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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
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.
Figure 1 - The Transformer
"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 to
adapt 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 inch
thick 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).
IEE Sri Lanka Centre, Chairman's Lecture 2000/01
Figure 2 - Faraday's first transformer:
Two coils wound on an iron toroid
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, in
fact 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
Figure 3 - Henry's Coils
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 Henry’s 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 was
induced; 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.
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Figure 4 - Ruhmkorff's inductor drawing
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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 companies were 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 that
only lamps consuming the identical power could be connected and if a single lamp burnt out the
complete line stopped lighting.
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Figure 5 - Series connection of arc lamps
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 heavycurrent 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.
Figure 7 Secondary Generator
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.
Figure 8 Zipernowski-Deri transformer
• 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 the
shape shown in figure 9.
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Figure 9 - Stanley's Paccinotti Ring
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Figure 10 - Dick and Kennedy's Transformer
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 that
time, 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
Figure 11 - "Block-shaped" transformer
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.
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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 threephase.
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 was
selected 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 consumers’ terminals.
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.
Figure 12 - Banking in Multiple
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 13 - Banking in Series
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Figure 14 - Banking on three-wire plan
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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 lamps
effectively 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 and
Figure 15 - Ferranti Transformer
secondary 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.
Figure 16 Early wound core
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 bundle
is 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
Figure 17a - National Transformer
Figure 17b - Winding & Core
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.
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Development of components of the transformer
The main components of the transformer are the core, winding, insulation and tank. I will now look
briefly at each of these components and some of their developments.
Transformer Core
The purpose of the transformer core is to provide a low reluctance path for the magnetic flux linking
the primary and secondary windings. In doing so the core experiences iron losses due to hysteresis and
eddy currents flow which manifests itself as heat. Research and development in electrical steels has
thus been on the reduction of these losses and also on the reduction of the noise emitted.
Core Materials
Early cores were made from bundles of soft-iron wire. The first transformers manufactures in the
1880s had cores made from high grade wrought iron and for a time Swedish steel was preferred.
Around 1900 it was realised that the addition of small amounts of silicon or aluminium to the iron
greatly reduced the magnetic losses. Thus began the technology of specialised electrical steel making.
However, low carbon steels continued to be used in certain instances up to around 1930.
Hot rolled steels
The addition of silicon reduces hysteresis loss, increases permeability and increases resistivity, thus
reducing eddy current loss. However the presence of silicon has the disadvantage that the steel
becomes brittle and hard so that the quantity of silicon has to be limited to not more than 4.5%.
Silicon steel for transformer cores was first used around 1906 and laminations of thickness around 0.35
mm were produced by a hot rolling process in which the grains are packed together in a random manner
so that the magnetic properties were independent of the direction of measurement. The specific loss
values of around 7 W/kg at 1.5 T were obtained at 50 Hz for these early hot rolled steels.
Cold rolled grain oriented steels (CRGO)
It had been recognised in the early 1920s that the silicon steel crystals were themselves anisotropic, but
it was not until 1934 that commercial use was made of this property. The first commercial grain
oriented cold-rolled steel was produced in 1939 and had a thickness of 0.32 mm and a loss of 1.5
W/kg at 1.5 T, 50 Hz.
High permeability grain oriented silicon sheet steels
In 1965 the Nippon Steel Corporation announced the production of the high permeability grain oriented
silicon steel and commercial production began in 1968. At flux densities of 1.7 T its permeability was
three times higher than the best cold-rolled grain oriented steel at the time. The low losses of these
steels were largely due to a reduction of around 40% in the hysteresis brought about by improved grain
orientation.
Laser irradiated super oriented steels
In 1980, Nippon Steel Corporation introduced the laser-etched high permeability grain oriented silicon
steel and by 1983 thickness had come down to 0.23 mm with losses as low as 0.85 W/kg at 1.7T, 50
Hz.
Amorphous steels
Amorphous steels have appeared relatively recently and their development stems from a totally
different source than the silicon core steels. The foil material is made from depositing molten steel
onto a fast rotating chilled drum. The amorphous or random grain structure is produced from cooling
the material at rates of up to 106 oC/s. The resulting material exhibits losses of 20-25% of those of the
best silicon steels. However with flux densities greater than about 1.56 T the loss rapidly increases
overtaking those of conventional steels. Foil thickness is around 0.025 mm. It was not until the mid1970s that the importance of their magnetic properties was recognised, but even after another twenty
years they are not very widely used mainly due to economic reasons and poor stacking factor.
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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.
Figure 18 - Basic form of construction
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. Each
lamination 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 shell
type transformer developed at the Ganz works
around the same time. The closed magnetic circuits
of these transformers were made of insulated wire.
Figure 19 - First Transformers with closed circuit
Figure 20 - "Hedgehog" Transformer
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 to
disseminate 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
Figure 21 - Flux direction at a corner
bolt holes in the core and the turning of the flux around
corners 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).
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Modern Core Building
Early core were constructed with square crosssection 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 crosssection in order to obtain optimum use of space
within the cylindrical windings (figure 22b).
Figure 22a - Early cores
Figure 22b - Seven Step
Stacked Core
Figure 23 - 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 two
plate 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.
Figure 23b - Step-lapped
Figure 24 - Wound Core
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.
(a) wound core in
original packing
(b) partly opened core
Figure 25 - Some steps in the construction of the wound core transformer
(c) Inserting winding
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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.
Figure 26 - Oval
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.
Figure 27 - Helical Coil
Single-layer coils were initially used for lowvoltage 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
Figure 28 to wind parallel helixes (figure 27).
Transposed conductor
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.
Figure 29 Disc winding
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 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 Winding
Figure 31 Foil winding
In 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 be
observed.
Figure 32 Foil winding in manufacture
Transformer Insulation
Today's transformers are almost entirely oil filled, but early transformers used asbestos, cotton and
low-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 shellacimpregnated 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.
IEE Sri Lanka Centre, Chairman's Lecture 2000/01
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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 350 oC 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 310 oC and auto-ignition temperature of 435 oC.
Rtemp is an improved version of mineral oil which has a flash point of 264 oC 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
Figure 34c Cast-resin transformer
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. The
maintenance 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 - LTL Transformer
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.
IEE Sri Lanka Centre, Chairman's Lecture 2000/01
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Recent Developments
Dryformer
Figure 34a - 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 nonmagnetic 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.
Figure 34b - Cross-section of winding
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 the
insulation 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 in
June 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.
IEE Sri Lanka Centre, Chairman's Lecture 2000/01
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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.
Figure 35 rectangular bar
The new generator (figure 36) has armature windings with a cylindrical crosssection 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.
Figure 37 - (a) cable, (b) winding ends
Figure 36 - Stator of Powerformer
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).
References
1. "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, ButterworthHeinemann 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, Bubier
Publishing 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
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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.html
14. Steed, J.C., "Amorphous core transformers", Power Engineering Journal, April 1994, vol 8, No 2, p92.