NATIONAL STANDARD
OF THE PEOPLE'S REPUBLIC OF CHINA
中華人民共和國國家標準
GB 1094.1-2013
Power Transformers - Part 1: General
電力變壓器 第1部分: 總則
1 Scope
This part of GB 1094 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;
——transformers, which have no windings with rated voltage higher than 1000 V;
——instrument transformers;
——traction transformers mounted on rolling stock;
——starting transformers;
——testing transformers;
——welding transformers;
——explosion-proof and mining transformers;
——transformers for deep water (submerged) applications.
When some standards do not exist for such categories of transformers (in particular transformer having no winding exceeding 1000V for industrial applications), this part may still be applicable either as a whole or in part.
This part does not address the requirements that would make a transformer suitable for mounting in a position accessible to the general public.
For those categories of power transformers and reactors which have their own standards, this part is applicable only to the extent in which it is specifically called up by cross-reference in the other standard. Such standards exist for:
——reactors (GB/T 1094.6);
——dry-type transformers (GB 1094.11);
——self-protected transformers (IEC 60076-13);
——gas-filled power transformers (IEC 60076-15);
——transformers for wind turbine applications (GB 1094.16);
——traction transformers and traction reactors (GB/T 25120);
——converter transformers for industrial applications (GB/T 18494.1);
——converter transformers for HVDC applications (GB/T 18494.2).
At several places in this part it is specified or recommended that an 'agreement' should be reached concerning alternative or additional technical solutions or procedures. Such agreement is 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.
2 Normative References
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB 1094.2 Power Transformers Part 2: Temperature Rise for Liquid-immersed Transformers (GB 1094.2-2013, IEC 60076 - 2: 2011, MOD)
GB 1094.3 Power transformers - Part 3: Insulation Levels, Dielectric Tests and External Clearances in Air (GB 1094.3-2003, IEC 60076-3: 2000, MOD)
GB 1094.5 Power Transformers - Part 5: Ability to Withstand Short Circuit (GB 1094.5-2008, IEC 60076-5: 2006, MOD)
GB/T 1094.10 Power Transformers - Part 10: Determination of Sound Levels (GB/T 1094.10-2003, IEC 60076-10: 2001, MOD)
GB 1094.11 Power Transformers - Part 11: Dry-type Transformers (GB 1094.11-2007, IEC 60076-11: 2004, MOD)
GB/T 2521 Cold-rolled Grain-oriented and Non-oriented Magnetic steel Strip (Sheet) (GB/T 2521-2008, IEC 60404-8-7: 1998 and IEC60404-8-4: 1998, MOD)
GB 2536 Fluids for Electrotechnical Applications - Unused Mineral Insulating oils for Transformers and Switchgear(GB 2536-2011, IEC 60296: 2003, MOD)
GB/T 2900.15 Electrotechnical Terminology - Transformer, Instrument Transformer, Voltage Regulator and Reactor (GB/T 2900.15-1997, neq. IEC 60050-421: 1990 and IEC 60050-321: 1986)
GB/T 4109 Insulated Bushings for Alternating Voltages above 1 000V (GB/T 4109-2008, IEC 60137: 2008, MOD)
GB/T 4798.4 Environmental Conditions Existing in the Application of Electric and Electronic Products - Part 4: Stationary Use at Non-weather-protected Locations (GB/T 4798.4-2007, IEC 60721-3-4: 1995, MOD)
GB 10230.1 Tap-changers - Part 1: Performance Requirements and Test Methods (GB 10230.1-2007, IEC 60214-1: 2003, MOD)
GB/T 19001 Requirements for Quality Management System (GB/T 19001-2008, ISO 9001: 2008, IDT)
3 Terms and Definitions
For the purposes of this document, the terms and definitions specified in GB/T 2900.15 and the following ones apply. Some are listed more than once and some are changed.
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.
Note: GB/T 2900.15-1997, 3.1.1, modified.
3.1.2
Auto-transformer
A transformer in which at least two windings have a common part.
[GB/T 2900.15-1997, 3.1.15]
Note: 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.
3.1.3
Series transformer
A transformer, other than an autotransformer, 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.
Note 1: GB/T 2900.15-1997, 3.1.8, modified.
Note 2: Series transformers were called booster transformers in earlier editions of this part.
3.1.4
Liquid-immersed type transformer
A transformer in which the core and windings are immersed in liquid.
[GB/T 2900.15-1997, 3.1.4]
3.1.5
Dry-type transformer
A transformer in which the core and windings are not immersed in an insulating liquid
[GB/T 2900.15-1997, 3.1.5]
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.
3.1.8
Design value
The expected value given by the number of turns in the design in the case of turns ratio or calculated from the design in the case of impedance, no-load current or other parameters.
3.1.9
Highest voltage for equipment applicable to a transformer winding
Um
The highest r.m.s. phase-to-phase voltage in a three-phase system.
[GB 1094.3-2003, 3.1]
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.
[GB/T 2900.15-1997, 5.5.1]
3.2.3
Neutral terminal
Neutral terminal includes:
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.
