IEC 60076-1:1993

IEC 60076-1
Edition 2.1 2000-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Power transformers –
Part 1: General
Transformateurs de puissance –
Partie 1: Généralités
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IEC 60076-1
Edition 2.1 2000-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Power transformers –
Part 1: General
Transformateurs de puissance –
Partie 1: Généralités
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CN
CODE PRIX
ICS 29.180 ISBN 2-8318-5034-7
60076-1 © IEC:1993+A1:1999 – 3 –
– 2 – 60076-1 © IEC:1993+A1:1999
CONTENTS
Page
FOREWORD . 4
Clause
1 Scope and service conditions. 5
1.1 Scope . 5
1.2 Service conditions. 5
2 Normative references. .6
3 Definitions. .7
3.1 General. .7
3.2 Terminals and neutral point. .8
3.3 Windings. . 9
3.4 Rating. 10
3.5 Tappings. 1 1
3.6 Losses and no-load current. 13
3.7 Short-circuit impedance and voltage drop . 13
3.8 Temperature rise . 14
3.9 Insulation. 14
3.10 Connections. 14
3.11 Kinds of tests. 15
3.12 Meteorological data with respect to cooling. 16
4 Rating. 16
4.1 Rated power . 16
4.2 Loading cycle. 17
4.3 Preferred values of rated power . 17
4.4 Operation at higher than rated voltage and/or at disturbed frequency. 17
5 Requirements for transformers having a tapped winding . 17
5.1 General – Notation of tapping range . 17
5.2 Tapping voltage – tapping current. Standard categories of tapping voltage
variation. Maximum voltage tapping . 18
5.3 Tapping power. Full-power tappings – reduced-power tappings . 2 1
5.4 Specification of tappings in enquiry and order. 22
5.5 Specification of short-circuit impedance . 22
5.6 Load loss and temperature rise. 23
6 Connection and phase displacement symbols for three-phase transformers . 23
7 Rating plates. 26
7.1 Information to be given in all cases. 26
7.2 Additional information to be given when applicable . 26
8 Miscellaneous requirements. 27
8.1 Dimensioning of neutral connection . 27
8.2 Oil preservation system . 27
8.3 Load rejection on generator transformers. 28
9 Tolerances. 28

60076-1 © IEC:1993+A1:1999 – 5 –
60076-1 © IEC:1993+A1:1999 – 3 –
Clause Page
10 Tests . 30
10.1 General requirements for routine, type and special tests. 30
10.2 Measurement of winding resistance . 3 1
10.3 Measurement of voltage ratio and check of phase displacement . 32
10.4 Measurement of short-circuit impedance and load loss . 32
10.5 Measurement of no-load loss and current . 32
10.6 Measurement of the harmonics of the no-load current. 33
10.7 Measurement of zero-sequence impedance(s) on three-phase transformers . 33
10.8 Tests on on-load tap-changers. 34
11 Electromagnetic compatibility (EMC). 34
Annex A (normative) Information required with enquiry and order. 35
Annex B (informative) Examples of specifications for transformers with tappings. 38
Annex C (informative) Specification of short-circuit impedance by boundaries. 40
Annex D (informative) Three-phase transformer connections. 4 1
Annex E (normative) Temperature correction of load loss. 44
Annex F (informative) Bibliography. 45
Figure 1a) – Constant flux voltage variation CFVV .20
Figure 1b) – Variable flux voltage variation VFVV.20
Figure 1c) – Combined voltage variation CbVV.2 1
Figure 2 – Illustration of 'clock number' notation – three examples .24
Figure C.1 – Example of specification of short-circuit impedance by boundaries.40
Figure D.1 – Common connections.4 1
Figure D.2 – Additional connections .42
Figure D.3 – Designation of connections of three-phase auto-transformers by connection
symbols. Auto-transformer Ya0. .43
Figure D.4 – Example of three single-phase transformers connected to form a three-phase
bank (connection symbol Yd5).43

