Transformer_history_2000.pdf

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The Institution of Electrical Engineers, Sri Lanka Centre

Chairman's Lecture 2000/01

Historical Development

of the

Transformer

Professor J R Lucas

Chairman, IEE Sri Lanka Centre 2000/01

Professor in Electrical Engineering, University of Moratuwa

14 November 2000

Hotel Galadari, Colombo, Sri Lanka


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The Institution of Electrical Engineers, Sri Lanka Centre

Chairman's Lecture - 14 November 2000

Historical Development of the Transformer

Professor J Rohan Lucas, Chairman IEE Sri Lanka Centre 2000/01

Fellow members of the IEE, Ladies and Gentlemen. It is a great honour for me to be able to deliver

this lecture today as your new Chairman. I have selected the topic "Historical Development of the

Transformer" as I feel that this is of both academic and general interest for an audience containing not

only electrical engineers but also computer engineers, electronic engineers, production engineers,

telecommunications engineers and well wishers.

The Introduction

Let me start in a lighter vein and tell you of an incident during the start of compilation of this talk. As

one of the sources for this talk, I went to the Internet and searched for "History + Transformer". The

first search gave me two groups of results. The first was about the history of Jefferson's transformers,

which when I went more deeply into was found to be mainly for toys. This obviously was not what I

was looking for. The next result filled me pleasure.

It had the title "The complete history of the transformers". From this I learnt that the transformers were

produced over a million years ago. I double checked it !! Yes, it said millions of years ago.

The transformers described and pictured did not look like any of the

transformers I had seen and I did not fully understand what they were

talking about. So I copied the first paragraph from the article.

"Millions of years ago, the planet of Cybertron was made by the

Quintessons as a production plant for the robots they needed. At the

start they experimented with creatures partially robot and partially

organic, later they made real robots. There were two types of robots;

military units and consumer goods - Transformers[1]". "They learned toadapt to anything." (Figure 1). From another article[2] I learnt that

transformers are "robots toys that transformed into vehicles and such".

I subsequently managed to find some more articles on "transformers" and especially on "distribution

transformers" from the Internet and from the library. I also managed to obtain some books and articles

from the Institution of Electrical Engineers in London. The rest of my presentation is based on these

and other such articles rather than on the "complete history" I found earlier.

The First Transformer

In 1831, Michael Faraday carried out a series of experiments convincingly demonstrating the principle

of electromagnetic induction. The first breakthrough in solving the problem of producing electricity

from magnetism occurred on 29th August 1831. On that day, he took a soft iron ring 7/8 of an inchthick and 6 inches in external diameter. Around one half of the ring's circumference (which side he

called A), he wound three coils of wire. Each coil had 24 feet of wire with the turns separated by wine

and calico. On the other side (side B), but separated from side A by a distance, he wound 60 feet of

wire in two separate coils in the same direction as the former coils. He connected the two coils on the

side B in series and carried the connecting wire over a magnetic needle. He then connected one of the

side A coils to a battery and closed the circuit on side A. The magnetic needle on side B immediately

sensed it, oscillated and then returned to its original position. He observed a further disturbance of the

needle only when he broke the battery connection on side A, but this was in the opposite direction.

Faraday's report of this momentary disturbance of the magnetic needle was the first demonstration of

what is known as electromagnetic induction today. Once he had got on the correct track, his

experiments progressed very rapidly. This was the forerunner of the modern electrical transformer

(figure 2).

Figure 1 - The Transformer


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IEE Sri Lanka Centre, Chairman's Lecture 2000/01 2

"Faraday's apparatus was designed to study

whether a direct current (dc), and the magnetic

field that was produced by a dc coil, induced

voltage in another coil. It took several years of

experimentation for Faraday to realise that

constant dc does not have such effect, but the

change, the increase or decrease of the current, infact generates voltage in the other coil. Naturally,

the apparatus was fed by a dc galvanic battery,

since no other power source was available at the

time [3]." "Faraday found that a current of

electricity flowing in a coil of wire wound

around a piece of iron would convert the iron

into a magnet and that, if this magnet were

inserted into another coil of wire, a galvanometer

connected to the terminals of the second coil

would be deflected. [4]."

Faraday's invention contained all the basic

elements of transformers - two independent coils and a closed iron core.