Note: GB/T 2900.15-1997, 5.5.2, modified.
3.2.4
Neutral point
The point of a symmetrical system of voltages which is normally at zero potential.
3.2.5
Corresponding terminal
Terminals of different windings of a transformer, marked with the same letter or corresponding symbol.
[GB/T 2900.15-1997, 2.1.28]
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 1: GB/T 2900.15-1997, 4.3.1, modified.
Note 2: 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 1: GB/T 2900.15-1997, 4.3.16, modified.
Note 2: The term 'phase winding' shall not be used for identifying the assembly of all coils on a specific leg.
3.3.4
High-voltage winding; HV winding
The winding having the highest rated voltage
[GB/T 2900.15-1997, 4.3.2]
3.3.5
Low-voltage winding; LV winding1
The winding having the lowest rated voltage.
[GB/T 2900.15-1997, 4.3.3]
Note: For a series transformer, the winding having the lower rated voltage may be that having the higher insulation level.
3.3.6
Intermediate-voltage winding1
A winding of a multi-winding transformer having a rated voltage intermediate between the highest and lowest winding rated voltages.
[GB/T 2900.15-1997, 4.3.4]
3.3.7
Auxiliary winding
A winding intended only for a small load compared with the rated power of the transformer.
[GB/T 2900.15-1997, 4.3.11]
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 1: GB/T 2900.15-1997, 4.3.12, modified.
Note 2: 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.
[GB/T 2900.15-1997, 4.3.13]
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.
Note: GB/T 2900.15-1997, 4.3.14, modified.
3.3.11
Energizing winding
The winding of a series transformer (or series transformer) which is intended to supply power to the series winding.
Note: GB/T 2900.15-1997, 4.3.15, modified.
3.3.12
Auto-connected windings
The series and common windings of an auto-transformer.
3.4 Rating
3.4.1
Rating
Those numerical values assigned to the quantities which define the operation of the transformer in the conditions specified in this part 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 3.5.9).
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
Ur
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.
Note 1: GB/T 2900.15-1997, 2.1.4, modified.
Note 2: The rated voltages of all windings appear simultaneously at no-load when the voltage applied to one of them has its rated value.
Note 3: For single-phase transformers intended to be connected in star to form a three-phase bank or to be connected between the line and the neutral of a three phase system, the rated voltage is indicated as the phase-to-phase voltage, divided by . Such as:
kV
Note 4: For single phase transformers intended to be connected between phases of a network, the rated voltage is indicated as the phase-to-phase voltage.
Note 5: For the series winding of a three-phase series transformer, which is designed as an open winding (see 3.10.5), the rated voltage is indicated as if the windings were connected in star.
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
Note: GB/T 2900.15-1997, 2.1.5, modified.
3.4.5
Rated frequency
fr
The frequency at which the transformer is designed to operate.
Note: GB/T 2900.15-1997, 2.1.6, modified.
3.4.6
Rated power
Sr
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 rate d power of the whole transformer.
3.4.7
Rated current
Ir
The current flowing through a line terminal of a winding which is derived from rated power Sr and rated voltage Ur for the winding.
Note 1: GB/T 2900.15-1997, 2.1.7, modified.
Note 2: For a three-phase winding the rated current is given by:
Note 3: 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 :
Note 4: For a single phase transformer not intended to be connected to form a three phase bank, the rated current is:
Note 5: The rated current of the open windings is the rated power divided by the number of phases and by the rated voltage of the open winding:
Where,
n——the number of phase.
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 a 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.
[GB/T 2900.15-1997, 2.1.12]
3.5.3
Tapping factor (corresponding to a given tapping)
The ratio:
Ud/Ur (tapping factor) or 100Ud/Ur (tapping factor expressed as a percentage).
Where,
Ur——the rated voltage of the winding (see 3.4.3);
Ud——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 1: For series transformers, the tapping factor is the ratio of the voltage of the series winding corresponding to a given tapping to Ur.
Note 2: GB/T 2900.15-1997, 2.1.13, modified.
3.5.4
Plus tapping
A tapping whose tapping factor is higher than 1.
[GB/T 2900.15-1997, 2.1.14]
3.5.5
Minus tapping
a tapping whose tapping factor is lower than 1.
[GB/T 2900.15-1997, 2.1.15]
3.5.6
Tapping step
The difference between the tapping factors, expressed as a percentage, of two adjacent tappings.
[GB/T 2900.15-1997, 2.1.16]
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.
[GB/T 2900.15-1997, 2.1.17]
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.
[GB/T 2900.15-1997, 2.1.18]
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 quantities
Those quantities the numerical values of which define the duty of a particular tapping (other than the principal tapping).
Note 1: Tapping quantities exist for any winding in the transformer, not only for the tapped winding (see 6.2 and 6.3).
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).
Note 2: GB/T 2900.15-1997, 2.1.20, modified.
3.5.10
Full-power tapping
A tapping whose tapping power is equal to the rated power.
