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Estimating energy saving potential from transformers and evaluating their impact on the feasibility of renewable energy systems Atkins A research report completed for the Department for Environment, Food and Rural Affairs DATE: March 2013
Published by the Department for Environment, Food and Rural Affairs Department for Environment, Food and Rural Affairs Nobel House 17 Smith Square London SW1P 3JR Tel: 020 7238 6000 Website: www.defra.gov.uk © Queen's Printer and Controller of HMSO 2007 This publication is value added. If you wish to re-use this material, please apply for a Click-Use Licence for value added material at: http://www.opsi.gov.uk/click-use/value-added-licence-information/index.htm Alternatively applications can be sent to Office of Public Sector Information, Information Policy Team, St Clements House, 2-16 Colegate, Norwich NR3 1BQ; Fax: +44 (0)1603 723000; email: hmsolicensing@cabinet-office.x.gsi.gov.uk Information about this publication is available from: SCP&W Evidence Base Defra Zone 5D, 5th Floor, Ergon House c/o Nobel House, 17 Smith Square London SW1P 3JR Email: scpevidence@defra.gsi.gov.uk
Estimating energy saving potential from transformer and evaluating their impact on the feasibility of renewable energy systems Interim Report to the Department for Environment, Food and Rural Affairs Date: March 2013 Final Authors Elizabeth Birchenough and Hugh Falkner Checked Mike Hales This research was commissioned and funded by Defra. The views expressed reflect the research findings and the authors‟ interpretation; they do not necessarily reflect Defra policy or opinions. Atkins mantra is to Plan, Design and Enable infrastructure projects. Our skills lie in the expansive area of infrastructure – the wiring of society – encompassing buildings, transport and utilities (including energy and water) as well as work for national and local governments and other industrial clients. The social and environmental framework is crucial and we view all our projects in the context of the communities and in which they will be undertaken
Table of Contents
Executive Summary ................................................................................................. 5
1 The UK Transformer Market ............................................................................ 7
1.1 The UK Transformer population ..................................................................... 7
1.2 Transformer asset life ...................................................................................... 7
1.2.1Factors impacting transformer life ................................................................. 8
1.2.2Transformer failure mechanisms .................................................................... 8
1.3 Transformer purchasers .................................................................................. 9
1.4 Typical UK transformer specifications ........................................................... 9
1.5 UK Transformer manufacturing capacity ..................................................... 11
2 Transformer design considerations ............................................................. 13
2.1 Fundamental transformer design relationships .......................................... 13
2.2 Conductor material selection ........................................................................ 15
2.3 Transformer ratings ....................................................................................... 16
2.4 Dry (cast resin) and Oil filled transformers.................................................. 16
2.5 Transformer efficiency trends ....................................................................... 18
2.6 Losses from Harmonic currents ................................................................... 18
2.7 Technology for improved efficiency ............................................................. 19
2.7.1Improvements in steel laminations .............................................................. 20
2.7.2Amorphous core transformers ..................................................................... 20
2.7.3Hexaformer transformer design topology.................................................... 21
3 Proposed Regulations for Distribution transformers ................................. 23
3.1 General Scope ................................................................................................ 23
3.2 Maximum power loss basis for distribution transformers ......................... 24
3.3 Proposed methodology for regulating large power transformers ............. 25
4 Renewable Energy Applications ................................................................... 26
4.1 Utilisation Considerations for Renewable Energy transformers ............... 26
4.2 Transformer De-energisation ........................................................................ 28
4.3 Wind Energy ................................................................................................... 28
4.4 Solar Photovoltaic .......................................................................................... 30
4.5 Tidal Generation ............................................................................................. 30
4.6 Wave Energy................................................................................................... 31
4.7 Hydro – run of the river.................................................................................. 32
1
4.8 Hydro – stored power .................................................................................... 32
4.9 Biomass .......................................................................................................... 33
4.10 Biogas ............................................................................................................. 33
5 Economics of transformer ownership in the UK ......................................... 34
5.1 Economic Selection of Transformers ........................................................... 34
5.2 Transformer loss recovery mechanism ....................................................... 34
5.3 Cost benefit analysis methodologies ........................................................... 34
5.4 Who pays for Renewable Energy transformer losses? .............................. 35
6 Optimising transformer design for renewable energy applications .......... 37
6.1 Distribution transformer losses when used in different applications ....... 37
6.2 Losses by application .................................................................................... 38
6.2.1Interpretation of results ................................................................................. 38
6.2.2Solar ................................................................................................................ 38
6.2.3Wind ................................................................................................................ 39
6.2.4Biomass .......................................................................................................... 40
6.2.5Methodology – Worked Example (Solar) ...................................................... 40
7 References ...................................................................................................... 42
2
List of Abbreviations
Abbreviation Definition
BS British Standards
CMR Continuous Maximum Rating
csa Cross Section Area
DER Distributed Energy Resources
DLAF Distribution Loss Adjustment Factor
DNC Declared Net Capacity
DNO Distribution Network Operator
ENA Energy Networks Association
EU-27 27 Member States of the European Union
EuP Energy Using Product Directive
HVDC High Voltage Direct Current
IEC International Electrotechnical Commission
IEEE Institute of Electrical and Electronic Engineers
MEEUP Methodology for the Ecodesign of Energy-using Products
MEPS Minimum Energy Performance Standards
MWh Mega Watt hour
NEMA National Electrical Manufacturers Association (US)
NETA New Electricity Trading Arrangements
NGET National Grid Electricity Transmission
OFAF Oil Forced, Air Forced (transformer cooling arrangements)
ONAF Oil Natural, Air Forced (transformer cooling arrangements)
ONAN Oil Natural, Air Natural (transformer cooling arrangements)
PTO Power Take Off
RE Renewable Energy
rms Root Mean Square
TCO (or TOC) Total Cost of Ownership (Total Ownership Cost)
TLAF Transmission Loss Adjustment Factor
TSO Transmission System Operator
TWh Terra Watt hours
UoSC Use of System Charge
VA Volt.Amperes (Apparent power)
3
Glossary of terms
A0Ck, C0Ck, ... etc: Indication of transformer losses class influenced by its design. The first
two letters refer to the no load losses, and the latter to the load losses. These values are
explained in detail in the EUP Preparatory study.