The Next Fifty Years, 1832-1882

For many years after Faraday's discovery, it had no

practical value. "Induction coils were used to produce

much higher voltage than galvanic batteries. In 1832.

when self-inductance was invented, Joseph Henry

noticed that with the interruption of current very high

(several hundred volts) voltage is induced in the coil

due to the rapid flux change[3]." Figure 3 shows the

coils used by Henry in his induction experiments."The coils are made of copper strips which have been

wrapped in silk insulation."

Continuous operation of the induction coils was ensured by the use of various vibrators.

Although other experimenters also repeated Henrys experiments and went on to build induction

coils operated with interrupted direct current to give shocks or sparks, there was no thought of

the transformer as an economical means of power distribution. The spark inductor was actually a

high-voltage pulse transformer, and cannot be identified with the heavy-current transformer of

today, and even less with its application.

However absurd it may sound from the physical aspect, spark inductors were regarded as dc

devices at the time! When turning the battery on, long-time but low-amplitude half-wave wasinduced; when breaking it, short-time, but high peak voltage was induced. Thus the starting

voltage could hardly be felt. When a spark gap was also present, only the break peak voltage

could produce a current, so that dc flowed in the secondary circuit. This way the positive and

negative pole was interpreted.

The development of spark inductors promoted the construction of the later transformers in the

area of production technology rather than theory. The important technical achievements were the

vacuum impregnation of high voltage coils, oil insulation, the disk-winding proposed by

Poggendorff, and the application of laminated iron core.

Figure 2 - Faraday's first transformer:

Two coils wound on an iron toroid

Figure 3 - Henry's Coils


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The first inductors only provided sparks that

were a few millimetres long, but in 1853,

Daniel Ruhmkorff increased the spark length

first to 200 mm then to 450 mm by improving

the vibrator and the insulation. [Figure 4]."

The Faraday and Rumkorff coils are types of

two classes of converters - the closed circuit

and opened transformers - prevailing just over

100 years ago. Even at that time it was

readily seen that[8] the Ruhmkorff type,

having a straight core, had to complete the

circuit through air; in strong contrast to the

Faraday type which had a complete magnetic

circuit of iron. Thus the transformers even

100 years ago were almost universally of the

Faraday class.

By the late 19th century, it was realised that the chief aim of the transformer builder was

to make the magnetic circuit entirely of iron,

to shorten the magnetic circuit as much as possible,

to increase the cross-sectional area as much as consistent with weight, and

to use iron of the greatest magnetic permeability.

In addition to the above features, due care was taken with regard to the insulation, and best ventilation

possible.

The War of the Currents

By 1888, the war of the currents - d.c. vs a.c.had heated up with Edison publishing a warning on the

mortal danger of ac. What drove Edison to this extreme measure could also have been the fact that

Westinghouse's ac companieswere fast catching up on Edison's dc companies. The chief advantage of

dc was that they could not only be used for street lighting but the availability of dc motors for traction

and manufacturing. Another advantage was that batteries could ensure continuity of supply when the

generators were not running. The chief disadvantage of dc was the lack of economical transmission due

to the absence of ready step-up and step-down devices.

The chief advantage of ac was availability of transformer for raising the voltage for distribution and

lowering it for safe use. This meant that ac could be sent on thin wires whereas dc required thick

copper conductors as distribution had to be at low voltage. The main disadvantage of ac was the

absence of an ac motor (Tesla's ac motor patented in 1888 was not ready yet).

The first alternating system

The "Jablochkoff candle", developed in the 1870s, was a simple and cheap flame-arc lamp without

mechanic regulator that definitely needed ac for its operation. This gave a boost to using ac. As arc

lamps spread, the need emerged that a generator should not only feed one lamp but all the lamps along

an avenue. The basic problem was the method of connecting lamps - in series or in parallel. Over one

hundred and twenty years ago, this was not unambiguous due to the nature of the load.

Arc lamps operated at 35 to 40 V, and with a low voltage network, the larger total current required by

the parallel consuming equipment caused very high line losses. Also the voltage drop in the line

limited the maximum distance between the generator and the flame-arc lamps to about 100 to 200m in

a parallel system. On the other hand, with a supply voltage of 1,000 to 1,500 V a series connected

system could operate 20 to 30 lamps with the line of lamps stretching several kilometres long.

This was true both for dc as well as for ac. However a problem with the series connection was thatonly lamps consuming the identical power could be connected and if a single lamp burnt out the

complete line stopped lighting.