60076-1 © IEC:1993+A1:1999 – 7 –
– 4 – 60076-1 © IEC:1993+A1:1999
INTERNATIONAL ELECTROTECHNICAL COMMISSION
___________
POWER TRANSFORMERS –
Part 1: General
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, the IEC publishes International Standards. Their preparation is
entrusted to technical committees; any IEC National Committee interested in the subject dealt with may
participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. The IEC collaborates closely with the International
Organization for Standardization (ISO) in accordance with conditions determined by agreement between the
two organizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical specifications, technical reports or guides and they are accepted by the National
Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
This International Standard has been prepared by IEC by technical committee 14: Power
transformers.
This consolidated version of IEC 60076-1 consists of the second edition (1993) [documents
14(CO)75 and 14(CO)77], its amendment 1 (1999) [documents 14/344/FDIS and 14/345/RVD]
and its corrigendum of June 1997.
The technical content is therefore identical to the base edition and its amendment and has
been prepared for user convenience.
It bears the edition number 2.1.
A vertical line in the margin shows where the base publication has been modified by
amendment 1.
IEC 60076 consists of the following parts, under the general title: Power transformers.
Part 1:1993, General.
Part 2:1993, Temperature rise.
Part 3:1980, Insulation levels and dielectric tests.
Part 5:1976, Ability to withstand short circuit.
Annexes A and E form an integral part of this standard.
Annexes B, C, D and F are for information only.

60076-1 © IEC:1993+A1:1999 – 9 –
60076-1 © IEC:1993+A1:1999 – 5 –
POWER TRANSFORMERS –
Part 1: General
1 Scope and service conditions
1.1 Scope
This part of International Standard IEC 60076 applies to three-phase and single-phase power
transformers (including auto-transformers) with the exception of certain categories of small
and special transformers such as:
– single-phase transformers with rated power less than 1 kVA and three-phase transformers
less than 5 kVA;
– instrument transformers;
– transformers for static convertors;
– traction transformers mounted on rolling stock;
– starting transformers;
– testing transformers;
– welding transformers.
When IEC standards do not exist for such categories of transformers, this part of IEC 60076
may still be applicable either as a whole or in part.
For those categories of power transformers and reactors which have their own IEC standards,
this part is applicable only to the extent in which it is specifically called up by cross-reference
*
in the other standard.
At several places in this part it is specified or recommended that an 'agreement' shall be
reached concerning alternative or additional technical solutions or procedures. Such
agreement is to be made between the manufacturer and the purchaser. The matters should
preferably be raised at an early stage and the agreements included in the contract
specification.
1.2 Service conditions
1.2.1 Normal service conditions
This part of IEC 60076 gives detailed requirements for transformers for use under the
following conditions:
a) Altitude
A height above sea-level not exceeding 1 000 m (3 300 ft).
b) Temperature of ambient air and cooling medium
A temperature of ambient air not below –25 °C and not above +40 °C. For water-cooled
transformers, a temperature of cooling water at the inlet not exceeding +25 °C.
___________
*
Such standards exist for dry-type transformers (IEC 60726), for reactors in general (IEC 60289), for traction
transformers and reactors (IEC 60310), and are under preparation for static convertor transformers.

