NOTE 1 For transformers having tappings, rated quantities are related to the principal tapping see 3.5.2, unless otherwise specified.. tapping voltage ratio of a pair of windings the rat
General
A power transformer is a static device featuring two or more windings that utilizes electromagnetic induction to convert one system of alternating voltage and current into another, typically with different values but at the same frequency, to facilitate the transmission of electrical power.
3.1.2 auto-transformer a transformer in which at least two windings have a common part
When indicating that a transformer is not auto-connected, terms like "separate winding transformer" or "double-wound transformer" should be used, as defined in IEC 60050-421:1990, 421-01-13 A series transformer, distinct from an autotransformer, features one winding designed to connect in series with a circuit to modify its voltage and/or shift its phase, while the other winding serves as the energizing winding.
NOTE Series transformers were called booster transformers in earlier editions of this standard
3.1.4 liquid-immersed type transformer a transformer in which the magnetic circuit and windings are immersed in liquid
3.1.5 dry-type transformer a transformer in which the magnetic circuit and windings are not immersed in an insulating liquid
3.1.6 liquid preservation system system in a liquid-filled transformer by which the thermal expansion of the liquid is accommodated
NOTE Contact between the liquid and external air may sometimes be diminished or prevented
3.1.7 specified value the value specified by the purchaser at the time of order
The design value refers to the anticipated value derived from the number of turns in the case of turns ratio, or calculated based on the design for parameters such as impedance and no-load current.
3.1.9 highest voltage for equipment U m applicable to a transformer winding the highest r.m.s phase-to-phase voltage in a three-phase system for which a transformer winding is designed in respect of its insulation
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
The neutral terminal in three-phase transformers and three-phase banks of single-phase transformers refers to the terminal connected to the common point, or neutral point, of a star or zigzag connected winding In the case of single-phase transformers, the neutral terminal is designated for connection to the neutral point of a network.
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
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)
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
HV winding* the winding having the highest rated voltage
LV winding* the winding having the lowest rated voltage
In electrical transformers, the 'primary winding' is the one that receives active power from the supply source, while the 'secondary winding' delivers active power to a load These designations are relevant only in the context of active power flow direction and do not indicate which winding has a higher voltage rating (refer to IEC 60050-421:1990, 421-03-06 and 07) Additionally, a 'tertiary winding' may exist, typically with a lower power rating than the secondary winding, which is also important to consider in transformer insulation levels.
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
3.3.7 auxiliary winding a winding intended only for a small load compared with the rated power of the transformer [IEC 60050-421:1990, 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
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
3.3.10 series winding the part of the winding of an auto-transformer or the winding of a series transformer which is intended to be connected in series with a circuit
3.3.11 energizing winding (of a series transformer) the winding of a series transformer which is intended to supply power to the series winding [IEC 60050-421:1990, 421-03-12, modified]
3.3.12 auto-connected windings the series and common windings of an auto-transformer
Rating
In accordance with IEC 60076, section 3.4.1, the rating refers to the numerical values assigned to the quantities that define the operation of the transformer These ratings are essential as they form the basis for the manufacturer's guarantees and the tests conducted on the transformer.
3.4.2 rated quantities quantities (voltage, current, etc.), the numerical values of which define the rating
For transformers with tappings, the rated quantities are associated with the principal tapping unless stated otherwise Quantities related to other specific tappings are referred to as tapping quantities.
NOTE 2 Voltages and currents are always expressed by their r.m.s values, unless otherwise specified
The voltage, denoted as U, is the value designated to be applied or generated at no-load across the terminals of an untapped winding or a tapped winding connected at the main tapping In the case of a three-phase winding, this voltage refers to the measurement between the line terminals.
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
For single-phase transformers designed for star connection in a three-phase bank or for connection between the line and neutral of a three-phase system, the rated voltage is specified as the phase-to-phase voltage divided by 3, such as 400/√3 kV.
NOTE 3 For single phase transformers intended to be connected between phases of a network, the rated voltage is indicated as the phase-to-phase voltage
In a three-phase series transformer with an open winding design, the rated voltage for the series winding is specified as if the windings were connected in a star configuration.
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
3.4.5 rated frequency f r the frequency at which the transformer is designed to operate
S r conventional value of apparent power assigned to a winding which, together with the rated voltage of the winding, determines its rated current
NOTE Both windings of a two-winding transformer have the same rated power which by definition is the rated power of the whole transformer
I r the current flowing through a line terminal of a winding which is derived from rated power S r and rated voltage U r for the winding
NOTE 1 For a three-phase winding the rated current I r is given by: r r r
= × rated current is indicated as line current divided by 3 , r I line 3
NOTE 3 For a single phase transformer not intended to be connected to form a three phase bank, the rated current is r r U r
For open windings of a transformer, the rated current is calculated by dividing the rated power by the number of phases and the rated voltage of the open winding.
Tappings
Tapping in a transformer refers to a specific connection within a tapped winding, which indicates a precise effective number of turns This configuration establishes a definite turns ratio between the tapped winding and any other winding that has a fixed number of turns.
One tapping is designated as the principal tapping, while other tappings are defined in relation to it through their specific tapping factors Definitions of these terms are provided below.
3.5.2 principal tapping the tapping to which the rated quantities are related
3.5.3 tapping factor (corresponding to a given tapping) the ratio: r
U (tapping factor expressed as a percentage) where
U r is the rated voltage of the winding (see 3.4.3);
U d is the voltage which would be developed at no-load at the terminals of the winding, at the tapping concerned, by applying rated voltage to an untapped winding
NOTE For series transformers, the tapping factor is the ratio of the voltage of the series winding corresponding to a given tapping to U r
3.5.4 plus tapping a tapping whose tapping factor is higher than 1
3.5.5 minus tapping a tapping whose tapping factor is lower than 1
3.5.6 tapping step the difference between the tapping factors, expressed as a percentage, of two adjacent tappings
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
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
The rated voltage ratio is defined to be at least 1, while the tapping voltage ratio may be less than 1 for specific tappings when the rated voltage ratio approaches 1.
