EN 50629:2015

SLOVENSKI STANDARD
01-september-2015
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Ta slovenski standard je istoveten z: EN 50629:2015
ICS:
27.015 (QHUJLMVNDXþLQNRYLWRVW Energy efficiency. Energy
2KUDQMDQMHHQHUJLMHQD conservation in general
VSORãQR
29.180 Transformatorji. Dušilke Transformers. Reactors
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN 50629
NORME EUROPÉENNE
EUROPÄISCHE NORM
June 2015
ICS 29.180
English Version
Energy performance of large power transformers (Um > 36 kV or
Sr ≥ 40 MVA)
Performance énergétique des transformateurs de grande Energiekennwerte von Großleistungstransformatoren (Um >
puissance (Um > 36 kV ou Sr ≥ 40 MVA) 36 kV oder Sr ≥ 40 MVA)
This European Standard was approved by CENELEC on 2015-06-25. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2015 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN 50629:2015 E
Contents Page
Foreword . 4
Introduction . 5
1 Scope . 6
2 Normative references . 7
3 Terms and definitions . 7
4 Efficiency and Efficiency Index calculation . 8
4.1 General . 8
4.2 Efficiency Index general formula . 8
4.3 Peak Efficiency Index . 9
5 Minimum Peak Efficiency Index values . 10
5.1 Standardised values of Minimum PEI . 10
5.2 Optimization of transformer losses according to application . 12
5.3 Rating plate data . 12
5.4 Transformer asset data . 12
5.5 Tolerances, measurement uncertainties and market surveillance. 12
5.5.1 Factory acceptance . 12
5.5.2 Verification procedure for market surveillance . 13
6 Transformers categories currently excluded . 13
7 Capitalisation of losses . 14
Annex A (normative) Minimum PEI for dry type large power transformers . 15
Annex B (informative) Peak Efficiency Index formula, graphs and calculations . 16
B.1 Calculation of k . 16
PEI
B.2 Graph of Efficiency Index and load factor with loss contributions . 17
B.3 Graphs of prescribed PEI values and rated power . 18
B.4 Independence of PEI to rated power. 19
B.5 Calculation of losses from PEI, k and S . 20
PEI r
Annex C (informative) Form for data requested . 21
C.1 Example of form for data requested . 21
C.2 Indications for filling the table . 22
Annex D (informative) Benchmark of Peak Efficiency Index . 23
D.1 General . 23
D.2 Benchmark figures . 23
D.3 Variations from the benchmark . 28
D.3.1 General . 28
D.3.2 Autotransformers. 28
D.3.3 Voltage and insulation level . 28
D.3.4 More than two windings . 28
D.3.5 Short-circuit impedance . 28
D.3.6 Tapping range . 29
D.3.7 Losses on taps different that rated tap . 29
D.3.8 Separate phases . 30
D.4 Exceptions from benchmark . 30
D.4.1 General . 30
D.4.2 Transformers with unusual combinations of windings and voltages . 30
D.4.3 Installation restrictions . 30
D.4.4 Offshore installation . 30
D.4.5 Transportation restrictions . 30
D.4.6 Transformers for temporary installation . 30
D.4.7 Converter transformers . 30
D.4.8 Dry-type and gas insulated transformers . 30
D.4.9 Other exemptions . 31
Annex E (informative) Capitalisation of losses . 32
E.1 General Theory, Concept of Capitalisation. 32
E.2 Impact of capitalisation values . 32
E.3 Capitalisation formula . 33
E.3.1 General . 33
E.3.2 Calculation of factor A . 34
E.3.3 Calculation of factor B . 35
E.3.4 Use of A and B for tender evaluation . 37
E.3.5 Determination of factors A and B . 37
Annex F (informative) Background on verification tolerances during market surveillance . 39
Annex ZZ (informative) Relationship between this European Standard and the requirements of
Commission Regulation (EC) No 548/2014 of 21 May 2014 on implementing Directive
2009/125/EC of the European Parliament and of the Council with regard to small, medium
and large power transformers . 40
Bibliography . 41

Foreword
This document (EN 50629:2015) has been prepared by CLC/TC 14, "Power transformers".
The following dates are fixed:
- latest date by which this document has (dop) 2016-06-25
to be implemented at national level by
publication of an identical national
standard or by endorsement
- latest date by which the national (dow) 2018-06-25
standards conflicting with this
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such patent rights.