60076-1 © IEC:1993+A1:1999 – 11 –
– 6 – 60076-1 © IEC:1993+A1:1999
Further limitations, with regard to cooling are given for:
– oil-immersed transformers in IEC 60076-2;
– dry-type transformers in IEC 60726.
c) Wave shape of supply voltage
A supply voltage of which the wave shape is approximately sinusoidal.
NOTE This requirement is normally not critical in public supply systems but may have to be considered in
installations with considerable convertor loading. In such cases there is a conventional rule that the
deformation shall neither exceed 5 % total harmonic content nor 1 % even harmonic content. Also note
the importance of current harmonics for load loss and temperature rise.
d) Symmetry of three-phase supply voltage
For three-phase transformers, a set of three-phase supply voltages which are
approximately symmetrical.
e) Installation environment
An environment with a pollution rate (see IEC 60137 and IEC 60815) that does not require
special consideration regarding the external insulation of transformer bushings or of the
transformer itself.
An environment not exposed to seismic disturbance which would otherwise require special
consideration in the design. (This is assumed to be the case when the ground acceleration
*
level a is below 2 m/s.)
g
1.2.2 Provision for unusual service conditions
Any unusual service conditions which may lead to special consideration in the design of a
transformer shall be stated in the enquiry and the order. These may be factors such as high
altitude, extreme high or low temperature, tropical humidity, seismic activity, severe
contamination, unusual voltage or load current wave shapes and intermittent loading. They
may also concern conditions for shipment, storage and installation, such as weight or space
limitations (see annex A).
Supplementary rules for rating and testing are given in other publications for:
– Temperature rise and cooling in high ambient temperature or at high altitude: IEC 60076-2
for oil-immersed transformers, and IEC 60726 for dry-type transformers.
– External insulation at high altitude: IEC 60076-3 and IEC 60076-3-1 for oil-immersed
transformers, and IEC 60726 for dry-type transformers.
2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this part of IEC 60076. At the time of publication, the editions
indicated were valid. All normative documents are subject to revision, and parties to agree-
ments based on this part of IEC 60076 are encouraged to investigate the possibility of
applying the most recent edition of the normative documents indicated below. Members of IEC
and ISO maintain registers of currently valid International Standards.
___________
*
See IEC 60068-3-3.
60076-1 © IEC:1993+A1:1999 – 13 –
60076-1 © IEC:1993+A1:1999 – 7 –
IEC 60050(421):1990, International Electrotechnical Vocabulary – Chapter 421: Power
transformers and reactors
IEC 60068-3-3:1991, Environmental testing – Part 3: Guidance. Seismic test methods for
equipments
IEC 60076-2:1993, Power transformers – Part 2: Temperature rise
IEC 60076-3:1980, Power transformers – Part 3: Insulation levels and dielectric tests
IEC 60076-3-1:1987, Power transformers – Part 3: Insulation levels and dielectric tests.
External clearances in air
IEC 60076-5:1976, Power transformers – Part 5: Ability to withstand short circuit
IEC 60137:1984, Bushings for alternating voltages above 1 000 V
IEC 60354:1991, Loading guide for oil-immersed power transformers
IEC 60529:1989, Degrees of protection provided by enclosures (IP Code)
IEC 60551:1987, Determination of transformer and reactor sound levels
IEC 60606:1978, Application guide for power transformers
IEC 60726:1982, Dry-type power transformers
IEC 60815:1986, Guide for the selection of insulators in respect of polluted conditions
IEC 60905:1987, Loading guide for dry-type power transformers
ISO 3:1973, Preferred numbers – Series of preferred numbers
ISO 9001:1987, Quality systems – Model for quality assurance in design/development,
production, installation and servicing
3 Definitions
For the purpose of this part of IEC 60076, the following definitions shall apply. Other terms
use the meanings ascribed to them in the International Electrotechnical Vocabulary (IEV).
3.1 General
3.1.1
power transformer
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 voltage and
current usually of different values and at the same frequency for the purpose of transmitting
electrical power [IEV 421-01-01, modified]

60076-1 © IEC:1993+A1:1999 – 15 –
– 8 – 60076-1 © IEC:1993+A1:1999
3.1.2
*
auto-transformer
a transformer in which at least two windings have a common part [IEV 421-01-11]
3.1.3
booster transformer
a transformer of which one winding is intended to be connected in series with a circuit in order
to alter its voltage and/or shift its phase. The other winding is an energizing winding
[IEV 421-01-12, modified]
3.1.4
oil-immersed type transformer
a transformer of which the magnetic circuit and windings are immersed in oil [IEV 421-01-14]
NOTE For the purpose of this part any insulating liquid, mineral oil or other product, is regarded as oil.
3.1.5
dry-type transformer
a transformer of which the magnetic circuit and windings are not immersed in an insulating
liquid [IEV 421-01-16]
3.1.6
oil preservation system
the system in an oil-immersed transformer by which the thermal expansion of the oil is
accommodated. Contact between the oil and external air may sometimes be diminished or
prevented
3.2 Terminals and neutral point
3.2.1
terminal
a conducting element intended for connecting a winding to external conductors
3.2.2
line terminal
a terminal intended for connection to a line conductor of a network [IEV 421-02-01]
3.2.3
neutral terminal
a) For three-phase transformers and three-phase banks of single-phase transformers:
The terminal or terminals connected to the common point (the neutral point) of a star-
connected or zigzag connected winding.
b) For single-phase transformers:
The terminal intended for connection to a neutral point of a network [IEV 421-02-02,
modified]
3.2.4
neutral point
the point of a symmetrical system of voltages which is normally at zero potential
3.2.5
corresponding terminals
terminals of different windings of a transformer, marked with the same letter or corresponding
symbol [IEV 421-02-03]
___________
*
Where there is a need to express that a transformer is not auto-connected, use is made of terms such as
separate winding transformer, or double-wound transformer (see IEV 421-01-13).

