1. Introduction

The core of first practical transformer was developedin 1885; it was made of carbon steel.

Later, carbon steel was substituted by silicon steel and today most of the power and distribution

transformer cores in service are of cold rolled grain oriented silicon steel (CRGO) laminations.

Due to global movement of environmental protection, energy saving and noise reduction have

been required for transformers, leading to a demand for low core loss and low magnetostriction

material. Amorphous alloy exhibit properties of low core loss and low magnetostriction,

compared to conventional grain oriented silicon steel. Amorphous alloy exhibits a structure in

which the metallic molecules exists in a random pattern. As opposed to the rigid grain oriented

structure of silicon steel, this unique structure enables easy magnetization and demagnetization.

When energized, the core material switches its magnetization 100 times per second. The extent

of energy losses that occur in the core is determined by how easily the core switch

magnetization; the easier the switching capability, the lower the losses. The key feature of the

amorphous core transformers is the sharp reduction in the no-load losses that occur in the core

of transformer. There are several amorphous alloys in market, among them iron-boron-silicon

alloy (Fe78B13Si9) has presented the best performance; the core loss in this alloy is about 1/10

of core loss in CRGO steel.

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2. Transformer

A transformer is an electrical device that transfers electrical energy between two or more

circuits through electromagnetic induction. A varying current in one coil of the transformer

produces a varying magnetic field, which in turn induces a voltage in a second coil. Power can

be transferred between the two coils through the magnetic field, without a metallic connection

between the two circuits. Faraday's law of induction discovered in 1831 described this effect.

Transformers are used to increase or decrease the alternating voltages in electric power

applications.

Since the invention of the first constant-potential transformer in 1885, transformers have

become essential for the transmission, distribution, and utilization of alternating current

electrical energy. A wide range of transformer designs is encountered in electronic and electric

power applications. Transformers range in size from RF transformers less than a cubic

centimeter in volume to units interconnecting the power grid weighing hundreds of tons.

Figure 2.1

2.1 Types of Transformer Transformers can be categorized in different ways, depending upon their purpose, use,

construction etc. The types of transformer are as follows,

2

1. Step Up Transformer & Step Down Transformer - Generally used for stepping up

and down the voltage level of power in transmission and distribution power system

network.

2. Three Phase Transformer & Single Phase Transformer -Former is generally used in

three phase power system as it is cost effective than later. But when size matters, it is

preferable to use a bank of three single phase transformer as it is easier to transport than

one single three phase transformer unit.

3. Electrical Power Transformer, Distribution Transformer & Instrument

Transformer -Power transformers are generally used in transmission network for

stepping up or down the voltage level. It operates mainly during high or peak loads and

has maximum efficiency at or near full load. Distribution transformer steps down the

voltage for distribution purpose to domestic or commercial users. It has good voltage

regulation and operates 24 hrs a day with maximum efficiency at 50% of full load.

Instrument transformers include C.T & P.T which are used to reduce high voltages and

current to lesser values which can be measured by conventional instruments.

4. Two Winding Transformer & Auto Transformer - Former is generally used where

ratio between high voltage and low voltage is greater than 2. It is cost effective to use

later where the ratio between high voltage and low voltage is less than 2.

5. Outdoor Transformer & Indoor Transformer -Transformers that are designed for

installing at outdoor are outdoor transformers and transformers designed for installing at

indoor are indoor transformers.

6. Oil Cooled & Dry Type Transformer -In oil cooled transformer the cooling medium

is transformer oil whereas the dry type transformer is air cooled.

7. Core type, Shell type & Berry type transformer -In core type transformer it has two

vertical legs or limbs with two horizontal sections named yoke. Core is rectangular in

shape with a common magnetic circuit. Cylindrical coils (HV & LV) are placed on both

the limbs. Shell type transformer: It has a central limb and two outer limbs. Both HV,

LV coils are placed on the central limb. Double magnetic circuit is present. Berry type

transformer: The core looks like spokes of wheels. Tightly fitted metal sheet tanks are

used for housing this type of transformer with transformer oil filled inside.

3

2.2 Construction

2.2.1 Cores

Laminated Steel Cores

Transformers for use at power or audio frequencies typically have cores made of high

permeability silicon steel. The steel has a permeability many times that of free space and the

core thus serves to greatly reduce the magnetizing current and confine the flux to a path which

closely couples the windings. Early transformer developers soon realized that cores constructed

from solid iron resulted in prohibitive eddy current losses, and their designs mitigated this

effect with cores consisting of bundles of insulated iron wires. Later designs constructed the

core by stacking layers of thin steel laminations, a principle that has remained in use. Each

lamination is insulated from its neighbors by a thin non-conducting layer of insulation.