This document has been prepared under a mandate given to CENELEC by the European Commission and
the European Free Trade Association, and supports requirements of Commission Regulation (EC).
For the relationship with requirements of Commission Regulation (EC) see informative Annex ZZ, which is an
integral part of this document.
Introduction
This European Standard has been prepared at the request of the European Commission under the mandate
EC 24/2011 and applies to large power transformers covered by the COMMISSION REGULATION (EU) N.
548/2014 of 21 May 2014.
For large power transformers (LPT) the strict definition of efficiency based on transmitted and absorbed active
power alone is not useful for evaluating the energy performance because the losses are either fixed (no load
loss), or depend on current (load loss) and therefore conventional efficiency would be zero if only reactive
power is transmitted (reactive power transmission is very important for network operation). The conventional
calculation of efficiency is therefore not helpful for comparing transformer designs which may be used over a
range of operating conditions.
In general for LPT it is not possible to give optimal values for load and no load losses for a particular rated
power because of the variety of applications which affect the energy performance.
In order to define an index that is specific to the transformer design, but applicable to a wide range of uses,
rather than a figure that varies from second to second depending on system conditions, it is essential to
characterize the energy performance of power transformers. For this reason a metric – Peak Efficiency Index
(PEI) – has been developed which is based on real power losses and total power transmitted and is
independent of load phase angle, load factor and rated power.
This document provides a standard method for evaluating the energy performance of power transformers
through the use of the Peak Efficiency Index, gives benchmark figures for PEI and the reasons why certain
transformers may have efficiencies which are higher or lower than the benchmark.
Setting a reasonable value of minimum Peak Efficiency Index will be effective in improving the overall
efficiency of the installed transformer population by eliminating transformers with poor efficiency, with the
exception of some transformers subject to specific limitations.
The use of a minimum value of Peak Efficiency Index sets a floor for transformer efficiency performance, but
the use of proper loss capitalisation for purchasing transformers is essential to select a transformer with the
optimal economically justified level of efficiency. Users not using loss capitalisation are strongly encouraged to
investigate the benefits of doing so.
For large units above 100 MVA the economically achievable efficiency of a transformer may be limited by the
technical parameters of the network (e.g. impedance), and specific transport and installation constraints. As
the units concerned are usually purchased by large transmission system owners, who typically use high
values of loss capitalization, those units above 100 MVA already tend to be state of the art as far as efficiency
is concerned.
For transformers with unusual configurations and/or very severe size or weight limitations it may be
unreasonable to meet the minimum efficiency requirement for either technical or economic reasons. In these
cases it will be acceptable to demonstrate that the highest reasonable level of efficiency has been achieved
(see Clause 6).
It is considered that the approach to energy performance set out in this document could also be applicable in
principle to transformers outside the scope of this standard.

1 Scope
This European Standard applies to new three-phase and single-phase power transformers with a highest
voltage for equipment exceeding 36 kV and a rated power equal or higher than 5 kVA, or a rated power equal
to or higher than 40 MVA regardless of the highest voltage for equipment.
The scope of this European Standard is the following:
- Defining the appropriate energy efficiency criteria;
- Setting of benchmark minimum efficiency levels for new transformers based on an assessment of the
energy efficiency of the European transformer population installed in the last 10 years;
- Proposing higher minimum efficiency levels for improving the energy efficiency of new transformers;
- Providing guidance for consideration of Total Cost of Ownership.
This European Standard provides also a form for efficiency data collection to inform future efficiency
benchmark levels.
NOTE 1 This standard covers the transformers under the EU Regulation N. 548/2014 and gives additional specific
guidance for single phase transformers, autotransformers, multi winding transformers and for transformers with OD and
OF cooling systems, necessary for the correct application of energy efficiency requirements to these categories of
transformers.