60076-1 © IEC:1993+A1:1999 – 17 –
60076-1 © IEC:1993+A1:1999 – 9 –
3.3 Windings
3.3.1
winding
the assembly of turns forming an electrical circuit associated with one of the voltages
assigned to the transformer
NOTE For a three-phase transformer, the 'winding' is the combination of the phase windings (see 3.3.3).
[IEV 421-03-01, modified]
3.3.2
tapped winding
a winding in which the effective number of turns can be changed in steps
3.3.3
phase winding
the assembly of turns forming one phase of a three-phase winding
NOTE The term 'phase winding' should not be used for identifying the assembly of all coils on a specific leg.
[IEV 421-03-02, modified]
3.3.4
high-voltage winding*
the winding having the highest rated voltage [IEV 421-03-03]
3.3.5
low-voltage winding*
the winding having the lowest rated voltage [IEV 421-03-04]
NOTE For a booster transformer, the winding having the lower rated voltage may be that having the higher
insulation level.
3.3.6
intermediate-voltage winding*
a winding of a multi-winding transformer having a rated voltage intermediate between the
highest and lowest winding rated voltages [IEV 421-03-05]
3.3.7
auxiliary winding
a winding intended only for a small load compared with the rated power of the transformer
[IEV 421-03-08]
3.3.8
stabilizing winding
a supplementary delta-connected winding provided in a star-star-connected or star-zigzag-
connected transformer to decrease its zero-sequence impedance, see 3.7.3 [IEV 421-03-09,
modified]
NOTE A winding is referred to as a stabilizing winding only if it is not intended for three-phase connection to an
external circuit.
3.3.9
common winding
the common part of the windings of an auto-transformer [IEV 421-03-10]
___________
*
The winding which receives active power from the supply source in service is referred to as a 'primary winding',
and that which delivers active power to a load as a 'secondary winding'. These terms have no significance as to
which of the windings has the higher rated voltage and should not be used except in the context of direction of
active power flow (see IEV 421-03-06 and 07). A further winding in the transformer, usually with lower value of
rated power than the secondary winding, is then often referred to as 'tertiary winding', see also definition 3.3.8.

60076-1 © IEC:1993+A1:1999 – 19 –
– 10 – 60076-1 © IEC:1993+A1:1999
3.3.10
series winding
the part of the winding of an auto-transformer or the winding of a booster transformer which is
intended to be connected in series with a circuit [IEV 421-03-11]
3.3.11
energizing winding
the winding of a booster transformer which is intended to supply power to the series winding
[IEV 421-03-12]
3.4 Rating
3.4.1
rating
those numerical values assigned to the quantities which define the operation of the trans-
former in the conditions specified in this part of IEC 60076 and on which the manufacturer's
guarantees and the tests are based
3.4.2
rated quantities
quantities (voltage, current, etc.), the numerical values of which define the rating
NOTE 1 For transformers having tappings, rated quantities are related to the principal tapping (see 3.5.2), unless
otherwise specified. Corresponding quantities with analogous meaning, related to other specific tappings, are
called tapping quantities (see 3.5.10).
NOTE 2 Voltages and currents are always expressed by their r.m.s. values, unless otherwise specified.
3.4.3
rated voltage of a winding (U )
r
the voltage assigned to be applied, or developed at no-load, between the terminals of an
untapped winding, or of a tapped winding connected on the principal tapping (see 3.5.2). For
a three-phase winding it is the voltage between line terminals [IEV 421-04-01, modified]
NOTE 1 The rated voltages of all windings appear simultaneously at no-load when the voltage applied to one of
them has its rated value.
NOTE 2 For single-phase transformers intended to be connected in star to form a three-phase bank, the rated
voltage is indicated as phase-to-phase voltage, divided by 3 for example U = 400 3 kV.
r
NOTE 3 For the series winding of a three-phase booster transformer which is designed as an open winding (see
3.10.5) the rated voltage is indicated as if the winding were connected in star, for example U = 23 3 kV.
r
3.4.4
rated voltage ratio
the ratio of the rated voltage of a winding to the rated voltage of another winding associated
with a lower or equal rated voltage [IEV 421-04-02]
3.4.5
rated frequency (f )
r
the frequency at which the transformer is designed to operate [IEV 421-04-03, modified]
3.4.6
rated power (S )
r
a conventional value of apparent power assigned to a winding which, together with the rated
voltage of the winding, determines its rated current
NOTE 1 Both windings of a two-winding transformer have the same rated power which by definition is the rated
power of the whole transformer.