The transformer universal EMF equation implies an acceptably large core cross-sectional area

in order to avoid saturation.

The effect of laminations is to confine eddy currents to highly elliptical paths that enclose little

flux, and so reduce their magnitude. Thinner laminations reduce losses, but are more laborious

and expensive to construct. Thin laminations are generally used on high-frequency

transformers, with some of very thin steel laminations able to operate up to 10 kHz.

Figure 2.2

Laminating the core greatly reduces eddy-current losses

One common design of laminated core is made from interleaved stacks of E-shaped steel sheets

capped with I-shaped pieces, leading to its name of 'E-I transformer'. Such a design tends to

exhibit more losses, but is very economical to manufacture. The cut-core or C-core type is

made by winding a steel strip around a rectangular form and then bonding the layers together. It

is then cut in two, forming two C shapes, and the core assembled by binding the two C halves

4

together with a steel strap. They have the advantage that the flux is always oriented parallel to

the metal grains, reducing reluctance.

Solid Cores

Powdered iron cores are used in circuits such as switch-mode power supplies that operate

above mains frequencies and up to a few tens of kilohertz. These materials combine high

magnetic permeability with high bulk electrical resistivity. For frequencies extending beyond

the VHF band, cores made from non-conductive magnetic ceramic materials called ferrites are

common.[58] Some radio-frequency transformers also have movable cores (sometimes called

'slugs') which allow adjustment of the coupling coefficient (and bandwidth) of tuned radio-

frequency circuits.

Toroidal Cores

Toroidal transformers are built around a ring-shaped core, which, depending on operating

frequency, is made from a long strip of silicon steel or permalloy wound into a coil, powdered

iron, or ferrite. A strip construction ensures that the grain boundaries are optimally aligned,

improving the transformer's efficiency by reducing the core's reluctance. The closed ring shape

eliminates air gaps inherent in the construction of an E-I core. The cross-section of the ring is

usually square or rectangular, but more expensive cores with circular cross-sections are also

available. The primary and secondary coils are often wound concentrically to cover the entire

surface of the core. This minimizes the length of wire needed and provides screening to

minimize the core's magnetic field from generating electromagnetic interference.

5

Figure 2.3

Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar

power level. Other advantages compared to E-I types, include smaller size (about half), lower

weight (about half), less mechanical hum (making them superior in audio amplifiers), lower

exterior magnetic field (about one tenth), low off-load losses (making them more efficient in

standby circuits), single-bolt mounting, and greater choice of shapes. The main disadvantages

are higher cost and limited power capacity (see Classification parameters below). Because of

the lack of a residual gap in the magnetic path, toroidal transformers also tend to exhibit higher

inrush current, compared to laminated E-I types.

Air Cores

If an alternating current is supplied to a coil, an alternating magnetic field is produced

surrounding it. If another coil is brought inside this magnetic field, an alternating emf is

induced across the second coil also as per Faraday's law of electromagnetic induction. This

induced emf in the second coil can be utilized to feed a load. As in this example the flux is

linked with both coils through air, so this arrangement can be referred as air core transformer.

Here, first coil serves as primary winding and second coil serves as secondary winding of the

said transformer. Whenever there is a need of changing voltage level from one level to another

in power network

we use electrical transformer.

Figure 2.4

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Whenever there is a need of electrical isolation from one circuit to other, we use

transformer.For transforming current and voltage to low measurable level we use current

transformer and potential transformer respectively. These are not only the applications of

transformers.

2.2.2 Windings

High-frequency transformers operating in the tens to hundreds of kilohertz often have windings

made of braided Litz wire to minimize the skin-effect and proximity effect losses. Large power

transformers use multiple-stranded conductors as well, since even at low power frequencies

non-uniform distribution of current would otherwise exist in high-current windings.[67] Each

strand is individually insulated, and the strands are arranged so that at certain points in the

winding, or throughout the whole winding, each portion occupies different relative positions in

the complete conductor. The transposition equalizes the current flowing in each strand of the

conductor, and reduces eddy current losses in the winding itself. The stranded conductor is also

more flexible than a solid conductor of similar size, aiding manufacture.

The windings of signal transformers minimize leakage inductance and stray capacitance to

improve high-frequency response. Coils are split into sections, and those sections interleaved

between the sections of the other winding.

