Transformers in Power Distribution Networks

APPLICATION NOTE
TRANSFORMERS IN POWER DISTRIBUTION
NETWORKS
Stefan Fassbinder
December 2015
ECI Publication No Cu0143
Available from www.leonardo-energy.org

Publication No Cu0143
Issue Date: December 2015
Page i
Document Issue Control Sheet
Document Title: Application Note – Transformers in Power Distribution Networks
Publication No: Cu0143
Issue: 03
Release: December 2015
Author(s): Stefan Fassbinder
Reviewer(s): Roman Targosz
Document History
Issue Date Purpose
1 August
2009
Initial publication as an Application Note
2 March
2012
Reworked by the author for adoption into the Good Practice Guide
3 November
2015
Review by Roman Targosz
Disclaimer
While this publication has been prepared with care, European Copper Institute and other contributors provide
no warranty with regards to the content and shall not be liable for any direct, incidental or consequential
damages that may result from the use of the information or the data contained.
Copyright© European Copper Institute.
Reproduction is authorised providing the material is unabridged and the source is acknowledged.

Page ii
CONTENTS
Summary ........................................................................................................................................................ 1
Introduction: why do we need a transformer? ............................................................................................... 2
The design and manufacturing of conventional and of special purpose transformers..................................... 6
Transformer tank and oil........................................................................................................................................6
Core ..................................................................................................................................................................8
Windings...............................................................................................................................................................14
Special types of transformers...............................................................................................................................16
Operational behaviour.................................................................................................................................. 18
Short circuit voltage..............................................................................................................................................18
Resistive load........................................................................................................................................................19
Inductive load .......................................................................................................................................................24
Capacitive load – care required!...........................................................................................................................25
Vector groups .......................................................................................................................................................28
Protection.............................................................................................................................................................31
Operating transformers in parallel .......................................................................................................................32
Energy Efficiency........................................................................................................................................... 36
New regulation governing transformer efficiencies.............................................................................................36
Optimizing the proportion between no-load and load losses..............................................................................43
Driving up costs by buying cheap .........................................................................................................................44
An example...........................................................................................................................................................46
Amorphous steel ..................................................................................................................................................48
Transformers used in renewable energy generation systems..............................................................................50
Other countries, other customs ...........................................................................................................................51
Outlook ................................................................................................................................................................52
Special solutions for special loads................................................................................................................. 53
Evil loads...............................................................................................................................................................53
Practical measures................................................................................................................................................57
Conclusion .................................................................................................................................................... 65

Page 1
SUMMARY
In electrical engineering terminology, transformers are regarded as electrical machines, although they only
convert one form of electricity into another form of electricity. Due to this relatively simple function, among
other reasons, their losses are lower than those of any equipment converting electricity into some other form
of energy. They are probably the most efficient machines ever devised by man. Transformer efficiencies are
around 80% for very small units used in domestic appliances and nearly 99% at the level of distribution
networks. The efficiency further increases with increasing unit power rating. The largest units achieve
efficiencies of up to 99.75% at rated load and even 99.8% at half load. At first glance, it looks rather unlikely
that there is any savings potential left that would be commercially significant, but in fact there is. It is true that
the payback periods are fairly long, but a transformer has a lifetime expectancy of well over 40 years and the
majority of all transformers are operated continuously at a high degree of loading. As a result, an improved
transformer design, primarily through the use of more active material, will usually pay off several times over
the lifespan of the transformer.

Page 2
INTRODUCTION: WHY DO WE NEED A TRANSFORMER?
Why do we need a transformer? Electric power has to be transmitted at the highest feasible voltage if power
losses in the electricity line are to be kept within reasonable limits. This is true in absolute terms: the higher
the transmission voltage, the lower the current and hence the smaller the (resistive) power loss in the line. It is
also true in relative terms. While a 3 V voltage drop in a motor vehicle’s 12-volt on-board electrical system is a
significant loss, it would hardly be noticed in a 230 /400 V distribution network and certainly would not impair
the function of any load. In a high voltage network, the same 3-volt drop would be almost immeasurably small.
Consider a transformer of certain dimensions. Now double each of the three dimensions: length, width, and
height, while retaining the same transformer structure. Clearly, the area of any face of the transformer will
increase fourfold. This will also apply to those surfaces available for dissipating heat losses, to the cross-
sectional area of the conductors, and to the cross-sectional area of the iron core—each of which is an
important transformer design parameter. If the linear dimensions are doubled however, all volumes will
increase by a factor of eight and so will the corresponding mass.
Assuming that the current densities in the conductors remain unchanged, the current carrying capacity (or
ampacity) of the conductors will increase fourfold, since the cross-sectional area of the conductors is now four
times as great. The current density measured in all transformers rated from 10 VA to 1 GVA is indeed
approximately 3 A/mm² for copper conductors and about 2 A/mm² when the conductor material is aluminium.
However, doubling the dimensions of the wire not only increases the conductor’s cross-section by a factor
four, it also doubles its length. The eightfold increase in the volume of the conductor material mentioned
above corresponds to an eightfold increase in the mass of copper or aluminium used. For a given current
density and temperature (though the effect of temperature is not critical in this simple analysis), every
kilogram of a particular conductor material will generate the same amount of heat loss. Therefore, a
transformer whose length, width, and height have all been doubled will weigh eight times as much, and the
heat losses it generates will consequently also rise by a factor of eight. This eightfold increase in heat loss must
nevertheless be dissipated by cooling surfaces whose area is only four times as great—a fact that we ignored
above. In practical applications, larger transformers therefore need additional cooling. The first step is to
introduce liquid cooling of the transformer windings. Further cooling can be achieved by increasing the area of
the transformer cooling surfaces. This type of cooling system is known as ONAN cooling (oil natural circulation,
air natural circulation). Forced cooling is used in transformers with ratings above about 40 MVA. In this type of
cooling, known as ONAF cooling (oil natural circulation, air forced circulation), liquid cooling is augmented by
cool air blown in by fans. Above about 400 MVA, it becomes necessary to use pumps to help circulate the oil
coolant. This form of cooling is abbreviated OFAF and stands for oil forced, air forced circulation. In
transformers with power ratings greater than 800 MVA, simply circulating the oil is no longer sufficient and
these transformers use ODAF cooling (oil directed, air forced cooling) in which a jet of cooling oil is directed
into the oil channels of the transformer windings.
Table 1 – Power densities and efficiencies of a range of real transformers from a miniature transformer to a
generator transformer.
Example transformers
found
S [kVA] Cu [kg]
S/Cu
[kVA/kg]
S/Cu4/3
[kVA/kg4/3
]
Current
Density
[A/mm²]
Energy
Efficiency
Minimum Transformer 0.001 0.014 0.070 0.291 7.000 45.00%
Small Transformer 0.100 0.500 0.200 0.252 3.000 80.00%
Industrial Transformer 40.000 48.200 0.830 0.228 3.397 96.00%
Distribution Transformer 200.000 200.000 1.000 0.171 98.50%
Bulk Supply Point Transformer 40000.000 10000.000 4.000 0.186 3.000 99.50%
Generator Transformer 600000.000 60000.000 10.000 0.255 99.75%
Geometric Mean Value --- --- --- 0.227 --- ---

Page 3
Figure 1 – Graph showing the copper content (blue) and the efficiencies (green) of the sample transformers
listed in Table 1 as well as the theoretical copper content derived from the formula (red).
The fourfold increase in the cross-sectional area of the transformer core permits a fourfold increase in the
voltage. This multiplied by the fourfold higher current in the fourfold greater cross-sectional conductor area
means a sixteen-fold (2
4
) rise in the rated output of a transformer whose mass is eight (2
3
) times as great. The
data in Table 1 show that this theoretically derivable correlation is indeed roughly confirmed in practice. If the
nominal power of the transformer is raised by a factor of 10
4
, the size of the transformer (i.e. its volume and
mass) only increases by a factor of 10
3
(since the length, width, and height have each increased by one power
of ten). This in turn means that the material costs and the costs of manufacturing and installing the
transformer system also rise by a factor of 10
3
.
Consider a high-power transformer rated at 1,100 MVA (currently the largest size of transformer being
manufactured). From an engineering point-of-view, it is perfectly possible to build even larger units. The
problem is that the only means of transporting these devices is by rail (Figure 2, Figure 3) and even then a
specially designed 32-axle low-loader wagon is required. A transformer of this size weighs in at around 460
tonnes. Approximately 60 tonnes of this total is copper.
If a transformer with 60 tonnes of copper has a power rating of 1,100 MVA, then one might imagine that a
small transformer containing 60 g should have an output of 1,100 VA. In fact, a transformer of this size only
manages about 11 VA. Similar scaling laws apply to motors and generators. For this reason—and of course,
because of the associated labour costs—it is more economical to generate electric power in large gigawatt
power stations and subsequently distribute this power to the regions within a 100 km radius, rather than
generating smaller quantities of electrical power locally and feeding them into the low-voltage distribution
network. This is where transformers come in. It is a commonly held misconception that a fully decentralized
electricity generation system would remove the need for the interconnected pan-European grid and its
transformers. Although grid loads would fall, the presence of the grid would be more important than ever
since it would have to compensate for sporadic and strongly fluctuating local loads. It would also be needed to
take up and distribute the unpredictable supply of solar and wind-generated power.
Specific copper content
of transformers
1E-02kg
1E-01kg
1E+00kg
1E+01kg
1E+02kg
1E+03kg
1E+04kg
1E+05kg
1E-03kVA 1E-01kVA 1E+01kVA 1E+03kVA 1E+05kVA 1E+07kVA
Transformer rated throughput 
Coppercontent
20%
30%
40%
50%
60%
70%
80%
90%
100%
Efficiency
Example transformers found
Theoretical Deduction
Energy Efficiency

Page 4
Figure 2 – A 32-axle low-loader rail wagon for transporting high-power transformers.
(Source: www.lokomotive-online.de/Eingang/Sonderfahrzeuge/Uaai/uaai.html)
Figure 3 – A high-power transformer ready for transport, shown here mounted on a small 24-axle low-loader
rail wagon.(www.lokomotive-online.de/Eingang/Sonderfahrzeuge/Uaai/uaai.html)
A kilogram of copper in a large machine causes more or less the same power losses as a kilogram of copper in
a small machine. However, each kilogram of copper in the generator of a large power station is responsible for
a power output of roughly 10 kVA, whereas a kilogram of copper in a bicycle dynamo would yield only 100 VA.
It is clear then that the efficiency of larger units is greater than that of smaller units, as already seen in Table 1
and Figure 1. Although transformers actually cause power losses, they are minimal in large transformers. It
could be argued that large transformers actually help to save power.
This effect also makes it more expedient to deploy a few large generators rather than a greater number of
smaller ones. Larger generators are significantly more efficient than smaller generators. However, since
generators also have to produce excitation power and suffer from mechanical losses, their efficiencies are
substantially lower than a transformer of equivalent size. The reduction in power loss that comes from
choosing a large generator rather than several smaller ones is larger than the losses that are incurred because
of the need to use four or five voltage transformation stages (see Figure 4).