Figure 4 - Ruhmkorff's inductor drawing


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Figure 6 - Multiple connection of arc lamps

This required that individual lamps should be made independent of the serial network. "Jablochkoff

was the first to realise in 1877 (in Paris) that instead of the direct connection of the lamps into the

serial main line, lamps should be fed through a two-coil induction device. He assumed that the

different operation of lamps, which were galvanically separated would not affect the other lamps.

Although he was mistaken, he still managed to improve the operation of the system, and what is even

more important, he started the research-development activity that led to the development of the heavy-

current transformer.

In 1882, Goulard and Gibbs patented a system of distributing power using

alternating current and two-coil induction devices. They used devices (then

known as secondary generators - figure 7) of the Ruhmkorff type in the first

alternating current distribution system and had a 1:1 ratio and were used with

their primaries in series. The farthest lamp fed, on the Torino-Lanzo railway

line, was at 40 km distance from the 2,000 V generator with 133 Hz

frequency. The series connection led to unsatisfactory regulation unless all

the transformers were equally loaded. This practice had long been

discontinued and the parallel Edison system had become widespread even by

1892.

Probably the first improvement of the Faraday coil (even though very slight)

was made by Kennedy in 1883 when he made the magnetic ring of wrought

iron wire instead of cast-iron, thus gaining greater magnetic permeability.

The Zipernowski and Deri Transformer

One of the best of the early ring-shaped transformer was

presented by Messrs Karoly Zipernowski and Miksa Deri in

1885. In this transformer, which was electrical excellent

though mechanically not very sound, the positions of the coils

and the iron were reversed. The primary and secondary coils,

both thoroughly insulated, was wound into a kind of solid core

and over-wound with a heavy layer of iron wire. They also

took a crucial step in March 1885.

This comprised of three major elements.

Rejecting series connection and connecting transformers

that supply the consuming equipment groups in parallel

to the main line,

Applying high-ratio transformers, separating high-voltage (1400-2000 V) wide supply network

from low-voltage (100 V) consumer networks, and

Developing a transformer with closed iron core, low drop (i.e. terminal voltage is almost

independent of the load), and low loss.

In 1885, George Westinghouse acquired the American rights under the patent and selected William

Stanley to develop the transformer. He made a transformer with a ring of finely laminated iron of theshape shown in figure 9.

Figure 5 - Series connection of arc lamps

Figure 7 -

Secondary Generator

Figure 8 -

Zi ernowski-Deri transformer


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Stanley's transformer was an adoption of the Paccinotti ring armature. Even at that time it was

difficult to ascertain what gains were expected to result from this construction as the leakage from the

teeth was anything but desirable. In the winter of 1885-86 William Stanley installed the first

experimental ac distribution system which supplied 150 lamps in the town of Great Barrington,Massachusetts.

Dick and Kennedy in 1886 introduced a transformer which showed the first really vital improvements

since the days of Faraday. Starting with Stanley's Paccinotti Ring, they wound the periphery with thin

sheet iron (figure 10). However, much of the efficiency which should have been gained by the

reduction of the length of the magnetic circuits was sacrificed due to the magnetic circuits not

following the direction of the lamination in the peripheral iron.

By the late nineteenth century, transformers had crept up very close to 100% efficiency (typically

about 97.5% for a 10 kW transformer).

First attention to transformer cost

Shortly thereafter, the question of cost of the transformer began to demand serious attention. Up to thattime, the closed circuit transformers had hand wound coils. The shape of the iron punchings made

them wasteful and expensive. The iron on the periphery was difficult to wind, and any repairs

necessitate tearing the whole thing to pieces. A further objection to this coil however, which applied

equally to all of the early ring-shaped transformers, was that the space occupied was quite

disproportionate to the work done. This together with the difficulty and expense of the winding,

eventually led to the abandonment of the ring transformer in its original form of an endless jointless

iron ring.

The first "block shaped" class of transformers

Around the same period, the "block shaped" class of

transformers commenced to attract attention. These

transformers were or many kinds, but no one differed very

substantially from each other. This class of transformers

generally had the coils and iron arranged about as shown in

figure 11. In these the coils are entirely surrounded by

laminated iron except at their ends, whilst the magnetic

circuits are comparatively short and in two directions, the

whole apparatus being mechanically simple, and easy to

assemble.