3.5.9 tapping quantities those quantities the numerical values of which define the 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 6.2 and 6.3).
– 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)
3.5.10 full-power tapping a tapping whose tapping power is equal to the rated power
3.5.11 reduced-power tapping a tapping whose tapping power is lower than the rated power
[IEC 60050-421:1990, 421-05-15] on-load tap-changer
OLTC a device for changing the tapping connections of a winding, suitable for operation while the transformer is energized or on load
DETC a device for changing the tapping connections of a winding, suitable for operation only while the transformer is de-energized (isolated from the system)
3.5.14 maximum allowable tapping service voltage the voltage at rated frequency a transformer is designed to withstand continuously without damage at any particular tap position at the relevant tapping power
NOTE 1 This voltage is limited by U m
The voltage is typically restricted to 105% of the rated tapping voltage, unless the purchaser's specifications explicitly require a higher voltage, as outlined in section 6.4 or as per the specifications in section 6.4.2.
Losses and no-load current
NOTE The values are related to the principal tapping (see 3.5.2), unless another tapping is specifically stated
No-load loss refers to the active power consumed when a rated voltage, known as tapping voltage, is applied at a rated frequency to the terminals of one winding, while the other winding or windings remain open-circuited.
The no-load current is defined as the root mean square (r.m.s.) value of the current that flows through a line terminal of a winding when the rated voltage, also known as tapping voltage, is applied at the rated frequency This occurs while the other winding or windings remain open-circuited.
NOTE 1 For a three-phase transformer, the value is the arithmetic mean of the values of current in the three lines
The no-load current of a winding is typically represented as a percentage of its rated current In the case of a multi-winding transformer, this percentage is calculated based on the winding with the highest rated power.
Load loss refers to the absorbed active power at a rated frequency and reference temperature, occurring in a pair of windings when rated current flows through the line terminals of one winding while the terminals of the other winding are short-circuited Any additional windings present are left open-circuited.
In a two-winding transformer, there is a single winding combination and a specific load loss value Conversely, a multi-winding transformer features multiple load loss values associated with various two-winding combinations, as outlined in Clause 7 of IEC 60076-8:1997 The overall load loss for the transformer is typically referenced to a designated winding load combination, which is generally not directly measurable during testing.
When the windings of a pair have varying rated power values, the load loss should be referenced to the rated current of the winding with the lower power rating, and it is essential to specify the reference power.
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
Short-circuit impedance and voltage drop
The short-circuit impedance of a pair of windings, denoted as \$Z = R + jX\$ in ohms, is measured at the rated frequency and reference temperature across the terminals of one winding while the other winding's terminals are short-circuited, with any additional windings left open-circuited For a three-phase transformer, this impedance is represented as phase impedance in an equivalent star connection.
NOTE 1 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 2 This quantity can be expressed in relative, dimensionless form, as a fraction z of the reference impedance Z ref , of the same winding of the pair In percentage notation: z = 100
Z = U (formula valid for both three-phase and single-phase transformers);
U is the voltage (rated voltage or tapping voltage) of the winding to which Z and Z ref belong;
S r is the reference value of rated power
The relative value is defined as the ratio of the applied voltage during a short-circuit measurement, which induces the rated or tapping current, to the rated or tapping voltage This applied voltage, known as the short-circuit voltage (IEC 60050-421:1990, 421-07-01), is typically expressed as a percentage.
The voltage drop or rise for a specified load condition is defined as the arithmetic difference between the no-load voltage of a winding and the voltage measured at the terminals of that winding under a specific load and power factor, with the voltage supplied to one of the other windings remaining constant.
– its rated value if the transformer is connected on the principal tapping (the no-load voltage of the 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 winding
In multi-winding transformers, the voltage variation is influenced by the load and power factor of both the specific winding and the other windings, as outlined in IEC 60076-8.
Zero-sequence impedance, as defined in IEC 60050-421:1990, refers to the impedance measured in ohms per phase at rated frequency It is the impedance between the line terminals of a three-phase winding, whether star-connected or zigzag-connected, when these terminals are connected together with the neutral terminal.
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).
Temperature rise
The difference between the temperature of the part under consideration and the temperature of the external cooling medium (see IEC 60076-2)
Insulation
For terms and definitions relating to insulation, see IEC 60076-3.
Connections
In a star connection, the windings of a three-phase transformer are configured so that each phase winding connects to a common neutral point, while the other ends are linked to their respective line terminals This arrangement is also applicable to single-phase transformers grouped in a three-phase bank, ensuring they share the same rated voltage.
NOTE Star connection is sometimes referred to as Y-connection
The delta connection is a configuration where the phase windings of a three-phase transformer, or the windings of single-phase transformers grouped in a three-phase bank, are connected in series to create a closed circuit.
NOTE Delta connection is sometimes referred to as D-connection
An open-delta connection refers to a winding configuration in which the phase windings of a three-phase transformer, or the windings of single-phase transformers grouped in a three-phase bank, are connected in series without completing one corner of the delta.
The Z-connection is a winding configuration that includes two sections: the first section is connected in a star formation, while the second section is connected in series between the first section and the line terminals Each phase of the second section is wound on a different limb of the transformer, corresponding to the part of the first section to which it is linked.
NOTE See Annex D for cases where the winding sections have equal voltages
3.10.5 open windings the phase windings of a three-phase transformer which are not interconnected within the transformer
Phase displacement in a three-phase winding refers to the angular difference between the phasors that represent the voltages at the neutral point and the terminals of two windings When a positive-sequence voltage system is applied to the high-voltage terminals, the windings are typically labeled in alphabetical or numerical order It is important to note that the phasors are assumed to rotate counter-clockwise.
NOTE 1 See Clause 7 and Annex D
In electrical engineering, the high-voltage winding phasor serves as the reference point, with the displacement of other windings represented using 'clock notation.' This notation indicates the position of the winding phasor relative to the high-voltage winding phasor, which is set at 12 o'clock, where rising numbers signify an increasing phase lag.
