GUIDE FOR PREPARATION OF SPECIFICATIONS FOR POWER TRANSFORMERS

528 GUIDE FOR PREPARATION OF SPECIFICATIONS FOR POWER TRANSFORMERS

Working Group A2.36

April 2013

GUIDE FOR PREPARATION OF SPECIFICATIONS FOR POWER TRANSFORMERS

WORKING GROUP A2.36 Task Force 1 Replaces TB 156 Convenor: T. Breckenridge (UK) Task Force Leader: M. Lamb (USA) Members N. Buthelezi (SA), A. Cancino (MX), L. Cornelissen (BE), E. de Groot (NL), M. Figura (PL), T. Fogelberg (SE), T.Gradnik (SI), AC Hall (UK), J. Lackey (CA), A. Manga (CA), A. Mjelve (NO), M. Oliva (SP), V. Podobnik (HR), S. Ryder (UK), K. Ryen (NO), C.Swinderman (USA), J.Velek (CZ), M. Zouiti (FR).

Copyright © 2013 “Ownership of a CIGRE publication, whether in paper form or on electronic support only infers right of use for personal purposes. Unless explicitly agreed by CIGRE in writing, total or partial reproduction of the publication and/or transfer to a third party is prohibited other than for personal use by CIGRE Individual Members or for use within CIGRE Collective Member organisations. Circulation on any intranet or other company network is forbidden for all persons. As an exception, CIGRE Collective Members only are allowed to reproduce the publication”. Disclaimer notice “CIGRÉ gives no warranty or assurance about the contents of this publication, nor does it accept any responsibility, as to the accuracy or exhaustiveness of the information. All implied warranties and conditions are excluded to the maximum extent permitted by law”. ISBN: 978-2-85873-222-7

WG A2-36

Guide for preparation of specifications for power transformers

Contents FOREWORD ................................................................................................................................. 1 1

SCOPE AND STANDARDS .................................................................................................. 4 1.1 Scope ............................................................................................................................ 4 1.2 Standards ...................................................................................................................... 4

2

DEFINITIONS ........................................................................................................................ 5

3

EXTENT OF SUPPLY ........................................................................................................... 6 3.1 General ......................................................................................................................... 6 3.2 Additional Requirements ............................................................................................... 6 3.3 Exclusions ..................................................................................................................... 6 3.4 Alternative offers ........................................................................................................... 6 3.5 Transfer of liability/ownership ....................................................................................... 7

4

PURPOSE OF THE EQUIPMENT ........................................................................................ 7 4.1 Some Examples ............................................................................................................ 7

5

SYSTEM OPERATING CONDITIONS .................................................................................. 7 5.1 General Description of Network .................................................................................... 7 5.2 Insulation Co-ordination ................................................................................................ 8 5.2.1 Method of System Earthing ....................................................................................... 8 5.2.2 Unusual Operating Conditions .................................................................................. 8 5.3 Short-Circuit Withstand ................................................................................................. 8 5.4 Over-Excitation.............................................................................................................. 9 5.5 DC Magnetisation........................................................................................................ 10 5.6 Harmonics ................................................................................................................... 10 5.7 Loading ....................................................................................................................... 10 5.8 Neutral Point Loading ................................................................................................. 11

6

SITE INFORMATION .......................................................................................................... 11

7

CONTRACT WORKS .......................................................................................................... 11 7.1 Installation Details ....................................................................................................... 11 7.2 Documentation Requirements .................................................................................... 11 7.3 Details of Additional Requirements ............................................................................. 12 7.4 Details of Requirements for Interfacing with Other Equipment, or to Achieve Interchangeability or Permit Parallel Operation .......................................................... 12

8

ENVIRONMENTAL CONSIDERATIONS ............................................................................ 12 8.1 General ...................................................................................................................... 12 8.2 Audible Sound ............................................................................................................. 13 8.3 Losses ......................................................................................................................... 13 8.4 Oil pollution ................................................................................................................. 14 8.5 Explosion or blast hazards .......................................................................................... 14 8.6 Fire hazards ................................................................................................................ 14 8.7 Visual impact ............................................................................................................... 15 8.8 Seismic impact ............................................................................................................ 15