Quoting from "Transformers" by Caryl D Haskins published in 1892, "Transformers are sold as a mere

commercial article, and lighting companies order a dozen of them as a cook might order a dozen eggs".The first ac transmission line in the US was put into operation in 1890 to carry electric energy

generated by water power a distance of 13 miles from Willamette Falls to Portland, Oregon. The first

transmission lines were single phase, and the energy was usually consumed for lighting purposes only.

Figure 11 - "Block-shaped" transformer

Figure 9 - Stanley's Paccinotti Ring Figure 10 - Dick and Kennedy's Transformer


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Even the first motors were single phase, but on May, 16 1888, Nikola Tesla presented a paper

describing two-phase induction and synchronous motors. The advantages of poly-phase motors were

apparent immediately, and a two-phase ac distribution system was demonstrated to the public at the

Columbian Exposition in Chicago in 1893. Thereafter, the transmission of electric energy by

alternating current, especially three-phase ac, gradually replaced dc systems. In January 1894, there

were five polyphase generating stations in the US of which one was two-phase and the others three-

phase.

Universal Electric Power System

In 1890, just five years prior to the start-up of the first large-scale power project at Niagara Falls, the

method of production and distribution of power was still undecided.

The project was to include transmission to Buffalo. Fourteen projects submitted for transmission were

considered. Four of the proposals were for compressed-air, with its basic industrial uses such as

hauling and lifting, through two feed diameter underground mains. One proposal was for hydraulic

transmission and another for mechanical transmission via steel cables in a chain of posts and pulleys.

Five of the proposals were for dc transmission and only two were for ac transmission. AC was a

novelty at that time, was not considered to be among the eight awards given.

After new proposals from General Electric and Westinghouse in August 1883, polyphase ac wasselected and power was first produced in Niagara Falls on August 6, 1895. The initial contract was for

generating 15,000 hp at 2200 V at 25 cycles.

The Transformer in Service

All supply undertakings had to solve the problem of transmitting electrical energy at high voltage from

the generating station to points nearer the consumers, then reducing the voltage and stabilising the

voltage at the consumersterminals.

For purposes of lighting, the preferred voltage was 50 t0 52 V although when the secondary circuit

length was long 100 to 104 V was used for reasons of economy.

When the number of lights to be carried on one circuit was

greater than the capacity of a single converter (transformer), a

number was arranged in multiple as shown in figure 12. When

this is done the transformers should always be of the same

capacity and same style. However, it was considered

preferable to divide all circuits when possible, using single

transformers, so that all the lamps in a building, may not be

dependent on a single source.

At times it became necessary to so bank transformers, as to

get an increased potential, as when a building was ready

wired for 100 V, lamps of 100 V but only transformers

available are designed to give a secondary potential of 50 V.

In this case the transformers must be banked with their

secondaries in series as shown in figure 13. The converters

must of course be of the same capacity.

Figure 12 - Banking in Multiple

Figure 13 - Banking in Series


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Transformers may be banked on the three-wire plan as

shown in figure 14. In this case the transformers are

banked with their primaries in multiple and their

secondaries in series, with the neutral or middle wire taken

between them. This was generally done with 100 V

transformers, and saves much wire, the greater portion of

the energy being distributed at 200 V with the lampseffectively burning two in series. The middle wire usually

had between one-half to one-third the carrying capacity of

the outside wires.

The Ferranti Transformer is a converter of European Manufacture.

It is a good example of European practice in 1892. It is seen that

the transformer is not enclosed within a water-proof case, as was

customary with converters of American manufacture at the time.

This is because in Europe the transformer was installed within

buildings. The frame which holds and supports the actual

converter is of cast iron and is so constructed to provide for

standing the transformer upon the floor. The primary andsecondary terminals are at opposite ends of the base and are so

constructed that they cannot be tampered with, or the wires loosened with an ordinary screwdriver.

The terminals are thoroughly insulated from the frame by means of sulphur and glass insulation, poured

while in a molten state, into the space between the frame and each terminal block.

The iron used in the construction of these transformers is extra soft Swedish sheet

and is unusually thin. A number of bundles of iron are brought together in

parallel as seen in figure 16 and are overwound and bound together by insulation

at their central portion. Over the insulation is wound secondary, and over this

again is placed the primary, generally in the form of ready wound coils, due

insulation being interposed. The soft iron is then turned back and over from each

end, the ends of the strips lapping one over the other, till the middle of the bundleis reached, when the last two ends are turned back and made fast. The remaining

half of the iron is then turned back similarly in the opposite direction, the iron,

when in position, enclosing the coils.