7

Figure 2.5

Power-frequency transformers may have taps at intermediate points on the winding, usually on

the higher voltage winding side, for voltage adjustment. Taps may be manually reconnected, or

a manual or automatic switch may be provided for changing taps. Automatic on-load tap

changers are used in electric power transmission or distribution, on equipment such as arc

furnace transformers, or for automatic voltage regulators for sensitive loads. Audio-frequency

transformers, used for the distribution of audio to public address loudspeakers, have taps to

allow adjustment of impedance to each speaker. A center-tapped transformer is often used in

the output stage of an audio power amplifier in a push-pull circuit. Modulation transformers

in AM transmitters are very similar.

Dry-type transformer winding insulation systems can be either of standard open-wound 'dip-

and-bake' construction or of higher quality designs that include vacuum pressure

impregnation (VPI), vacuum pressure encapsulation (VPE), and cast coil

encapsulation processes. In the VPI process, a combination of heat, vacuum and pressure is

used to thoroughly seal, bind, and eliminate entrained air voids in the winding polyester resin

insulation coat layer, thus increasing resistance to corona. VPE windings are similar to VPI

windings but provide more protection against environmental effects, such as from water, dirt or

corrosive ambients, by multiple dips including typically in terms of final epoxy coat.

2.2.3 Cooling

To place the cooling problem in perspective, the accepted rule of thumb is that the life

expectancy of insulation in all electrics, including all transformers, is halved for about every

7 °C to 10 °C increase in operating temperature, this life expectancy halving rule holding more

narrowly when the increase is between about 7 °C to 8 °C in the case of transformer winding

cellulose insulation.

Small dry-type and liquid-immersed transformers are often self-cooled by natural convection

and radiation heat dissipation. As power ratings increase, transformers are often cooled by

forced-air cooling, forced-oil cooling, water-cooling, or combinations of these. Large

transformers are filled with transformer oil that both cools and insulates the windings.

Transformer oil is a highly refined mineral oil that cools the windings and insulation by

circulating within the transformer tank. The mineral oil and paper insulation system has been

extensively studied and used for more than 100 years. It is estimated that 50% of power

8

transformers will survive 50 years of use, that the average age of failure of power transformers

is about 10 to 15 years, and that about 30% of power transformer failures are due to insulation

and overloading failures. Prolonged operation at elevated temperature degrades insulating

properties of winding insulation and dielectric coolant, which not only shortens transformer life

but can ultimately lead to catastrophic transformer failure. With a great body of empirical study

as a guide, transformer oil testing including dissolved gas analysis provides valuable

maintenance information. This underlines the need to monitor, model, forecast and manage oil

and winding conductor insulation temperature conditions under varying, possibly difficult,

power loading conditions.

Figure 2.6

The tank of liquid filled transformers often has radiators through which the liquid coolant

circulates by natural convection or fins. Some large transformers employ electric fans for

forced-air cooling, pumps for forced-liquid cooling, or have heat exchangers for water-

cooling. An oil-immersed transformer may be equipped with a Buchholz relay, which,

depending on severity of gas accumulation due to internal arcing, is used to either alarm or de-

energize the transformer. Oil-immersed transformer installations usually include fire protection

measures such as walls, oil containment, and fire-suppression sprinkler systems.

2.2.4 Insulation Drying

Construction of oil-filled transformers requires that the insulation covering the windings be

thoroughly dried of residual moisture before the oil is introduced. Drying is carried out at the 9

factory, and may also be required as a field service. Drying may be done by circulating hot air

around the core, or by vapor-phase drying (VPD) where an evaporated solvent transfers heat by

condensation on the coil and core.

For small transformers, resistance heating by injection of current into the windings is used. The

heating can be controlled very well, and it is energy efficient. The method is called low-

frequency heating (LFH) since the current used is at a much lower frequency than that of the

power grid, which is normally 50 or 60 Hz. A lower frequency reduces the effect of inductance,

so the voltage required can be reduced. The LFH drying method is also used for service of

older transformers.

2.2.5 Bushings

Larger transformers are provided with high-voltage insulated bushings made of polymers or

porcelain. A large bushing can be a complex structure since it must provide careful control of

the electric field gradient without letting the transformer leak oil.

Bushings are highly reliable components. However, if failed, the typical result is ex-tended

downtime of a power transformer with not only heavy financial losses but also challenges

imposed on the system operator to compensate for the lost unit.