Page 5
Figure 4 – Transformers in a public power supply network:
Yellow: sub-station transformer
Red: generator transformer
Blue: grid-coupling transformer
Green: distribution transformer.
Figure 5 – Three transformer stages are also used in railway traction power systems to step the voltage down
from the generator voltage to that required to drive the motors.
0.4kV
20kV
10kV
380kV
220kV
110kV
50 Hz50 Hz
3~3~
Transmission grids
Distribution networks
Structure of Public Electricity Supply in Germany
27 kV, nuke
21 kV, e. g. coal
10 kV, e. g. hydro
0.5 kV, e. g. wind
15kV
110kV
161622
//33 HzHz
1~1~
1.5kV
21 kV, e. g. coal
10 kV, e. g. hydro
Structure of Railway Electricity Supply in Germany

Page 6
THE DESIGN AND MANUFACTURING OF CONVENTIONAL AND OF SPECIAL
PURPOSE TRANSFORMERS
There is a common conception that the refining of transformer design has been exhausted and as a result just
a bit dull. Not true. There is, in fact, a great deal more to these so-called passive devices than meets the eye.
While transformers may be simple in principle, designing and optimizing them for specific applications requires
a great deal of detailed expertise and considerable experience. Without such knowledge and experience, it
would not be possible to create the transformers we see with efficiencies of up to 99.75%. Even if you are not
responsible for designing or building a transforming, purchasing the right transformer for a specific application
still requires a solid understanding of transformer fundamentals and transformer characteristics.
TRANSFORMER TANK AND OIL
The oil-immersed transformer is the most common type of distribution transformer. There are approximately
2 million oil-immersed distribution transformers in service in the EU with power ratings up to 250 kVA. There
are a further 1.6 million rated above 250 kVA. There are also estimated to be about 400,000 cast-resin
transformers in use.
Figure 6 – Structure of a modern oil-immersed transformer.

Page 7
Figure 7 – The interior of the distribution transformer (here a museum exhibit) exposed to view. (Stadtwerke
Hannover)
Figure 8 – At one time, the yoke frames were made of wood. The winding taps and terminal leads are clearly
visible.

Page 8
Figure 9 – Manufacturing a wide copper foil winding. (Wieland Werke AG, Ulm)
The most widespread design found today is the hermetically sealed transformer with flexible corrugated walls
that deform to compensate for the thermal expansion of the oil. These transformers do not need an expansion
tank with a dehydrating breather. Nor do they require all of the maintenance procedures that need to be
performed on large transformers with attached radiators. Most of today’s distribution transformers remain
maintenance-free for the duration of their scheduled service life of 20 to 30 years. There are numerous cases
of units in service for 30 to 40 years. With service lives that span decades rather than years, many older
transformers no longer comply with current technical requirements. As a result, transformers that are
technically outdated but not actually defective (Fig. 6) tend to be left in service (Figure 7).
The oil serves both as a cooling and electrical insulating agent. Flashover distances (clearances) and creep
paths can be reduced to about one fifth of their values in air. Moreover, the active portion of the transformer
(i.e. the pre-assembled core-and-coil unit) has a relatively small area requiring cooling. Heat transfer from a
core-and-coil assembly to a liquid medium is approximately 20 times better than to air. The surface of the
corrugated tank (Figure 17), in contrast to that of the active section, can be enlarged as required to ensure an
adequate rate of heat transfer to the ambient air. Oil-immersed transformers are therefore more compact
than air-cooled designs.
CORE
In spite of the fact that the manufacture of transformers is a highly labour-intensive process, materials used in
both the core and the coils contribute significantly to the cost of a power transformer. Selecting the right sheet
steel for the laminations, accurate stacking with frequent staggering (every two sheets), and minimization of
the residual air gap are all key parameters in reducing open-circuit currents and no-load losses. Today,
practically all core laminations are made from cold-rolled, grain-oriented steel sheet despite the significantly
higher cost of this type of steel. Note that the thinner the laminations, the lower the eddy currents.

Page 9
Table 2 – Historical development of core sheet steels.
It is worthwhile mentioning the revolutionary technology of amorphous steel here, which further reduces the
no-load losses (more on this technology at the end of this publication). Minimizing noise levels requires
application of the right amount of pressure to the yoke frame that holds the yoke laminations in place (Figure
10, Figure 22). Applying the greatest possible pressure is not necessarily the best approach. One of the key
aspects in core construction is ensuring the absence of eddy current loops. Even in small transformers with
ratings above about 1 kVA (depending on the manufacturer), the clamping bolts are electrically insulated on
one side (see Figure 10 and Figure 11) for this reason. These benefits would also be apparent in transformers
with power ratings below 100 VA. Given the advantages that insulated fastening bolts can yield in relatively
small transformers, the benefits gained in much larger distribution and high-power transformers is obvious.
An interesting real-life case in which a transformer was earthed twice via its yoke clamping bolts illustrates just
how important it is to take these apparently innocuous elements into consideration. The transformer was
fitted with an earth conductor on the high voltage side that ran from one of the yoke clamping bolts to the
earthing system; a similar earth conductor was installed on the low-voltage side. However, the technical
expert examining the transformer discovered a current of 8 A in each of the earth conductors. The two
conductors formed a current loop that was short-circuiting the insulation of the bolt. It was only because the
engineer had a detector for magnetic leakage fields that he was able to discover the current in the loop.
Figure 10 – A small three-phase transformer has a very similar structure to a distribution transformer. While
these small transformers do not generally need to be equipped with round coils...
Year Material
Thick-
ness
Loss
(50Hz)
at flux
density
1895 Iron wire 6.00W/kg 1.0T
1910 Warm rolled FeSi sheet 0.35mm 2.00W/kg 1.5T
1950 Cold rolled, grain oriented 0.35mm 1.00W/kg 1.5T
1970 Cold rolled HiB sheet 0.30mm 0.80W/kg 1.5T
1975 Amorphous iron 0.03mm 0.20W/kg 1.3T
1980 Cold rolled HiB sheet 0.23mm 0.70W/kg 1.5T
1983 Laser treated HiB sheet 0.23mm 0.60W/kg 1.5T
1987 Plasma treated HiB sheet 0.23mm 0.60W/kg 1.5T
1991 Chemically etched HiB sheet 0.23mm 0.60W/kg 1.5T

Page 10
Figure 11 – …the yoke frames are similar in shape to those used in distribution transformers and the single-side
insulation for the yoke clamping bolts is essential.
This would not have been a problem when yoke frames were still being manufactured from wood (Figure 8),
were it not for the fact that the frame (whether made of wood, or steel as is the case today) and the yoke
laminations are frequently drilled (or punched) to accept the clamping bolts. These holes have to be large
enough so that an insulating bushing can be pushed over the shaft of the bolt to ensure that the bolt does not
come into contact with the burred edges of the yoke plates and only touches one side of the yoke frame. If
multiple contact points occur, it essentially short-circuits the relevant section of the yoke. In addition, cutting
bolt holes effectively reduces the cross-sectional area of the core, and eddy currents are also induced in the
bolt, which, for obvious reasons, cannot be manufactured from laminated sheet. Clamping bolts made of
stainless steel are sometimes chosen. This is because, perhaps surprisingly, stainless steel is not in fact
ferromagnetic although it consists predominantly of iron and nickel—both ferromagnetic elements. The
magnitude of the magnetic field in these stainless steel bolts is therefore lower, thus reducing eddy current
losses. In addition, stainless steel is much better at suppressing eddy currents because its electrical
conductivity is only about one seventh of that of conventional steels. However, stainless steel bolts can in no
way replace the sheet iron that was removed when punching the bolt holes, which is to some extent possible
when conventional steel bolts are used. These two effects can be illustrated in the following experiment
performed on a small transformer (Figure 12 and Figure 13).
Transformers of this size are typically not fitted with insulating flanged bushings. Inserting the bolts results in a
reduction in the magnetizing reactive power of up to 7%. This is because the bolts are to some extent able to
replace the sheet iron lost through the creation of the bolt holes. However, no-load losses increase by 20%
partly as a result of eddy currents in the bolts, but, primarily, because of the earth loops created when the
bolts are inserted.
A better means of clamping the yoke laminations, though more costly than employing stainless steel bolts, is
to use a clamping frame that wraps around the yoke (Figure 22). However, it is essential to ensure that the
clamping ring does not form a closed electrical circuit that could short-circuit the yoke. An experimental set-up
using a small single-phase transformer demonstrates the potential consequences of an electrically closed
clamping ring (Figure 14 and Figure 15).

Page 11
Figure 12 – Unfortunately, a less stringent approach is taken in the case of single-phase transformers.
Figure 13 – The sheet metal casing and fixing screws slightly reduce the magnetizing reactive power, but the
no-load active power is significantly greater.
Figure 14 – Not the most intelligent fastenings for a small transformer—the no-load power increases to more
than six times that measured without the fastening clamps in place. (Figure 15)

Page 12
Figure 15 – The no-load active power measured for the same transformer without the fastening clamps.
Figure 16 – A Swiss tubular tank transformer (photo: Rauscher & Stoecklin) 1958. This type of transformer is
still being widely built in newly industrialized countries where labour costs are not an issue.