9

TRANSPORT ...................................................................................................................... 15 9.1 General ....................................................................................................................... 16 9.2 Design for transport .................................................................................................... 16 9.3 Impact recorders ......................................................................................................... 17 9.4 Transportation with or without oil ................................................................................ 17 9.5 Loose equipment......................................................................................................... 18 9.6 Inventory ..................................................................................................................... 18 9.7 Handling and markings ............................................................................................... 18 9.8 Documentation ............................................................................................................ 18 i

WG A2-36 9.9 9.10 9.11 10

Guide for preparation of specifications for power transformers

Storage ........................................................................................................................ 19 Storage without oil ...................................................................................................... 19 Storage with oil............................................................................................................ 20 SAFE WORKING SYSTEMS ......................................................................................... 20

11 TECHNICAL REQUIREMENTS ..................................................................................... 21 11.1 General ....................................................................................................................... 21 11.2 Normal and Abnormal Operating Conditions .............................................................. 21 11.2.1 Gas and Oil Actuated relays ............................................................................... 21 11.2.2 Overloads ............................................................................................................ 21 11.2.3 Geomagnetic Induced Current (GIC) Effects ...................................................... 22 11.3 Design Requirements ................................................................................................. 22 11.3.1 Flux density ......................................................................................................... 22 11.3.2 Voltage regulation ............................................................................................... 22 11.3.3 Cooling ................................................................................................................ 23 11.3.4 Control detail ....................................................................................................... 23 11.3.5 System Earthing .................................................................................................. 23 11.4 Core ............................................................................................................................ 23 11.5 Tank ............................................................................................................................ 24 11.5.1 Handling facilities ................................................................................................ 24 11.5.2 Cover ................................................................................................................... 24 11.5.3 Oil-tight Joints ..................................................................................................... 24 11.5.4 Vacuum and pressure requirements ................................................................... 25 11.5.5 Valves ................................................................................................................. 25 11.5.6 Circulating and eddy-currents ............................................................................. 26 11.5.7 Access Openings ................................................................................................ 26 11.5.8 Conservator Tanks .............................................................................................. 26 11.5.9 Tank Earthing ...................................................................................................... 26 11.5.10 Pressure relief ..................................................................................................... 27 11.6 Insulating Fluid ............................................................................................................ 27 11.7 Bushings ..................................................................................................................... 27 11.8 Secondary Wiring and Control Cabinets ..................................................................... 28 11.9 Fittings ......................................................................................................................... 28 11.10 Tap Changers ......................................................................................................... 29 11.11 Monitoring ............................................................................................................... 31 11.12 Interchangeability ......................................................................................................... 31 11.13 Standardisation ....................................................................................................... 31 11.14 Exclusions ............................................................................................................... 32 12 MANAGING QUALITY ................................................................................................... 32 12.1 Quality Inspection and Test Plan (QITP) .................................................................... 33 12.2 Quality Inspection and Test Plan (Outline) ................................................................. 33 12.3 Quality Assurance Plan ............................................................................................... 35 12.4 Quality Assurance Manuals ........................................................................................ 35 12.5 Final Quality File ......................................................................................................... 35 13 FACTORY ACCEPTANCE TESTS AND FINAL INSPECTIONS .................................. 37 13.1 General ....................................................................................................................... 38 13.2 Standards and testing specifications .......................................................................... 38 13.3 Testing environment ................................................................................................... 38 13.4 Measurement accuracy ............................................................................................... 38 13.5 Tolerances .................................................................................................................. 39 13.6 Summary of tests ........................................................................................................ 39 13.6.1 Routine tests ............................................................................................................ 39 13.6.2 Type tests .......................................................................................................... 40 13.6.3 Special tests ........................................................................................................... 40 13.6.4 Additional tests .................................................................................................... 41 13.7 Test sequence............................................................................................................. 41 13.8 Test results and test reports ....................................................................................... 42 13.9 Site acceptance tests - erection tests ......................................................................... 42 ii

WG A2-36 13.9.1 13.9.2 13.9.3 13.10 13.11 13.12 13.13

Guide for preparation of specifications for power transformers General ............................................................................................................... 42 Required tests ..................................................................................................... 43 Commissioning tests ........................................................................................... 43 Energisation ............................................................................................................ 44 Trial operation ......................................................................................................... 44 Special tests ............................................................................................................ 44 Site test reports ....................................................................................................... 45