Transformers considered to be out of the scope of this document are the following:
- instrument transformers, specifically designed to supply measuring instruments, meters, relays and
other similar apparatus,
- transformers with low-voltage windings specifically designed for use with rectifiers to provide a DC
supply,
- transformers specifically designed to be directly connected to a furnace,
- transformers specifically designed for offshore applications and floating offshore applications,
- transformers specially designed for emergency installations,
- transformers and auto-transformers specifically designed for railway feeding systems,
- earthing or grounding transformers, this is, three-phase transformers intended to provide a neutral
point for system grounding purposes,
- traction transformers mounted on rolling stock, this is, transformers connected to an AC or DC contact
line, directly or through a converter, used in fixed installations of railway applications,
- starting transformers, specifically designed for starting three-phase induction motors so as to
eliminate supply voltage dips,
- testing transformers, specifically designed to be used in a circuit to produce a specific voltage or
current for the purpose of testing electrical equipment,
- welding transformers, specifically designed for use in arc welding equipment or resistance welding
equipment,
- transformers specifically designed for explosion-proof and underground mining applications,
- transformers specifically designed for deep water (submerged) applications,
- medium Voltage (MV) to Medium Voltage (MV) interface transformers up to 5 MVA,
- large power transformers where it is demonstrated that for a particular application, technically feasible
alternatives are not available to meet the minimum efficiency requirements set out by EU
REGULATION N. 548/2014,
- large power transformers which are like for like replacements in the same physical location/installation
for existing large power transformers, where this replacement cannot be achieved without entailing
disproportionate costs associated to their transportation and/or installation.
For dry type large power transformers Minimum PEI values have been published in European Regulation and
these values are included in Annex A.
NOTE 2 To retain consistency, the same list of exclusions in the EU Regulation N. 548/2014, has also been
reproduced here. Within the above EU exclusion list, some had been excluded simply because no PEI data was available
to CENELEC at the time on which to base appropriate PEI levels. Consequently, as such information becomes available in
the future, it may be possible to derive suitable PEI Levels. Accordingly these particular categories are listed in Clause 6
as suitable for future consideration.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
EN 60076 (all parts), Power transformers (IEC 60076, all parts)
EN 60076-19, Power transformers — Part 19: Rules for the determination of uncertainties in the measurement
of the losses on power transformers and reactors (IEC/TS 60076-19)

3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 60076-1:2011 and the following
apply.
3.1
Large Power Transformer
LPT
power transformer with a highest voltage for equipment exceeding 36 kV and a rated power equal or higher
than 5 kVA, or a rated power equal to or higher than 40 MVA regardless of the highest voltage for equipment
3.2
load factor
k
ratio of actual input current over the rated current of transformer
Note 1 to entry: Normally 0 ≤ k ≤ 1.
3.3
transmitted apparent power
kS
r
product of the load factor and the rated power
3.4
Efficiency Index
EI
ratio of the transmitted apparent power of a transformer minus electrical losses to the transmitted apparent
power of the transformer
3.5
Peak Efficiency Index
PEI
highest value of efficiency index that can be achieved at the optimum value of load factor
3.6
load factor of Peak Efficiency Index
k
PEI
load factor at which Peak Efficiency Index occurs
3.7
declared value
regulatory value given in Table 1 which is to be used for market surveillance activities
Note 1 to entry: According to EN 60076-1, ‘declared value’ and ‘guaranteed value’ refer to two different concepts.
‘Guaranteed Values’ relate to the values cited in the commercial contract, whereas ‘declared values’ are those values
which are cited to establish compliance with EU Regulation N. 548/2014.
4 Efficiency and Efficiency Index calculation
4.1 General
The energy performance of a transformer can be stated in a variety of ways, principally by giving:
a) The no-load and load losses at rated load or at a particular reference power;
b) The efficiency at a defined power factor and particular load factor, for example 50 % or 100 % of rated
load;
c) The Peak Efficiency Index and the load at which it occurs.
The general definition of efficiency raises some complications such as whether the electrical consumption of
the cooling equipment of transformer at no-load or at a particular load shall be included in the calculation.
For the scope of this standard the Peak Efficiency Index has been chosen to set benchmark efficiency figures
because it does not impose a particular load factor (which may vary greatly depending on the application) and
because it does not depend explicitly on the rated power of the transformer. Peak efficiency is an intrinsic
parameter of the transformer that does not depend on whether the transformer has alternative ratings
depending on cooling modes.
The Peak Efficiency Index includes the losses associated with the cooling system that is in service in the no-
load condition. If additional cooling is required at the load factor where PEI occurs, then the additional cooling
loss required for this cooling shall be computed in the calculation of PEI. Any further additional cooling and
associated cooling loss necessary to achieve rated power is excluded.
NOTE 1 This applies to transformers equipped with heat-exchangers which need pumps and fans to provide heat
dissipation (e.g. ODAF, ODWF, OFWF, OFAF, OFAN).