Auxiliary Losses (Paux): The active power consumed by the auxiliary components of the
transformer (e.g. fans, pumps).
Availability factor (AF): Assumed fraction of time (usually taken over a year) that the plant
is available for operation, here assumed for simplicity to be 100%.
K-factor: A de-rating factor for a standard transformer used to supply non-linear loads, such
that the total loss on harmonic load does not exceed the fundamental design loss of the
transformer. For these applications, specially constructed or K-rated transformers should be
used (EN 50464-3).
Load Factor (a): The ratio of the energy generated by a unit during a given period of time to
the energy it would have generated if it had been running at its maximum capacity for the
operation duration within that period of time (IEC 60050 International Electrotechnical
Vocabulary). The Load Factor is a specific transformer parameter, and should not be
confused with the Utilisation Factor. Utilisation factor refers to the percentage availability that
a complete system is operating. For example, the utilisation factor for a wind turbine may be
25% for 2 hours. The load factor on the transformer associated with that same wind turbine
over the same time window may be 30%, because the transformer will be powering auxiliary
circuits even if the turbine is stationary.
Load form factor (Kf): The ratio of the root mean squared (rms) power to the average
power (= Prms / Pavg).
Load loss multiplier (ae x ae): This multiplier allows for direct calculation of conductor
losses, (which is proportional to current squared).
Load losses (Pk): The absorbed active power at rated frequency and reference
temperature, associated with a pair of windings when rated current is flowing through the line
terminals of one of the windings, and the terminals of the other winding are short-circuited.
Further windings, if existing, are open-circuited. (IEC 60076-1).
No load losses (Po): The active power absorbed when a given voltage at rated frequency is
applied to the terminals of one of the windings, the other winding(s) being open-circuited
(IEC 60076-1).
Power factor (PF): The ratio of the active power (kW) to the apparent power (kVA).
Rated Power (S): The value of apparent power assigned to a winding which, together with
the rated voltage of the winding, determines its rated current. The rated power refers to
continuous loading. This is a reference value for guarantees and tests concerning load
losses and temperature rises (IEC 60076-1: Power Transformers – Part1: General).
Transformer availability factor (AF): This determines the availability of the transformer on
a given instant of time (usually considered on a yearly basis).
4
Executive Summary
The Proposed EUP regulations and DER transformers
Distribution transformers within the EU-27 have combined annual energy losses estimated at
38TWhpa, and so represent an interesting target for improving energy efficiency and hence
reducing carbon emissions. A 2011 European Commission Energy Using Product (EUP)
Preparatory study1 reported on the basis of possible regulations to reduce the losses in new
distribution transformers. These regulations are now (March 2013) being considered for
introduction as early as July 2014.
Within the UK, it is estimated that 46,000 distribution transformers will be purchased over the
next five years, of which 40,000 will be replacements to existing systems, and 6,000 for
Distributed Energy Resource (DER) applications. These transformers will have a lifetime of
25 - 50 years, and so purchasing decisions made now will have a growing impact on total
energy losses from the UK and European transformer stock.
The current EU installed capacity of Distribution transformers in DER applications is only
very small (50,000 units total), but represents an important and growing part of sales. The
draft regulations for Distribution transformers published in the October 2012 Consultation
forum draft2 exclude DER transformers as a special category.
Transformer losses
Transformer losses comprise two components, the no load and no load losses:
No load (core) losses, which occur for the whole time that the transformer is energised.
These are constant and independent of load. The majority of these losses are related to
hysteresis, which is proportional to flux density, and therefore dependent on the construction
and materials used in the core.
Load (resistive) losses, which is proportional to the square of the load current.
The proposed regulations for distribution transformers state maximum power losses for each
of these components, not the total combined loss, hence limiting the design flexibility
available to the designer.
Utilisation factors of DER in the UK and the impact of the proposed EUP regulations
Transformer Utilisation factors vary widely with application. For high utilisation sources such
as hydro or biomass, transformer load losses are particularly important. But for low
utilisation sources such as solar, then no load losses are proportionately more important.
Ideally, distribution transformers should be available that are optimised for these very
different duties, as this would both reduce energy losses and give a better return on
investment. However, it is found that the costs of type testing and the need to produce high
volumes mean that such special designs are not cost effective. Without external
intervention, such as through regulation, suppliers will continue not to offer these designs,
5
and the User will not specify them.
In practice, the fundamental equations governing the design and performance of
transformers means that any changes to one of the loss components will have an impact on
the other. In addition, it is essential to maintain the correct short circuit impedance, as this is
a network requirement in order to limit the prospective fault current and related protection
equipment. There is therefore only limited scope for altering designs without incurring
excessive costs.
It is observed that the proposed regulations will not worsen the situation regarding energy
losses from distribution transformers used in DER applications. However, the regulation of
these two loss components separately rather than on a total efficiency basis could preclude
the development of transformers designed specifically for some DER applications. It is such
a total efficiency approach that is being considered for large power transformers, a market
where custom designs are the norm.