14 LIST OF GUARANTEES AND WARRANTIES .............................................................. 45 14.1 Guaranteed performances .......................................................................................... 45 14.2 Other types of guarantee ............................................................................................ 45 15 CONTRACT DOCUMENTATION ................................................................................... 46 15.1 An Enquiry document should include the following details, where applicable: ........... 46 15.2 A Tender should include: ............................................................................................ 46 15.3 A manufacturer should provide the following documents as part of a Contract: ........ 46 16

EXAMPLES OF TECHNICAL AND OTHER INFORMATION SCHEDULES RELATING TO A TRANSFORMER SPECIFICATION ..................................................................... 47

APPENDIX A - Loss evaluation, penalties, bonuses and rejection ................................... 63

iii

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Guide for preparation of specifications for power transformers

FOREWORD

CIGRÉ Technical Brochure 156 was first published under the auspices of CIGRÉ Study Committee 12 in 2000. This first revision of the original document has been prepared by CIGRÉ WG A2-36 “Transformer Procurement Process” and directly replaces Technical Brochure 156. The original document was produced by Working Group 12.15 and published in April 2000. The members of the original Working Group 12.15 were: A. C. Hall (UK) Convener, A. Alvarez (ES), S. Bhumiwat (TH), G. Cannavale (IT), A. Carlson (SE), V.M. Chornogotsky (UA), J. Elovaara (FI), G. Gomez (ES), A. Higgins (IR), J. Kulikowski (PL), R. De Lhorbe (CA), Z. Luspay (HU), T.L. Machado (BR), G. Moore (IR), K. Newman (UK), H.J. Klein Nibbelink (NL), J-O. Persson (SE), A Petersen (AU), J-F. Ravot (CH), Y Shafir (UE), C.M. Sharma (IN), V. Sokolov (UA), J-P. Taisne (FR), E. Troyan (UA), J-P. Uehlinger (CH). This document is a general update from the existing CIGRÉ Technical Brochure 156 and introduces additional sections based on current market issues such as transportation. References to IEC standards have been updated to reflect the current documents many of which have be brought under the IEC 60076-X umbrella since the original publication date. The title of the previous guide restricted the application to above 123 kV and above 100 MVA. The principles in this document are equally applicable at lower voltages and lower rated power where purchasers often require more support than larger user and so the limitations in the title have been removed to widen the scope of application. This document has been prepared as an aid to purchasers and manufacturers of power transformers in the preparation of specifications for purchasing transformers. A typical guide has been produced under the headline of “transformer procurement process” and is one of a number of key steps in the process. A typical procurement process is shown in figure 1 and clearly shows where this guide fits into the whole procurement process with various quality factors other than just the cost. It should be borne in mind when preparing transformer specifications that this document will determine all of the future operating characteristics of the unit and it is where many future problems in terms of reliability of the transformer ultimately begin. It is of vital importance for the specification to be correct in order that the manufacturer can deliver the transformer the purchaser really needs. The key word is communication and the specification is the foundation of the technical communication. Getting the specification right is the first step, and ensuring the requirements are communicated and understood by the manufacturer is also important, but that is where the design review comes into play. With the globalisation of the transformer market place since the original issue of this guide, specifications are often written in a language which is foreign to the potential manufacturer; therefore the purchaser needs to consider carefully the wording of the document. Complicated language can often be misinterpreted by a non-native speaker, so it is extremely important to try and use simple internationally understood language wherever possible. The aim of the document is to highlight some of the important topics that should be considered for inclusion in an enquiry document. Wherever possible, guidance and other practical information about such topics is provided to explain their purpose and significance in transformer enquiries. Clearly it is not possible to address all the issues that may arise. Neither is the document in any way intended to be a complete and

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Guide for preparation of specifications for power transformers

applicable specification. It is strongly recommended that purchasers use recognised standards and application guides on which to base their specifications. This document refers principally to standards and guides published by the International Electrotechnical Commission (IEC), but other regional or national relevant standards and guides may be used. In addition, purchasers can refer to manufacturers and other specialist providers for assistance in preparing specifications. This can be especially important if the technical features of the required equipment are outside the purchaser’s previous scope and experience. The format and content of technical specifications are not constant. They vary according to particular equipment needs and also to other factors such as purchasers purchasing, economic and operating policies as well as technology innovations by manufacturers and material suppliers. Significant changes are often necessary as the result of experience of previous purchases, either during manufacture or in service or both. In other words, it is both customary and necessary to review specifications often to ensure that they are up to date with purchaser’s requirements and obligations and able to utilise the best manufacturing and operating practices. Purchasers should also remember that the constantly changing legislative and regulatory environment in which power system operators find themselves often requires changes in policy or solutions being purchased and this should be reflected in the specification. Certain sections of this specification include a ‘Preface’. This is used solely to draw the purchaser’s attention to some particular aspects of the section or to explain its purpose. Additional information may also be obtained from application guides and codes of practice. Because the document is advisory and not purchase or site specific, or intended for use directly as a purchasing specification, words such as ‘should’, ‘may’ and ‘could’ are used throughout the document. Purchasers should therefore, strengthen the wording of appropriate clauses in their specifications by using words such as ‘shall’ and ‘must’ to indicate mandatory requirements”.