The connection symbol is a standard notation that represents the interconnections of high-voltage, intermediate-voltage (if applicable), and low-voltage windings, along with their corresponding phase displacements This notation is expressed through a combination of letters and clock-hour figures.
Test classification
3.11.1 routine test a test to which each individual transformer is subjected
A type test is conducted on a transformer that serves as a representative model for other transformers, ensuring compliance with specified requirements not addressed by routine tests A transformer is deemed representative if it is constructed according to the same designs, utilizing identical techniques and materials, and manufactured in the same facility.
NOTE 1 Design variations that are clearly irrelevant to a particular type test would not require that type test to be repeated
NOITE 2 Design variations that cause a reduction in values and stresses relevant to a particular type test do not require a new type test if accepted by the purchaser and the manufacturer
Transformers with a capacity below 20 MVA and a voltage rating of U m ≤ 72.5 kV may allow for considerable design variations, provided there is evidence of compliance with type test requirements Additionally, a special test, which is distinct from type or routine tests, can be conducted if mutually agreed upon by the manufacturer and the purchaser.
Special tests may be conducted on individual transformers or on all transformers of a specific design, as requested by the purchaser in their inquiry and order for each special test.
Meteorological data with respect to cooling
3.12.1 temperature of cooling medium at any time the maximum temperature of the cooling medium measured over many years
3.12.2 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
3.12.3 yearly average temperature one-twelfth of the sum of the monthly average temperatures
Other definitions
3.13.1 load current the r.m.s value of the current in any winding under service conditions
3.13.2 total harmonic content the ratio of the effective value of all the harmonics to the effective value of the fundamental (E 1 , I 1 ) total harmonic content E 1 n E i i i
(for voltage) total harmonic content l 1 n l i i i
E i represents the r.m.s value of voltage of the i th harmonic
I i represents the r.m.s value of current of the i th harmonic
3.13.3 even harmonic content the ratio of the effective value of all the even harmonics to the effective value of the fundamental (E 1 , I 1 ) even harmonic content E 1 n E i i i
(for voltage) even harmonic content l 1 n l i i i
E i represents the r.m.s value of voltage of the i th harmonic
I i represents the r.m.s value of current of the i th harmonic
General
The service conditions outlined in section 4.2 define the standard operational parameters for transformers specified by this standard For atypical service conditions that necessitate special design considerations, refer to section 5.5 These conditions may include factors such as high altitude, extreme temperatures of the cooling medium, tropical humidity, seismic activity, severe contamination, unusual voltage or load current waveforms, high solar radiation, and intermittent loading Additionally, considerations for shipment, storage, and installation, including weight and space limitations, are addressed in Annex A.
Supplementary rules for rating and testing are given in the following publications:
– temperature rise and cooling in high external cooling medium temperature or at high altitude: IEC 60076-2 for liquid-immersed transformers, and IEC 60076-11 for dry-type transformers;
– external insulation at high altitude: IEC 60076-3 for liquid-filled transformers, and IEC 60076-11 for dry-type transformers.
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. b) Temperature of cooling medium
The temperature of cooling air at the inlet to the cooling equipment not exceeding:
30 °C monthly average of the hottest month;
20 °C yearly average and not below:
–25 °C in the case of outdoor transformers;
–5 °C in the case of transformers where both the transformer and cooler are intended for installation indoors
At any time, monthly average and yearly average are defined in 3.12
The purchaser may specify a higher minimum temperature of cooling medium in which case the minimum temperature of cooling medium shall be stated on the rating plate
This paragraph permits the use of an alternative insulating liquid that does not meet the minimum temperature requirement of -25 °C in situations where this threshold is not suitable.
For water-cooled transformers, a temperature of cooling water at the inlet not exceeding:
Further limitations, with regard to cooling are given for:
– liquid-immersed transformers in IEC 60076-2;
– dry-type transformers in IEC 60076-11
For transformers equipped with both air/water and water/liquid heat exchangers, it is important to note that the cooling medium's temperature is based on the external air temperature, rather than the potentially elevated water temperature in the intermediate circuit.
The relevant temperature for cooling equipment is measured at the inlet, not the outside air temperature Users must ensure that any potential air recirculation from the cooler's output is considered when evaluating the cooling air temperature Additionally, the shape of the supply voltage waveform is an important factor to consider.
A sinusoidal supply voltage with a total harmonic content not exceeding 5 % and an even harmonic content not exceeding 1 % d) Load current harmonic content
Total harmonic content of the load current not exceeding 5 % of rated current
Transformers designed for loads with total harmonic content exceeding 5% of the rated current, or those intended for power electronic or rectifier applications, must comply with the IEC 61378 series standards.
Transformers can function at their rated current without significant loss of lifespan when the current harmonic content is below 5% However, it is important to recognize that any harmonic loading can lead to an increase in temperature rise, potentially surpassing the rated limits Additionally, the symmetry of the three-phase supply voltage plays a crucial role in the performance of transformers.
Three-phase transformers require a set of approximately symmetrical three-phase supply voltages, where the highest phase-to-phase voltage does not exceed 1% above the lowest phase-to-phase voltage continuously, or 2% higher for short durations of about 30 minutes under exceptional conditions.
An environment with a pollution rate (see IEC 60137 and IEC/TS 60815) that does not require special consideration regarding the external insulation of transformer bushings or of the transformer itself
In environments with ground acceleration levels below 2 m/s² (approximately 0.2 g), seismic disturbances are minimal, allowing for standard design considerations Refer to IEC 60068-3-3 for further guidance on this topic.
When a transformer is installed in an enclosure not provided by the manufacturer and located away from the cooling equipment, such as in an acoustic enclosure, it is essential to ensure that the surrounding air temperature does not exceed 40 ºC at any time.
Environmental conditions within the following definitions according to IEC 60721-3-4:1995: – climatic conditions 4K2 except that the minimum external cooling medium temperature is –25 ºC;
For transformers intended to be installed indoors, some of these environmental conditions may not be applicable
Rated power
General
The purchaser must either specify the rated power for each winding or provide adequate information to the manufacturer to ascertain the rated power during the inquiry stage.