Figure 2.7

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2.3 Losses in Transformer-As the electrical transformer is a static device, mechanical loss in transformer normally does

not come into picture. We generally consider only electrical losses in transformer. Loss in any

machine is broadly defined as difference between input power and output power. When input

power is supplied to the primary of transformer, some portion of that power is used to

compensate core losses in transformer i.e. Hysteresis loss in transformer and Eddy current

loss in transformer core and some portion of the input power is lost as I2R loss and dissipated

as heat in the primary and secondary windings, because these windings have some

internal resistance in them. The first one is called core loss or iron loss in transformer and the

later is known as ohmic loss or copper loss in transformer. Another loss occurs in

transformer, known as Stray Loss, due to Stray fluxes link with the mechanical structure and

winding conductors.

Copper Losses in Transformer

Copper loss is I2R loss, in primary side it is I12R1 and in secondary side it is I2

2R2 loss, where

I1 & I2 are primary and secondary current of transformer and R1 and R2 are resistances of

primary & secondary winding. As the both primary & secondary currents depend upon load of

transformer, copper loss in transformer vary with load.

Core Losses in Transformer

Hysteresis loss and eddy current loss, both depend upon magnetic properties of the materials

used to construct the core of transformer and its design. So these losses in transformer are

fixed and do not depend upon the load current. So core losses in transformer which is

alternatively known as iron loss in transformer can be considered as constant for all range of

load.

Eddy Current Loss - In transformer, we supply alternating current in the primary, this

alternating current produces alternating magnetizing flux in the core and as this flux

links with secondary winding, there will be induced voltage in secondary, resulting

current to flow through the load connected with it. Some of the alternating fluxes of

transformer; may also link with other conducting parts like steel core or iron body of

transformer etc. As alternating flux links with these parts of transformer, there would be

11

a locally induced emf. Due to these emfs, there would be currents which will circulate

locally at that parts of the transformer. These circulating current will not contribute in

output of the transformer and dissipated as heat. This type of energy loss is called eddy

current loss of transformer.

Eddy current losses in transformer is denoted as

Hysteresis Loss in Transformer - The magnetic core of transformer is made of ′Cold

Rolled Grain Oriented Silicon Steel′. Steel is very good ferromagnetic material. This

kind of materials are very sensitive to be magnetized. That means, whenever magnetic

flux would pass through, it will behave like magnet. Ferromagnetic substances have

numbers of domains in their structure. Domains are very small regions in the material

structure, where all the dipoles are paralleled to same direction. In other words, the

domains are like small permanent magnets situated randomly in the structure of

substance. These domains are arranged inside the material structure in such a random

manner, that net resultant magnetic field of the said material is zero. Whenever external

magnetic field or mmf is applied to that substance, these randomly directed domains get

arranged themselves in parallel to the axis of applied mmf. After removing this external

mmf, maximum numbers of domains again come to random positions, but some of them

still remain in their changed position. Because of these unchanged domains, the

substance becomes slightly magnetized permanently. This magnetism is called "

Spontaneous Magnetism". To neutralize this magnetism, some opposite mmf is required

to be applied. The magneto motive force or mmf applied in the transformer core is

alternating. For every cycle due to this domain reversal, there will be extra work done.

For this reason, there will be a consumption of electrical energy which is known as

Hysteresis loss of transformer.

Hysteresis loss in transformer is denoted as,

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3. Amorphous Core Transformer

An amorphous metal transformer (AMT) is a type of energy efficient transformer found on

electric grids. The magnetic core of this transformer is made with a ferromagnetic amorphous

metal. The typical material (Metglas) is an alloy of iron with boron, silicon, and phosphorus in

the form of thin (e.g. 25 µm) foils. These materials have high magnetic susceptibility, very

low coercively and high electrical resistance. The high resistance and thin foils lead to low

losses by eddy currents when subjected to alternating magnetic fields. On the downside

amorphous alloys have a lower saturation induction and often a

higher magnetostriction compared to conventional crystalline iron-silicon electrical steel.

In a transformer the no load loss is dominated by the core loss. With an amorphous core, this can be

70–80% lower than with traditional crystalline materials. The loss under heavy load is dominated

by the resistance of the copper windings and thus called copper loss. Here the lower saturation

magnetization of amorphous cores tend to result in a lower efficiency at full load. Using more

copper and core material it is possible to compensate for this. So high efficiency AMTs can be

more efficient at low and high load, though at a larger size. The more expensive amorphous core

material, the more difficult handling and the need for more copper windings make an AMT more

expensive than a traditional transformer

3.1 Amorphous Metal

The amorphous metal used by ABB is a metallic alloy of iron, boron and silicon (Fe-B-Si)

produced by solidifying alloy melts at rates rapid enough to prevent crystallization of the metal.

