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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.

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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.

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.

(For detailed information see relevant reference list)

TAMINI GROUP FOR IRON AND STEEL AND ELECTRO-METALLURGICAL INDUSTRIES

2. 190 MVA AC-EAF transformer 34.5/1.5-0.850 kV and 59.4 MVAR series reactor for USA

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ELECTRIC ARC FURNACE TRANSFORMERS AC TECHNOLOGY

INTRODUCTION AND OPTIONS

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, inorder 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 e nergy consumption.

The increase of the productivity is very high.

Recent years have also seen the deve lopment 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

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.

Reactor

High Current Connection

A.C. Furnace

Furnace Transformer

Step-down Transformer

AC EAF Basic Diagram

4. A 123 MVA AC-EAF transformer for France coupled with the saturable reactor see fig. 7

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AC TECHNOLOGY

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 tap-

changer 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-

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 we re

already used many years ago in the small furnaceinstallations ( 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 transformermanufacturer 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.

H.V.

- Δ

L.V.

A. Tap-changer on Primary Side Diagram

H.V. L.V.

B. Autotransformer Diagram

H.V.

L.V.

M a i n T r a n s f o r m e r

B o o s t e r T r a n s f o r m e r

C. Booster Transformar Diagram

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9

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 ene rgy consumption.

A new solution, shown in the diagram, specially

interesting for high power furnaces, has been

developed by Tamini for a 80 MVA transformerinstalled 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

R e a c t o r

H.V.

L.V.

M a i n T r a n s f o r m e r

B o o s t e r T r a n s f o r m e r

6. A 150 MVA EAF transformer with built-in reactor OLTC’s for voltage and reactance control. (Schematic diagram as fig. D)

D. Series Reactor with On-load Tap-changer on Booster Transformer

AC TECHNOLOGY

A.C. Power Supply A.C. Windings

D.C. Windings

D.C. Control

Smoothing Reactor

A.C. Power Outlet to Eaf

E. Saturable Reactor, 1-Phase Diagram

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AC TECHNOLOGY

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 transformersis 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

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

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 threeindependent 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 three7. A 146 MVAR three-phase saturable reactor for France.

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).

C T 3

C T 2

C T 1

1 2

3 4

5 6

O L .T C .

1 W

O L .T C .

1 V

O L .T C .

1 U

2 V

2 V

2 W

2 W

2 U 2

U 1 U

1 V

1 W

8. Two 90 MVA 33/0.99-0.45 kV AC-EAF transformers for South Africa

F. A schematic diagram of 90 MVA EAF transformer for South Africa

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DC EAF TECHNOLOGY

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 motor-

driven 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.

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-phase-

bridge 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 pulsesor 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.

L.V.H.V.

H.V.



Rectifier Transformer

Step Down Transformer

Smoothing Reactor

D.C. Furnace

RECTIFIER

9. A 100 MVA 33/0.77-0.45 kV DC-EAF transformer for China.

10. Two 70 MVA 30/0.82-0.67 kV DC-EAF transformers for Germany

G. DC EAF Basic Diagram

H. 2x3 Phase Bridge Connection With Wye-Delta Secondary Windings

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STEP-DOWN TRANSFORMERS FOR IRON AND STEEL AND ELECTRO-METALLURGICAL WORKS

Tamini does not design or m anufacture 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.

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.

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 sometimesthere 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 asatisfactory 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

11. A 2x35 MVA 15/6x9.3 kV transformer for a compensator of an EAF steel plant in USA

REDUCTION OF ELECTRIC DISTURBANCES (Flicker)

12. One of three single-phase step-down transformers for a three-phase bank 120/63/27 MVA -400/20/20 kV for Spain.

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regulation 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

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.

1U

1W 1V

1N

TI 1 TI 9

TI 10

TI 13

TI 2TI 11

TI 8

V

U W

2U

2V

2W

k

k

k–

+

–

+

– + 50

51

37

38

68

69

5657

7273

71 7255 5453

52

58

59

63 62 6160

6766

6564

3W

3V

3U

T 14

T 16

T 17

T 15

I. Diagram of the 100 MVA EAF regulating transformer for France

17

FURNACE TRANSFORMERS AND REACTORS DESIGN AND FEATURES

FURNACE TRANSFORMERS

The content of this section refers specially to electric

arc furnace transformers for iron and steel works

which are subject to e xceptional 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 anypossible further improvements.

Electrical Stresses

These are mainly due to the v ariation 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

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 USA13. A 100 MVA AC-EAF regulating transformer, 220 kV with threeindependent tap changers.

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Basic Design Description

The description refers to the normal design for

furnace transformers including the on-load tap-

changer; 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 heavymechanical 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 typeOLTC 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

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 connectionshowever 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


5. One of two 140/157 MVA 34.5/1.35-0.85 kV AC-EAF transformers for USA

. A 48 MVA 30/0.3 kV AC-EAF transformer withh reactor in the same tank for Poland

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21


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 withalarm 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

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 thefollowing 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

. A 140 AC-EAF TRANSFORMER 115.000/1300-650V

. A 80 MVA 15/1.1-0.66 kV AC-EAF transformer for USA

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23

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 v alues.

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

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 typeof 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.19. A 65 MVAR reactor for a 190 MVA AC-EAF transformer for USA

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ALL SALES AND ADMINISTRATIVE

ACTIVITIES OF THE TAMINI GROUP

ARE DIRECTED FROM THE GROUP

HEADQUARTERS

HEADQUARTERSTAMINI Trasformatori s.r.l.via Cesare Battisti 3720077 Melegnano MI - Italytel. +39.02.982051 fax +39.02.98230322www.tamini.it

TRANSFORMER PRODUCTION FACILITIESTamini Melegnano: via Emilia 3720077 Melegnano MI

Tamini Legnano: via P. Ovidio Nasone20025 Legnano MI

Verbano TrasformatoriCorso Risorgimento 20928100 Novara NO

V.T.D. Trasformatorivia Gasdotto 6

36078 Valdagno VI

UK and Eire OperationsTCM Tamini Limited55, Shrivenham Hundred Business ParkWatchfield, Swindon SN6 8TYtel. +44.1793.780306fax +44.1793.787888

North American OperationsTamini Transformers USA2803 Butterfield Road, Suite 385, Oak Brook, IL 60523 USAtel. 630.368.9907fax 630.368.9910www.tamini.com

TCM TAMINI LIMITED. SwindonTCM TAMINI LIMITED. Swindon

RomaRoma

MilanoMilano

LondonLondon

TAMINI USATAMINI USA

GEOGRAPHICAL LOCATION

VERBANO TRASFORMATORI Novara VERBANO TRASFORMATORI Novara

TAMINI MelegnanoTAMINI Melegnano

V.T.D. Trasformatori V.T.D. Trasformatori

TAMINI LegnanoTAMINI Legnano

MILANO

S. DonatoMilanese

Paullo

S. GiulianoMilanese

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