The transformer will feature a designated rated power for each winding, clearly indicated on the rating plate This rated power signifies continuous loading and serves as a benchmark for guarantees and tests related to load losses and temperature increases.
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
For multi-winding transformers, the purchaser shall specify the required power-loading combinations, stating, when necessary, the active and reactive outputs separately
When a transformer is supplied with its rated voltage on the primary winding and the rated current flows through the secondary winding terminals, it effectively receives the corresponding rated power for those windings.
The transformer must continuously handle its rated power, including specified combinations for multi-winding transformers, while adhering to the conditions outlined in Clause 4 Additionally, it should not surpass the temperature-rise limits established in IEC 60076-2 for liquid-immersed transformers.
NOTE 1 The interpretation of rated power according to this subclause implies that it is a value of apparent power input to the transformer - including its own absorption of active and reactive power The apparent power that the transformer delivers to the circuit connected to the terminals of the secondary winding under rated loading differs from the rated power The voltage across the secondary terminals differs from rated voltage by the voltage drop (or rise) in the transformer Allowance for voltage drop, with regard to load power factor, is made in the specification of the rated voltage and the tapping range (see Clause 7 of IEC 60076-8:1997)
National practices may be different
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.
Preferred values of rated power
For transformers up to 20 MVA, values of rated power should preferably be taken from the R10 series given in ISO 3:1973, Preferred numbers – series of preferred numbers:
NOTE National practices may be different.
Minimum power under alternative cooling modes
Where the user has a particular requirement for a minimum power under a particular cooling mode other than the cooling mode for rated power, this shall be stated in the enquiry
The transformer must continuously support the specified minimum power, including the designated combinations of winding rated powers for multi-winding transformers, while adhering to the temperature-rise limitations outlined in IEC 60076-2 for liquid-immersed transformers.
Transformers must function at a specified minimum percentage of their rated power when the forced cooling system (ONAN) is not operational, ensuring the auxiliary supply loss is managed effectively.
Loading beyond rated power
According to the relevant standards, a transformer and its components can sometimes handle loads exceeding their rated power The calculation method for permissible loading is detailed in IEC 60076-7 for liquid-immersed transformers and IEC 60076-12 for dry-type transformers.
Purchasers must specify any unique loading requirements that exceed rated power, as well as conditions for operation at elevated external cooling medium temperatures or lower temperature rise limits in their inquiry and contract Additionally, any extra tests or calculations needed to confirm adherence to these specific requirements should also be clearly outlined.
NOTE 1 This option is intended to be used in particular to give a basis for design and guarantees concerning temporary emergency loading of power transformers
The bushings, tap-changers, current transformers and other auxiliary equipment shall be selected so as not to restrict the loading capability of the transformer
NOTE 2 The relevant component standards IEC 60137 for bushings and IEC 60214-1 for tap-changers should be consulted for the loading capability of those components
NOTE 3 These requirements do not apply to transformers for special applications, which do not need a loading capability beyond rated power For these transformers, if such a capability is required, it should be specified.
Cooling mode
The user shall specify the cooling medium (air or water)
If the user has particular requirements for the cooling method(s) or cooling equipment, this shall be stated in the enquiry
For additional information see IEC 60076-2.
Load rejection on transformers directly connected to a generator
Transformers designed for direct connection to generators must endure a voltage of 1.4 times their rated capacity for 5 seconds at the terminals where the generator connects, particularly under load rejection conditions.
Rated voltage and rated frequency
Rated voltage
The purchaser must specify the rated voltage, or for special applications, provide adequate information to the manufacturer to ascertain the rated voltage during the inquiry phase.
The transformer shall have an assigned rated voltage for each winding which shall be marked on the rating plate.
Rated frequency
The rated frequency shall be specified by the purchaser to be the normal undisturbed frequency of the network
The rated frequency is the basis for the guaranteed values such as losses, impedance, and sound level.
Operation at higher than rated voltage and/or at other than rated
IEC 60076-8 outlines methods for determining appropriate rated voltage values and tapping ranges to address various loading scenarios, including loading power, power factor, and corresponding line-to-line service voltages.
A transformer must operate continuously at rated power without damage under 'overfluxing' conditions, where the voltage-to-frequency ratio (V/Hz) can exceed the rated values by up to 5%, unless otherwise specified by the purchaser.
At no load, transformers shall be capable of continuous operation at a V/Hz of 110 % of the rated V/Hz
At a current K times the transformer rated current (0 ≤ K ≤ 1), the overfluxing shall be limited in accordance with the following formula:
If the transformer is to be operated at V/Hz in excess of those stated above, this shall be identified by the purchaser in the enquiry.
Provision for unusual service conditions
The purchaser shall identify in his enquiry any service conditions not covered by the normal service conditions Examples of such conditions are:
– external cooling medium temperature outside the limits prescribed in 4.2;
– altitude in excess of the limit prescribed in 4.2;
– humidity in excess of the limit prescribed in 4.2;
– high harmonic content of the load current exceeding the requirements of 4.2;
– distortion of the supply voltage waveform exceeding the limits of 4.2;
– unusual high frequency switching transients, see Clause 13;
– seismic qualification which would otherwise require special considerations in the design, see 4.2;
– extreme mechanical shock and vibrations;
– regular frequent energisation in excess of 24 times per year;
– V/Hz in excess of 5.4.3 above; connected to the generator without protection on the lower voltage side;
– corrosion protection, according to the kind of installation and the installation environment (see 4.2), the purchaser should choose classes of protection in ISO
12944 or by agreement between purchaser and manufacturer;
– load rejection conditions for generator transformers more severe than those given in 5.3 above
Transformer specification for operation under such abnormal conditions shall be subject to agreement between the supplier and purchaser
IEC 60076-2 outlines additional requirements for the rating and testing of transformers intended for non-standard service conditions, including high cooling air temperatures and altitudes exceeding 1,000 meters.
Highest voltage for equipment U m and dielectric tests levels
For line terminals, unless otherwise specified by the purchaser, U m shall be taken to be the lowest value that exceeds the rated voltage of each winding given in IEC 60076-3
For transformer windings with a highest voltage for equipment exceeding 72.5 kV, the purchaser must indicate whether the neutral terminals will be directly earthed during operation If the neutral terminals are not to be earthed, the purchaser should specify the U m for these terminals.