Such rapid solidification leaves a vitrified solid with a random (amorphous) atomic structure,

essentially as in the liquid phase. This differs from the atomic structure of conventional regular

grain-oriented 

13

(RGO) silicon steel (a Fe-Si alloy), which has an organized crystalline structure. The largest

volume usage of amorphous metal is in the cores of electrical distribution transformers. These

materials offer, in concert, excellent magnetic characteristics and economy in production costs.

In fact, the advent of Fe-B-Si amorphous metal alloys in the mid-1980s has been the most

important advancement in materials for distribution transformers in the second half of the 20th

century.

Figure 3.1

The disordered structure of amorphous steel and the ordered crystalline structure of regular

grain-oriented steel

Solidification rates of 106 K/s are necessary to produce Fe-B-Si amorphous metals. The high

heat extraction rates constrain the solid in the form of a thin ribbon, about 25 μm thick. Since

the material is thin, the application of amorphous metal is restricted to wound transformer

cores. Amorphous metal cores have been in use for over 20 years in liquid-filled transformers,

and this technology is now being applied to dry type transformers.

3.2 Significantly lowered core losses

The most notable characteristic of an amorphous metal in a transformer is that it yields a much

lower core loss than even the best grades of RGO steel, by up to 70 percent. In a transformer

core material, there are two major types of losses: hysteresis loss and eddy current loss. The

former reflects the ease of magnetization of the material when the core is energized, and the 14

latter results from internal currents generated in the material. The absence of a crystalline

structure in amorphous metal allows easy magnetization of the material, leading to lower

hysteresis losses. The eddy current losses are also lower in amorphous metal due to a

combination of its thinness and a high electrical resistivity of 130 μΩcm–1, compared with 50

μΩcm–1 in RGO steels.

3.3 Optimization Via Anneals 

The low losses in amorphous metal are achieved through optimized anneals, ie, exposure to a

temperature close to the material's Curie temperature (668 K) for a selected amount of time in

the presence of an externally applied magnetic field. Annealing is an essential step for

amorphous metals.

Magnetic anisotropy in a ferromagnetic material is a measure of the ease of magnetization

away from a given direction. In RGO steels, the crystal structure primarily defines this

anisotropy, whereby there are predefined easy axes for magnetization. The random atomic

architecture in amorphous metals precludes such magnetocrystalline anisotropy. However, a

magnetic anisotropy may be induced to define easy directions for magnetization in these

materials. As a result of the very rapid cooling rates, amorphous metals have high quenching

stresses, resulting in stress-induced anisotropy. With annealing, the quenching stresses are

relaxed and by applying an external magnetic field of typically 1,000 A/m, a preferred axis of

magnetization (along the ribbon length) is introduced in the material. A consequence of the

anneal step is that the previously ductile amorphous metal becomes brittle, which requires more

careful material handling in subsequent processing steps.

3.4 Lower design induction level Due to the presence of boron, amorphous metal has a lower saturation induction (1.56 T) than

RGO steels (2.1 T). Therefore, the design induction with amorphous metal is lower than that

available from RGO steels. As a result, amorphous core transformers often have a larger core

cross-sectional area, resulting in larger coils and transformer footprint. Transformer sound level

Transformers with amorphous metal cores generate about 3 to 5 dB higher sound levels than

those with RGO steel cores. Techniques to mitigate these higher sound levels are the focus of

ongoing ABB research activities. Sound is generated within transformer cores due to an

intrinsic characteristic of the core material called magnetostriction. When the magnetization

direction has to rotate under an applied field, the material undergoes a dimensional change and 15

sound is generated. In RGO steels, the crystalline axes for easy magnetization are well aligned

between grains. This is not so in the case of amorphous metal, since the quenching stresses are

never fully relaxed from annealing an amorphous metal. Therefore, during operation of a

transformer, a greater degree of magnetization rotation is called for from amorphous metal than

from RGO steels. The dimensional change is consequently higher, leading to greater sound

levels.

3.5 Stable losses over time 

In the 1990s it was shown that the performance stability of amorphous metal at the transformer

operating temperature is very high and that significant changes in its loss performance would

take more than 1,000 years. Those aging tests were carried out with small toroids protected

from oxygen. Since then, the composition of the amorphous metal alloy has changed slightly

and manufacturing methods have been further developed. In order to guarantee the stability of

today’s material (and to see the influence of an ambient air) ABB repeated accelerated aging

measurements at high temperature (490 K) with a full-size core under air atmosphere for more

than 200 days. But the results were similar, suggesting that no degradation of losses during the

transformer lifetime needs to be expected.