Page 13
Figure 17 – The oil-immersed transformer has been the standard since about 1930. The typical corrugated tank
design was introduced around 1965. (Photo: Pauwels)
Figure 18 – A typical, commercially available cast-resin transformer.
Eddy currents can also be induced in electrically conducting parts that are not actually located within the
transformer core but simply situated in its immediate vicinity. This is particularly relevant in the case of
ferromagnetic materials that attract stray magnetic fields. In larger transformers, the insides of the tank are
sometimes fitted with so-called flux traps made from core sheet steel that attract stray magnetic fields and
through which the field flux lines will preferentially flow rather than through solid, non-laminated, structural
steel parts. In some dry-type transformers, the clamping bolts (Figure 20 and Figure 22) and other screws are
made from glass-cloth laminate. In oil-immersed transformers, one occasionally finds nuts and bolts made
from a moulded synthetic resin/compressed wood compound, but this material is of insufficient strength to be
used for coil clamping bolts.

Page 14
WINDINGS
In distribution transformers, the low-voltage coil is usually foil-wound because of the low number of windings
and the high cross-sectional area of the conductor. The length of the finished coil is approximately equal to the
width of the foil (Figure 9).
Several strip-wound coils arranged adjacently in the axial direction can be used for smaller sized transformers
or when higher voltages are involved. The high voltage coil is also usually constructed in this way. Round wire
windings are used in smaller transformers; shaped wire windings are used in larger devices.
Figure 19 – In small transformers, such as the 40 kVA device shown here, the coil windings can be
approximately rectangular in section reflecting the rectangular geometry of the core. (Photo: Riedel)
Figure 20 – The upper yoke frame and the coils clamped tightly in the axial direction. (Photo: Rauscher &
Stoecklin)
Figure 21 – In larger transformers, the rectangular core is adapted to more or less match the circular
geometrical form of the coil.

Page 15
The coils in small transformers are rectangular in section (Figure 10 and Figure 11). The same type of coil
geometry is sometimes found in special types of low-rating distribution transformers. Elliptical coils are used in
larger transformers and circular-section coils are used in transformers with the highest power ratings
(generally 1 MVA and higher). In addition, if the coils were not circular before, they certainly are if they ever
suffer a short circuit. Such a change in coil geometry is the result of the magnetic forces acting between the
conductors. These forces play no role at the transformer’s nominal current density, but increase
proportionately with the product of the currents in the low-voltage and high voltage windings.
Since the currents in the LV and HV coils flow in opposite directions (Figure 21), the coils repel each other. If a
short circuit does occur and the winding current is correspondingly large, the outer coil will try to expand
outward and, since the circle is the geometrical form that encloses the greatest possible area for a given
circumference, the coil will seek to adopt a circular shape. Such a shape offers the maximum average distance
from the inner coil.
The inner coil, which is usually the low-voltage winding, will be pressed against the core. Since the low-voltage
coil is typically a copper foil winding, a short circuit will often result in a core that looks as if it has been copper
clad. This is the reason why the coils are very tightly clamped in the axial direction (Figure 20). It is also why
any taps in the high voltage winding, which allow for any variation in the input voltage (typically two steps of
+2.5% above the nominal voltage, and two steps of -2.5% below), are located in the central section of the
winding (Figure 22) and not at its upper or lower ends. This ensures that the effective axial height of that
portion of the high voltage coil that carries current is essentially constant as is the relative height of the HV and
LV coils.
Without the tight clamping, a number of windings at the upper or lower end of the coil will be lost if a short
circuit causes a significant force in the axial direction between the coils. In transformers that have been in
service for a long time, the coils may no longer be as rigidly clamped as they were at the time of manufacture
and the insulating materials may be showing signs of age. A short circuit in such a transformer or a breakdown
of the insulation material because of a lightning strike often causes the device to fail completely. At
installations where short circuits or lightning strikes only occur every few decades, a transformer can remain
operational for as long as 60 years before finally having to be replaced for economic reasons.
The rectangular core is altered to approximately match the geometry of the circular coils as shown in Figure
21. The yokes have exactly the same cross-sectional area. Five-leg cores are normally only used in high-power
transformers since this allows the cross-sectional area of the yoke to be halved. This slightly reduces the total
height of the transformer, making transport somewhat easier. Looked at mathematically, the five-leg core has
only four legs (three + two half-legs), because the two outermost return legs only need to carry half of the flux
in this type of core. We will take a look at the special case of a five-leg core in a distribution transformer later
on.
The structure of the transformer’s active part can be seen in Figure 22 and Figure 23, though in these diagrams
the active portion is not depicted large enough to illustrate the staggering of the core laminations. This detail
has therefore been shown in the magnified image on the right in Figure 23. Normally every two, sometimes
every four, laminations are staggered by, for example, 15 mm relative to the previous two or four core
laminations. The yoke laminations are staggered to the left and to the right, while in the legs, the laminations
are displaced upward and downward. In addition, the upper and lower asymmetrical tips of the central leg
differ in that one tip is located more to the left and the other more to the right. Offsetting the joints in this way
improves magnetic contact between the abutting surfaces.

Page 16
Figure 22 – The structure of a transformer’s core-and-coil assembly (active part). The design shown here is the
more elegant solution with unperforated yokes.
Figure 23 – The yoke frame and coil clamping bolts have been removed and the upper yoke lifted off to expose
the inner structure.
SPECIAL TYPES OF TRANSFORMERS
The quality and performance specifications that transformer oil has to fulfil are extremely high. The oil in a
hermetically sealed transformer tank has to provide forty or more years of service and it generally cannot be
subjected to tests during that time. Irrespective of its quality however, the mineral oils used in transformers
are of course combustible. It was for this reason that several decades ago oil-immersed transformers were
forbidden for use in interior locations and sites subject to high risk in the event of a fire. Mineral cooling oil
was replaced in such locations by polychlorinated biphenyls (PCBs), a group of substances that are classified as
non-combustible or nearly non-combustible. Unfortunately it was subsequently realized, especially in the wake
of the Seveso disaster, that these substances form highly toxic dioxins when partially oxidized.
The search for alternatives led to the use of low-flammability, non-toxic, synthetic silicone oils. However, those
silicone oils never really became established, at least not for the size of transformer being discussed here. As a
result, dry-type transformers enjoyed a revival. The new models no longer used paper and varnish for
Yoke frames
Coil clamping
bolts
Yoke
lamination
retaining strap
Wooden coil-
clamping
blocks
HV coil
Yoke
lamination
clamping bolts
LV coil
Tapping points

Page 17
insulation, but were manufactured as cast-resin transformers. Depending on the required degree of
protection, these cast-resin transformers can be used unenclosed or with the appropriate protective
enclosure.
As with other types of transformers, the conductor materials used in distribution transformers can be either
copper or aluminium. Though more expensive, copper is usually chosen because it enables more compact (as
well as more robust) designs. Because the volume of conductor material is less if copper is used, the volume of
the winding space is correspondingly smaller. This results in a somewhat heavier but slightly smaller device.
Aluminium, however, is the preferred material in cast-resin transformers because its greater thermal
expansion coefficient is closer to the generally very high expansion coefficient exhibited by organic materials,
and this helps to reduce the thermal stresses within the rigid winding assembly.
One very special type of transformer was developed in 1987: gas-cooled transformers. They had in fact already
been the subject of research some 25 years earlier. When gas cooling is involved, physicists tend to think
immediately of hydrogen as it has a very high heat capacity. However, heat capacity is generally expressed
relative to mass, and the density (i.e. the mass per unit volume) of hydrogen is almost one tenth of that of air.
If on the other hand, the key parameter is the speed of circulation in a cooling circuit, then heat capacity per
volume is more relevant since the resistance to flow is proportional to the square of the volume flow in any
given system. The gaseous material finally selected was sulphur hexafluoride (SF6), a well-known substance
that was already in use as an insulating material in switchgear and that has a density five times that of air and
with considerably better dielectric strength.
Although the heat capacity of a kilogram of SF6 is only half that of a kilogram of air, its heat capacity per litre is
2.5 times greater. That means that if SF6 is used as the coolant, it only needs to circulate at 40% of the speed
used in air-cooled devices in order to produce the same cooling effect. As a result, the fan power can be
reduced to about 32% of that needed in an equivalent forced-air cooling system. Two prototype transformers
each with a power rating of 2 MVA, a corrugated tank, and internal forced cooling (i.e. cooling Class GFAN—
gas-forced, air natural) were built and successfully operated in an explosion hazard area within a chemical
manufacturing plant.
The dielectric strength and the cooling capacity of SF6 can be increased by raising the pressure and
compressing the gas. A hand-welded tubular tank transformer, similar to the one shown in Figure 16, was built
to test this effect. This type of transformer design used to be common but its construction is far too labour-
intensive for it to be economical today. Nevertheless, the test device allowed the test engineers to
demonstrate that the observed temperature rise agreed approximately with that expected from
computational analysis. The transformer with GNAN cooling handled 630 kVA at an overpressure of 3 bar and
was of an acceptable size. Having completed these trails, the project team set about developing a more
economical method of production. Apparently, these transformers sell well in the Far East, or at least sold well
for a time, where they were used in high-rise buildings. Widespread use in the domestic market failed because
of the very stringent regulations governing the construction and use of pressure vessels. The principle behind
the technology had, however, been shown to work.