60076-1 © IEC:1993+A1:1999 – 21 –
60076-1 © IEC:1993+A1:1999 – 11 –
NOTE 2 For a multi-winding transformer, half the arithmetic sum of the rated power values of all windings
(separate windings, not auto-connected) gives a rough estimate of its physical size as compared with a two-
winding transformer.
3.4.7
rated current (I )
r
the current flowing through a line terminal of a winding which is derived from rated power S
r
and rated voltage U for the winding [IEV 421-04-05, modified]
r
I
NOTE 1 For a three-phase winding the rated current is given by:
r
S
r
I = A
r
3 × U
r
NOTE 2 For single-phase transformer windings intended to be connected in delta to form a three-phase bank the
rated current is indicated as line current divided by 3 , for example:
I A
=
r
3.5 Tappings
3.5.1
tapping
in a transformer having a tapped winding, a specific connection of that winding, representing
a definite effective number of turns in the tapped winding and, consequently, a definite turns
ratio between this winding and any other winding with fixed number of turns
NOTE One of the tappings is the principal tapping, and other tappings are described in relation to the principal
tapping by their respective tapping factors. See definitions of these terms below.
3.5.2
principal tapping
the tapping to which the rated quantities are related [IEV 421-05-02]
3.5.3
tapping factor (corresponding to a given tapping)
The ratio:
U U
d d
(tapping factor) or 100 (tapping factor expressed as a percentage)
U U
r r
where
U is the rated voltage of the winding (see 3.4.3);
r
U is the voltage which would be developed at no-load at the terminals of the winding, at
d
the tapping concerned, by applying rated voltage to an untapped winding.
NOTE This definition is not appropriate in relation to a series winding of a booster transformer (see 3.1.3), and in
that case the percentage notation would be referred to the voltage of the energizing winding or of the winding of an
associated system transformer. [IEV 421-05-03, modified]
3.5.4
plus tapping
a tapping whose tapping factor is higher than 1 [IEV 421-05-04]
3.5.5
minus tapping
a tapping whose tapping factor is lower than 1 [IEV 421-05-05]

60076-1 © IEC:1993+A1:1999 – 23 –
– 12 – 60076-1 © IEC:1993+A1:1999
3.5.6
tapping step
the difference between the tapping factors, expressed as a percentage, of two adjacent
tappings [IEV 421-05-06]
3.5.7
tapping range
the variation range of the tapping factor, expressed as a percentage, compared with the value
'100'
NOTE If this factor ranges from 100 + a to 100 – b, the tapping range is said to be: +a %, –b % or ±a %, if a = b.
[IEV 421-05-07]
3.5.8
tapping voltage ratio (of a pair of windings)
the ratio which is equal to the rated voltage ratio:
– multiplied by the tapping factor of the tapped winding if this is the high-voltage winding;
– divided by the tapping factor of the tapped winding if this is the low-voltage winding.
[IEV 421-05-08]
NOTE While the rated voltage ratio is, by definition, at least equal to 1, the tapping voltage ratio can be lower
than 1 for certain tappings when the rated voltage ratio is close to 1.
3.5.9
tapping duty
the numerical values assigned to the quantities, analogous to rated quantities, which refer to
tappings other than the principal tapping (see clause 5, and IEC 60606) [IEV 421-05-09,
modified]
3.5.10
tapping quantities
those quantities the numerical values of which define the tapping duty of a particular tapping
(other than the principal tapping)
NOTE Tapping quantities exist for any winding in the transformer, not only for the tapped winding, (see 5.2
and 5.3).
The tapping quantities are:
– tapping voltage (analogous to rated voltage, 3.4.3);
– tapping power (analogous to rated power, 3.4.6);
– tapping current (analogous to rated current, 3.4.7). [IEV 421-05-10, modified]
3.5.11
full-power tapping
a tapping whose tapping power is equal to the rated power [IEV 421-05-14]
3.5.12
reduced-power tapping
a tapping whose tapping power is lower than the rated power [IEV 421-05-15]
3.5.13
on-load tap-changer
a device for changing the tapping connections of a winding, suitable for operation while the
transformer is energized or on load [IEV 421-11-01]