A distinction should be made between those transformers that are connected to the DNO
network and those within privately owned renewable energy system. Whereas transformers
connected to the DNO network need to comply with DNO specifications, those that are
privately owned do not. This gives flexibility for optimising secondary voltages to match that
of the energy source, selection of dimensions to fit limited space in for example wind turbine
towers, and in the selection of prospective fault levels and hence impedances. Although not
explicitly excluded from the proposed EUP regulations, it is assumed that because they may
have non-standard impedances, such privately owned transformers may be excluded from
the regulations. This report focuses on the use of DER transformers that are connected to
the DNO network.
A new methodology for regulating large power transformers
It is proposed that large power transformers are regulated on the basis of total efficiency,
rather than the separate Minimum Energy Performance Standards (MEPS) for load and no-
load losses methodology proposed for smaller distribution transformers. This “total losses”
approach is currently only applicable to these larger transformers, as these are the only
types that are currently custom designed for each application.
The importance of harmonic currents on DER transformers
Many Renewable Energy sources have a power electronic interface to the electricity
network, which will generate harmonics. Harmonics lead to additional heating through core
losses and in the conductors through eddy currents and the skin effect. These lead to a de-
rating of the transformer, known as the “K” factor, which is explained in more detail later on
within the report. As with the Energy Using Product (EUP) Preparatory Study, this is
considered to be application rather than product specific, and so is outside the scope of any
regulations and not considered further.
61 The UK Transformer Market
1.1 The UK Transformer population
The population of distribution transformers in the UK is estimated to be 684,000 units, with
an average rating of 234kVA3. The vast majority of these are installed on Distribution
Network Operator (DNO) networks and are of oil-filled construction. Dry (also called cast
resin) distribution transformers are used primarily in buildings where the perceived fire
hazard of transformer oil precludes their use.
The UK has a higher redundancy built into the design of both the transmission and
distribution networks than much of the rest of Europe (EU-27), and so has a lower average
load factor. Consequently no load losses are relatively of more importance in the UK than
for the rest of the Europe.
The UK is characterised by a higher proportion of pole mounted transformers than many
other countries, which are found particularly in remote rural areas. As the rural load has
increased, many pole mounted transformers are being upgraded to a traditional three phase
supply with ground mounted transformers. More generally, DNO‟s are moving towards
ground mounted transformers because it allows for larger units, and improved access for
maintenance.
1.2 Transformer asset life
The typical average asset life for transformers in the UK is estimated as follows;
40 to 50 years for transmission system transformers
40 years for distribution transformers
20 to 25 years for light industrial transformers
20 to 50 years for DER applications
The SEEDT3 report also states that approximately 6% of the installed transformers are over
40 years old and therefore technically at the end of their asset life. It is common practice for
transmission and distribution transformers to be monitored and replaced on the basis of age
and condition.
Atkins internal data on DNO planned asset replacements shows that 40,000 units of the
current stock will need replacing over the next 5 years.
It is projected that there will be an increase in installed renewable generation capacity from
the current 6.1GW, to an estimated peak of 82GW, with the majority being associated with
wind energy4 . It is always difficult to predict growth in demand, but there are infrastructure
plans in place for major improvements in rail electrification, and the prospect of increased
use of electric vehicles and heat pumps.
To estimate the potential number of transformers needed in the next 5 years, an average
rating of 4.5MVA is suggested as a typical size for a wind turbine, giving a new market
exceeding 6300 units. There will of course be many other technologies contributing to the
renewable energy mix, but in the short term wind will dominate, as it is an established
technology.
7On this basis, the future UK market for new distribution transformers will be in the order of
46,000 over the next 5 years.
1.2.1 Factors impacting transformer life
Lifespan can be affected by a number of factors, such as the number of times the
transformer has been switched on and off, fluctuation of the demand on the transformer
terminals, its local environment (dusty and hot environment for example) and the transformer
loading.
Expected lifetimes of renewable energy sources will vary, up to 25 years for a wind farm or
solar installation to perhaps twice this or more for a large hydro or biomass scheme.
However, there is insufficient data to understand whether the life of transformers in
renewable energy applications will be any different from those in conventional distribution
applications.
It is likely that at end of life a wind-farm will be re-powered with higher rated turbines, and so
the transformers would also be replaced. Many transformers are also mounted in the
nacelle or the tower as an integral part of the power system package, and so would have a
lifetime the same as the wind turbine. In addition there is a growing second hand market for
reconditioned wind turbines, in which case the transformers may also have an extended
lifetime.
Some early offshore wind turbine applications suffered failures, but lessons have been learnt
and this is no longer such an issue.
1.2.2 Transformer failure mechanisms
For all types of transformers, failure and hence replacement may be required for any of the
following reasons5:
Insulation Failures – Insulation failures are the leading cause of failure. This category
excludes those failures where there was evidence of a lightning or line surge. There are
actually four factors that are responsible for insulation deterioration: pyrolosis (heat),
oxidation, acidity, and moisture. From studies in the US, it was found that the average age
of the transformer that failed due to insulation failure was 18 years.
Design / Manufacturing Errors – This category includes conditions such as: loose or
unsupported leads, loose blocking, poor brazing, inadequate core insulation, and inferior
short circuit strength. This is the second most common cause of transformer failures.
Oil Contamination – This category relates to cases where oil contamination can be
established as the cause of failure. This includes sludging and carbon tracking, which is
caused by ingress of water or other pollutants, and by internal arcing.