Page 18
OPERATIONAL BEHAVIOUR
Transformers inevitably affect the power networks to which they are connected. However, to a certain extent
some of the operating parameters of a transformer can have a beneficial—and in some cases even essential—
influence on the operation of the supply network. In what follows, we will be investigating how to
manufacture and select transformers to optimize these parameters.
In a distribution transformer with a short circuit voltage of 6% that is operating at its rated current, there will
be a drop of 6% in the voltage across the device’s internal impedances. That means that when the transformer
is operating at its rated load, the voltage is 6% lower than the open-circuit voltage. There are additional
voltage drops along the wires and cables that lead away from the transformer as well as in the upstream
power supply network. In total, it is reasonable to expect voltage losses totalling approximately 10%. While
10% may sound excessive, the situation is not as bad as it appears. To see why requires a precise definition of
the term short circuit voltage.
SHORT CIRCUIT VOLTAGE
The characterization of the operating behaviour of a transformer relies on its rated voltage and its rated power
output. The next most important parameter is the short circuit voltage. To anyone training to become an
electrical technician, the expression short circuit voltage might initially appear to be a misnomer. After all,
when a short circuit occurs, the voltage is generally defined as being zero. However, this is not the case when
the short circuit is on the output side and the voltage is on the input side of a transformer. The short circuit
voltage (Usc) is the voltage applied to the primary winding in the event that:
1) The secondary winding of the transformer is short-circuited
2) The voltage applied to the primary winding is large enough to generate the rated current in the
secondary winding
The short circuit voltage is usually not expressed in volts but rather as a percentage of the rated voltage (usc).
In large transformers, the short circuit voltage can reach values of 18-22%. In contrast, the rated short circuit
voltages in distribution transformers are typically between 4% and 6%. The actual value is measured during the
final testing of the device and is printed on the transformer's rating plate (Figure 24). However, deviations
from the typical values of 4% to 6% usually have little practical relevance.
Figure 24 – Whether it is called Tension court circuit, Kortsluitspanning or Short circuit voltage, the actual value
is measured during final testing and is printed on the transformer’s rating plate.

Page 19
The short circuit voltage thus characterizes the voltage drop within the transformer. If the short circuit voltage
is known, the short circuit current Isc can be readily calculated. For example, if all upstream impedances are
ignored, the short circuit current in a transformer with usc = 6% is:
𝐼𝑆𝐶 =
𝐼 𝑁
𝑢 𝑠𝑐
=
𝐼 𝑁
6%
≈ 16.7 𝐼 𝑁
For a transformer with usc = 6%, six percent of the rated input voltage is therefore needed to generate the
rated (nominal) current IN in the short-circuited secondary winding. However, only a small part (uR) of the
voltage drop in a medium-sized distribution transformer is due to the ohmic resistances in the windings. The
largest factor contributing to the voltage drop is by far the reactive/inductive voltage drop uX. This stems from
the leakage inductance caused by the portion of the magnetic flux that bypasses the core (leakage flux) and
permeates only a single winding. This leakage flux does not flow in the primary and secondary windings
simultaneously. Rather it flows in the main leakage channel between the high voltage winding, which is
generally located on the outside, and the low-voltage winding on the inside [c.f. Section 2 Design]. The leakage
flux is therefore part of the magnetic flux of the outer but not the inner winding. This also has the incidental
effect that the short circuit voltage cannot be influenced by the non-linearity of the iron core. uX is also
referred to as the leakage reactance voltage. The principal function of the main leakage channel is cooling; its
secondary function is insulation. In addition, it also serves to maintain the so-called leakage reactance, which is
in effect a defined short circuit voltage.
As shown in Figure 28, the sum of the squares of the inductive voltage drop uX and the ohmic voltage drop uR
equals the square of the overall voltage drop usc (c.f. Pythagoras’ theorem concerning the sides of a right-
angled triangle). Fortunately, the ohmic voltage drop uR is, as already mentioned, the smaller portion. The
larger the transformer, the smaller this is. A simple calculation proves this point: If a transformer in the 630
kVA range has an efficiency of 98.5% when operating at its rated load, then the total ohmic voltage drop across
the two windings can be no more than 1.5% of the rated voltage. In practice however, the value is lower, for
example 1%, because the 1.5% includes losses other than the ohmic losses in the coils. Our usc of 6% is
therefore made up of uR = 1% and uX = 5.91% of the rated voltage (6² =1² + 5.91²).
RESISTIVE LOAD
A transformer’s power rating is always specified relative to its resistive load. The ohmic resistance of the
winding contributes linearly to the (rated) load, while the inductive resistance (reactance) of the leakage
inductance contributes quadratically. Therefore the resistance of the winding contributes only 1% to the load
resistance, with the remaining 5.91% having only a negligible effect on the total voltage drop across the
transformer and the load. We now want to determine precisely how small this effect actually is.
The non-ideal behaviour of a transformer can be illustrated by an equivalent circuit model (Figure 25). The
model assumes that the input and output windings have the same number of turns. As this is obviously not
usually the case, the values associated with one side of the transformer are moved (i.e. referred) to the other
side by multiplying them by the ratio of the number of turns on the two windings. The behaviour of the
transformer can then be calculated for the relevant reference side. In Figure 25, all elements have been
referenced to the load (i.e. secondary) side.

Page 20
Figure 25 – General single-phase equivalent circuits for a two-winding transformer.
If we are only interested in the modulus (absolute magnitude) of the voltage drop at the rated load, which by
definition is an ohmic load, then we can adopt the simplified expression:
𝑈2 = 𝑈1
′
− 𝐼𝑙𝑎𝑠𝑡 (𝑅 𝐶𝑢1
′
+ 𝑅 𝐶𝑢2 +
(𝑋1𝜎
′
+ 𝑋2𝜎)2
𝑅 𝐿𝑎𝑠𝑡
)
where:
U2 = secondary voltage,
𝑈1
′
=
𝑛2
𝑛1
𝑈1 = primary voltage referred to the secondary side,
𝑅 𝐶𝑢1
′
= (
𝑛2
𝑛1
)
2
𝑅 𝐶𝑢1 = resistance of primary winding referred to the secondary side,
RCu2 = resistance of secondary winding,
𝑋𝑙𝜎
′
= (
𝑛2
𝑛1
)
2
𝑋𝑙𝜎 = leakage reactance of primary winding referred to the secondary side,
X2 = leakage reactance of secondary winding,
n1 = number of turns in primary winding,
n2 = number of turns in secondary winding.
ILoad and RLoad are related to one another in accordance with Ohm’s law:
𝑅𝑙𝑜𝑎𝑑 =
𝑈2
𝐼𝑙𝑜𝑎𝑑
and cannot therefore be changed independently of one another.
Load
X1‘ X2
RFe
Xm
RCu1‘ RCu2
Load
X1‘ X2
RFe
Xm
RCu1‘ RCu2
Load
X1‘ X2
RFe
Xm
RCu1‘ RCu2

Page 21
Figure 26 – No-load currents in a high-quality Swiss distribution transformer with a power rating of 630 kVA to
which a voltage of 400 V has been applied to the low-voltage side. The currents are relative to the rated output
current of 909 A.
The parameters
RFe = core resistance
XH = magnetizing reactance
have not been taken into account in this simplified model. With the exception of small transformers, the
magnitudes of these quantities are usually so large that the currents flowing through these elements are
insignificant, at least as far as the effect of the transformer on connected loads is concerned. (Their relevance
for the transformer’s internal losses is far greater, as will be shown in Section 4). XM is the magnetizing
reactance under open-circuit conditions (in this case, XM is referred to the secondary voltage, as if the
excitation voltage was being applied to the output side of the transformer, which is perfectly possible, and was
indeed the case when making the measurements for Figure 26.) The no-load current in a good-quality
distribution transformer is only around 0.5% of the rated current (Figure 26) and more than half of the no-load
current is attributable to the magnetization current. Consequently, as the magnetizing reactance XM is the
main cause of the open-circuit current, its magnitude must be at least 200 times that of the load impedance.
RFe is a fictitious resistance that represents the iron (or core) losses and whose magnitude, if good-quality iron
is used, is generally substantially greater than XM. The shunt impedance of these two elements that determine
the open-circuit behaviour of the transformer is therefore significantly more than 100 times greater than the
load impedance. In contrast, for a transformer with a short circuit current of 6%, the short circuit impedance
(i.e. the effective sum of X1σ’, X2σ, RCu1’ and RCu2), which are all in series with the load, is only 0.06 times as large
as the load impedance. The short circuit impedance is therefore more than 100/0.06 (i.e. almost 2,000) times
smaller than the shunt impedance comprising RFe and XM. As a result, the currents flowing through RFe and XM
can be ignored when describing the transformer’s behaviour with respect to its connected load. This is even
more the case when the transformer is under short circuit conditions. The expression for the leakage
reactance can therefore be simplified to:
𝑋 𝜎 = 𝑋1𝜎
′
+ 𝑋2𝜎 = 2𝑋2𝜎

Page 22
A similar expression applies for the resistive components of the short circuit impedance.
𝑅 𝐾 = 𝑅1
′
+ 𝑅2 ≈ 2𝑅2
Figure 27 – Phasor diagram showing the voltage drops in a transformer and its rated (ohmic) load.
Figure 28 – Diagram showing the voltage drops in the transformer itself, shown here at a magnification of
about ten times that in Figure 27, Figure 29, and Figure 30.
InputvoltageU1referredtothesecondaryside(100%)
Inductive drop uX in the transformer
Ohmic drop
uR in the
transformer
Total voltage
drop usc in the
transformer
(e.g. 6%)
InductivedropuXinthetransformer
Ohmic drop uR in the transformer

Page 23
This approach provides a simple means of describing the operation of a non-ideal transformer and enables
quantities of interest to be calculated with sufficient accuracy. Although Figure 25 does not model the
substantial non-linearity of the iron core and the resulting strong distortion of the magnetizing current
waveform, the fact that the magnetizing current is so small means that this equivalent circuit model is
applicable in practice. In fact, the magnetizing current can usually be ignored when compared to the rated
current, as was assumed above. In that case, the voltage drops found in a transformer driving a load are as
shown in Figure 27. The phasor diagram can be thought of as an instantaneous snapshot. The individual
voltages are represented as vectors that precess around the fixed origin (below), in a manner analogous to the
generation of an AC voltage in rotating machines. The input voltage (green) serves as the reference and is
always shown as its peak positive value, i.e. as a vertically aligned voltage vector. The vector sum of all the
voltage drops must equal the applied voltage. Graphically, this means that if the voltage drop vectors are
placed in sequence starting at the origin (base of the green vector), they must arrive at the same point as the
tip of the green vector representing the applied voltage. It then becomes apparent that in this transformer
with a short circuit voltage drop (usc) of 6% and an ohmic voltage drop (uR) of 1%, the modulus of the output
voltage at rated load is almost 99% (and not approximately 94%) of the open circuit voltage. The only
difference is the phase shift on the output side relative to that of the input voltage, but that has no effect on
the load.
The voltage drops within the transformer are barely discernible in the diagram. The resistive (ohmic)
component in particular is almost inconspicuous. This is good, since this component represents the ohmic
losses. If one wants to visualize these voltage drops, they need to be magnified and shown without the input
and output voltage vectors (see Figure 28).
Figure 29 – Inductive load.
Voltagedropacrossload(99%)
Inductive drop uX in the transformer
Ohmic drop
uR in the
transformer
Total voltage
drop usc in the
transformer
(e.g. 6%)