NOTE 2 If the loss capitalisation method is used in the transformer procurement process, then it may be expected that
the Peak Efficiency Index will occur at approximately the loading where the ratio between load and no-load losses is equal
to the ratio between the capitalisation rates for load and no-load loss, except where this has been modified by the relative
cost of reducing load and no-load losses (See Annex D).
4.2 Efficiency Index general formula
The Efficiency Index at load factor k is calculated in accordance with Formula 1:
kS - (P +P )−(k²P+P (k))
r c0 k ck
𝐸𝐸(k) =   (pu) (1)
kS
r
Where
P is the no load loss measured at rated voltage and rated frequency, on the rated tap;
P is the electrical power required by the cooling system for no load operation, derived from the
c0
type test measurements of the power taken by the fan and liquid pump motors;
P is the measured load loss at rated current and rated frequency on the rated tap corrected to
k
reference temperature according to EN 60076-1;
P (k) is the additional electrical power required (in addition to P ) by the cooling system for
ck c0
operation at k times the rated load derived from the type test measurements of the power
taken by the fan and liquid pump motors;
S is the rated power of the transformer or autotransformer on which P is based;
r k
k is the load factor.
NOTE 1 This approach respects the philosophy of EN 60076 (all parts) which refers the rated power to the rated
voltage and current of one of the transformer windings.
The derivation of P at k involves establishing the total power consumption of the fans and the pumps (from
ck PEI
type test measurements) and then ascribing a proportion of this total cooling loss to that required at PEI
loading. The proportion used is the ratio of the average electrical loss of the fans and pumps used at k and
PEI
average yearly ambient temperature (20 °C unless otherwise specified) to the total electrical loss of the
pumps and fans installed.
If fans and pumps have variable speed drives, an additional type test measurement may be required to
determine P at k .
ck PEI
NOTE 2 No routine measurements of cooling power consumption are required.
For the PEI calculation, the following shall be considered.
a) The reference temperature for liquid immersed transformers with rated average winding temperature
rise less than or equal to 65 K for OF or ON, or 70 K for OD is 75 °C;
b) For transformers with other rated average winding temperature rise, the reference temperature is
equal to the rated average winding temperature rise + 20 °C, or rated winding temperature rise +
yearly external cooling medium average temperature, whichever is higher.
If a purchaser needs to compare transformer with different insulation systems and different average winding
temperature rises, the reference temperature should be according to b) above.
For the scope of this document and for sake of simplicity it is conventionally assumed that:
- the voltage and load current systems are symmetrical and sinusoidal;
- the line voltage is equal to the rated voltage.
4.3 Peak Efficiency Index
Under the assumption that the cooling at no load is sufficient to operate at k (this assumption is used to
PEI
simplify the calculation), the load factor which maximises the Efficiency Index is given by:
P +P
0 c0
k =� (pu) (2)
PEI
P
k
For symbols meaning refer to 4.2, Formula 1.
The formula to be used for Peak Efficiency Index calculation is therefore Formula 3, which is obtained from
Formula (1) by replacing k with k as defined in Formula (2) and by assuming P (k )=0:
PEI ck PEI
2(P +P )
0 c0
PEI = 1 − (pu) (3)
P +P
c0
S�
r
P
k
For symbols meaning refer to 4.2, Formula 1.
NOTE 1 Demonstration of the mathematical derivation is given in B.1.
NOTE 2 An example is given in B.2.2.
As mentioned in 4.1, if additional cooling is required at the load factors where PEI occurs, then the assumption
P (k ) = 0 does not hold, then the term P (k ) shall be added to P in the formula for PEI for the
ck PEI ck PEI c0
transformers in the scope of this standard.
2[P +P +P (𝑘 )]
0 c0 ck 𝑃𝑃𝑃
PEI = 1 − (pu) (4)
�P +P +P (𝑘 )�
0 c0 ck 𝑃𝑃𝑃
S�
r
P
k
NOTE 3 An example is given in B.2.3.
⁄
NOTE 4 The value of Formula 3 depends on the ratio 𝑆 �𝑃 which does not vary significantly if S is changed (for
r
𝑟 𝑘
example by changing cooling mode) provided P is measured at S .
k r
5 Minimum Peak Efficiency Index values
5.1 Standardised values of Minimum PEI
The Minimum Peak Efficiency Index values for liquid immersed transformers are given in Table 1 and those
for dry type LPT are given in Annex A.