Fire / Explosion – Internal failures that result in a fire or explosion. This is a very rare event
and requires a number of safety measures to have failed. There are also some recorded
incidents of catastrophic failures arising from external damage or vandalism. The attempted
theft of metals has increased significantly in the last five years, with substations being seen
as an easy target.
8Line surge – This category includes switching surges, voltage spikes, line faults / flashovers
and other network abnormalities. Attention should be given to external surge protection,
such as spark gaps and surge arrestors, or to adequate coil clamping and short circuit
strength.
Maintenance / Operation – Inadequate or improper maintenance and operation can be a
major cause of transformer failures, which includes overloading, loose connections and
moisture. This category also includes disconnected or improperly set controls, loss of
coolant, accumulation of dirt and oil, and corrosion.
All solar and many wind energy and ocean energy machines use inverters to synthesise a
sinusoidal “mains” frequency waveform. The performance of modern inverters is excellent,
but transformers need to be de-rated according to the harmonic content present. Active or
passive filters can be used to help mitigate generated harmonics, but these come at a cost
and they will have a small energy loss. The faster rate of voltage change from the use of
inverters puts additional stress on the transformer insulation, which may lead to premature
failure on transformers with old or already compromised insulation systems.
1.3 Transformer purchasers
Different purchaser groups have different approaches to specifying transformer losses:
Utilities will use some variant on the total lifetime costs as discussed in chapter 5,
and so do have an interest in transformer losses to a greater or lesser extent.
Research for this work showed that with a few notable exceptions, renewable energy
developers do not pay much attention to transformer losses.
Building developers, especially of speculative developments, are concerned primarily
with lowest first cost, and so show little interest in losses.
1.4 Typical UK transformer specifications
The UK distribution transformer market is characterised by manufacturers offering almost
identical ranges of standard transformers to specified technical standards. Any significant
variations from these standard types would themselves need type testing, and so would be
very costly. Offering just these standard ranges, which are made in relatively large numbers,
is also critical in keeping costs low in this price competitive market.
Within the UK market6, the primary and secondary voltage levels and MVA ratings are
generally standard across the various networks, (with the exception of some parts of
London, the industrial Midlands and Northern Ireland).
400/132kV – 240MVA (GB)
275/132kV – 240MVA (GB)
132/66kV – 30, 45, 75MVA (typical in London and the industrial Midlands)
132/33kV – 30, 60, 90MVA (GB)
9 132/22kV – 60MVA (London, typically for the Underground network)
110/33kV – 60, 90MVA (Northern Ireland)
132/11kV – 60, 45, 30, 15MVA (GB)
66/22kV – 25, 36, 45MVA (London, typically for the Underground network)
66/11kV – 30 (2x15MVA), 28, 17, 15MVA (typical in London and in the industrial
Midlands)
33/11kV – 40, 24, 20, 15, 12, 10, 8MVA (UK)
33/6.6kV – 20, 24MVA (typical in London and in the industrial Midlands)
Ground mounted transformers (11000/420V) – three phase – 100, 200, 315, 500,
800, 1000kVA (UK)
Pole mounted transformers (11000/420V) – three phase – 50, 100, 200, 315kVA
(UK)
Note: it is common practice to define the primary and secondary voltages at the “no load”
condition. Hence for the distribution transformers the “no load” secondary voltage above is
given as 420V. This will drop to 400V under normal load conditions, which is the declared
load at LV consumer terminals in the UK. The primary voltage is normally the “source”
winding, the secondary voltage being the “load” winding.
For transformers used within the renewables industry, the rating will usually be determined
by the nameplate rating of the generation equipment. Therefore typical values for wind
turbine transformers are 250kVA, 500kVA, 750kVA, 1MVA, 1.5MVA, 2MVA, 2.5MVA, 4MVA.
With smaller transformers, DER developers will select the nearest available transformer to
match their turbine, rather than incurring the expense of designing and type testing a
bespoke transformer.
UK transformers historically were constructed in compliance with BS171, which has been
obsolete since approx 1980. Transmission transformers were designed to ENA BEBS T2
which was a bespoke CEGB standard. These standards have been replaced with the
introduction of IEC EN 60076 – Power Transformers. All new transformers are constructed
using this standard as the basis of their design for the UK market.
Oil in transformers may be replaced with a synthetic ester-based dielectric coolant. Although
this has beneficial properties, being far less flammable than traditional transformer oil, and
less polluting in the event of a spillage, it is less efficient as a cooling medium. Therefore
synthetic coolant filled transformers are larger than the equivalent oil filled type, and may
require additional pumps and fans.
The bespoke CEGB standards called for many unique design features related to installation,
testing and maintenance. For example, the exact way in which terminal blocks were made in
control panels were defined precisely, down to the thread on the brass locking nuts. The
CEGB specifications naturally favoured traditional UK manufacturers who had a guaranteed
market with the CEGB. These transformers tended to be more expensive to manufacture
than equivalent units manufactured in Europe and elsewhere. With the deregulation, the UK
supply industry could no longer justify bespoke designs, which opened up the market to
European manufacturers working to the new IEC standard. Unfortunately, the UK
manufacturers were not able to compete, leading to a sharp decline apart from the very
specialist quality market seen today. The IEC standard is perfectly respectable, however it
does allow the manufacturer some flexibility relating to the quality and robustness of
construction, which has resulted in much lighter tanks for example. For the DER market,
where the expected life may only be 20 years, this may not be an issue, but for transmission
transformers, located outdoors for 40+ years, issues may develop relating to rusting and
leakage.
10The existing constraints on the network operators are very heavily skewed towards
minimising capital costs, which has opened up the market to imported transformers from
outside Europe.
