice of the components of
the plants Tamini has designed and successfully
supplied different types of transformer-reactor
connection diagrams (see diagram page 5).
In particular, the “Booster-type” transformers (with
OLTC), having the reactor (with OLTC) connected
on the tertiary side (see diagram D page 8).
This solution allows the optimisation of
voltage/current values in the tertiary circuit,
at purpose of selecting the most convenient type
of tap changer.
REFERENCE STANDARDS
The EAF transformers and reactors are designed,
manufactured and tested according to the IEC, IEEE
and CSA standards, as well as to the major national
standards in force in the countries of destination.
23

MILANO
TANGENZIALE OVEST
TANGENZIALE EST
VIA EMILIA
TANGENZIA LE EST
TCM TAMINI LIMITED. Swindon
TAMINI USA
Linate
GEOGRAPHICAL LOCATION
S. Donato
Milanese
A1
London
Roma
Milano
VERBANO TRASFORMATORI Novara
S. Giuliano
Milanese
VIA EMILIA
TAMINI Legnano
V.T.D. Trasformatori
TAMINI Melegnano
PAULLESE PAULLESE
MELEGNANO
TRANSFORMERS
TRANSFORMERS
Paullo
ALL SALES AND ADMINISTRATIVE
ACTIVITIES OF THE TAMINI GROUP
ARE DIRECTED FROM THE GROUP
HEADQUARTERS
HEADQUARTERS
TAMINI Trasformatori s.r.l.
via Cesare Battisti 37
20077 Melegnano MI - Italy
tel. +39.02.982051 fax +39.02.98230322
www.tamini.it
TRANSFORMER PRODUCTION FACILITIES
Tamini Melegnano: via Emilia 37
20077 Melegnano MI
Tamini Legnano: via P. Ovidio Nasone
20025 Legnano MI
Verbano Trasformatori
Corso Risorgimento 209
28100 Novara NO
V.T.D. Trasformatori
via Gasdotto 6
36078 Valdagno VI
UK and Eire Operations
TCM Tamini Limited
55, Shrivenham Hundred Business Park
Watchfield, Swindon SN6 8TY
tel. +44.1793.780306
fax +44.1793.787888
North American Operations
Tamini Transformers USA
2803 Butterfield Road, Suite 385, Oak Brook, IL 60523 USA
tel. 630.368.9907
fax 630.368.9910
www.tamini.com

TAMINI GROUP
C. Battisti, 37 • 20077 Melegnano (MI)
Tel. 39-02-982051 • Fax 39-02-98230322
www.tamini.it