Unless otherwise specified by the purchaser, dielectric test levels shall be taken to be the lowest applicable value corresponding to U m , given in IEC 60076-3.
Additional information required for enquiry
Transformer classification
The kind of transformer, for example, separate winding transformer, auto-transformer or series transformer shall be specified by the user.
Winding connection and number of phases
The required winding connection shall be specified by the user in accordance with the terminology given in Clause 7 to suit the application
Purchasers must specify the need for a delta connected stabilizing winding In the case of star-star connected transformers or autotransformers, if the design features a closed magnetic circuit for zero sequence flux and a delta winding is not requested, the requirements should be discussed between the manufacturer and the purchaser, as outlined in IEC 60076-8.
NOTE A closed magnetic circuit for zero sequence flux exists in a shell-form transformer, and in a core-form transformer with an unwound limb or limbs
Purchasers must specify any high and low limits for zero sequence impedance, as these requirements can affect the core configuration and the necessity for a delta winding If the zero sequence criteria necessitate a delta connected winding that was not explicitly requested by the purchaser, the manufacturer must clearly indicate this in the tender documents.
The transformer manufacturer shall not use a delta connected test winding if no delta winding has been specified, unless specifically agreed by the purchaser
Users must specify their requirements for either a single-phase transformer or a three-phase unit; if not, the manufacturer will clarify the type of transformer being offered in the tender document.
Sound level
Where the purchaser has a specific requirement for a guaranteed maximum sound level, this shall be given in the enquiry and should preferably be expressed as a sound power level
The sound level, unless stated otherwise, is defined as the no-load sound level with all necessary cooling equipment operating at rated power If the purchaser specifies an alternative cooling mode, the sound level for each mode must be guaranteed by the manufacturer and measured during testing.
The sound level of transformers during operation is affected by the load current, as outlined in IEC 60076-10 If the buyer requests a measurement test for the sound level at load current or seeks a guarantee for the overall noise level of the transformers, including load noise, this requirement must be clearly specified in the inquiry.
The sound level measured in the test according to IEC 60076-10 shall not exceed the guaranteed maximum sound level The guaranteed maximum sound level is a limit without tolerance.
Transport
If transport size or weight limits apply, they shall be stated in the enquiry
Any additional special conditions for transportation must be specified in the inquiry This may involve restrictions on the use of insulating liquid or different environmental conditions anticipated during transport compared to those expected during service.
The transformer must be engineered to endure a constant acceleration of at least 1 g in all directions, alongside the vertical acceleration due to gravity, without sustaining any damage This capability should be validated through static force calculations that consider a constant acceleration value.
If the transport is not the responsibility of the manufacturer and an acceleration in excess of
During transport, an acceleration of 1 g is anticipated, with specific accelerations and frequencies outlined in the inquiry If the customer requests higher accelerations, the manufacturer must provide calculations to demonstrate compliance.
If the transformer is intended to be used as a mobile transformer, this shall be stated in the enquiry
NOTE The use of impact or shock recorders during transportation for large transformer is common practice.
Components and materials
All materials and components used in transformer construction must meet the applicable IEC standards, unless otherwise specified Specifically, bushings should adhere to IEC 60137, tap-changers must comply with IEC 60214-1, and insulating liquids should follow IEC 60296 for mineral oil or as agreed for alternative liquids.
General – Notation of tapping range
The following subclauses apply to transformers in which only one of the windings is a tapped winding
In a multi-winding transformer, the statements apply to the combination of the tapped winding with either of the untapped windings
For transformers specified in accordance with 6.4.2, the notation shall be as specified by the purchaser in item 3 of that subclause
In auto-connected transformers, tappings may be positioned at the neutral, resulting in a simultaneous change in the effective number of turns in both windings For these transformers, tapping details must be agreed upon unless specified according to section 6.4.2 The stipulations of this subclause should be applied as much as possible.
The principal tapping is typically positioned at the center of the tapping range, while other tappings are designated by their respective tapping factors The total number of tappings and the transformer ratio's variation range can be succinctly represented by the percentage deviations of the tapping factors from 100.
EXAMPLE A transformer with a tapped 160 kV winding with a tapping range of ±15 % having
21 tappings, symmetrically arranged around the rated voltage, is designated:
If the tapping range is specified asymmetrically around the rated voltage, this is designated as:
Regarding the full presentation on the nameplate of data related to individual tappings, see Clause 8
Certain tappings are classified as 'reduced-power tappings' because of limitations in tapping voltage or current The points where these restrictions occur are referred to as 'maximum voltage tapping' and 'maximum current tapping.'
Tapping voltage – tapping current Standard categories of tapping voltage variation Maximum voltage tapping
voltage variation Maximum voltage tapping
The notation for tapping range and tapping steps reflects the variation in the transformer's ratio; however, it does not fully define the assigned values of tapping quantities To provide a complete understanding, additional information is required, which can be presented in a table detailing tapping power, voltage, and current for each tap, or through descriptive text that specifies the 'category of voltage variation' and any limitations on the range of 'full-power tappings'.
The categories of tapping voltage variation are defined as follows: a) Constant flux voltage variation (CFVV)
The tapping voltage remains constant across untapped windings, while the voltages in tapped windings are proportional to their respective tapping factors This relationship is illustrated in Figure 1a Additionally, the concept of Variable Flux Voltage Variation (VFVV) is also relevant in this context.
The tapping voltage in the tapped winding remains constant across different taps, while the tapping voltages in any untapped winding are inversely proportional to the tapping factor This relationship is illustrated in Figure 1b, highlighting the concept of combined voltage variation (CbVV).
In various applications, especially with transformers that have an extensive tapping range, a combination of principles is utilized across different segments of the range, known as combined voltage variation (CbVV) The transition point in this system is referred to as the 'maximum voltage tapping'.