1.5 UK Transformer manufacturing capacity
Few standard distribution transformers are now manufactured in the UK, with DNO‟s
typically buying high volumes of a few models made to standard specifications. By contrast
there are many small to medium manufacturers of specialist types.
Historically the UK had several manufacturers of large transformers (greater than 36kV), but
with the exception of Brush transformers, these were all closed in the late 1990‟s and early
2000‟s. Now Alstom (Stafford) is the only recognised transformer manufacturer in the UK
that constructs specialist power transformers. In addition there are a number of smaller
companies who manufacture specialist transformers below 36kV.
As part of the evidence collecting process for this report, contact was made with 12 UK
distribution transformer manufacturers. It was found that the majority of these transformer
manufacturers produce specialist transformers for specific applications such as traction
transformers and generator neutral earthing transformers. Critically there is no capacity for
the production of standard distribution transformers of the type used within the renewable
energy or power distribution sectors. This means that UK manufacturers will not be
impacted the forthcoming EC Energy Using Product Directive on transformers. As part of this
project, the major transformer producers were contacted for market information, but with
notable exceptions there was disappointingly little response. This was taken as a reflection
of the fact that most transformer activity in the UK is either relating to specialist types outside
the scope of the regulations, and the large amount of refurbishment activity.
ABB Power Technology Products Dundee Transformer Division is the focal point for all
ABB transformer projects in the UK. Products made include various ranges of both TrafoStar
power transformers for transmission networks and distribution transformers for local supply
networks.
Eaton (Cutler-Hammer) can supply a wide range of power and distribution transformers.
Included are low temperature rise energy efficient single- and three-phase energy efficient
types. All are NEMA TP-1-1996 compliant.
Square D offers a complete line of transformers that can significantly reduce energy losses.
The Type EE energy efficient transformers are optimised for maximum efficiency at the most
common loading levels.
The company's Watchdog 115°C or 80°C rise transformers offer significant energy savings
in less common applications where the average daily loading exceeds 50% of nameplate
rating.
Another interesting product from Square D is its Type EE NEMA TP-17 compliant ENERGY
STAR transformers. These comply with TP-1 for optimum energy efficiency at 35% load, are
more efficient than standard 150°C rise transformers at all load levels, have identical
dimensions to standard 150°C rise general purpose transformers, and are reported to have
been cost optimised to allow an energy savings payback in 3 - 5 years.
11Merlin Gerin hermetically sealed transformers giving reduced maintenance have been
added to the Fast Trans range. Once installed, these transformers will operate without
further attention required to the fluid, making them ideal for petrochemical complexes, water
plants and sewage treatment works, as well as many industrial applications.
122 Transformer design considerations
2.1 Fundamental transformer design
relationships
The transformer designer is able to adjust the design for best efficiency at different load
points. But any changes might impact other parameters, and so there is only limited scope
for change. In particular, any reduction in the impedance of a transformer will increase the
fault level on the secondary side. The control of fault level is essential for the correct
operation of the network protection schemes. Maintaining a correct fault level is also crucial
in maintaining the stability of voltage at the consumer‟s terminals as the network load varies,
especially for large motors and rail traction supplies. Key variables are:
Footprint
Weight
Voltage regulation and short circuit capability
Cost
Materials used in the core and windings
Insulation and cooling medium
The formula governing transformer design, which is called the EMF Equation of Transformer,
is:
E = 4.44 BANf
Where
E = Electromotive Force induced in the transformer winding
B = flux density
A = Cross Section Area (csa) of core
N = Number of turns
f = Frequency
Some key relationships flow from this and additional physical considerations, which explain
why in practice transformers for the same duty are all very similar;
Higher frequency enables a smaller transformer. This is why US 60Hz regulations
are not appropriate for EU 50Hz applications.
The steel core has a definite knee-point around 1.6-1.8Tesla, above which the
Watt/kg losses get rapidly much higher, and so there is in practice little room for
changing the flux density without using alternative materials.
The number of turns is determined by the voltage ratio, and also the flux, giving a
V/turn for the selected core.
The area of the core is fixed through the relationship Φ (flux) = B.A, where B and A
are already constrained as mentioned above.
13The following table gives a summary of the impacts of measures to change load or no-load
losses.
To decrease no-load losses Impact on Load Total cost
losses
Use lower loss core material No change but Higher
could increase if
lower flux density
Decrease flux density by increasing core cross Higher Higher
sectional area
Decrease flux density by decreasing volts per Higher Higher
turn
Decrease flux path length by decreasing Higher Lower
conductor cross sectional area
To decrease load losses Impact on No load Total cost
losses
Use lower loss conductor materials No change Higher
Decrease current density by increasing conductor Higher Higher
cross sectional area
Decrease current path length by decreasing core Higher Lower
cross sectional area
Decrease current path length by increasing volts Higher Higher
per turn
Table 1: The impact of measures to reduce load or no load losses
The Short circuit impedance is determined by the required fault level on the network, and
so cannot be altered by the designer. This limits the scope for altering the design of
distribution transformers, although transformers within privately owned renewable energy
generation networks will not be subject to this constraint.
The short circuit impedance of a transformer can be approximated as follows:8
X = 8 π2 f N2 l d / h
Where
X = Short circuit impedance (Ω)
f = Network frequency (Hz)
N = Number of turns per winding
l = Average turn length (m)
d = Winding diameter (m)
h = Winding height (m)
The interplay of this equation and the fundamental transformer design equation, E = 4.44
BANf, allows for the adjusting of losses between load and no load losses. Any adjustment
in the number of turns, wire diameter or core dimensions will impact both losses, and so the
two factors are closely inter-related. It is not possible in practical terms to alter one loss
without impacting the other loss.