The T1 values are based on the results of the survey described in Annex D set at approximately the level of
the lower quartile (25 %). This is considered to be an achievable level of efficiency which is likely to be
economically justified.
The T2 values are set at approximately the median level (50 %) of the surveyed transformer efficiencies and
represent an ambitious target which needs to be applied with due consideration of economic efficiency. The
intention is that these levels represent an ambition that needs to be validated.
These figures take into consideration the need to remove distortions introduced into the data collected by
specific designs, the uneven spread of data over the size range, and the necessity for coordination with the
efficiency standard for transformers with rated voltages lower than 36 kV and rated powers below 40 MVA (the
resulted graph of PEI values versus the rated power values is reported in B.3).
Table 1 — Values of minimum Peak Efficiency Index for liquid immersed transformers
S PEI – T1 PEI – T2
r
(MVA) (%) (%)
≤ 4 99,465 99,532
5 99,483 99,548
6,3 99,510 99,571
8 99,535 99,593
10 99,560 99,615
12,5 99,588 99,640
16 99,615 99,663
20 99,639 99,684
25 99,657 99,700
31,5 99,671 99,712
40 99,684 99,724
50 99,696 99,734
63 99,709 99,745
80 99,723 99,758
≥ 100 99,737 99,770
NOTE  For clarity the PEI values in the table are expressed in
percent.
For rated powers different from the ones reported in Table 1, the corresponding PEI value can be obtained by
linear interpolation from the two adjacent values.
The PEI requirements apply to transformers and auto-transformers.
For auto-transformers the reference power for PEI values is the rated power.
For transformer with rated power lower than 4 MVA it might not be possible (technically and economically
justified) to reach the PEI value given for 4 MVA.
Three phase or single phase transformers shall be evaluated against the rated power of the individual
transformer.
The PEI requirements in Table 1 apply to autotransformers and separate winding transformers having three
windings as follows. Assuming that ratings are x/y/z, then:
- if x and y are equal and z is lower than or equal to one third of x or y, the PEI shall be the one
corresponding to x or y rating and the losses of winding z shall not be considered for PEI calculation
(e.g. 600/600/65 MVA)
- if x is equal to the sum of y and z, the PEI shall be the one corresponding to x rating and the three
winding losses shall be considered for PEI calculation (e.g. 100/60/40 MVA)
- In all other cases, the PEI shall be the one corresponding to maximum of the three ratings. Load loss
shall be measured for each winding pairs and the load combination to be used for PEI calculation is:

x x
𝑥 / y / z
y + z y + z
NOTE 1 In general, transformers of similar design criteria, but with more than two windings, have higher total
losses and lower PEI values. This formula also allows for the verification of PEI requirements in transformers
other than two winding transformers, such as three winding transformers and autotransformers. For the
computation of the load loss for each winding, the criteria given in IEC 60076-8 can be taken as reference.
NOTE 2 E.g. for a 60/60/30 MVA, it is essential that the PEI limit be that of 60 MVA and the load combination for
load loss calculation be:
60 60
60 / 60 / 30    =>    60 / 40 / 20
60+30 60+30
The approach used for three winding transformers can be applied in principle to transformers with more than
three windings.
For transformers with re-connectable windings PEI calculation shall be made based on loss measurements
taken at the highest rated voltage(s).
Specific power transformers with factors such as size and weight limitations, transportation restrictions,
unusual combinations of windings and voltages (see also D.4) may not meet PEI value of Table 1.
NOTE 3 Where these transformers do not meet the minimum PEI in Table 1, then it is important to show by using a proper
method, as for example the capitalisation method given in Clause 7, that the transformer has the highest economically
justified efficiency within the limitations of the intended application.
5.2 Optimization of transformer losses according to application
The minimum PEI value prescribed, which shall be met in all cases, can be obtained with different
combinations of no load and load losses. This is equivalent to different load factors at which PEI occurs. The
ratio of the load and no load losses needs to be tailored to the application in order to obtain the best actual
efficiency in service. In order to achieve this, the following methods are available:
- Providing capitalisation values for no load and load losses that reflect the anticipated loading (this is
the recommended method);
- Prescribing maximum values for no load and load losses;
- Specifying minimum efficiency index at a specific load factor.
NOTE 1 The associated PEI may occur at a different load factor.