CFVV applies for tappings with tapping factors below the maximum voltage tapping factor
VFVV applies for tappings with tapping factors above the maximum voltage tapping factor
Figure 1a – Constant flux voltage variation (CFVV)
Optional maximum current tapping shown
Figure 1b – Variable flux voltage variation (VFVV)
Optional maximum current tapping shown
Figure 1c – Combined voltage variation (CbVV)
The change-over point is indicated within the plus tapping range, representing both a maximum voltage tapping (U A) and a constant maximum current tapping (I B) that does not exceed this point Additionally, an optional maximum current tapping is available in the CFVV range.
U A , I A tapping voltage and tapping current in the tapped winding
U B , I B tapping voltage and tapping current in the untapped winding
Abscissa tapping factor, percentage (indicating relative number of effective turns in tapped winding)
1 indicates full-power tappings throughout the tapping range
2 indicates 'maximum-voltage tapping', 'maximum current tapping' and range of reduced power tappings
Figure 1 – Different types of voltage variation
Tapping power Full-power tappings – reduced-power tappings
General
The purchaser shall specify the requirements for tapping either according to 6.4.2 or 6.4.3
The purchaser shall specify if the tap changer or tap changers are intended to be operated on load or de-energized
In applications involving variable flux voltage variation (VFVV), the design ratio can typically align with the specified ratio at only two points within the regulation range The buyer must indicate the specific locations for this alignment, such as the extreme taps, principal and maximum tap, or principal and minimum tap If no specifications are provided, the alignment will default to the two extreme taps.
Subclause 6.4.2 mandates that users identify the specific winding to be tapped along with the corresponding tapping powers Meanwhile, Subclause 6.4.3 outlines the overall voltage and current requirements, placing the responsibility on the manufacturer to choose the appropriate winding or windings for tapping This specification can lead to various transformer design options Additionally, IEC 60076-8 provides comprehensive information on tapping arrangements and voltage drop calculations.
Constructional specification
To define the transformer design, it is essential to determine the tapped winding, the number of steps and tapping range, and the voltage variation category By default, the tapping range is assumed to be symmetrical around the principal tapping with equal steps, unless specified otherwise Additionally, it is important to indicate any unequal steps in the tender Furthermore, the design must clarify if maximum current limitations apply to reduced power tappings and specify which tappings are affected.
Instead of items c) and d), tabulation of the same type as used on the rating plate may be used to advantage (see example in Annex B).
Functional specification
This type of specification is intended to allow the purchaser to specify operational requirements and not the category of voltage variation or which winding is to be tapped
This method of specification is not applicable to separate-winding transformers up to and including 2 500 kVA with a tapping range not exceeding ±5 %
When making an enquiry, the purchaser must provide additional information beyond the rated voltage and rated power specified in Clause 5 This includes the direction of power flow, which may be bidirectional, as well as the number of tapping steps and the size of each tapping step expressed as a percentage of the rated voltage at the principal tapping It is important to indicate if the tapping range is not symmetrical around the principal tapping or if the tapping steps vary in size across the range.
NOTE 1 It may be that the range of variation and the number of steps is more important than achieving the exact voltage at the principal tap In this case the range of variation and the number of steps may be specified For example +5 % to –10 % in 11 steps c) Which voltage shall vary for the purpose of defining rated tapping voltage
NOTE 2 The rated tapping voltage is needed to determine the impedance base for each tap Where the functional method of specification is adopted, the rated tapping voltage cannot be used to determine the rated tapping power d) Any requirements for fixing the ratio of turns between two particular windings on a more than two winding transformer e) Minimum full load power factor (this affects the voltage drop of the transformer) f) Whether any tapping or range of tappings can be reduced power tappings
The manufacturer selects the configuration of windings and the specific tapped winding(s) for the transformer It is essential that the transformer can deliver the rated current on the secondary winding across all tapping positions while adhering to the operational conditions, ensuring that the temperature rise remains within the limits specified by IEC 60076-2.
The transformer must be engineered to endure the voltages and fluxes generated by the specified loading conditions, including any overload scenarios Upon request, a calculation demonstrating compliance with this requirement will be provided to the purchaser.
An example is given in Annex B (example 4)
Users can submit a set of loading cases that include values for active and reactive power, clearly indicating the direction of power flow, along with the corresponding on-load voltages These cases should reflect the extreme voltage ratio values at both full and reduced power, as outlined in the "six-parameter method" of IEC 60076-8 Using this information, the manufacturer will choose the appropriate tapped winding and specify the rated and tapping quantities in their tender proposal An agreement on the design tapping quantities must be established between the manufacturer and the purchaser.
Specification of short-circuit impedance
For transformers without tappings that exceed a voltage variation of ±5% from the principal tapping, the short-circuit impedance of the windings should be specified only at the principal tapping This can be expressed in ohms per phase (Z) or as a percentage (z) relative to the rated power and voltage of the transformer Alternatively, the impedance may be defined using one of the specified methods.
For transformers with tappings that exceed a voltage variation of ±5% from the principal tapping, the impedance values, denoted as Z or z, must be specified The principal impedance should be measured during short-circuit impedance and load loss tests, as outlined in Clause 11.4, and must adhere to the tolerances specified in Clause 10 When expressed as a percentage, the impedance z should be referenced to the rated tapping voltage and the rated power of the transformer at the principal tapping.
The user must choose an impedance value that balances the need to limit voltage drop and overcurrent during fault conditions Economic optimization of the design, considering losses, suggests a specific range of impedance values Additionally, when operating in parallel with an existing transformer, it is essential to match the impedance as outlined in Clause 6 of IEC 60076-8:1997.
When an enquiry specifies both the impedance at the principal tapping and its variation across the tapping range, it significantly restricts transformer design, particularly regarding the arrangement and geometry of the windings Additionally, the transformer specification must consider that substantial impedance changes between taps can either diminish or amplify the impact of the tappings.
Maximum and minimum impedances, represented as z or Z, can be defined for each tapping across the entire tapping range using a graph or table (refer to Annex C) It is essential that the boundaries are sufficiently spaced to allow for the double-sided tolerances specified in Clause 10 to be applied to a median value Additionally, measured values must remain within these boundaries, which are established as limits without any tolerance.