3.6 Energy-savings potential

 Comparison of US DOE mandated minimum efficiency standards across a wide range of

transformer ratings compares the efficiency of AMDT with the mandated minimum efficiency

standards for the same by the US Department of Energy (DOE), across a wide range of

transformer ratings. The improved energy efficiencies attained are quite clear. A quick back-of-

the-envelope calculation may be used to highlight the energy savings potential from the

deployment of AMDT. As mentioned, use of amorphous metal cores can reduce transformer

core no-load losses by about 70 percent, when compared with RGO steel cores. Assuming that

about 1 percent of the installed US generating capacity of 1.4 TW is lost in distribution

transformer no-load losses, this reduction of losses from the use of amorphous cores suggests a

potential annual energy savings of about 85 billion kWh.

16

Figure 3.2

The energy savings are sizable, even if only a fraction of all distribution transformers are

replaced by AMDTs. This savings from existing generating capacity allows deferral or

cancellation of plans for additional generation to meet ever-growing demand. The

environmental benefits associated with reductions in noxious gas emissions and in the CO2

footprint are clear, as are the economical benefits accrued from energy and cost savings. The

17

consequent social benefits are self-evident. Similar summary estimates for energy savings and

CO2 reductions have been made for representative major nations, as presented. If

CO2 emissions are to be taxed by $25/ton, this amounts to $2.5 billion annually.

Table 3.1

 Annual energy savings potential and impact on CO2 generation from the use of AMDTs 

3.7 Transformer losses

As in any transformer, the losses in amorphous metal transformers consist of two parts: the no-

load loss (P0) generated in the core, and the load loss (Pk) mainly occurring in the transformer

windings. P0 is always present and constant during normal operation, whereas Pk only occurs

during transformer operation and is load dependent. The no-load loss of ABB's AMDT is only

30 percent of the no-load loss of a standard transformer. Thus, the use of an amorphous metal

core could prevent the emission of 140,000 tons of CO2 – equivalent to 60,000 kg of oil –

during an operation period of 20 years for a 1,000 kVA transformer.

18

Figure 3.3

Comparison of typical no-load loss values of standard and amorphous metal core liquid-

immersed and dry-type 1,000 kVA transformers

3.8 Transformer costs

When selecting a transformer, a variety of costs may be considered: first costs, lifecycle costs,

or costs including all additional infrastructure expenses. Life-cycle costs include capitalization

of the transformer losses. This is usually done by using the TOC (total ownership cost)

approach, in which specific valuations are assigned to P0 and Pk. These valuations, among other

considerations, depend on the cost of electricity, on the cost of providing the lost power and on

the utilization of the transformer. The values used by most utilities range between $5 and

$10/W for P0 and between $1 and $2/W for Pk.

TOC = CT + A · P0+ B · Pk ($)

where TOC = total ownership cost, CT = transformer purchase price, A = capitalization factor

for no-load loss, and B = capitalization factor for load loss.

AMDTs have a higher first cost. However, if life-cycle costs are considered, they are still the

most economical choice.

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4. A Comparison with Conventional CRGO-Core Transformers

Limitations with amorphous alloy are low saturation limit, more hardness and more

cost as compared to CRGO steel (Boyd E.L. and Borst J.D. 1984, Nicols DeCristofaro

1998). The saturation limit for amorphous alloy (Fe78B13Si9) is 1.69 Tesla; however

it is 2.03Tesla for CRGO steel (BHEL 2009). Therefore, amorphous alloy is more

suitable for miniature size transformers and distribution transformers, as they are

designed for lower flux densities. Distribution transformers are designed for low core

loss; the core loss in a material depends on flux density (Bm). The core loss (Pc) in the

amorphous alloy is given by (Lee Ji-kwang 1999) -

Pc = [6.5(f/1000)1.51 Bm1.74 ] watts/Kg. Here, ‘f’ is the supply

frequency

For economic design of a transformer, the cross sectional area of the core should be

smaller; for smaller cross sectional area the chosen flux density is high and therefore,

the core loss increases. If large rating power transformers are designed for low flux

density then cross sectional area of core becomes large; for larger cross-sectional area

of a core, costs of the core and winding are higher, therefore the cost of the transformer

becomes higher (Dasgupta 2011, Sawhney 2006, Say M.G. 1977). Therefore,

amorphous alloy is not a suitable choice for large rating power transformers, if

economy is an issue. Amorphous core transformers are energy efficient transformers

with increased costs (Kennedy B 1998); the cost of amorphous core transformers is 20