Another transformer manufactured in 1890 is the National Transformer

manufactured by the National Electric Manufacturing Co. of Wisconsin. Its general appearance is

shown in figure 17a and its internal character by figure 17b.

The National transformer is of the ring type and the entire winding is surrounded by iron, all of the wire

in the transformer thus being active. A novel feature of this is the fuse and connection box on the

lower side of the case, where the opening of the fuse box door simultaneously breaks the connection

between the primary and the fuse contacts.

Figure 14 - Banking on three-wire plan

Figure 15 - Ferranti Transformer

Figure 16 -

Earl wound core

Figure 17a - National Transformer Figure 17b - Winding & Core


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Core Construction

Over the years, various arrangements of cores and coils have been worked out. However, all these

arrangements can be considered to fall into two general classes - shell form and core form.

Figure 18a and 18b show the basic single phase shell-form construction and the core-form construction

respectively.

In the shell-form construction, the coils are more or less rectangular in shape and the iron is built

through the opening and around the outside of the coils to form a shell around the straight part. Eachlamination of the iron, when assembled, forms a rectangle with two windows through which the coils

pass. The flux-return paths of the core are external to and enclose the windings

In the core-form transformer, the iron is in the shape of two cores, or legs, surrounded by the coils and

joined at the ends by yokes.

Both the shell-form of construction and the ring-form

of construction have existed from the days of the

early transformers.

Figure 19(a) shows the core type transformer

developed by Gaulard and Gibbs and used by

Westinghouse in 1885. Figure 19(b) shows a shelltype transformer developed at the Ganz works

around the same time. The closed magnetic circuits

of these transformers were made of insulated wire.

There were also the transformers with open

magnetic circuit in those days (figure 20). The core was almost

invariably of wire, straight and non-continuous, the magnetic

circuit being completed through the air. The iron wire was

permitted to extend considerably beyond the coils, the wires

being bent into a radiating form, so that each individual wire

was separated from its neighbours. This construction served todisseminate the lines of force (magnetic flux) through the

surrounding air.

Around the beginning of the twentieth century, core-type

transformers were built with rectangular cut laminations

(figure 21a) where the direction of flux did not correspond to

the rolling direction at the joints, and bolt holes distorted the

flux path from the orientation.

One of the disadvantages of grain oriented steel is that any

factor which requires the flux to deviate from the grain

direction increases the core loss. Such factors included the

bolt holes in the core and the turning of the flux aroundcorners of core limbs. In order to limit the extent to which the flux path cuts across the grain direction

the corners of the laminations are now cut on a 45o mitre. Also from the latter part of the 1970s

manufacturers have adopted totally boltless cores (figure 21b).

Figure 18 - Basic form of construction

Figure 19 - First Transformers with closed circuit

Figure 21 - Flux direction at a corner

Figure 20 - "Hedgehog" Transformer


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Modern Core Building

Early core were constructed with square cross-

section and then two-step (figure 22a). Since then,

core designs have been constantly improved and

core laminations have been built up to form a limb

or leg having as near as possible a circular cross-

section in order to obtain optimum use of spacewithin the cylindrical windings (figure 22b).

Stacked Core

The transformer core is usually built by

stacking laminations, two or three per lay. The lay-down sequence

must take into account of the need to alternate the lengths of plates

to provide the necessary overlaps at the mitred corners (figure

23a)..

Step-lapped joints

In the simple stacked core, a simple arrangement consisting of only twoplate configurations are used. Because much of the loss in modern

transformer cores arises from the yoke to limb joints, manufacturers have

come out with an arrangement where a joint may have as many as seven

different plate lengths so that the mitre can have a seven step overlap.

This joint ensures a smoother transfer of the flux and thus provides a lower

corner loss.

From around the 1990s, wound cores made from amorphous steel foil

(made from depositing molten steel onto a fast rotating chilled drum) have

been used by some utilities, mainly on an experimental basis for the smaller

distribution transformers. This material exhibits losses of the order of only

20 to 30% of those of the best silicon steels. However the transformers

tend to be about 10% heavier and have a much higher capital cost. The life

cycle cost is still marginally higher. The foil is ultra thin and special

techniques of material handling have evolved where by the amorphous core

transformer can bee assembled in a lesser time than the conventional

stacked core transformer. Figure 24 shows a wound core. Although not

discernible in the photo, each loop of core steel has an overlapped joint at

the upper end.