The maximum and minimum impedances specified must accommodate an impedance tolerance that is at least equal to the tolerances outlined in Clause 10 However, if needed, a tighter tolerance can be established through mutual agreement between the manufacturer and the purchaser.
When basing the impedance on the rated tapping voltage and the rated power of a transformer at the principal tapping, the relationship between ohms per phase (Z) and percentage impedance (z) varies for each tap and depends on the specified winding for voltage variation It is crucial to ensure the specified impedance is accurate, especially for transformers with tapping powers that differ from the rated power at the principal tapping.
Load loss and temperature rise
For transformers with a tapping range within ±5% and a rated power not exceeding 2,500 kVA, load loss guarantees and temperature rise tests apply only to the principal tapping However, if the tapping range exceeds ±5% or the rated power is above 2,500 kVA, the guaranteed losses must be specified for the principal tapping position, unless the purchaser requests otherwise during the inquiry stage In such cases, the manufacturer must clarify which additional tappings, beyond the principal tapping, will have guaranteed load losses based on the relevant tapping current values The temperature rise limits are applicable to all tappings, considering the appropriate tapping power, voltage, and current.
The temperature-rise test is conducted on a single tapping, typically the 'maximum current tapping' with the highest load loss, unless specified otherwise The maximum total loss on this tapping serves as the test power for assessing liquid temperature rise The current for the selected tapping is used as the reference for measuring winding temperature rise above the liquid For detailed rules and testing procedures related to the temperature rise of liquid-immersed transformers, refer to IEC 60076-2.
The temperature-rise test is designed to verify that the cooling equipment can effectively dissipate the maximum total loss at any tapping point Additionally, it ensures that the temperature increase of any winding, relative to the external cooling medium, remains within the specified maximum limit.
For an autotransformer, the maximum current in the series and common windings typically occurs at two distinct tap positions To comply with IEC 60076-2 requirements during testing, an intermediate tap position may be chosen to ensure both windings are evaluated simultaneously.
In certain tapping configurations, the tapping winding may not conduct current in the maximum current tapping position Consequently, to assess the temperature rise of the tapping winding, an alternative tapping may be chosen, or an additional test can be arranged.
Connection and phase displacement symbols for three-phase transformers
Connection symbol
The connection type of phase windings in a three-phase transformer, or in single-phase transformers grouped in a three-phase bank, is denoted by specific letters The high-voltage (HV) winding is indicated by capital letters Y, D, or Z, while the intermediate and low-voltage (LV) windings are represented by lowercase letters y, d, or z.
In star-connected or zigzag-connected windings, if the neutral point is accessible, it is denoted as YN (yn) or ZN (zn) This designation also applies to transformers where the neutral connections of each phase winding are individually brought out and then combined to create a service neutral point.
For an auto-connected pair of windings, the symbol of the lower voltage winding is replaced by the letter a
Open windings in a three-phase transformer, where the windings are not interconnected but each phase winding has its terminals exposed, are designated as III for high voltage (HV) or iii for intermediate or low voltage windings.
In a transformer, letter symbols representing various windings are arranged in descending order of rated voltage, regardless of the intended power flow Each winding connection letter for intermediate and low-voltage windings is accompanied by its phase displacement, indicated by a 'clock number.'
Examples of connections in general use, with connection diagrams, are shown in Annex D.
Phase displacement in clock number notation
The following conventions of notation apply
The connection diagrams illustrate the arrangement of the high-voltage winding positioned above the low-voltage winding, with the directions of the induced voltages indicated in the upper section of the windings, as shown in Figure 2.
The phasor diagram for the high-voltage winding is aligned with phase I at the 12 o'clock position The orientation of the low-voltage winding's phase I phasor is determined by the induced voltage relationship corresponding to the specified connection The clock number symbol indicates the hour at which the low voltage is directed.
The sense of rotation of the phasor diagrams is counter-clockwise, giving the sequence I –
NOTE This numbering is arbitrary Terminal marking on the transformer follows national practice Guidance may be found in IEC/TR 60616.
Open windings do not have a clock number notation because the phase relationship of these windings with other windings depends on the external connection
A stabilizing or test winding, which can be either delta or star-connected and is not terminated for external three-phase loading, is denoted by the symbols '+d' or '+y' based on its connection type.
If a transformer is specified with a reconfigurable winding connection, the alternative coupling voltage and connection is noted in brackets after the delivered configuration as indicated by the following examples:
The transformer can operate at dual high voltage levels of 220 kV or 110 kV, both requiring a star connection It is supplied in a 220 kV configuration, while the low voltage side is connected in a delta configuration at 10.5 kV.
If LV can be 11 kV in star and 6,35 kV in delta and the transformer is delivered in 11 kV star configuration and HV is 110 kV star connected:
If the LV vector group is reconfigurable without changing the rated voltages (11 kV in this example) and the transformer is delivered in d11 and the HV is 110 kV star connected:
Examples are shown below and their graphical representations are shown in Figures 2 and 3
Figure 2 – Illustration of 'clock number' notation
– A transformer with the high-voltage winding rated 20 kV, delta-connected, the low- voltage winding rated 400 V star-connected with neutral brought out The LV winding lags the HV by 330°
The three-winding transformer features a high-voltage winding rated at 123 kV, connected in a star configuration with an accessible neutral It includes an intermediate-voltage winding of 36 kV, also star-connected with a neutral brought out, which is in phase with the high-voltage winding but is not auto-connected Additionally, there is a third winding rated at 7.2 kV, configured in delta and lagging by 150°.
– A group of three single-phase auto-transformers designed for a 400 kV HV and a
The system features a 130 kV intermediate voltage with 22 kV tertiary windings The auto-connected windings are configured in a star connection, while the tertiary windings utilize a delta connection Notably, the delta winding lags the high-voltage winding by 330°.
In cases where the delta winding is utilized solely as a stabilizing winding and not connected to three line terminals, it is denoted by a plus sign Consequently, no phase displacement notation is applicable for this stabilizing winding.