[GB/T 2900.15-1997, 2.1.24]
3.5.11
Reduced-power tapping
A tapping whose tapping power is lower than the rated power.
[GB/T 2900.15-1997, 2.1.25]
3.5.12
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.
[GB/T 2900.15-1997, 5.6.1]
3.5.13
De-energized tap-changer; 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).
Note: GB/T 2900.15-1997, 5.6.2, modified.
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 Um.
Note 2: This voltage will normally be limited to 105 % of the rated tapping voltage unless a higher voltage is required by the purchaser’s specification of the tapping (see 6.4) either explicitly or as a result of a specification according to 6.4.2.
3.6 Loss and no-load current
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 a rated voltage (tapping voltage) at a rated frequency is applied to the terminals of one of the windings, the other winding or windings being open- circuited.
Note: GB/T 2900.15-1997, 2.1.33, 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 a rated frequency to that winding, 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 lines.
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.
Note 3: GB/T 2900.15-1997, 2.1.34, modified.
3.6.3
Load loss
The absorbed active power at a rated frequency and reference temperature (see 11.1), 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 7 of GB/T 13499-2002). 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 shall be mentioned.
Note 3: GB/T 2900.15-1997, 2.1.31, modified.
3.6.4
Total losses
The sum of the no-load loss and the load loss.
Note 1: The power consumption of the auxiliary plant is not included in the total losses and shall be stated separately.
Note 2: GB/T 2900.15-1997, 2.1.30, 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(Ω) 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).
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 Zref, of the same winding of the pair. In percentage notation:
Where,
Formula valid for both three-phase and single-phase transformers.
U——the voltage (rated voltage or tapping voltage) of the winding to which Z and Zref belong;
Sr——the reference value of rated power.
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 (see GB/T 2900.15-1997, 2.1.37) of the pair of windings. It is normally expressed as a percentage.
Note 3: GB/T 2900.15-1997, 2.1.37, modified.
3.7.2
Voltage drop of 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 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.
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 GB/T 13499).
[GB/T 2900.15-1997, 2.1.40]
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.
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).
Note 4: GB/T 2900.15-1997, 2.1.41, modified.
3.8 Temperature rise
Temperature rise
The difference between the temperature of the part under consideration and the temperature of the external cooling medium (see GB 1094.2).
Note: GB/T 2900.15-1997, 2.1.46, modified.
3.9 Insulation
For terms and definitions relating to insulation, see GB 1094.3.
3.10 Connections
3.10.1
Star 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
Note 1: GB/T 2900.15-1997, 4.4.1, modified.
Note 2: Star connection is sometimes referred to as Y-connection.
3.10.2
Delta 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.
Note 1: GB/T 2900.15-1997, 4.4.2, modified.
Note 2: Delta connection is sometimes referred to as D-connection.
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.
[GB/T 2900.15-1997, 4.4.4]
3.10.4
Zigzag connection
A winding connection consisting of two winding sections, the first section connected in star, the second connected in series between the first section and the line terminals: the two sections are arranged so that each phase of the second section is wound on a different limb of the transformer to the part of the first section to which it is connected.
Note 1: See Annex C for cases where the winding sections have equal voltages.
Note 2: zigzag connection is sometimes referred to as Z-connection.
3.10.5
Open windings
The phase windings of a three-phase transformer which are not interconnected within the transformer.
Note: GB/T 2900.15-1997, 4.4.5, 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
Note 1: GB/T 2900.15-1997, 2.1.27, modified.
Note 2: See Clause 7 and Annex C.
Note 3: 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).
Note: GB/T 2900.15-1997, 2.1.26, modified.
3.11 Test classification
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 the specified requirements not covered by the routine tests: a transformer is considered to be representative of others if it is built to the same drawings using the same techniques and materials in the same factory.
Note 1: Design variations that are clearly irrelevant to a particular type test would not require that type test to be repeated.
Note 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.
Note 3: For transformers below 20 MVA and Um≤72.5 kV, significant design variations may be acceptable if supported by demonstration of compliance with type test requirements.
3.11.3
Special test
A test other than a type test or a routine test, agreed by the manufacturer and the purchaser.
Note: Special tests can be carried out on one transformer or all transformers of a particular design, as specified by the purchaser in the enquiry and order for every special test.
3.12 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.
3.13 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 (E1, I1).
Total harmonic content:
Total harmonic content:
Where,
Ei——the r.m.s. value of voltage of the ith harmonic;
Ii——the r.m.s. value of current of the ith harmonic.
3.13.3
Even harmonic content
The ratio of effective value of all the even harmonics to the effective value of the fundamental (E1, I1)
Even harmonic content:
Even harmonic content:
Where,
E2i——the r.m.s. value of voltage of the 2ith harmonic;
I2i——the r.m.s. value of current of the 2ith harmonic.