Forced cooling will allow a transformer to operate at higher power levels, but carries a
capital cost penalty. It also represents a source of additional power loss associated with
pumps and fans, which is not included in the current standard.
14Where transformers are located within buildings, generally in densely populated city centres,
there is an additional load associated with air handling to ventilate the transformers. This
load is considered as part of the building services, and is therefore covered by building
regulations. Alternatively, in some cases the losses will be used during cooler months to
provide some heating for the buildings.
1
Figure 1: Transformer efficiency and different losses for a 75kVA oil filled transformer
Figure 1 shows how the total transformer losses vary with load, with minimum losses always
occurring when the load and no load losses are equal. The load point corresponding to
minimum total energy loss represents the optimal load point.
2.2 Conductor material selection
Both aluminium and copper are used for transformer conductor, with the choice being
determined by both economic and technical factors.
Copper is generally the preferred material, as for the same rated power, copper gives a
more compact package, with attendant reductions in structural materials and core losses.
On a volume basis, the conductivity of aluminium is approximately 62% that of copper, giving
copper the advantage in physically constrained applications. Copper also has greater
mechanical strength and so is better able to resist short circuit conditions, offers better
electrical connections and is less prone to corrosion and stress fatigue.
However, the lower weight of aluminium makes it attractive for small pole mounted
transformers where the cost of the supporting structure is important. In general, below
200kVA aluminium is a popular choice, and above 200kVA copper is the popular choice.
152.3 Transformer ratings
The nameplate continuous rating of a transformer, which is defined in the IEC standard, is
based on the output power corresponding to a 65K temperature rise above ambient
temperature, known as Continuous Maximum Rating (CMR). In addition, it will have a short
term overload capacity of typically 20% above CMR for 8 hours, and an even higher short
term emergency rating. It is accepted practice to design a transformer to regularly go into
the overload region for short periods of time, but this additional thermal stress will reduce the
life of the transformer.
A poor load power factor will adversely impact transformer losses through additional current
flow for the same output power, (VA). However, as with harmonic heating, this is a load
impact that will apply to all transformers irrespective of their rated efficiency.
In theory, users could chose to circumvent new regulations by deliberately buying under-
sized transformers and running them at higher loads than intended. Given that a transformer
is effectively specified in terms of a temperature rise, deliberately under-sizing a transformer
in order to be able to select a transformer that has a rated efficiency below the MEPS
legislation may start to occur.
2.4 Dry (cast resin) and Oil filled transformers
There are two generic types of transformer sold worldwide and in the EU and UK: dry and oil
filled.
Oil filled transformers
The majority of transformers in the UK are oil filled. The oil acts as an insulator and coolant.
Typically, 25% of the transformer weight is attributed to the oil, so for the largest
transformers, the oil volume will be several thousand litres. As the oil density fluctuates with
temperature, it is common practice to fit a conservator, which acts as a header tank to
ensure an even pressure and fill level is maintained within the tank. Transformer oil is slightly
hydroscopic, so it is important that any air within the conservator is kept dry, which is usually
done by fitting a bag of dry air in the tank, which expands and contracts as the temperature
of the oil fluctuates or by allowing the conservator to breathe through a passive or active
drier.. Small transformers may not be fitted with conservators (sealed transformers). In this
case, the design of the oil containment is such that changes in pressure are absorbed by
some permitted flexing.
New Distribution transformers are usually delivered to site 90% full of oil, with the balance
added once installed The oil is then pumped in once the transformer is installed. There is a
risk of spillage during this operation. Oil filled transformers are generally located within a
bund, designed to contain oil spillages, and may also be surrounded by fire walls. The
choice of fire protection depends on the fire risk assessment. In enclosed spaces, carbon
dioxide was traditionally used, but this has now largely been replaced by modern nitrogen
mixes. There is also a significant health and safety risk to personnel caught in a discharge
cloud of carbon dioxide. Where transformers are open to atmosphere, fire protection is
normally a water drench system.
Transformer oil must be tested periodically, to ensure it has not deteriorated. It is possible to
clean the oil by pumping it round a filter system, but this is a time consuming exercise, and
has to be done with the transformer out of service. If the transformer oil deteriorates, it can
eventually lead to transformer failure.
16Oil filled transformers are generally fitted with additional instrumentation, which provides
alarms under a range of conditions, such as oil level low, oil temperature high, winding
temperature high, gas bubbles in oil (this is detected by a Buchholtz relay) or pressure relief
valves.
Figure 2. Oil filled transformer (Voltimum)
Oil filled transformers are cooled by heat transfer from the oil to atmosphere. Traditionally,
transformers are fitted with banks of radiators. The oil circulates by natural convection, which
can be augmented with pumps when the oil temperature reaches a predefined level. The
effectiveness of heat loss from the radiators can be enhanced by the use of fans to increase
the air speed across the radiators. Transformers in hot climates are frequently shaded from
direct sunlight to reduce solar gain, and may be fitted with larger radiators. It is also
frequently necessary to surround transformers and radiators with fencing to prevent
unauthorised access, and reduce the possibility of wildlife attempting to nest on the
convenient warm ledges. Birds nests are a particular hazard, as bird droppings are
corrosive, and the nesting materials can cause short circuits across the transformer
terminations.
Altering the design of fire walls or enclosures would enhance the natural flow of air around
the transformers, and hence reduce the need for auxiliary cooling.
There are a number of synthetic oil substitutes available, which are widely used where the
fire risk is considered high. However these fluids are not as efficient cooling media as
traditional mineral oil, and therefore the transformers are physically larger for the same
rating. Synthetic oils may also be specified where there is a significant environmental risk,
such as water pumping stations, as they are less polluting in the event of spillage.