The symbol would be the same for a three-phase auto-transformer with the same connection, internally with the exception of the voltage notation See example below
This article discusses a three-phase autotransformer engineered for a high voltage of 400 kV and an intermediate voltage of 130 kV, featuring 22 kV tertiary windings The autotransformer utilizes star connections for the auto-connected windings, while the tertiary windings are configured in a delta connection, with the delta winding lagging the high-voltage winding by 330°.
If the delta winding is used solely as a stabilizing winding and not connected to three line terminals, it is denoted with a plus sign In this case, no phase displacement notation is applicable for the stabilizing winding.
A three-phase generator step-up transformer is designed for a 20 kV network with an 8.4 kV generator side The generator side windings are configured in a delta connection, while the network side windings are arranged in a star configuration Notably, the delta winding lags the high-voltage winding by 330°.
Reconnectable windings
If a transformer is specified with a reconfigurable winding connection, the alternative coupling voltage and connection is noted in brackets after the delivered configuration as indicated by the following examples:
The transformer can operate at dual high voltage levels of 220 kV or 110 kV, both requiring a star connection It is supplied in a 220 kV configuration, while the low voltage side is connected in a delta configuration at 10.5 kV.
If LV can be 11 kV in star and 6,35 kV in delta and the transformer is delivered in 11 kV star configuration and HV is 110 kV star connected:
If the LV vector group is reconfigurable without changing the rated voltages (11 kV in this example) and the transformer is delivered in d11 and the HV is 110 kV star connected:
Examples
Examples are shown below and their graphical representations are shown in Figures 2 and 3
Figure 2 – Illustration of 'clock number' notation
– A transformer with the high-voltage winding rated 20 kV, delta-connected, the low- voltage winding rated 400 V star-connected with neutral brought out The LV winding lags the HV by 330°
The three-winding transformer features a high-voltage winding rated at 123 kV, connected in a star configuration with an accessible neutral It includes an intermediate-voltage winding of 36 kV, also star-connected with a neutral brought out, which is in phase with the high-voltage winding but is not auto-connected Additionally, there is a third winding rated at 7.2 kV, configured in delta and lagging by 150°.
– A group of three single-phase auto-transformers designed for a 400 kV HV and a
The system operates at an intermediate voltage of 130 kV, featuring 22 kV tertiary windings The auto-connected windings are configured in a star connection, while the tertiary windings utilize a delta connection Notably, the delta winding lags the high-voltage winding by 330°.
In cases where the delta winding is utilized solely as a stabilizing winding and not connected to three line terminals, it is denoted by a plus sign Consequently, no phase displacement notation is applicable for this stabilizing winding.
The symbol would be the same for a three-phase auto-transformer with the same connection, internally with the exception of the voltage notation See example below
This article discusses a three-phase autotransformer engineered for a high voltage of 400 kV and an intermediate voltage of 130 kV, featuring 22 kV tertiary windings The autotransformer utilizes star connections for the auto-connected windings, while the tertiary windings are configured in a delta connection, with the delta winding lagging the high-voltage winding by 330°.
In cases where the delta winding is utilized solely as a stabilizing winding and not connected to three line terminals, it is denoted with a plus sign Consequently, there is no phase displacement notation associated with the stabilizing winding.
A three-phase generator step-up transformer is designed for a 20 kV network with an 8.4 kV generator side The generator side windings are configured in a delta connection, while the network side windings are arranged in a star configuration Notably, the delta winding lags the high-voltage winding by 330°.
Figure 3 – Illustration of 'clock number' notation for transformers with open windings
– A three-phase transformer designed for a 20 kV delta connected HV and with a 10 kV open winding
– A three-phase three winding transformer designed for a 220 kV star connected HV with a 40 kV open winding and a 10 kV third winding delta connected
– A three-phase series transformer designed for a 400 kV network and with a 40 kV excitation winding delta connected
Connection and phase displacement symbols for single phase transformers
Connection symbol
The connection of phase windings in single-phase transformers is denoted by the capital letter "I" for the high-voltage (HV) winding, while the intermediate and low-voltage (LV) windings are represented by the lowercase letter "i."
In a transformer, letter symbols representing various windings are arranged in descending order of rated voltage, regardless of the intended power flow Each winding connection letter for intermediate and low-voltage windings is accompanied by its phase displacement, indicated by a 'clock number.'
For an auto-connected pair of windings, the symbol of the lower voltage winding is replaced by the letter a.
Phase displacement in clock number notation
The clock number of single-phase transformers is established similarly to that of three-phase transformers, with the possibility of being 0 when both windings are in phase or 6 when they are in opposition.
Windings not intended to be loaded
The presence of a test or additional winding, which remains unconnected to external loading, is denoted by the symbol '+i' following the symbols of loadable windings, as illustrated in the example below.
Reconnectable windings
If a transformer is specified with a reconfigurable winding connection, the alternative coupling voltage and connection is noted in brackets after the delivered configuration as indicated by the following examples
– If HV can be 220 kV or 110 kV (dual voltage) but with the same connection required for both voltages
– If LV can be 11 kV in 0 and 5,5 kV in 6 and the transformer is delivered in 11 kV 0 configuration and HV is 110 kV:
– If the LV vector group is reconfigurable without changing the rated voltages (11 kV in this example) and the transformer is delivered in i0 and the HV is 110 kV:
Examples are shown below and some of their graphical representations are on Figure 4
The same convention as in Figure 2 applies
Figure 4 – Illustration of 'clock number' notation
– A transformer with the high-voltage winding rated 20 kV, the low-voltage winding rated
400 V, the LV winding being in phase with the high voltage
– A three-winding transformer: with a 123 kV HV, an intermediate voltage winding of
36 kV, in phase with the HV winding but not auto-connected, and a 7,2 kV third winding, lagging by 180°
– A single-phase autotransformer designed for a 400 kV HV and a 130 kV intermediate voltage with 22 kV tertiary windings all in phase
If the third winding is not intended to be loaded, the symbol would indicate this by a plus sign No phase displacement notation would then apply for the third winding