4 Service Conditions
4.1 General
The service conditions set out in 4.2 represent the normal scope of operation of a transformer specified to this standard. For any unusual service conditions which require special consideration in the design of a transformer see 5.5. Such conditions include high altitude, extreme high or low external cooling medium temperature, tropical humidity, seismic activity, severe contamination, unusual voltage or load current wave shapes, high solar radiation and intermittent loading. They may also concern conditions for shipment, storage and installation, such as weight or space limitations (see Annex D).
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: GB 1094.2 for liquid-immersed transformers, and GB 1094.11 for dry-type transformers;
——external insulation at high altitude: GB 1094.3 for liquid-filled transformers, and
GB 1094.11 for dry-type transformers.
4.2 Normal service conditions
This part gives detailed requirements for transformers for use under the following conditions:
a) Altitude
A height above sea-level not exceeding 1000 m.
b) Temperature of cooling medium
The temperature of cooling air at the inlet to the cooling equipment not exceeding:
40℃ at any time;
30℃ monthly average of the hottest month;
20℃ yearly average.
and not below:
–25℃ in the case of outdoor transformers;
–25℃ in the case of outdoor transformers;
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.
Note 1: This paragraph above is intended to allow the use of an alternative insulating liquid which does not meet minimum temperature requirements in circumstances where the minimum temperature of –25 ℃ is not appropriate.
For water-cooled transformers, a temperature of cooling water at the inlet not exceeding:
25℃ at any time;
20℃ yearly average.
At any time and yearly average are defined in 3.12.
Further limitations, with regard to cooling are given for:
——liquid-immersed transformers in GB 1094.2;
——dry-type transformers in GB 1094.11.
Note 2: For transformers with both air/water and water/liquid heat exchangers, the temperature of cooling medium refers to the external air temperature rather than the water temperature in the intermediate circuit which may exceed the normal value
Note 3: The relevant temperature is at the inlet to the cooling equipment rather than the outside air temperature, this means that the user should take care that if the installation can create conditions where air recirculation from the output of the cooler can occur, that this is taken into account when assessing the cooling air temperature.
c) Wave shape of supply voltage
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.
Note 4: Transformers where total harmonic content of the load current exceeds 5% of rated current, or transformers specifically intended to supply power electronic or rectifier loads should be specified according to GB/T 18494 series.
Note 5: Transformers can operate at rated current without excessive loss of life with a current harmonic content of less than 5 %, however it should be noted that the temperature rise will increase for any harmonic loading and may exceed rated rise.
e) Symmetry of three-phase supply voltage
For three-phase transformers, a set of three-phase supply voltages which are approximately symmetrical. Approximately symmetrical shall be taken to mean that the highest phase to phase voltage is no more than 1% higher than the lowest phase to phase voltage continuously or 2% higher for short periods (approximately 30min) under exceptional conditions.
f) Installation environment
An environment with a pollution rate (see GB/T 4109 and GB/T 26218.1) 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 require special consideration in the design. (This is assumed to be the case when the ground acceleration level is below 3ms-2 horizontally; or below 1.5ms-2 vertically) See GB/T 2424.25.
Where the transformer is installed in an enclosure not supplied by the transformer manufacturer remotely from the cooling equipment, for example in an acoustic enclosure, the temperature of the air surrounding the transformer is not exceeding 40℃ at any time.
Environmental conditions within the following definitions according to GB/T 4798.4:
——climatic conditions 4K2 except that the minimum external cooling medium temperature is –25℃;
——special climatic conditions 4Z2, 4Z4, 4Z7;
——biological conditions 4B1;
——chemically active substances 4C2;
—— mechanically active substances 4S3;
——mechanical conditions 4M4.
For transformers intended to be installed indoors, some of these environmental conditions may not be applicable.
5 Rating and General Requirements
5.1 Rated power
5.1.1 General
The rated power for each winding shall either be specified by the purchaser or the purchaser shall provide sufficient information to the manufacturer to determine the rated power at the enquiry stage.
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.
For multi-winding transformers, the purchaser shall specify the required power-loading combinations, stating, when necessary, the active and reactive outputs separately.
When the transformer has rated voltage applied to a primary winding, and rated current flows through the terminals of a secondary winding, the transformer receives the relevant rated power for that pair of windings.
The transformer shall be capable of carrying, in continuous service, the rated power (for a multi-winding transformer: the specified combination(s) of winding rated power(s)) under conditions listed in Clause 4 and without exceeding the temperature-rise limitations specified in GB 1094.2 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 GB/T 13499-2002).
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.
5.1.2 Preferred values of rated power
Values of rated power should preferably be taken from the R10 series (……100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000……).
5.1.3 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 shall be capable of carrying, in continuous service, the specified minimum power (for a multi-winding transformer: the specified combination(s) of winding rated power(s) under conditions listed in Clause 4, and under the specified cooling mode, without exceeding the temperature-rise limitations specified in GB 1094.2 for liquid immersed transformers.
Note: An example of this is where the transformer is required to operate at a particular minimum percentage of rated power with the forced cooling out of service (ONAN) to allow for the loss of auxiliary supply.