Both dry and oil filled transformers on load emit a low pitched noise, arising from the
vibrations in the core and windings. This can be reduced to some extent by the use of anti-
vibration mounts, but where noise is likely to cause an environmental nuisance, additional
screening has to be installed. This may degrade the natural flow of cooling air, requiring the
possible use of auxiliary cooling.
Dry (cast resin) transformers
In the UK, dry type transformers are widely used in commercial buildings where the
developer wants a low first cost solution, partly driven by the perception by insurers that they
17are less of a fire risk. There are also reduced civil works costs, as this type of transformer
does not require bunding or fire protection.
Dry type transformers can suffer from an increase in electrical stresses within their windings
if there is frequent switching in and out of the transformer. This is because partial
discharges occur within the void between the coil and the cast resin. For liquid filled
transformers, the electrical stresses are reduced during frequent switching on and off due to
the insulating liquid‟s ability to regenerate after a discharge has occurred within it.
Figure 3. Dry Type Transformer, (R&B Switchgear Services)
2.5 Transformer efficiency trends
From 1970 to 2000, the general efficiency for liquid filled transformers improved by
approximately 0.5%, while that for dry type transformers has actually decreased by
approximately 0.9% for a unit size of 250kVA. The reason for the decrease in efficiency of
dry type transformers is that purchasers of dry type transformers do not consider losses as
part of their evaluation criteria when purchasing new transformers. It has been reasoned in
a number of technical papers that the efficiency of dry type transformers could have been
improved by a further 1.3% on its 1970 efficiency value if manufacturers had the incentive to
improve the dry type transformer efficiency. This would have made dry type transformers
comparable with oil filled types in efficiency terms9.
2.6 Losses from Harmonic currents
Many RE sources have a power electronic network interface, which will generate harmonics.
Most standards only consider the fundamental sinusoidal waveform, but in real life a
transformer will be subject to harmonic currents. These increase the losses by the following
mechanisms:
Additional core losses
Eddy current losses in conductors
18 Skin effect losses in conductors
The significance of harmonics in power systems has increased substantially due to the use
of power electronic devices. An important consideration when evaluating the impact of
harmonics is their effect on power system components and loads. The increased losses due
to harmonic distortion can cause excessive winding loss and hence abnormal temperature
rise.
Existing standards give a procedure to determine the capability of an existing transformer
subject to non-sinusoidal load currents based on conservative assumptions. The eddy
current loss generated by the electromagnetic field is assumed to vary with the square of the
RMS current and the square of the frequency (harmonic order h). Due to the skin effect, the
electromagnetic flux may not totally penetrate the strands in the winding at high frequencies.
A standard solution for reducing the skin-effect problem is therefore to use multiple parallel
conductors.
The temperature rise due to harmonics is estimated based on constant harmonic load
currents, and the average daily or monthly temperatures to which a transformer would be
subjected while in service.
When a non-linear load is supplied from a transformer, it is sometimes necessary to de-rate
the transformer capacity to avoid overheating and subsequent insulation failure. The „K‟
factor is used to de-rate transformers subject to harmonic distortion. The K-factor is a
weighting of the harmonic load currents according to their effects on transformer heating. A
K-factor of 1.0 indicates a linear load (no harmonics). The higher the K-factor, the greater
the harmonic heating effects.
It is noted that the EUP Preparatory Study reasonably states that this does not need taking
account of in any regulations, as it applies uniformly to all transformers and so there is no
room for further regulatory intervention.
The use of inverters on many types of RE generation plant means that harmonic losses are
becoming more of an issue. In the same way that IEC 60034-2-3 is now looking at testing
induction motors under harmonic distortion from VSDs, it is suggested that transformer tests
should consider these losses. However, more work is needed in the transformer community
before it will be timely to make any suggestions.
2.7 Technology for improved efficiency
It has been stated by a number of key transformer manufacturers in both Europe and the US
that the losses associated with distribution transformers can technically be reduced by
between 30% and 40% below current levels. This increase in energy efficiency has a big
impact on the cost per unit for the transformer.
On average, 70% of all losses associated with transformers are no-load losses, and so this
is the focus of improving efficiencies for transformers. Fortunately, improved core
technologies are available that allow for cost effective reductions in no load losses.
192.7.1 Improvements in steel laminations
Developments in conventional steels have reduced losses through the following changes:
Improved magnetic permeability
Magnetic domain refinement
Reduced core steel sheet thickness,
Increased lamination stacking factors through increased perfection in steel surface
quality
Thickness uniformity
Inter-laminar insulation.
In practice, the regulations as currently stated are effectively dictating the use of these lower
loss laminations, as it will be hard to achieve the specified minimum no load losses with
older lamination designs.
2.7.2 Amorphous core transformers
Ultra-efficient transformer cores made with amorphous metal alloys such as Honeywells‟
METGLAS make lower core losses possible.
Amorphous metal distribution transformers (or AMDTs) have been available for over 20
years. These can be defined as bulk, structural, metallic materials whose microstructure in
the solid state is, unlike that of conventional metals, non-crystalline, amorphous or 'glassy'.
As a result of this novel microstructure, amorphous alloys exhibit unique combinations of
properties such as magnetic performance, hardness, strength and corrosion resistance. It is
possible to improve power quality and mitigate harmonics with the devices
The US has been at the forefront of recent applications in amorphous core transformers,
which is where most applications are found. This kind of technology is slowly starting to
penetrate the UK market place, and is being used by several UK based manufacturers such
as Wilson Power Solutions.