Figure 25 shows some steps in the construction of a wound core amorphous steel transformer .

Figure 25 - Some steps in the construction of the wound core transformer

Figure 22a - Early cores

Figure 22b - Seven Step

Figure 23 - Stacked Core

Figure 23b - Step-lapped

Figure 24 - Wound Core

(c) Inserting winding

p p(a) wound core inori inal packin


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The Oval cross-section of core is commonly used with a foil winding (figure 26).

This is because the distribution of current axially in the foil winding corresponds

exactly to that of the high voltage winding and axial forces are very much less even

in the event of heavy fault currents. Thus the circular cross section may be

dispensed with to reduce the construction cost of distribution transformers as the

oval shape lends itself to a higher space factor with a correspondingly lower number

of steps. Thus the two step construction may be used without any adverse effects.

Transformer Winding

Transformer windings have mostly been made from hard drawn copper except in countries where

aluminium is readily available due to its generally superior properties.

The early transformer windings were made of round conductors and were double cotton covered wire.

These were wound on a lathe and the coils were given a final coating of shellac and baked in an oven.

Circular cross-section wire is now generally restricted to the plain enamel covered form used for the

high voltage winding in distribution transformers. Wire of circular cross-section cannot be wound into

windings having a good space factor and thus either rectangular-section wire or strip is used. This

rectangular cross-section conductor is usually paper insulated.

Single-layer coils were initially used for low-

voltage windings. To obtain higher power ratings,

single-layer coils with several concentric layers

were manufactured. The helical winding was

introduced in the mid 1920s. This is built from a

large number of parallel strands. It is also possible

to wind parallel helixes (figure 27).

Each strand in a helical winding may also be transposed (figure 28). By means of continuous

transposing, each strand will on average enclose the same flux, thereby preventing circulating currents

with associated increase in losses. This technique of designing helical windings has been refined over

the years and several coaxial shells of this type of winding are also utilised.

The disc winding (figure 29) was utilised right from the beginning as the

high voltage winding in transformers requiring many turns. The voltage

distribution for rapid transients, lightning overvoltages was a difficult

problem for these windings in the early days.

In today's high-voltage disc windings the turns

are interleaved between different discs so that

a higher series capacitance and consequently

a better impulse voltage distribution are

obtained.

Figure 30 shows several methods by which series capacitance can be

increased. The first uses an electrostatic shield connected to the line end

and inserted between the two hv discs nearest to the line end. The

second winds a dummy strand connected to the line lead but terminating

in the first disc. The shield itself is usually made by wrapping a

pressboard ring of the appropriate diameter with thin metal foil. The

third usually involves winding two or more strands in parallel and then

reconnecting the ends of every second or fourth disc after winding to give

the interleaving required. It has an advantage over the first two methods

in that it does not waste any space as every turn remains active.

However it is more costly.

Figure 26 - Oval

Figure 27 - Helical CoilFigure 28 -

Transposed conductor

Figure 29 -

Disc winding

Figure 30 -

Winding Stress Control


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Multilayer winding

Around 1960, the multi layer winding was introduced. In this winding, the impulse voltage is shared

between the layers. This gives a much better distribution of voltages than a disc winding where a large

part of the voltage is across the first coils.

Foil WindingIn modern distribution transformers, foil winding is frequently

used. In this form of construction the winding turn, of copper

or aluminium foil, occupies the full width of the layer (figure

31). The arrangement represents a very cost effective method

of manufacturing low voltage windings and also enables a

transformer to be built with a high degree of electromagnetic

balance and hence good mechanical short circuit strength. With

the foil winding, the axial forces during a short circuit is

limited to one-tenth of the force which occurs in copper strip

winding. Diamond dotted paper is frequently used as

interlayer insulation. Figure 32 shows a foil winding in

manufacture with diamond dotted presspaper insulation being

used between layers. The oval shape of the coil can also beobserved.