NOTE 2 Typical values of the ratio between load and no load loss can range between 3 for heavily loaded transformers
(such as GSU) up to 6-7 and sometimes more than 10 for lightly loaded units (i.e. transmission and distribution
transformers).
5.3 Rating plate data
In addition to EN 60076-1 requirements, the following values shall be shown on the rating plate:
- PEI based on measurements;
- k at which PEI occurs in pu;
PEI,
- P , the no load loss measured at rated voltage and rated frequency, on the rated tap;
- P , the electrical power required by the cooling system for no load operation derived from the type
c0
test measurements of the power taken by the fan and liquid pump motors;
- P , the measured loss at rated current and rated frequency on the rated tap corrected to reference
k
temperature according to EN 60076-1;
- Nature of the conductor (for instance copper, aluminium, copper/aluminium);
- Mass of the conductor;
- Nature of the core material (for instance silicone steel, amorphous steel);
- Mass of the active part, if different from the untanking mass.
5.4 Transformer asset data
In Annex C a pro-forma for further data collection is given. The scope of the pro-forma is to collect data for
improving the present standard and to determine any issues that may arise in the general application of T2
values. A completed copy of the pro-forma shall be supplied to the purchaser of the transformer preferably in
electronic format.
NOTE Such data collection is required for an effective review by the European Commission of the efficiency of the
installed base of large power transformers and to provide efficiency benchmarks. The practical method to collect data in
Annex C is currently under definition.
5.5 Tolerances, measurement uncertainties and market surveillance
5.5.1 Factory acceptance
No tolerances are applicable to the minimum PEI value.
The PEI calculated from the contractually guaranteed and the measured values of load and no-load losses
shall be equal to or higher than the PEI value specified in this standard.
The test report of the transformers shall report the PEI value and the corresponding k
PEI.
The tolerances for the measured values to the guaranteed values of load and no-load losses are subject to
agreement between manufacturer and purchaser within the limits prescribed by EN 60076-1.
The losses shall be measured in compliance with the methodologies stated in the EN 60076 series of
standards in order to achieve an acceptable level of uncertainty.
5.5.2 Verification procedure for market surveillance
During the market surveillance, with reference to ANNEX III of the EU Regulation N. 548/2014, market
surveillance authority will measure no load and load loss. PEI value shall then be calculated by the market
surveillance authority by this formula:
2(P +P )
A0 Ac0
PEI = 1 − (pu) (5)
P +P
A0 Ac0
1,05 ∙ S�
r
P
Ak
Where
P is the no load loss measured at rated voltage and rated frequency, on the rated tap by the
A0
market surveillance authority;
P is the electrical power required by the cooling system for no load operation as determined by
Ac0
the market surveillance authority from the type test measurements of the power taken by the
fan and liquid pump motors;
P is the measured load loss at rated current and rated frequency on the rated tap corrected to
Ak
reference temperature according to EN 60076-1 by the market surveillance authority;
S is the rated power of the transformer or autotransformer on which P is based;
r k
NOTE The factor 1,05 represents the verification tolerance on loss components which is allowed to market authority
during surveillance checks according to ANNEX III of EU Regulation N.548/2014.
The test results of the loss measurements are expressed as a numerical quantity which is not an exact
number but suffers from uncertainty. How wide this margin of uncertainty is depends on the quality of the test
installation, particularly its measuring system, on the skill of the staff and on measurement difficulties
presented by the test object. The submitted test result shall contain the most correct estimate that is possible,
based on the measurements that have been carried out. This value shall be accepted as it stands. The
uncertainty margin shall not be involved in the judgement of compliance for guarantees with no positive
tolerance or tolerance ranges for performance data of the test object. However, a condition for acceptance of
the whole test is that the measurements themselves have to fulfil certain requirements of quality (see
EN ISO 9001).
The measurement uncertainty applicable to the market surveillance authority shall be:
- The expanded uncertainty, as defined in EN 60076-19 and referring to a coverage factor k = 2 (i.e. to
a confidence level of about 95 % assuming a normal distribution).
- The measurement uncertainty defined in this way, expressed as a relative value shall not exceed 5 %.
This procedure is detailed in Annex F.

6 Transformers categories currently excluded
The following categories of transformers are considered to be relevant for energy performance evaluation
criteria, but since no data was available, a decision on minimum peak efficiency levels was not taken. In the
future, after appropriate data has been collected, they will be considered.