Page 24
Figure 30 – Capacitive load.
INDUCTIVE LOAD
The situation changes, however, if we are dealing with an inductive load. This adds linearly (i.e. completely) to
the inductive voltage drop within the transformer and quadratically to the much smaller ohmic component. In
this case, the output voltage is calculated by means of the following equation:
𝑈2 = 𝑈1
′
− 𝐼𝑙𝑜𝑎𝑑 (𝑋1𝜎
′
+ 𝑋2𝜎 +
(𝑅 𝐶𝑢1
′
+ 𝑅 𝐶𝑢2)2
𝑅𝑙𝑜𝑎𝑑
)
This can be rewritten using the simplifications introduced earlier:
𝑈2 = 𝑈20 − 𝐼𝑙𝑎𝑜𝑑 (𝑋 𝜎 +
𝑅 𝐶𝑢
2
𝑅𝑙𝑜𝑎𝑑
)
where U20 is the no-load voltage on the low-voltage side. When the rated current is flowing, the output voltage
does indeed drop by almost 6%, at least as far as the absolute value (magnitude) of the voltage is concerned.
This is because the rated load is defined as an ohmic load (see Figure 25). By introducing power-factor
compensation in parallel with the load, we can reduce the overall load being driven by the transformer. This
not only reduces the copper loss, it also reduces the size of the voltage drop, since this is predominantly
inductive in nature. If we are dealing with an alternating inductive load, the compensation circuit must be
controllable. The more rapidly the load changes, the faster the controller must be able to respond. These
control systems can be used to avoid the flicker that is caused by rapidly changing, strongly inductive loads
such as spot welding machines or three-phase induction motors (without a power converter). Anyone who has
Voltagedropacrossload(99%)
Inductive
drop uX in
the
transformer
Ohmic drop
uR in the
transformer
Total voltage drop usc in the
transformer (e.g. 6%)

Page 25
lived for a time next to a building site where a tower crane has been operating (without power-factor
correction) will know exactly what this refers to
1
. Luckily, mobile units are now available that can be used to
counteract such flicker sources
2
.
The same approach can be used to determine whether the primary cause of the voltage drop at a domestic
power socket is the leakage inductance of the distribution transformer or the ohmic resistance of the power
cables. For instance, when a fan heater with a current input of 9 A is operating, the voltage at a particular
power socket is found to decrease from 230.7 V to 226.3 V. If a large inductive load that draws over four times
as much current (see Figure 27) is connected, we see that there is no discernible increase in the voltage drop.
With reference to the phasor diagrams shown in Section 3.2, it can be concluded that the ohmic resistance is
by far the larger component—at least in the case of this particular cable and transformer. However, this test
must be performed with great caution. Since this test involves a substantial overload, the duration of the test
must be kept short. Failure to do this will trigger the 16-ampere circuit breaker, which is normally rated for a
power factor range from 1 to 0.6. A circuit breaker is not really the best means of switching off what is
essentially a purely inductive load.
Figure 31 – Large inductive load on a domestic power socket.
CAPACITIVE LOAD – CARE REQUIRED!
Things get really exciting when dealing with capacitive loads. In principle, this is the situation described earlier
in which the transformer is excited from the output side, i.e. compensation of the inductive magnetizing
reactive power by the capacitive load. If the load is so small that it is just sufficient to provide compensation,
then only the transformer will be excited—a perfectly normal situation, irrespective of the winding. The
picture changes however, as the capacitive load increases. The passage of the load current through the
leakage reactance results in a voltage drop with a phase lead of 90°. Its passage through the capacitive load is
accompanied by a voltage drop lagging by 90°. In other words, these two voltage drops have opposite signs
and therefore subtract from one another rather than add. At 100% of the rated input voltage, we have a
1
Fassbinder, Stefan: Netzstörungen durch passive und aktive Bauelemente, VDE Verlag, Berlin / Offenbach
2002, p. 188
2
Bolliger, R.: Wenn die Lichter flackern [‘When the lights flicker’]. ET Schweizer Zeitschrift für angewandte
Elektrotechnik 11/2003, p. 26

Page 26
voltage drop of almost 6% across the leakage reactance and therefore almost 106% of the open circuit voltage
across the load. The result is an overvoltage on the output side at full load current, despite applying only the
rated voltage on the input side (Figure 30). As the load—or more precisely its impedance—must be assumed
to be constant, the current rises in accordance with the increased voltage, creating an even greater voltage
drop within the transformer and hence an even greater voltage at the output. Ultimately, the voltage stabilizes
at a level slightly above rather than just below 106%. This was exactly what was observed when a medium-
voltage customer of an electric utility company wanted to have a power factor correction system of 1,400 kVAr
connected on the MV side, because that was the side where active and reactive power were metered. It
turned out, however, to be cheaper to buy a low-voltage power factor correction unit in combination with a
1,600 kVA transformer. Note that although this solution was cheaper, it was not more cost-effective, as the
method saves only the additional price that the electric utility company would have charged for the reactive
power, but not the costs generated by letting this reactive current circulate around the installation and
through the transformer.
A test run on the customer’s system mentioned above resulted in the phase voltage increasing from 230 to
255 V. This was because the transformer was driving a capacitive load only and was working more or less at
full load. In this particular case, the problem was solved by reconnecting the input side of the transformer at
an input voltage of 22 kV, even though only 20 kV were actually being applied. This enabled the output voltage
to be lowered to near its rated value. Since the excitation current was coming from the output side, this
proved to be the only way to prevent over-excitation of the transformer. This is one of the rare cases in which
the output side is the reference side.
If overloading is severe, the situation will eventually escalate. Current overload creates a voltage overload in
both the load and in the transformer. Overvoltage across the load then generates further current overloading,
which will drive the overvoltage even higher. Taken together, the leakage reactance and the capacitive load
form an LC oscillator circuit, whose resonance frequency f0 can be calculated as follows:
𝑓0 =
1
2𝜋√𝐿𝐶
At this frequency, the inductive leakage reactance and the capacitive reactance of the load are of equal
magnitude but opposite sign and thus cancel each other out. This is not, however, the case when the
transformer is passing its rated current. In the example transformer discussed earlier (for which usc = 6%), the
reactance of the load was about 16 times greater than that of the internal reactance of the transformer. The
resonance frequency is then:
𝑓0 = 50 𝐻𝑧 ∗ √16 = 200 𝐻𝑧

Page 27
Figure 32 – The output current of a transformer with uR = 1% and usc = 6% (for which Isc ≈ 16.7 * IN) can in
theory climb to 100 * IN at a capacitive overload of 16.7 times the rated load, rather than just the 16.7 * IN that
would arise in a short circuit.
Figure 33 – Detail from Figure 32 (see box bottom left) with the current values typically found in practice.
However, if a ripple-control signal of similar frequency is present, the ripple-control signals through the series
resonant circuit comprising the transformer’s leakage reactance and the capacitive load, will be shorted and
lost, and therefore fail to reach the low-voltage level. In a transformer with a short circuit voltage usc = 4%, the
critical point is shifted to 224 Hz. If we were dealing with capacitive loads that could be varied (e.g. by means
of a VAr controller), the critical point would also vary in a potentially uncontrollable manner.
If the transformer is subjected to a capacitive overload of 16 times the transformers rated load, which can be
achieved, say, by connecting a 1,600 kVAr power-factor correction unit to a 100 kVA transformer, we find that
the resonant frequency drops to 50 Hz. In this case, the inductive leakage reactance would be compensated
for by the capacitive reactance of the load. Since the current would be limited only by the resistance of the
0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
RelativeloadcurrentI/IN
Relative load admittance Y/YN 
Current with resistive load
Current with inductive load
Current with capacitive load

Rated load
Short-circuit current
ISC=16.7*IN

16.7*rated load (ZSC=ZLoad or YSC=YLoad, resp.) 

Page 28
winding, it would rise to almost six times the short circuit current or 100 times the rated current (see Figure
32). In Figure 32, the actual current is plotted against the ratio of the magnitude of the conductance G to the
rated conductance GN:
𝐺 𝑁 =
𝐼 𝑁
𝑈 𝑁
depending on the phase angle of the ohmic, inductive, or capacitive load. For
𝐺 =
𝐺 𝑁
6%
≈ 16.7 𝐺 𝑁
the impedance of the load is equal to the internal impedance of the transformer. However, that should not be
interpreted to mean that the output voltage always corresponds to half the open-circuit voltage. If that were
the case, one would not need to include the phase angle in the calculations. The curves for the ohmic,
inductive and capacitive loads will only converge at infinite conductance (which corresponds to short circuit
conditions).
Fortunately, capacitive loads of that magnitude do not occur in practice. Nevertheless, this thought
experiment shows just how rapidly things change when the capacitive load starts to increase. A more detailed
look at the part of Figure 32 that covers realistic operating currents (see Figure 33) confirms remarks made
earlier: the transformer’s output voltage is 99% of the open-circuit voltage for the ohmic rated load. If the
transformer drives an inductive load of equal size, the output voltage drops to 94%. If the load is capacitive in
nature (and of equal magnitude), the voltage is about 107%. Care is therefore paramount when dealing with
capacitive loads.
Note however, that the transient overloading of a transformer with a capacitive load does occur and is in fact
purposely used on occasions to eliminate voltage dips and the resulting flicker in ohmic and ohmic-inductive
impulse loads. For this to occur, the required correction capacitance has to be connected to the transformer at
the same time as the critical load. The resulting voltage rise compensates for the voltage dip that would have
been caused by the critical load alone.
Considerable care has to be taken when dimensioning such systems. Although temporarily connecting the
capacitive load to an ohmic load (e.g. a spot welding machine) causes the voltage dip to disappear, the total
load current increases and places the transformer under greater stress than would have been the case with
the critical ohmic load on its own. It is only when the flicker is generated by an inductive load, such as the
tower crane referred to in a previous example, that this type of compensation actually reduces the load on the
transformer. In both cases, it is recommended that the load and the compensation unit be connected jointly to
the transformer so that compensation is immediate and proactive rather than delayed and reactive.
VECTOR GROUPS
Distribution transformers are normally designed with the Dyn5 vector group. That means that the transformer
has a delta-connected HV winding, a star-connected LV winding, and with the star point brought out as a
neutral terminal. The input and output voltages have a relative phase-shift of 150° (5 x30°). Phase shifts are
restricted to steps of 30°, hence the index 5 to signify 150°. At least that is the case if one limits oneself to
zigzag connections in which the delta-connected and the star-connected parts of the relevant winding have
the same voltage. This transformer classification scheme simply reflects the fact that in the past the use of
other ratios would have made little sense. Today, special-purpose transformers are available that offer a 1:3
voltage ratio and therefore phase-shift steps of 15°. However, understandably, no one has attempted to
market them as, say, transformers with vector group Dyn4½. This type of transformer is used to generate