5.1.4 Loading beyond rated power
A transformer and its component parts in accordance with this part are able under some circumstances to carry loading beyond rated power. The method for calculating the permissible loading can be found in GB/T 1094.7 for liquid immersed transformers and in GB/T 1094.12 for dry-type transformers.
Any specific requirements for loading beyond rated power, operation at higher external cooling medium temperatures or reduced temperature rise limits shall be specified by the purchaser in the enquiry and the contract. Any additional tests or calculations required to verify compliance with these specific requirements shall also be specified.
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 t 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 GB/T 4109 for bushings and GB 10230.1 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.
5.2 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 GB 1094.2.
5.3 Load rejection on transformers directly connected to a generator
Transformers intended to be connected directly to generators in such a way that they may be subjected to load rejection conditions shall be able to withstand 1.4 times rated voltage for 5s at the transformer terminals to which the generator is to be connected.
5.4 Rated voltage and rated frequency
5.4.1 Rated voltage
The rated voltage shall either be specified by the purchaser or for special applications the purchaser shall provide sufficient information to the manufacturer to determine the rated voltage at the enquiry stage.
The transformer shall have an assigned rated voltage for each winding which shall be marked on the rating plate.
5.4.2 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.
5.4.3 Operation at higher than rated voltage and/or at other than rated frequency
Methods for the specification of suitable rated voltage values and tapping range to cope with a set of loading cases (loading power and power factor, corresponding line- to-line service voltages) are described in GB/T 13499.
Within the prescribed values of (Um), for the transformer windings, a transformer shall be capable of continuous operation at rated power without damage under conditions of 'overfluxing' where the value of voltage divided by frequency (V/Hz) exceeds the corresponding value at rated voltage and rated frequency by no more than 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
5.5 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;
——restricted ventilation;
——altitude in excess of the limit prescribed in 4.2;
——damaging fumes and vapours;
——steam;
——humidity in excess of the limit prescribed in 4.2;
——dripping water;
——salt spray;
——excessive and abrasive dust;
——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;
——superimposed DC current;
——seismic qualification which would otherwise require special considerations in the design, see 4.2;
——extreme mechanical shock and vibrations;
——solar radiation;
——regular frequent energization in excess of 24 times per year;
——regular frequent short-circuits;
——regular frequent short-circuits;
——if a generator step up transformer is intended to be used in back -feed mode when not 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.
——Operation at extreme low temperature;
——special wiring modes;
——special installation positions and arrangement modes;
——installation at public places;
——special grounding mode of neutral points.
Transformer specification for operation under such abnormal conditions shall be subject to agreement between the supplier and purchaser.
Supplementary requirements, within defined limits, for the rating and testing of transformers designed for other than normal service conditions listed in Clause 4, such as high temperature of cooling air or altitude above 1000 m are given in GB 1094.2.
5.6 Highest voltage for equipment Um and dielectric tests levels
For line terminals, unless otherwise specified by the purchaser, Um shall be taken to be the value that equals to or slightly exceeds rated voltage of each winding.
For transformer windings with a highest voltage for equipment greater than (>) 72.5 kV, the purchaser shall specify whether any neutral terminals for that winding are to be directly earthed in service or not, and if not, the Um for the neutral terminals shall be specified by the purchaser.
Unless otherwise specified by the purchaser, dielectric test levels shall be taken to be the lowest applicable value corresponding to Um, given in GB 1094.3.
5.7 Additional information required for enquiry
5.7.1 Transformer classification
The kind of transformer, for example, separate winding transformer, auto-transformer or series transformer shall be specified by the user.
5.7.2 Winding connection and number of phases
The required winding connection shall be specified by the user in accordance with Clause 7.
If a stabilizing winding is required, it shall be specified by the purchaser. For star-star connected transformers or autotransformers, if the design has a closed magnetic circuit for zero sequence flux and no delta winding is specified, then the requirement shall be discussed between the manufacturer and the purchaser (see GB/T 13499).
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.
If there are requirements for high and low limits for the zero sequence impedance, this shall be stated by the purchaser and may influence the core configuration and the requirement for a delta winding. If the zero sequence requirements dictated the use of a delta connected winding that was not directly specified by the purchaser, then this shall be clearly stated by the manufacturer 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.
If there is a particular requirement for either a bank of single phase transformer or a three phase unit, then this shall be specified by the user; otherwise the manufacturer shall make it clear in the tender document what type of transformer is being offered.
5.7.3 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.
Unless otherwise specified, the sound level shall be taken as the no load sound level with all cooling equipment required to achieve rated power in operation. If an alternative cooling mode is specified (see 5.1.3) the sound level for each alternative mode may be specified by the purchaser and if specified shall be guaranteed by the manufacturer and measured on test.
The sound level in service is influenced by the load current (see GB/T 1094.10). If the purchaser requires a load current sound level measurement test or a guarantee of the total noise level of the transformers, including load noise, this shall be stated in the enquiry.
The sound level measured in the test according to GB/T 1094.10 shall not exceed the guaranteed maximum sound level. The guaranteed maximum sound level is a limit without tolerance.