Transformer Insulation

Today's transformers are almost entirely oil filled, but early transformers used asbestos, cotton andlow-grade pressboard in air. Shellac insulated paper in the late nineteenth century was a tremendous

step forward. Mineral oil started to be used for the insulation and cooling of transformers in 1906. The

oil in turn was cooled with water-filled cooling tubes inserted in the transformer tank. The shellac-

impregnated paper could not match the thermal capabilities of the newly developed oil-filled

transformers. These used kraft paper and pressboard insulation system. Paper and pressboard account

for the greatest part of insulation material used in power transformers when used in transformer oil but

not very good dielectrics in the absence of oil. Other forms of paper used are thermally upgraded

paper and diamond dotted paper. The next most commonly used material is wood.

Petroleum oils have been used in electrical equipment since the latter part of the nineteenth century.

Ferranti recognised their benefits for the transformer as long ago as 1891. These mineral oils are still

used with of course better refining and better selection.

Silicone liquid is frequently employed in transformers where there is a desire to avoid fire hazard.

Figure 31 Foil winding

Figure 32 Foil winding in manufacture


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Silicone liquids are synthetic materials, the most well known being polydimethylsiloxane,

characterised by thermal stability and chemical inertness. Silicone liquid has a very high flash point

and in a tank below 350oC will not burn even when its surface is subjected to a flame. Distribution

transformers with silicone liquid have been in operation for several years.

A synthetic ester fluid has been developed to meet high-voltage insulation oil specifications and is

finding increasing application as a dielectric fluid in transformers. These have very high flash points of

around 310o

C and auto-ignition temperature of 435o

C.

Rtemp is an improved version of mineral oil which has a flash point of 264oC and is suitable for

indoor applications.

Treated sunflower oil is under development as a transformer oil. Although it is environmentally very

friendly, it is still about 10 times dearer than normal mineral oil.

Although 'dry' design using Sulphur-hexafluoride gas for insulation has been

produced for a number of years and proved attractive especially in the Asian

region, they suffered from the fact that SF6 is a greenhouse gas listed for

emission reduction.

Alternative 'dry' designs based on moulded epoxy and glass fibre insulation

systems are also available (cast-resin type), but the need to keep internal field

strengths to 3 kV/mm limited the voltage rating of such designs to about 36kV

Transformer Tank

Very early transformers at the end of the nineteenth century were generally of the cast iron type. Since

then, transformers have almost invariably been constructed from welded plates.

Detachable radiators of corrugated sheet steel were introduced at the beginning of the 1920s. In the

1930 radiators with cooling fans were adopted, thereby making it possible to build a naturally-cooled,

three-phase transformer with a rating of 45 MVA. Forced cooling with oil pump and fan on the air side

was introduced in the 1950s.

Around 1970s hermetically sealed type with no gas cushion was introduced. In these transformers the

expansion/contraction of the oil is handled by the deeper (50 to 400 mm deep) corrugations (fins) rather

than by a separate conservator tank. The corrugated fins (1.2 to 1.75 mm thick) in the present design

have replaced the rather heavy tanks (4 - 20 mm thick) which exist with cooling tubes or radiators.

These hermetically sealed transformers are used for distribution

transformers and small power transformers only, as insufficient

cooling is provided by the fins for the larger transformers. Themaintenance of the hermetically sealed transformer is virtually none,

as moisture or air cannot enter the tank. However a check must be

made for corrosion and oil leakage.

Figure 33 shows a distribution transformer presently been made by

Lanka Transformers Limited. This is of the hermetically sealed type

with corrugated fins. It has a copper foil for the low voltage winding

with diamond dotted paper insulation separating the layers. The high

voltage winding is of round enamelled copper. The core is of oval

cross-section having only 2 steps.Figure 33 - LTL Transformer

Figure 34c -

Cast-resin transformer


15/16


Recent Developments

Dryformer

The Dryformer is an innovative high-voltage transformer design that eliminates the need for oil based

on the use of high-voltage cross-linked polyethylene (XLPE) power cable instead of oil/paper in the

construction of the transformer windings. (figure 34a)

The new concept is the result of the marraige of high

voltage cable technology and transformer technology.

Figure 34b shows cable conductors and the vertical non-

magnetic steel rods that provide mechanical support

against axial forces. Developed by ABB, the first

Dryformer was delivered in early December 1999 to a

Swedish Utility and is rated at 20 MVA, 140kV/6.6kV.