Page 29
greater than 12-pulse rectifiers such as those used to reduce harmonic emissions in electrolysis plants and in
very large converter drives.
A transformer that is star-connected on both sides, whether with neutral points (YNyn) or without (Yy), can
only have the vector group Y(N)y(n)0 or Y(N)y(n)6 as the sole variation possible in star-star transformers. Either
the three start of winding points are joined to form a neutral point and the end of windings are brought out as
phase conductor connections, or vice versa (i.e. reversing the polarity of each of the coils).
Is this in fact the sole possible variation? Strictly speaking, no. It is possible to re-label the phase conductors
and define, say, conductor L1 on the input side as L2 or L3 on the output side. This would lead to something
that could be designated as a Y(N)y(n)4 or a Y(N)y(n)8 vector group—or one could simply call it a mistake. The
mistake would become apparent only if the output sides of a correctly wired and an incorrectly wired
transformer were to be connected in parallel. In all probability such a connection would destroy the
installation. This is hardly the sort of failure detection method that one would use intentionally!
Some in the electrical industry refer to the brought-out neutral point as a PEN connection. This is incorrect. In
spite of what is sometimes assumed, the carefully insulated neutral point brought out from an oil-immersed
transformer is never actually connected to the inside of the tank. While conceivable in theory, it is unlikely that
a customer would ever want a transformer configured in such a manner. If a transformer was customized in
this way, it would no longer be possible to operate it in the modern, standardized
3
multi-feed power
distribution networks in use today.
In principle, transformers with other vector groups could conceivably be used to feed into public low-voltage
networks. However, these transformers cost more and have no advantage over the standard vector group
(Dyn5). At least that has been the case up until now. Recent developments have begun to reshape the power
engineering landscape and we will be looking at these later on.
Since medium-voltage networks do not generally have a neutral conductor, only a delta winding is feasible on
the HV side of the transformer. If the windings were star-connected, this would reduce the voltage across a
coil by a factor of 3 making it easier to control. This is precisely why all grid coupling transformers (i.e.
transformers that interconnect EHV and HV power networks) are configured with the YNyn0 vector group.
However these networks always have a neutral conductor, even if this only means that the neutral points on
each side of the transformer have been earthed. As a rule, a medium-voltage power network does not have a
neutral conductor. If a loadable neutral is needed on the output side in order to be able to tap two different
AC output voltages, then current will flow in only one LV coil generating magnetic flux and only in that one
limb under single-phase supply conditions.
The operating principle of a transformer dictates that this flux must be compensated by a corresponding
counter flux generated by the HV coil in that same limb. This can only occur however if the input voltage is
directly applied to both ends of the input winding. This is only possible in the case of a delta winding or a
connected neutral point.
The delta connection could be approximated using a parallel circuit. Likewise, the star connection without a
neutral terminal could be modelled by an equivalent series circuit (as shown in the modified equivalent circuit
diagram in Figure 34). In the case of an open neutral point on the input side and the single-phase load on the
output side, the result would be like a loaded coil in series with an unloaded coil (the unloaded coil
3
Cf. amendment of EN 50174-2 (VDE 0800 Part 174-2) from January 2002 für the September 2001 edition

Page 30
representing a reactor with a very high magnetizing reactance XM that reduces the load current). As a result,
almost all of the voltage across the quasi-series circuit drops across this unloaded coil, which excites the core
and drives the core into magnetic saturation. The consequence of this is a dramatic increase of the iron losses
and of the magnetic leakage losses.
Although the magnetizing reactance falls, it does not fall far enough for it not to cause a substantial shift in the
voltages. The voltage across the load collapses to a fraction of the rated voltage while a substantial
overvoltage is present on the unloaded output winding. If the winding is in fact not completely unloaded, but
actually feeds a relatively small load, then this load will have to cope with a continuous 3-fold overvoltage.
This is a situation that brings with it a genuine risk of damaging or destroying the load and of fire damage. This
is why Yyn0 and Yyn6 vector groups are generally not used if the only brought-out neutral point (i.e. the one
on the output side) is loaded. The input windings of distribution transformers are usually delta-connected for
this reason.
Figure 34 – Equivalent circuit representing single-phase loading of a Yyn transformer. Since the load impedance
is considerably less than XM and RFe, the total impedance of the upper circuit is much smaller than that of the
lower circuit.
This is the case, unless you happen to be dealing with a TT system of a type still frequently found in Belgium.
These TT systems are fed by a transformer with the usual Dyn5 vector group but for which the output voltage
is only 133/230 V. In this case, we could have used Yyn0 or the Yyn6 vector groups equally well, since the
neutral point on the low-voltage side is brought out, but is not connected and therefore not loaded. Its only
use is for measurement and testing procedures such as monitoring earth faults. The voltage at the AC power
socket is between the two phase conductors. Therefore the current from the power socket (which for example
in Germany would be a single-phase current) flows as a two-phase current through two low-voltage windings
and expects a corresponding current through the two high voltage coils on the relevant limbs. If the high
voltage side is star-connected without a neutral terminal, there is nothing to prevent this current from flowing.
But a single-phase load on the output side would mean that the current in the HV winding would have to first
flow through a loaded coil and then through an unloaded one, the latter acting, as already described, as a
reactor that attenuates the current. This sort of behaviour needs to be taken into account if the LV neutral
point is earthed with the intent of reducing the earth-fault loop impedance. This impedance is in any case
much higher in TT systems than in TN systems because of the resistance of the earth path in the impedance
loop. Clearly the high load-imbalance impedance of the transformer will prevent any noticeable reduction in
the impedance of the earth fault loop.
RLoad
RFe
X1‘ X2
Xm
RCu1‘ RCu2
RFe
X1‘ X2
Xm
RCu1‘ RCu2
L1
N
L2

Page 31
A voltage tester will light up whenever it touches any active conductor in an AC power socket, though only
weakly, because the voltage on each active conductor is only about 133 V relative to earth. In an IT earthing
system, the circuit for the voltage tester is closed via the stray coupling capacitances. An ordinary tungsten-
filament lamp would no longer be able to light up. However in a TT system, this current—although sufficient to
illuminate the lamp and to cause a dangerous electric shock—would not be enough to trigger an overcurrent
protection device. This is the well-known problem of the TT system and one that is exacerbated by this type of
(Belgian) supply. Dangerous shock currents can also arise in IT earthing systems, making RCD protection
necessary in both TT and IT systems. This however depends upon the extent and the age of the supply network
and the possible capacitive leakage currents, especially those in modern loads.
In addition, neither variant allows for the possibility of driving single-phase and three-phase AC loads designed
for a 400 V supply. For example, the popular German flow heater requiring a three-phase 400 VAC connection
would not function in parts of Belgium (unless of course the user was prepared to accept only a third of the
device’s rated output). In contrast, an electric cooker designed for a three-phase supply works without
difficulty. Cookers are usually designed to be able to cope with these supply networks by having a means of
reconnecting the terminals on the terminal board so that 230 V is always supplied to each of the three load
groups. The cooker does not actually need 400 V to operate. However, this approach will not work in the case
of the flow heater, since the individual heating elements are usually dimensioned for 400 V, i.e. for a delta
connection. While it would be possible in principle to design a flow heater to run on a 230 V/400 V delta/star
supply, there is a big difference between providing 7.5 kW for an electric cooker or 27 kW for a flow heater.
Providing the latter level of power becomes complicated if no 400 V supply is available.
PROTECTION
The distribution transformers used in public power supply networks are generally not protected on the output
side. The input side, in contrast, is equipped with HV HRC (high voltage high-rupturing capacity) fuse links.
However, if such a fuse is subjected to an overcurrent in the range between one and three-times the fuse’s
current rating, it will tend to overheat but will not interrupt the fault current.
But before a cynic turns round and says that HRC is obviously an abbreviation of Hopelessly Redundant
Component, we need to set the record straight. This type of fuse provides protection against a short circuit
fault, but not against overloading. This kind of protection is usually perfectly adequate, because by properly
planning the network based on parameters gained from years of experience, and by designing-in sufficient
levels of reserve power, it is possible to prevent overloading in, say, Germany. It simply does not occur.
However, overloading is the normal state in other regions of the world. Transformers are pretty tough devices
that can put up with a lot. The question of whether this makes economic sense is something that we will be
examining a little later. The crucial areas in which protection is needed are short-circuiting or arcing on the
output side and the rare occurrence of an internal fault within the transformer. Turn-to-turn faults in the LV
foil windings in particular can result in some spectacular damage. Arcing can cause some of the oil to vaporize
or can cause it to decompose into gaseous components. The resulting pressure wave swells the sides of the
transformer tank. The tank tries to attain a spherical form that offers greater volume per surface area in much
the same way coils react when a short circuit occurs. If such a damaged transformer is subsequently
disassembled, the conductor material found in the base of the tank has the form of egg-sized, egg-shaped
globules of red copper or pale silver aluminium speckled with soot. The coils, and in many cases the tank as
well, are now simply scrap, with only the core and transformer accessories still usable.
If the fault had persisted for even a few seconds, the tank would have burst, leaking copious amounts of oil
that would have ignited and acted as a fire accelerant. It is therefore quite clear, that short circuit protection is
essential, but overload protection is not. The development of electronic control systems means that it is now
common to have remote monitoring of the oil temperature. This not only helps to prevent hazardous