to 30 percent higher than that of conventional CRGO core transformers (Puneet K

Singh 2010). Cost of a transformer depends on cost of core, cost of winding and

manufacturing cost. Costs of core and winding are affected by shape and size of cross-

sectional area of core. A core having square or rectangular crosssection is called 1-step

core. For square or rectangular cross-section of a core, cost of core is lower but the cost

of winding is higher, than that of circular multi-stepped cross-section of core. Selection

of number of steps in a core depends on KVA rating of transformer. As the rating of

transformer increases, the number of steps in a core increases. For more number of

steps, the diameter of circumscribing circle reduces for an iron area of the core, so cost

of copper winding reduces, and copper losses are also reduced. However; with the

increase in number of steps, the assembly cost of the core increases. Therefore for low

rating transformers the square section of the core is economical and for medium and

large rating transformers multi-stepped CRGO core is economical. On the other hand,

20

for the amorphouscore transformers of medium and large ratings, the square or

rectangular section of core is adopted by manufacturers (Lee Ji-Kwang 1999, Nicols

DeCristofaro 1998, Schulz R.N. 1998), the reason is the higher cost of amorphous-core

assembly, as compared to CRGO-core assembly.

Stacking factor (Ks) is defined by the ratio between the cross-sectional area of

ferromagnetic material and total cross-sectional area of the core. With sheets of CRGO

steel it is 0.9 or higher. For amorphous alloy it is in between 0.8 and 0.9 (Bendito et al

1999). With reduced stacking factor the gross cross-sectional area of core becomes

larger, resulting overall size to be big. The disadvantage with bigger size is the higher

cost, but the advantage is better cooling.

Cost of a transformer is also assessed as Total owning cost (TOC) (Amoiralis 2009).

TOC is sum of initial cost of a transformer and cost of energy losses during operation.

For low TOC, the losses in a transformer should be low. As the time passes, the TOC

increases. Amorphous-core transformers have higher initial cost with a reduced cost of

energy losses; therefore they become economical after a certain period of time.

For medium and large rating transformers electromagnetic forces on the windings must

also be considered as they are very high (Martin J. Heathcote 1988). Radial

electromagnetic forces on the winding are proportional to square of the current (Fr α

I2). Under short circuit condition the current in a transformer may be 20 times of rated

current, so the electromagnetic force may be 400 times higher than that of normal

condition. For square section of core, the shape of the coil is also square, for which

radial electromagnetic forces are not uniform around the periphery of the coil. Non

uniform radial electromagnetic forces may distort the shape of a coil in a transformer.

Therefore square or rectangular section of core is not advisable for medium and large

rating transformers. For a multi-stepped core, the shape of the coil is circular, and the

radial electromagnetic forces are uniform around periphery of the coil.

Low voltage and high voltage windings are accommodated in the window area (Aw).

The ratio between height (Hw) and width of the window (Ww) is chosen 2 or above;

for low leakage reactance this ratio should be high (Dasgupta 2011,Sawhney 2006, Say

M.G. 1977). The window space factor (Kw) is defined by the ratio between the

conductor cross section area and the window area. Insulation and clearance reduce the

available area for actual conductor cross section to kw.Aw. Window design

considerations for conventional CRGO core transformers and amorphous core

transformers are same.

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For a convention CRGO core transformers, the magnetising current is 1% to 2% of

rated current. The magnetising current drawn by a transformer depends on two factors;

one is the nature of B-H curve, and other is number of joints in the core assembly.

Comparing between B-H characterics of amorphous alloy and conventional CRGO

steel, magnetizing current for amorphous alloy is about one third than that of CRGO

steel (BHEL 2009). During construction of conventional CRGO core assembly for a

transformer, Mitred joints are used in corners.

Magnetostriction is less in amorphous alloy, compared to CRGO steel (Bendito et al

1999). Therefore, amorphous core transformers have low noise level, so they are

friendlier to environment .

Average life of an amorphous core transformer is 30 years; however it is 25 years in

case of conventional transformers. Therefore, the average life of amorphous core

transformer is more than the average life of conventional transformer.

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5. B-H Curve – Amorphous metals have high relative permeability, so they are suitable for magnetization, as

illustrated in Figure

Figure 5.1

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6. Applications of Amorphous TransformerThe main application of AMTs are the grid distribution transformers rated at about 50–1000

kVA. These transformers typically run 24 hours a day and at a low load factor (average load

divided by nominal load). The no load loss of these transforms makes up a significant part of

the loss of the whole distribution net. Amorphous iron is also used in specialized electric

motors that operate at high frequencies of perhaps 350 Hz or more.