The Dryformer has substantial benefits for both

customers and the environment. The absence of oil

eliminates the risk of contamination of soil or ground

water, and minimises the risk of fire and explosion. The

net result is that, in principle, the new design can be

installed anywhere - close to lakes and rivers, in

underground caverns or densely populated areas.

By using state of the art technology, XLPE cable can have field strengths up to 15 kV/mm. However,

the electric field is fully contained within the XLPE cable and the cable surface is at ground potential.

From a manufacturing perspective, the Dryformer has the considerable advantage of having theinsulation system built up at the cable factory (unlike in oil/paper insulation where a thorough drying

out process using a combination of high temperature and vacuum and quick assembly is required).

Powerformer

The Powerformer (trademark of ABB) was developed by ABB recently to combine the functions of a

conventional generator and a step-up transformer. Thus it is a high voltage generator which can be

connected directly to the power network without the need of a step-up transformer. The novelty of the

new generator concept is the use of proven power cable as stator winding. Although this is not a

transformer, this has been included here as it a does away with the necessity for a generator

transformer.

The first generator (11 MVA, 45 kV, 600 rpm) to feature this concept was successfully inaugurated inJune 1998 at the Porjus hydropower plant in the Swedish national grid. Another generator rated at 136

kV, 42 MVA, 3000 rpm for a thermal power station is scheduled to be commissioned in Autumn 2000

also in Sweden.

Figure 34a - Dryformer

Figure 34b - Cross-section of winding


16/16


The conventional generator design is based on rectangular armature slots and

conductor bars (figure 35a) and the maximum output voltage is limited to the

order of 25-30 kV but is usually fixed at around 13.8 kV. In contrast, the

powerformer operates at a relatively high voltage and low current.

The new generator (figure 36) has armature windings with a cylindrical cross-

section based on proven solid dielectric power cables, like in the dryformer.

Thanks to the cable design, the electric field has an even distribution and is totally confined within the

cable itself.

The winding cable consists of a conductor (1), an inner semi-conductive layer (2), a solid dielectric (3)

and an outer semi-conductive layer (4) as shown in figure 37. The solid dielectric is cross-linked

polyethylend (XLPE).

References1. "The Complete History of The Transformers", Internet article, www.xs4all.nl/~wjtbeek/history1.html

2. "Transformer history", Internet article, http://sabretron.fsn.net/TransformersHistory.htm

3. Jeszensky, S., "History of Transformers", IEEE Power Engineering Review, December 1996, p 9-12.

4. Gibbs, J. B., "Transformer Principles and Practice", McGraw-Hill Book Company, New York, Second

Edition, 1950

5. Mosser, H.P., "Transformerboard", Scientia Electra, April 1979

6. Franklin, A.C., and Franklin, J.S.C., "J & P Transformer Book", Eleventh Edition, 1995, Butterworth-

Heinemann Ltd, Oxford

7. "Power Transformer Handbook", edited by Hochart, Bernard; English Edition 1987, Butterworths, Oxford

8. Haskins, C. D., "Transformers - Their theory, construction and application, simplified", 1892, BubierPublishing Company, Mass.

9. Foran, Jack, "The day they turned the falls on: the invention of the universal electrical power system", Internet

article, Case studies in Science, http://ublib.buffalo.edu/libraries/projects/cases/niagra.htm

10. Rao, S., "Power Transformers and Special Transformers", Khanna Tech Publications, Delhi, 1991, Second

edition.

11. Leijon, M., Owman, F., Karisson, T., Lindahl, S., Parkegren, C., Sorqvist, T., and Miller, R., "Powerformer:

Electric Power Generation for the Twenty-First Century", Internet article, Nemesis GPI - Powerformer,

http://www.nemesis.at/publication/gpi_99_2/articles/abb.html

12. Leijon, M, and Andersson, T., "High and Dry", IEE Review, July 2000, Vol 46, No 4, pp 9-14.

13. Leijon, M., et Al "A Major Breakthrough in Transformer Technology", Synopsis for CIGRE 2000. Group 12:2,

Internet site: http://www.elforsk.se/cigre/synop122.html14. Steed, J.C., "Amorphous core transformers", Power Engineering Journal, April 1994, vol 8, No 2, p92.

Figure 35 -

rectangular bar

Figure 36 - Stator of PowerformerFigure 37 - (a) cable, (b) winding ends

Documents

Transformer_history_2000.pdf