60076-1 © IEC:1993+A1:1999 – 25 –
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3.6 Losses and no-load current
NOTE The values are related to the principal tapping, unless another tapping is specifically stated.
3.6.1
no-load loss
the active power absorbed when rated voltage (tapping voltage) at rated frequency is applied
to the terminals of one of the windings, the other winding or windings being open-circuited
[IEV 421-06-01, modified]
3.6.2
no-load current
the r.m.s. value of the current flowing through a line terminal of a winding when rated voltage
(tapping voltage) is applied at rated frequency, the other winding or windings being open-
circuited
NOTE 1 For a three-phase transformer, the value is the arithmetic mean of the values of current in the three
phases.
NOTE 2 The no-load current of a winding is often expressed as a percentage of the rated current of that winding.
For a multi-winding transformer this percentage is referred to the winding with the highest rated power.
[IEV 421-06-02, modified]
3.6.3
load loss
the absorbed active power at rated frequency and reference temperature (see 10.1),
associated with a pair of windings when rated current (tapping 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
NOTE 1 For a two-winding transformer there is only one winding combination and one value of load loss. For a
multi-winding transformer there are several values of load loss corresponding to the different two-winding
combinations (see clause 6 of IEC 60606). A combined load loss figure for the complete transformer is referred to
a specified winding load combination. In general, it is usually not accessible for direct measurement in testing.
NOTE 2 When the windings of the pair have different rated power values the load loss is referred to rated current
in the winding with the lower rated power and the reference power should be mentioned.
3.6.4
total losses
the sum of the no-load loss and the load loss
NOTE The power consumption of the auxiliary plant is not included in the total losses and is stated separately.
[IEV 421-06-05, modified]
3.7 Short-circuit impedance and voltage drop
3.7.1
short-circuit impedance of a pair of windings
the equivalent series impedance Z = R + jX, in ohms, at rated frequency and reference
temperature, across the terminals of one winding of a pair, when the terminals of the other
winding are short-circuited and further windings, if existing, are open-circuited. For a three-
phase transformer the impedance is expressed as phase impedance (equivalent star
connection).
In a transformer having a tapped winding, the short-circuit impedance is referred to a
particular tapping. Unless otherwise specified the principal tapping applies.
NOTE This quantity may be expressed in relative, dimensionless form, as a fraction z of the reference impedance
Z , of the same winding of the pair. In percentage notation:
ref
Z
z = 100
Z
ref
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– 14 – 60076-1 © IEC:1993+A1:1999
where
u
Z = (Formula valid for both three-phase and single-phase transformers).
réf
S
r
U is the voltage (rated voltage or tapping voltage) of the winding to which Z and Z belong.
ref
S is the reference value of rated power.
r
The relative value is also equal to the ratio between the applied voltage during a short-circuit measurement which
causes the relevant rated current (or tapping current) to flow, and rated voltage (or tapping voltage). This applied
voltage is referred to as the short-circuit voltage [IEV 421-07-01) of the pair of windings. It is normally expressed
as a percentage. [IEV 421-07-02, modified]
3.7.2
voltage drop or rise for a specified load condition
the arithmetic difference between the no-load voltage of a winding and the voltage developed
at the terminals of the same winding at a specified load and power factor, the voltage supplied
to (one of) the other winding(s) being equal to:
– its rated value if the transformer is connected on the principal tapping (the no-load voltage
of the former winding is then equal to its rated value);
– the tapping voltage if the transformer is connected on another tapping.
This difference is generally expressed as a percentage of the no-load voltage of the former
winding.
NOTE For multi-winding transformers, the voltage drop or rise depends not only on the load and power factor of
the winding itself, but also on the load and power factor of the other windings (see IEC 60606). [IEV 421-07-03]
3.7.3
zero-sequence impedance (of a three-phase winding)
the impedance, expressed in ohms per phase at rated frequency, between the line terminals
of a three-phase star-connected or zigzag-connected winding, connected together, and its
neutral terminal [IEV 421-07-04, modified]
NOTE 1 The zero-sequence impedance may have several values because it depends on how the terminals of the
other winding or windings are connected and loaded.
NOTE 2 The zero-sequence impedance may be dependent on the value of the current and the temperature,
particularly in transformers without any delta-connected winding.
NOTE 3 The zero-sequence impedance may also be expressed as a relative value in the same way as the
(positive sequence) short-circuit impedance (see 3.7.1).
3.8 Temperature rise
The difference between the temperature of the part under consideration and the temperature
of the external cooling medium. [IEV 421-08-01, modified]
3.9 Insulation
For definitions relating to insulation see IEC 60076-3.
3.10 Connections
3.10.1
star connection (Y-connection)
the winding connection so arranged that each of the phase windings of a three-phase
transformer, or of each of the windings for the same rated voltage of single-phase
transformers associated in a three-phase bank, is connected to a common point (the neutral
point) and the other end to its appropriate line terminal [IEV 421-10-01, modified]