The use of ABB's amorphous core transformers can have a significant impact on energy

consumption in many different applications, such as utilities, wind power generation and

photovoltaic power plants.

Transformers in the utility distribution grid 

For utilities, the reduction of no-load loss is a major focus, since the average loading of a

distribution transformer is generally low. AMDTs are the perfect choice to achieve this goal.

Although first costs of AMDTs are higher, they are often the preferred choice if TOC is

considered. This is shown in for 1,000 kVA transformers having the no-load loss values

depicted in and with capitalization factors A = $10/W and B = $2/W. Additional cost savings

may be achieved if dry-type transformers need to be actively cooled, or if CO2taxation is

considered.

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Figure 6.1

Transformers for wind power generation

Energy is produced from the turbines on a wind farm for only short periods throughout the day.

Therefore, transformer no-load losses need to be minimized in wind power applications. As

discussed above, the advantage of AMDTs in such applications becomes evident from TOC

considerations.

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EcoDry transformers for photovoltaic power plants

To promote solar energy, many countries have established a feed-in tariff system for electricity

generated by photovoltaics (PV). For example, for a free-standing PV plant, which began

operation in 2010, the tariff is 0.26 euros/kWh (about $0.34/kWh) in Germany and 0.32 euros/

kWh (about $0.42/kWh) in Spain. The operator of the PV plant wishes to maximize his income

by maximizing the plant output and minimizing losses. A transformer in a PV power plant

experiences a heavily varying load, depending on the time of day, the season and weather

conditions. At night, the transformer consumes the no-load loss that the operator has to pay for,

unless the transformer is disconnected from the grid. The dry-type EcoDry transformer is the

ideal choice for such applications.

A simple model allows the calculation of the benefits. In the model, a certain number of sunny

days per year is assumed, with a number of hours per day with an output of 95 percent and a

number of hours with an output of 15 percent (morning and evening). The remaining days of

the year are assumed to be cloudy and the output of the PV plant is 15 percent during the whole

day. These conditions result in a certain specific annual output (kWh/kWp). For example, a

specific output of 1,700 kWh/kWp is realized by having 240 sunny days with 6 hours at 95

percent load and 4 hours at 15 percent load.

If the price difference between the standard transformer, which has low first cost but high

losses, and the EcoDry transformer is considered as an additional investment, the return on the

investment can be calculated as shown in fig. Electricity costs of 0.08 euros/kWh (about

$0.10/kWh) and revenues of 0.28 euros/kWh (about $0.37/kWh) are assumed in these

calculations.

Figure 6.226

7. Advantages and Disadvantages

More efficient transformers lead to a reduction of generation requirement and, when using electric

power generated from fossil fuels, less CO2 emissions. This technology has been widely adopted by

large developing countries such as China and India where labour cost is low. AMT are in fact more

labour-intensive than conventional distribution transformer, a reason that explain a very low

adoption in the comparable (by size) European market. These two countries can potentially save

25–30 TWh electricity annually, eliminate 6-8 GW generation investment, and reduce 20–30

million tons of CO2 emission by fully utilizing this technology

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ConclusionsCompared to conventional transformers, amorphous core transformers have- low core losses,

low magnetising current, less zero sequence current, less noise, higher inrush current, more

harmonic problem, bigger size, higher initial cost, higher efficiency, and longer life.

Advantages with amorphous core transformer are more, compared to disadvantages. At present,

initial cost of amorphous core transformer is comparatively higher; it becomes economical after

a certain period of time. In future, as the manufacturing will increase, the initial cost will be

reduced because of reduction in manufacturing cost. Amorphous core transformers are energy

efficient transformers; If all conventional transformers are replaced by amorphous core

transformers, a considerable amount of energy will be saved for a nation.

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References Amoiralis, Marina and Antonios, 2009: Transformer design and optimization: A

literature survey, IEEE trans. on Power delivery vol. 24 No. 4, pp. 1999- 2024.

Bendito Antonio, Misael Elias and Claudio Shyinti Kiminami, 1999: Single phase 1-

KVA Amorphous core Transformer, IEEE trans. on magnetics, vol- 35,No.4, July.

BHEL, 2009: Transformers Tata Mc-Graw Hill publication, India.

Boyd E.L. and Borst J.D. 1984: Design concepts for an amorphous metal distribution

transformer, IEEE Trans. Power Apparatus and Systems, vol.103,no.11,pp.3365-3372.

Dasgupta I. 2011: Design of transformers, Tata McGraw Hill publishers.

Kennedy B. 1998: Energy Efficient Transformers, McGraw-Hill.

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