5.7.4 Transport
5.7.4.1 Transport limitation
If transport size or weight limits apply, they shall be stated in the enquiry.
If any other special conditions apply during transportation, they shall be stated in the enquiry.
This might include a restriction on the transportation with insulating liquid or different environmental conditions expected to be experienced during transportation than those expected in service.
5.7.4.2 Transport acceleration
The transformer shall be designed and manufactured to withstand a constant acceleration of at least 3g in all directions without any damage, demonstrated by static force calculations based on a constant value of acceleration.
If the transport is not the responsibility of the manufacturer and an acceleration in excess of 3g is expected during transport, the accelerations and frequencies shall be defined in the enquiry. If higher accelerations are specified by the customer, the manufacturer shall demonstrate compliance by means of calculation.
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.
5.8 Components and materials
All components and materials used in the construction of the transformer shall comply with the requirements of the relevant standards where they exist unless otherwise agreed or specified. In particular bushings shall comply with GB/T 4109, tap-changers shall comply with GB 10230.1, and insulating liquid shall comply with GB 2536 for mineral oil or as agreed for other liquids. The electrical steel strip shall comply with GB/T 2521.
6 Requirements for Transformers Having a Tapped Winding
6.1 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 6.4.2c).
In auto-connected transformers, tappings are sometimes arranged at the neutral which means that the effective number of turns is changed simultaneously in both windings. For such transformers, unless they are specified in accordance with 6.4.2, the tapping particulars are subject to agreement. The requirements of this subclause should be used as far as applicable.
Unless otherwise specified, the principal tapping is located in the middle of the tapping range. Other tappings are identified by their tapping factors. The number of tappings and the range of variation of the transformer ratio may be expressed in short notation by the deviations of the tapping factor percentages from the value 100 (for definitions of terms, see 3.5).
Example: A transformer with a tapped 220 kV winding with a tapping range of ±10 % having 17 tappings, symmetrically arranged around the rated voltage, is designated:
(220±8×1.25%)/35 kV
If the tapping range is specified asymmetrically around the rated voltage, this is designated as:
KV
Regarding the full presentation on the nameplate of data related to individual tappings, see Clause 8.
Some tappings may be 'reduced-power tappings' due to restrictions in either tapping voltage or tapping current. The boundary tappings where such limitations appear are called 'maximum voltage tapping' and 'maximum current tapping' [see Figures 1a), 1b) and 1c)].
6.2 Tapping voltage – tapping current (standard categories of tapping voltage variation. Maximum voltage tapping)
The short notation of tapping range and tapping steps indicates the variation range of the ratio of the transformer. But the assigned values of tapping quantities are not fully defined by this alone. Additional information is necessary. This can be given either in tabular form with tapping power, tapping voltage and tapping current for each tapping, or as text, indicating 'category of voltage variation' and possible limitations of the range within which the tappings are 'full-power tappings'.
The categories of tapping voltage variation are defined as follows:
a) Constant flux voltage variation (CFVV)
The tapping voltage in any untapped winding is constant from tapping to tapping. The tapping voltages in the tapped winding are proportional to the tapping factors. See Figure 1a).
b) Variable flux voltage variation (VFVV)
The tapping voltage in the tapped winding is constant from tapping to tapping. The tapping voltages in any untapped winding are inversely proportional to the tapping factor. See Figure 1b).
c) Combined voltage variation (CbVV)
In many applications and particularly with transformers having a large tapping range, a combination is specified using both principles applied to different parts of the range: combined voltage variation (CbVV). The change-over point is called 'maximum voltage tapping'. For this system the following applies:
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. See Figure 1c).
a) Constant Flux Voltage Variation (CFVV)
b) Variable Flux Voltage Variation (VFVV)
c) Combined Voltage Variation (CbVV) (from left to right)
From Figure 1c), the change-over point is shown in the plus tapping range. It constitutes both a maximum voltage tapping (UA), and a maximum current tapping (IB constant, not rising above the change-over point). An additional, optional maximum current tapping (in the CFVV range) is also shown.
Key:
UA and IA——the tapping voltage and tapping current in the tapped winding;
UB and IB——the tapping voltage and tapping current in the untapped winding;
SAB——the tapping power;
Abscissa——the tapping factor, percentage (indicating relative number of effective turns in tapped winding);
1——full-power tappings throughout the tapping range
2——'maximum-voltage tapping', 'maximum current tapping' and range of reduced power tappings.
Figure 1 Different Types of Voltage Variation
6.3 Tapping power (Full-power tappings – reduced-power tappings)
The following shall apply unless the voltage and current at each tapping is otherwise specified.
All tappings shall be full-power tappings, that is, the rated tapping current at each tapping shall be the rated power divided by the rated tapping voltage at each tap except as specified below.
In separate-winding transformers up to and including 2500 kVA with a tapping range not exceeding ±5%, the rated tapping current at all minus tappings shall be equal to the rated tapping current at the principal tapping. This means that the principal tapping is a 'maximum current tapping'.