- transformers with low-voltage windings specifically designed for use with rectifiers to provide a DC
supply (converter transformers),
- transformers specifically designed to be directly connected to a furnace,
- transformers specifically designed for offshore applications and floating offshore applications,
- transformers specially designed for emergency installations,
- transformers and auto-transformers specifically designed for railway feeding systems,
- gas insulated transformers.
7 Capitalisation of losses
Capitalisation of losses is considered to be the most economically correct method to optimize transformer
design in accordance with the customer needs. The method is described in Annex E.
It is recognized that to get the benefits of standardization of transformer design, the capitalisation of losses is
normally considered over the fleet of transformers rather than for individual units.
If the efficiency of a transformer purchased using a loss capitalisation formula exceeds the benchmark peak
efficiency proposed for that transformer, it should be used, as the benchmark simply sets a lower floor for
transformer efficiency – it does not attempt to set an optimal value.
Annex A
(normative)
Minimum PEI for dry type large power transformers
The following data are reproduced from Table I.8 of the COMMISSION REGULATION (EU) No 548/2014 of
21 May 2014, although not based on data collected by CENELEC.

Table A.1 — Minimum Peak Efficiency Index requirements for dry-type large power transformers
Tier 1 (01.07.2015) Tier 2 (01.07.2021)
Rated Power (MVA)
Minimum Peak Efficiency Index (%)
≤ 4 99,158 99,225
5 99,200 99,265
6,3 99,242 99,303
8 99,298 99,356
10 99,330 99,385
12,5 99,370 99,422
16 99,416 99,464
20 99,468 99,513
25 99,521 99,564
31,5 99,551 99,592
40 99,567 99,607
50 99,585 99,623
≥ 63 99,590 99,626
Minimum PEI values for MVA ratings that fall in between the ratings given in Table A.1 shall be calculated by
linear interpolation.
Annex B
(informative)
Peak Efficiency Index formula, graphs and calculations
B.1 Calculation of k
PEI
This part of Annex B shows the mathematical derivation of the load factor which maximizes the Efficiency
Index.
Efficiency Index general formula is:
kS - (P +P )-(k²P +P (k))
r c0 k ck
EI =
kS
r
Under the assumption that the cooling at no load is sufficient to operate at k , the P term in the general
PEI ck
formula is neglected.
Hence, the function EI(k) is at its maximum when its derivative is null, then:
'
k.S - (P +P )-k².P
' r c0 k
EI (k)=� �
k.S
r
'
'
�k.S - (P +P )-k².P� .k.S−�k.S - (P +P )-k².P�.[k.S ]
' r c0 k r r c0 k r
0 0
So  EI (k)=
[ ]
k.S
r
�S - 2k.P�.k.S−�k.S - (P +P )-k².P�.[S ]
' r r r r
k c0 k
i.e.  EI (k)=
2 2
k .S
r
2 2
k.S - 2k .P .S -k.S + (P +P ).S +k².P .S
' r k r c0 r k r
r 0
i.e.  EI (k)=
2 2
k .S
r
- k .P .S + (P +P ).S
' k r c0 r
i.e.  EI (k)=
2 2
k .S
r
- k .P + (P +P )
k c0
i.e.  EI(k)=
k .S
r
(P +P )
P
' c0 k
i.e.  EI (k)= −
S
k .S
r
r
(P +P )
P
c0
k 0
it comes that η’(k) = 0 when: =
S
r k .S
r
(P +P ) S
2 c0
0 r
i.e.  k =
S P
r
k
(P +P )
c0
So the Peak Efficiency is a maximum when k=k =�
PEI
P
k
And then
2(P +P )
0 c0
PEI = EI(k ) = 1 −
PEI
P +P
c0
S�
r
P
k
B.2 Graph of Efficiency Index and load factor with loss contributions
B.2.1 General
This paragraph shows in a graphical way what is the influence of P , P , P and P on the efficiency index
0 c0 k ck
EI(k) by means of two numerical examples. Such examples demonstrate also the rationale behind the
prescription given in this standard about including either P or [P + P (k )] in the formula for PEI
c0 c0 ck PEI
calculation.
B.2.2 Example of a typical ONAN or ONAN/ONAF transformer
Table B.1
Transformer data
S 40000 kVA
r
P 20 kW
P 160 k
...