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3.10.2
delta connection (D-connection)
the winding connection so arranged that the phase windings of a three-phase transformer, or
the windings for the same rated voltage of single-phase transformers associated in a three-
phase bank, are connected in series to form a closed circuit [IEV 421-10-02, modified]
3.10.3
open-delta connection
the winding connection in which the phase windings of a three-phase transformer, or the
windings for the same rated voltage of single-phase transformers associated in a three-phase
bank, are connected in series without closing one corner of the delta [IEV 421-10-03]
3.10.4
zigzag connection (Z-connection)
the winding connection in which one end of each phase winding of a three-phase transformer
is connected to a common point (neutral point), and each phase winding consists of two parts
in which phase-displaced voltages are induced
NOTE These two parts normally have the same number of turns. [IEV 421-10-04, modified]
3.10.5
open windings
phase windings of a three-phase transformer which are not interconnected within the
transformer [IEV 421-10-05, modified]
3.10.6
phase displacement of a three-phase winding
the angular difference between the phasors representing the voltages between the neutral
point (real or imaginary) and the corresponding terminals of two windings, a positive-
sequence voltage system being applied to the high-voltage terminals, following each other in
alphabetical sequence if they are lettered, or in numerical sequence if they are numbered.
The phasors are assumed to rotate in a counter-clockwise sense [IEV 421-10-08, modified]
NOTE The high-voltage winding phasor is taken as reference, and the displacement for any other winding is
conventionally expressed by the 'clock notation', that is, the hour indicated by the winding phasor when the H.V.
winding phasor is at 12 o'clock (rising numbers indicate increasing phase lag)
3.10.7
connection symbol
a conventional notation indicating the connections of the high-voltage, intermediate-voltage (if
any), and low-voltage windings and their relative phase displacement(s) expressed as a
combination of letters and clock-hour figure(s) [IEV 421-10-09, modified]
3.11 Kinds of tests
3.11.1
routine test
a test to which each individual transformer is subjected
3.11.2
type test
a test made on a transformer which is representative of other transformers, to demonstrate
that these transformers comply with specified requirements not covered by routine tests
NOTE A transformer is considered to be representative of others if it is fully identical in rating and construction,
but the type test may also be considered valid if it is made on a transformer which has minor deviations of rating or

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other characteristics. These deviations should be subject to agreement between the manufacturer and the
purchaser.
3.11.3
special test
a test other than a type test or a routine test, agreed by the manufacturer and the purchaser
3.12 Meteorological data with respect to cooling
3.12.1
monthly average temperature
half the sum of the average of the daily maxima and the average of the daily minima during a
particular month – over many years
3.12.2
yearly average temperature
one-twelfth of the sum of the monthly average temperatures
4 Rating
4.1 Rated power
The transformer shall have an assigned rated power for each winding which shall be marked
on the rating plate. The rated power refers to continuous loading. This is a reference value for
guarantees and tests concerning load losses and temperature rises.
If different values of apparent power are assigned under different circumstances, for example,
with different methods of cooling, the highest of these values is the rated power.
A two-winding transformer has only one value of rated power, identical for both windings.
When the transformer has rated voltage applied to a primary winding, and rated current flows
through the terminals of a secondary winding, the tran
...