In transformers with a tapping range wider than ±5%, restrictions may be specified on values of tapping voltage or tapping current which would otherwise rise considerably above the rated values. When such restrictions are specified, the tappings concerned will be 'reduced-power tappings'. This subclause describes such arrangements.
When the tapping factor deviates from 1, the tapping current for full-power tappings may rise above rated current on one of the windings. As Figure 1a) illustrates, this applies for minus tappings, on the tapped winding, under CFVV, and for plus tappings on the untapped winding under VFVV [Figure 1b)]. In order to limit the corresponding reinforcement of the winding in question, it is possible to specify a maximum current tapping. From this tapping onwards the tapping current values for the winding are then specified to be constant. This means that the remaining tappings towards the extreme tapping are reduced-power tappings [see Figures 1a), 1b) and 1c)].
Under CbVV, the 'maximum voltage tapping', the change-over point between CFVV and VFVV shall at the same time be a 'maximum current tapping' unless otherwise specified. This means that the untapped winding current stays constant up to the extreme plus tapping [Figure 1c)].
6.4 Specification of tappings in enquiry and order
6.4.1 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.
Where variable flux voltage variation VFVV is used, it is normally only possible for the design ratio to match the specified ratio at two positions over the regulation range. The purchaser shall specify where the design ratio shall match the specified ratio, e.g. extreme taps, principal and maximum tap or principal and minimum tap. If not otherwise specified, the two extreme taps shall be the ratios to match.
Note: Subclause 6.4.2 requires the user to specify which winding is to be tapped and particular tapping powers. Subclause 6.4.3 defines overall voltage and current requirements and requires the manufacturer to select which winding or windings will be tapped. Such a specification may result in a variety of possible transformer designs. GB/T 13499 gives details of tapping arrangements and voltage drop calculations.
6.4.2 Constructional specification
The following data are necessary to define the design of the transformer:
a) which winding shall be tapped;
b) the number of steps and the tapping step (or the tapping range and number of steps). Unless otherwise specified, it shall be assumed that the range is symmetrical around the principal tapping and that the tapping steps in the tapped winding are equal. If for some reason, the design has unequal steps, this shall be indicated in the tender;
c) the category of voltage variation and, if combined variation is applied, the change-over point ('maximum voltage tapping', see 6.2);
d) whether maximum current limitation (reduced power tappings) shall apply, and if so, for which tappings.
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 E).
6.4.3 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 2500 kVA with a tapping range not exceeding ±5%.
The following information shall be given by the purchaser in the enquiry in addition to the rated voltage and rated power defined in Clause 5:
a) Direction of power flow (can be both directions).
b) The number of tapping steps and the size of the tapping step expressed as a percentage of the rated voltage at the principal tapping. If the tapping range is not symmetrical about the principal tapping then this shall be indicated. If the tapping steps are not equal across the range then this shall be indicated.
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 will choose the arrangement of windings, the winding or windings that are tapped. The transformer shall be able to supply the rated current on the secondary winding on all tapping positions consistent with the above operating conditions, without exceeding the temperature rise requirements defined by GB 1094.2.
The transformer shall be designed to withstand without damage the voltages and fluxes arising from the above specified loading conditions (including any specified overload conditions). A calculation showing that this condition is satisfied shall be supplied to the purchaser on request.
An example is given in Annex E (Example 4).
Alternatively, the user shall submit a set of loading cases with values of active and reactive power (clearly indicating the direction of power flow), and corresponding on-load voltages. These cases shall indicate the extreme values of voltage ratio under full and reduced power (see “the six-parameter method” of GB/T 13499). Based on this information, the manufacturer will then select the tapped winding and specify rated quantities and tapping quantities in his tender proposal. An agreement shall be reached between the manufacturer and the purchaser on the design tapping quantities.
6.5 Specification of short-circuit impedance
For transformers with no tappings exceeding a voltage variation of ±5% from the principal tapping, the short-circuit impedance of a pair of windings shall be specified at the principal tapping only, either in terms of ohms per phase Z or in percentage terms z referred to the rated power and rated voltage of the transformer (see 3.7.1). Alternatively, the impedance may be specified in accordance with one of the methods below.
For transformers with tappings exceeding a voltage variation of ±5% from the principal tapping, impedance values expressed in terms of Z or z shall be specified for the principal tapping and the extreme tapping(s) exceeding ±5%. On such transformers, these values of impedance shall also be measured during the short-circuit impedance and load losses test (see 11.4) and shall be subject to the tolerances given in Clause 10. If the impedance is expressed in percentage terms z, this shall be referred to the rated tapping voltage (at that tapping) and the rated power of the transformer (at the principal tapping).
Note 1: The selection of an impedance value by the user is subject to conflicting demands: limitation of voltage drop versus limitation of overcurrent under system fault conditions. Economic optimization of the design, bearing in mind loss, leads towards a certain range of impedance values. Parallel operation with an existing transformer requires matching impedance (see GB/T 13499).