Page 32
situations from arising, but it also helps to optimize the operation of the supply network, since the difference
between the temperature of the oil and the ambient temperature provides an indication of the degree of
transformer loading.
The issue of short circuit protection again underscores the problems described earlier concerning the use of
inappropriate vector groups. If, for instance, there is a single-pole-to-earth fault on the output side and an
unconnected neutral point on the input side, and the magnetizing reactance of a limb not involved in the short
circuit is in the way of the short circuit, then the short circuit current that flows will be too small to trigger the
fuse. The extreme shift in the voltage system of almost a factor of √3 causes the limb in question to become
overexcited and magnetically saturated. As a result, that portion of the voltage that exceeds the rated voltage
is more or less only affected by the leakage reactance of the corresponding limb of the core. The amount of
current flowing can therefore be well above the rated current for the transformer itself and for the fuse, yet
still too small to trigger the fuse. The question then is which of the two blows first. It is worth repeating our
call to spend time carefully choosing the right vector group so that if a short circuit does occur, it is definitely
big enough to be identified as such by the fuse system, which can then react as it should and interrupt the
fault current. If that is not possible, then other protective and monitoring systems need to be put in place.
OPERATING TRANSFORMERS IN PARALLEL
Connecting transformers in parallel is of course possible in principle. This obviously means that the voltage
ratings of the two coils to be operated in parallel must be identical. Any off-load tap changers or strap panels
must also have the same settings. As an example, we will assume that we want to operate two transformers,
which have the same output power rating, with their primary windings in parallel and the secondary windings
also connected in parallel. Each transformer has a voltage changer with a range of ±5% of the rated voltage,
but one is set to +5%, while the other is set to -5%. One transformer has a short circuit voltage of usc = 4%; for
the other usc = 6%. In this case, the two short circuit impedances are in series and both transformers are
coupled via an impedance of 4% + 6% = 10% of the load impedance relative to the rated load of one of the
transformers.
Similarly, the difference between the parallel voltages is also 10% (of the rated voltage). As a result, the
transformer that exhibited the 10% higher open-circuit voltage will drive the rated current through the other
transformer, which will in turn transform this current back onto the input side. All windings on both
transformers would therefore be carrying the rated current, one forward and the other backwards, and
without supplying any electrical power. If a load was then connected, the voltage across the parallel output
terminals would decrease slightly. This would reduce the load on the backward-feeding transformer, but the
forward-feeding transformer would be overloaded.
Even if the asymmetric input voltage settings were to be corrected, the assumption made above should never
have been made. Even when all the voltages are identical, one should never operate transformers that have
different short circuit voltages with their output sides connected in parallel, since the load will still be
distributed unequally. Now assume the following: our two example transformers are both connected to the
same voltage source and have the same tap changer positions or that some other measures have been taken
to ensure that the open-circuit terminal voltages on the LV sides of both transformers are of the same
magnitude and have the same phase relationship. The two secondary windings are now connected in parallel
and drive a load that corresponds to the sum rated output of the two transformers. In this situation, each
device would be working at full capacity (but no more), if, that is, each played its part. But they do not. The
one with a short circuit voltage of 6% will only be loaded to
4
/5 of its rated load, the one with usc = 4% has to
deal with
6
/5 of its rated load, i.e. with a 20% overload.
Once again however, that’s not quite the whole truth. The size of the transformers (i.e. the ratio of their rated
outputs) also plays a role. Experts generally state that transformers that differ in size by more than a factor of

Page 33
three should not have their secondary windings connected in parallel. In fact, since 1997, the recommendation
is that the size difference should not exceed a factor of two
4
. As was already briefly mentioned, the ohmic
voltage drop across the winding decreases as the size of the transformer increases. In a small transformer, the
short circuit voltage usc contains substantially more uR and slightly less uX (Figure 35).
As the size of the transformer increases, uR gradually becomes increasingly negligible, at least in terms of what
we are considering here. But as Figure 27, Figure 29, and Figure 30 show, the size of the voltage drop in the
transformer depends on whether the device is driving an ohmic or an inductive load. If the voltage drop in the
transformer has only a small ohmic component, it will be reduced more if the transformer is attached to an
inductive load than when it is attached to an ohmic load, and vice versa. As a result, transformers with
different uR/uX ratios may well have exactly the same open-circuit voltage, but when operating under load,
there will be a slight difference in their voltages. Depending on the phase angle of the load, either the reactive
voltage or the ohmic voltage will drop more strongly. Consequently, if the transformer is operating under load
(i.e. not under open-circuit conditions), connecting the secondary windings in parallel will generate a
circulating current. The magnitude and direction of the circulating current will depend on the phase angle of
the load—something that is hard to predict.
As a consequence, connecting a large and a small transformer in parallel requires the introduction of a safety
factor, though this in turn makes the whole argument somewhat circular as the following example illustrates.
There is little point in providing a large 630 kVA transformer with a small assistant transformer with a rated
power of say 63 kVA or 100 kVA. The large transformer should have been dimensioned so that it offers at least
that amount of reserve capacity. If an assistant is required, the safety factor mentioned above means that the
actual capacity required is more like 250 kVA. But that would satisfy the 3:1 ratio rule, making the safety factor
superfluous to requirements—and we are back to where we started.
Figure 35 – Differing ratios of the active to the reactive voltage drop in a large transformer with a 630 kVA
Class C rating according to HD 428 (left) and a smaller transformer with a 50 kVA Class B rating (right).
In contrast to an electric motor, the most economical operating point for a transformer is well below its rated
load, so it makes sense to design in plenty of reserve capacity during the planning phase. In the example
discussed above, it is worth budgeting for a transformer with a rating of 1,000 or even 1,250 kVA—or better
4
www.a-eberle.de/pdf/info_12.pdf
uX=3.91%
uR = 0.9%
uX=2.95%
uR = 2.7%

Page 34
still—introducing system redundancy by including a pair of 630 kVA transformers, each of which is capable in
an emergency of handling the load on its own. Having reserve capacity also helps to settle the nerves. First,
conversion or retrofitting costs are far lower if there is a need to handle greater loads at some later date.
Secondly, power losses are reduced and, thirdly, the voltage drop in the transformer is lower as a result of
using either two 630 kVA devices in parallel or from using a single large device. Indeed this can be the
optimum solution for flicker problems since it gets things right at the start rather than attempting to deal with
the problem by grafting on a solution later
5
. Of course, the price for this improved resistance to flicker in the
supply system is a higher short circuit power. The short circuit power rises linearly with the size of the
transformer (provided the rated short circuit voltage is constant) and is the sum of the short circuit powers of
the individual transformers if they are connected in parallel. This needs to be considered when configuring the
downstream distribution network.
The term short circuit power needs to be used with care. According to the definition, the short circuit power is
calculated by multiplying the open-circuit (i.e. no-load) voltage by the short circuit current. The operating
states no-load and short circuit are however mutually exclusive. Short circuit power is a purely fictional
computational parameter, but never-the-less one that is useful in estimating what could happen in the event
of a short circuit.
There is one further and very obvious condition for operating transformers in parallel: the vector group codes
of the units to be connected in parallel must be the same. The important aspect here is that the digits
following the letter codes are identical. If they were not the same, one would end up connecting windings with
different phase relationships—a situation that is obviously unacceptable. For instance, two transformers with
the vector group codes Dd0 and YNyn0 can certainly be operated in parallel. But if the neutral point is loaded,
the single-phase or non-linear load will be borne by only one of the transformers, since the other does not
have a neutral point. This needs to be taken into consideration. Additionally, the short circuit voltages on the
rating plates refer to symmetric, linear, three-phase loads. If another type of load is connected, quite different
values will apply. The size of the deviation will then depend strongly on the vector groups involved
6
. By this
point, things can have begun to get quite confusing. Connecting transformers with different vector groups in
parallel is certainly not to be recommended, and should only really be seen as an emergency measure.
When we discuss operating transformers in parallel, we normally mean that the output sides are connected in
parallel, as the input sides are usually connected either directly or indirectly in parallel. It is possible, however,
that a distribution transformer is fed from different MV systems, which in turn are fed from the same HV
system but via HV transformers with differing vector group codes (and therefore having different phase
relationships). In this case, the sum of the vector group code digits for the HV transformer and the
downstream distribution transformer must be the same for each HV transformer so that the voltages at the
secondary windings have the same phase.
But as is so often the case, that’s not quite the whole truth. Vector group codes are not the only significant
elements. We also need to take the power transmission networks into account. The case capacitance (i.e. the
5
See [1], p. 51
6
Fender, Manfred: Vergleichende Untersuchungen der Netzrückwirkungen von Umrichtern mit Zwischenkreis
bei Beachtung realer industrieller Anschlußstrukturen [‘Comparative studies of the effects of converters with
intermediate circuits on power quality in real industrial installations], Ph.D. thesis, Wiesbaden 1997