Figure 4: Amorphous Core for a three phase transformer
20Amorphous core technology does have some disadvantages that have historically limited its
impact:
The cost of high efficiency rated equipment can be twice as great as a standard
efficiency transformer.
Availability in UK currently is not as good as for conventional types.
Amorphous metal cores are up to 30% bigger than conventional cores
Brittle, and so careful handling required during manufacture, installation and
servicing.
Improved high frequency performance, so better if subject to high harmonic currents.
Only available in sizes up to 20MVA.
An important consideration is that AMDT transformers are generally designed with a
utilisation factor of around 50% as this is the most efficient value for AMDT materials.
2.7.3 Hexaformer transformer design topology
This design is for a hexagonal core which is completely different from that of conventional
transformers. Swedish manufacturer Hexaformer ® AB, http://www.hexaformer.se offers this
design, which has quite a long tradition; the first design of such transformer was introduced
about 100 years ago. Hexaformer produces low voltage transformers with voltages of up to
1000 V, and also high voltage transformers up to 24 kV - from 5 kVA up to 2,500 kVA.
Hexaformer specifies the following advantages of this design:
No-load losses can be reduced by up to 50 %.
The Hexaformer starts smoothly and inrush currents are comparatively weak. It is
therefore frequently possible to disregard inrush current when dimensioning fuses
and connections.
There is barely any discernible noise.
The weight can be reduced by up to 30 % compared with conventional designs.
The volume can be reduced by up to 40 % compared with conventional designs.
The magnetising current is reduced.
The three coils work under identical conditions, which give the phases a symmetrical
character.
Due to the fact that there is less loss, there is also less need for cooling.
The Hexaformer is expected to have a long lifespan
21Figure 5. Hexaformer transformer, (ABB)
These transformers are quite popular in Sweden, other Nordic countries and increasingly in
Spain. A recent development (June 2012) has been the introduction of Amorphous
materials being used for the core which reduces the losses associated with the Hexaformer
even further. This type of transformer should be considered for smaller scale renewable
projects where generation may be very intermittent due to its low no-load loss
characteristics. If and when the maximum kVA size of the transformer increases, then the
number of potential applications for this type of transformer will increase. Market impact is
also limited by the longer construction time needed to build these transformers.
There are around 2,000 Hexaformer transformers installed today on the distribution power
grid. These are found mainly in Northern Europe, however a number of units have just been
ordered by ESB Ireland.
223 Proposed Regulations for Distribution
transformers
3.1 General Scope
The scope of the “Working document on a Commission Regulation implementing Directive
2009/125/EC with regard to small, medium and large power transformers” comprises the
following:
“This working document pursuant to Directive 2009/125/EC establishes ecodesign
requirements related to small, medium and large power transformers with a minimum power
rating of 1 kVA used in 50Hz electricity transmission and distribution.
This Regulation shall not apply to the following categories of transformers:
Instrument transformers.
Traction transformers on rolling stock.
Starting transformers
Testing transformers
Welding transformers
Explosion-proof and underground mining transformers.
Transformers for deep water (submerged) applications.
Definitions
Transformers are considered as energy related products within the meaning of Article 2 (1)
of Directive 200 9/125/EC.
For the purpose of this working document and its annexes the following definitions shall
apply.
(1) “Power transformer” means a static piece of apparatus with two or more windings which,
by electromagnetic induction, transforms a system of alternating voltage and current into
another system of alternating voltage and current usually of different values and at the same
frequency for the purpose of transmitting electrical power.
(2) “General purpose s mall power transformer” means a power transformer with a highest
voltage for equipment not exceeding 1 kV ..
(3) “Medium power transformer” means a power transformer with a high voltage winding with
a rated voltage higher than 1 kV, but not exceeding 36 kV.”
(Additional note for clarification: Large power transformers may be used in any one of the
following applications:
Generator step up transformer
Transmission transformer, including interbus transformers
Primary substation transformers, which for the UK market are Bulk Supply Points
and Grid Supply Points)
23It is of note that while the Preparatory Study on which these proposed regulations are based
included DER (Distributed Energy Resource or Renewable Energy) transformers as a
separate type, in the draft regulations this distinction is not made. DER transformers will
therefore be subject to the same regulations as standard Distribution transformers.
An important distinction should be made between those transformers that are connected to
the DNO network and those within privately owned renewable energy system. Whereas
transformers connected to the DNO network need to comply with DNO specifications, those
that are privately owned do not. This gives flexibility for optimising secondary voltages to
match that of the energy source, selection of dimensions to fit limited space in for example
wind turbine towers, and in selection of prospective fault levels and hence impedances.
Although not explicitly excluded from the proposed EUP regulations, it is assumed that
because they may have non-standard impedances, such privately owned transformers may
be excluded from the regulations.
3.2 Maximum power loss basis for distribution
transformers
The following table shows the maximum allowable power loss for load and no load losses for
different sizes of transformer. It is noted that in 2018 the more lenient levels for pole
mounted transformers disappear.
Tier 1 (from July 2014) Tier 2 (from July 2018)
RATED
POWER
(kVA) Maximum load Maximum no-load Maximum Maximum
losses (in Watts) losses (in Watts) load losses no-load
Short-circuit (in Watts) losses (in
impedance in Watts)
()
Pole Pole (Pole mounted sub-category
mounted mounted disappears)
100 (4%) 1250 145 145 1250 116
400 (4%) 3850 430 3250 344
1000 (6%) 7600 770 7600 616
2000 (6%) 15000 1450 15000 1160
Table 2. Draft maximum energy loss requirements for oil filled distribution transformersYou can also read