Note 2: If an enquiry contains a specification of not only the impedance at the principal tapping but also its variation across the tapping range, this can impose an important restriction on the design of the transformer (the arrangement of the windings in relation to each other and their geometry). The transformer specification and design also need to take into account that large changes in impedance between taps can reduce or exaggerate the effect of the tappings.
Alternatively maximum and minimum impedances in terms of z or Z may be specified for each tapping across the whole tapping range. This may be done with the aid of a graph or a table.(See Annex F). The boundaries shall where possible be at least as far apart as to permit the double-sided tolerances of Clause 10 to be applied on a median value between them. Measured values shall not fall outside the boundaries, which are limits without tolerance.
Note 3: The specified maximum and minimum impedances shall allow an impedance tolerance at least as great as the tolerances given in Clause 10 but where necessary a tighter tolerance may be used by agreement between manufacturer and purchaser.
Note 4: Basing the impedance on the rated tapping voltage and the rated power of the transformer at the principal tapping means that the relationship between ohms per phase Z and percentage impedance z will be different for each tap and will also depend on which winding the voltage variation is specified. Great care is therefore needed to ensure that the specified impedance is correct. This is particularly important for transformers specified with tapping powers different to rated power at principal tapping.
6.6 Load loss and temperature rise
Load loss and temperature rise shall be in accordance with:
a) If the tapping range is within ±5%, and the rated power not above 2500 kVA, load loss guarantees and temperature rise refer to the principal tapping only, and the temperature rise test is run on that tapping.
b) If the tapping range exceeds ±5% or the rated power is above 2500 kVA, the guaranteed losses shall be stated on the principal tapping position, unless otherwise specified by the purchaser at the enquiry stage. If such a requirement exists, it shall be stated for which tappings, in addition to the principal tapping, the load losses are to be guaranteed by the manufacturer. These load losses are referred to the relevant tapping current values. The temperature-rise limits are valid for all tappings, at the appropriate tapping power, tapping voltage and tapping current.
The temperature-rise type test shall be carried out on one tapping only, unless otherwise specified. It will, unless otherwise agreed, be the 'maximum current tapping' (which is usually the tapping with the highest load loss). The maximum total loss on any tapping is the test power for determination of liquid temperature rise during the temperature rise test, and the tapping current for the selected tapping is the reference current for determination of winding temperature rise above liquid. For information about rules and tests regarding the temperature rise of liquid-immersed transformers, see GB 1094.2.
In principle, the temperature-rise type test shall demonstrate that the cooling equipment is sufficient for dissipation of maximum total loss on any tapping, and that the temperature rise over external cooling medium temperature of any winding, at any tapping, does not exceed the specified maximum value.
Note 1: For an autotransformer, the maximum current in the series and common windings are usually at two different tap positions. Therefore an intermediate tap position may be selected for test to allow the requirements of GB 1094.2 to be met on both windings during the same test.
Note 2: For some tapping arrangements, the tapping winding is not carrying current in the maximum current tapping position. Therefore, if the temperature rise of the tapping winding needs to be determined, another tapping may be selected or an extra test may be agreed.
7 Connection and Connection Symbols
7.1 Connection and connection symbols for three-phase transformers and for single phase transformers connected in a three phase bank
7.1.1 Connection symbol
The star, delta, or zigzag connection of a set of phase windings of a three-phase transformer or of windings of the same voltage of single-phase transformers associated in a three-phase bank shall be indicated by the capital letters Y, D or Z for the high-voltage (HV) winding and small letters y, d or z for the intermediate and low-voltage (LV) windings.
If the neutral point of a star-connected or zigzag-connected winding is brought out, the indication shall be YN (yn) or ZN (zn) respectively. This also applies to transformers where the neutral end connections for each phase winding is brought out separately but are connected together to form a neutral point for service.
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 (that are not connected together in the transformer but have both ends of each phase winding brough
Foreword I
1 Scope
2 Normative References
3 Terms and Definitions
4 Service Conditions
5 Rating and General Requirements
6 Requirements for Transformers Having a Tapped Winding
7 Connection and Connection Symbols
8 Rating Plates
9 Safety, Environmental and other Requirements
10 Tolerances
11 Tests
12 Electromagnetic Compatibility (EMC)
13 High Frequency Switching Transients
Annex A (Informative) Technical Differences between This Part and IEC 60076-1: 2011 and Their Reasons
Annex B (Informative) Vacuum Deflection Test and Pressure Deflection Test for Liquid Immersed Transformers in IEC 60076-1:
Annex C (Informative) Examples of Three-phase Transformer Connections
Annex D (Informative) Technical Requirements to be Provided with Enquiry and Order
Annex E (Informative) Examples of Specifications for Transformers with Tappings
Annex F (Informative) Specification of Short-circuit Impedance by Boundaries
Annex G (Normative) Temperature Correction of Load Loss
Annex H (Informative) Facilities for Future Fitting of Condition Monitoring Systems to Transformers
Annex I (Informative) Environmental and Safety Considerations
Bibliography