Page 35
capacitance per unit length)
7
of underground cable is very large, while its series inductance (i.e. inductance per
unit length) is rather small. In overhead power lines, on the other hand, the capacitance is smaller and the
inductance is greater. Phase relationships will therefore vary depending on the specific load conditions.
Let us assume that we have two distribution transformers that are connected in parallel on their output sides.
They have the same rated outputs, the same vector groups, the same short circuit voltages, the same output
voltages, and even approximately the same copper losses. In other words, they are ideally suited to be
operated in parallel. One of the transformers is fed via a relatively long underground MV cable, the other via a
relatively long overhead MV line. As we assume that the voltages supplied at the start of the two cables have
the same phase and the modulus of the impedance is similar for the two cables, it is reasonable to expect that
there will not be any appreciable imbalance in the distribution of the common load. However, the phases of
the voltages arriving at the input sides of the two parallel distribution transformers (i.e. at the ends of the MV
supply cables) can indeed be different. The phase relationships measured at the LV bushings are therefore also
different and if the bushings are connected in parallel, a circulating current will begin to flow and the two
transformers will appear to heat up even under open circuit (no-load) conditions. However this is not actually
the case since the transformers are not really under no-load conditions. In fact, they drive the circulating
current. The power losses are almost purely reactive in nature. The only exception is the ohmic losses in the
two transformers as well as in their connection cables on the high voltage and low-voltage sides as far as the
coupling points.
The underground cable also represents a substantial capacitive load for the HV feeder transformer and, as
already described, this can cause an increase in the output voltage (Figure 33). In contrast, the capacitance of
the overhead line is probably small enough to be compensated (at least at full load) by the leakage inductance
of the distribution transformer that is being fed. The voltages can therefore differ not only in terms of their
absolute magnitudes but also with respect to phase. The question of whether this difference could become
critical is something that has to be calculated for a wide range of load cases during the planning phase. That is
not as simple as it sounds, given the large number of transformer and supply system parameters that need to
be taken into account.
Summary: Conditions for operating transformers in parallel
 Same voltage across the windings to be connected in parallel
 Same rated short circuit voltages
 Same vector group codes
 Ensure supply networks have the same phase relations
 If the transformers are not connected in parallel on the input side, ensure that the supply networks
have approximately the same short circuit power levels
 Maximum size ratio of transformers operated in parallel: 3:1
7
Fassbinder, Stefan: ‘Erdkabel kontra Freileitung’ [‘Underground vs overhead power transmission cables’], in
de, vol. 9/2001, p. gig9, appears in DKI reprint s180 ‘Drehstrom, Gleichstrom, Supraleitung – Energie-
Übertragung heute und morgen’ [‘Three-phase AC, DC and superconducting systems – Power transmission now
and in the future’] from the German Copper Institute (DKI), Düsseldorf

Page 36
ENERGY EFFICIENCY
In 1999, the Swiss journal Bulletin SEV/VSE
8
carried a cover story entitled Replacing old transformers pays off
9
.
The article showed that as a result of the significant improvements in the efficiency of modern transformers,
there are now sound economic reasons (in addition to important environmental arguments) why older
transformers should be decommissioned even when they are still functioning properly. In this section, we
explain how these efficiency improvements have been achieved and their current and future significance for
those responsible for purchasing and deploying transformers.
NEW REGULATION GOVERNING TRANSFORMER EFFICIENCIES
On July 1st 2015 the new European Regulation N 548/14
10
on power transformers entered into force. This was
a world premier of a regulation stipulating a minimum energy performance for large power transformers.
The regulation establishes eco-design requirements for power transformers with a minimum power rating of 1
kVA used in 50 Hz electricity transmission and distribution networks or for industrial applications. The
regulation indicates that transformers are strategic assets in the electrical networks, playing an important role
in achieving the ambitious energy efficiency targets set by most industrialized countries. Considering Europe
only, 16.7 TWh (corresponding to 3.7 megatons of CO2) will be saved in 2025 through the reduction of no-load
and load losses of transformers falling under this regulation.
The requirements for distribution transformers are formulated in the form of maximum load and no-load
losses (in W).
Requirements for three-phase liquid-immersed medium power transformers with one winding are listed
below, for respectively Um (maximum voltage) ≤ 24 kV and for Um ≤ 1.1 kV:
8
SEV/Electrosuisse: Swiss Association for Electrical Engineering, Power and Information Technologies
VSE: Association of Swiss Electricity Utility Companies
9
Borer Edi: ‘Ersatz von Transformatoren-Veteranen macht sich bezahlt’ [‘Replacing old transformers does
pay’], in Bulletin SEV/VSE, vol. 4/1999, p. 31
10
http://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=OJ:L:2014:152:FULL&from=EN

Page 37
Tier 1 (from 1 July
2015)
Tier 2 (from 1 July
2021)
Rated
Power
(kVA)
Maximum
load
losses Pk
(W) (*)
Maximum
no-load
losses Po
(W) (*)
Maximum
load
losses Pk
(W) (*)
Maximum
no-load
losses Po
(W) (*)
≤ 25
Ck (900) Ao (70) Ak (600) Ao – 10 %
(63)
50
Ck (1 100) Ao (90) Ak (750) Ao – 10 %
(81)
100
Ck (1 750) Ao (145) Ak (1 250) Ao – 10 %
(130)
160
Ck (2 350) Ao (210) Ak (1 750) Ao – 10 %
(189)
250
Ck (3 250) Ao (300) Ak (2 350) Ao – 10 %
(270)
315
Ck (3 900) Ao (360) Ak (2 800) Ao – 10 %
(324)
400
Ck (4 600) Ao (430) Ak (3 250) Ao – 10 %
(387)
500
Ck (5 500) Ao (510) Ak (3 900) Ao – 10 %
(459)
630
Ck (6 500) Ao (600) Ak (4 600) Ao – 10 %
(540)
800
Ck (8 400) Ao (650) Ak (6 000) Ao – 10 %
(585)
1 000
Ck (10
500)
Ao (770) Ak (7 600) Ao – 10 %
(693)
1 250
Bk (11
000)
Ao (950) Ak (9 500) Ao – 10 %
(855)
1 600
Bk (14
000)
Ao (1 200) Ak (12
000)
Ao – 10 %
(1080)
2 000
Bk (18
000)
Ao (1 450) Ak (15
000)
Ao – 10 %
(1 305)
2 500
Bk (22
000)
Ao (1 750) Ak (18
500)
Ao – 10 %
(1 575)
3 150
Bk (27
500)
Ao (2 200) Ak (23
000)
Ao – 10 %
(1 980)
Table 3 – Eco-design regulation for three-phase liquid-immersed medium power transformers.
Most of DSO owned transformers are of the liquid immersed type, but the category “Distribution
transformers” also covers the dry type. The requirements for three-phase dry-type medium power
transformers with one winding and a maximum voltage Um ≤ 24 kV and Um ≤ 1,1 kV respectively, are listed
below:

Page 38
Tier 1 (1 July 2015) Tier 2 (1 July 2021)
Rated
Power
(kVA)
Maximum
load
losses Pk
(W) (*)
Maximum
no-load
losses Po
(W) (*)
Maximum
load
losses Pk
(W) (*)
Maximum
no-load
losses Po
(W) (*)
≤ 50
Bk (1 700) Ao (200) Ak (1 500) Ao – 10 %
(180)
100
Bk (2 050) Ao (280) Ak (1 800) Ao – 10 %
(252)
160
Bk (2 900) Ao (400) Ak (2 600) Ao – 10 %
(360)
250
Bk (3 800) Ao (520) Ak (3 400) Ao – 10 %
(468)
400
Bk (5
500)
Ao (750) Ak (4
500)
Ao – 10
% (675)
630
Bk (7
600)
Ao (1
100)
Ak (7
100)
Ao – 10
% (990)
800
Ak (8
000)
Ao (1
300)
Ak (8
000)
Ao – 10
% (1 170)
1 000
Ak (9
000)
Ao (1
550)
Ak (9
000)
Ao – 10
% (1 395)
1 250
Ak (11
000)
Ao (1
800)
Ak (11
000)
Ao – 10
% (1 620)
1 600
Ak (13
000)
Ao (2
200)
Ak (13
000)
Ao – 10
% (1 980)
2 000
Ak (16
000)
Ao (2
600)
Ak (16
000)
Ao – 10
% (2 340)
2 500
Ak (19
000)
Ao (3
100)
Ak (19
000)
Ao – 10
% (2 790)
3 150
Ak (22
000)
Ao (3
800)
Ak (22
000)
Ao – 10
% (3 420)
Table 4 – Eco-design regulation for three-phase dry type medium power transformers.
To harmonize the requirements of the EU regulation on power transformer efficiency with European
standards, Cenelec Technical Committee No. 14 for Power Transformers adopted 3 new standards:
• EN 60076-19: Rules for the determination of uncertainties in the loss measurements on power
transformers and reactors. These rules are important for market surveillance verification.
 EN 50588-1: Medium power transformers, 50 Hz, with the maximum rated voltage not exceeding 36
kV - Part 1: General requirements. This standard introduces efficiency classes for medium power
transformers.
 EN 50629: Energy performance of large power transformers (Um > 36 kV or Sr ≥ 40 MVA). This
standard introduces efficiency requirements for large power transformers.
Transformers are now far more efficient than in the past. The EN50588-1 standard reflects this transformer
technology development by setting new loss classes. The following tables are defined for step-down or step-up
transformers with one winding and a maximum voltage Um ≤ 24 kV or Um ≤ 1.1 kV respectively.

Page 39
Rated
power
AAAo AAo LWA Ao LWA
kVA W W dB(A) W dB(A)
≤ 25 35 63 36 70 37
50 45 81 38 90 39
100 75 130 40 145 41
160 105 189 43 210 44
250 150 270 46 300 47
315 180 324 48 360 49
400 220 387 49 430 50
500 260 459 50 510 51
630 300 540 51 600 52
800 330 585 52 650 53
1000 390 693 54 770 55
1250 480 855 55 950 56
1600 600 1080 57 1200 58
2000 730 1305 59 1450 60
2500 880 1575 62 1750 63
3150 1100 1980 63 2200 64
Table 5 – No load loss (P0) and sound power level for liquid immersed transformers.
The sound power level LWA for transformers AAA0 has to be agreed between the manufacturer and the
purchaser.

Page 40
Rated power Ak Bk Ck Short-circuit Impedance
kVA W W W %
≤ 25 600 725 900 4
50 750 875 1100 4
100 1250 1475 1750 4
160 1750 2000 2350 4
250 2350 2750 3250 4
315 2800 3250 3900 4
400 3250 3850 4600 4
500 3900 4600 5500 4
630 4600 5400 6500 4 or 6
800 6000 7000 8400 6
1000 7600 9000 10500 6
1250 9500 11000 6
1600 12000 14000 6
2000 15000 18000 6
2500 18500 22000 6
3150 23000 27500 6
Table 6 – Load loss (Pk) and short circuit impedance for liquid immersed transformers.

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