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Guide for preparation of specifications for power transformers

Identify Transformer Requirements

Prepare and Issue Technical Specifications

Determine List of Tenderers

Assess Tender Returns Technically

Issue Tender

Pre Contract Design Review

Place Contract

Yes

Assess Capability of Manufacturer Any previous Service Experiences of manufacturer?

Post Contract Design Review

On Going Manufacturers Inspections

Is Tenderer Capable of Manufacturing?

No

Factory Acceptance Testing

Do Not Use Manufacturer

On Site installation and Commissioning

Shipping to Site

Concerns over suppliers’ capability?

Post Contract Clarification of Requirements

End of Procurement Process

No

Continue to Use Manufacturer

Yes Re-assess Capability of Manufacturer

Enhanced inspection and review Is Manufacturer Capable of Manufacturing?

No

Yes Yes

Can issue be resolved with additional supervision?

No

Figure 1 – Example of a transformer procurement process

3

Terminate Contract

WG A2-36

1 1.1

Guide for preparation of specifications for power transformers

SCOPE AND STANDARDS Scope

As a minimum the scope of a specification for a large transformer is meant to include design and development; procurement of components and materials; manufacturing; and acceptance testing at the manufacturer’s works. Depending on the delivery terms, the scope of a specification may be extended to include transport, in whole or in part; installation; commissioning, usually understood to include acceptance testing after installation and before first energisation; and warranty/service provision. In some cases the scope of a specification may be further extended to include provision of spares components or maintenance equipment. 1.2

Standards

As was noted in the Foreword to this guide, it is strongly recommended that purchasers use Standards as the basis for specifications. These may include international standards and national standards, such as IEC and ISO standards. In any case, the purchaser needs to consider suitability of particular Standards or even the applicable revision of a Standard for the specific application. Power transformers should comply with the requirements of the specification and the standards listed therein, for instance such as those listed below: IEC 60044

Current transformers

IEC 60050

International Electrotechnical Vocabulary

IEC 60050(421) International Electrotechnical Vocabulary - Chapter 421: Power transformers and reactors IEC 60060

High Voltage test techniques

IEC 60060-1

General definitions and test requirements

IEC 60060-2

Measuring systems

IEC 60071-1

Insulation coordination - Part 1: Definitions, principles and rules

IEC 60071-2

Insulation coordination - Part 2: Application guide

IEC 60076-1

Power transformers - Part 1: General

IEC 60076-2

Power transformers - Part 2: Temperature Rise for liquid-immersed transformers

IEC 60076-3

Power transformers - Part 3: Insulation levels, dielectric tests and external clearances in air

IEC 60076-4

Power transformers - Part 4: Guide to the lightning impulse and switching impulse testing - Power transformers and reactors

IEC 60076-5

Power transformers - Part 5: Ability to Withstand Short-circuits

IEC 60076-6

Power transformers - Part 6: Reactors

IEC 60076-7

Power transformers - Part 7: Loading guide for oil-immersed power transformers

IEC 60076-8

Power transformers – Part 8: Application Guide

IEC 60076-10

Power transformers – Part 10: Determination of sound levels

IEC 60076-18

Power transformers – Part 18: Measurement of frequency response

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Guide for preparation of specifications for power transformers

IEC 60137

Bushings for Alternating Voltages above 1000V

IEC 60214-1

Tap-changers - Part 1: Performance requirements and test methods

IEC 60214-2

Tap-changers - Part 2: Application Guide

IEC 60270

High-voltage test techniques - Partial discharge measurements

IEC 60296

Fluids for electrotechnical applications - Unused mineral insulating oils for transformers and switchgear

IEC 60422

Mineral Insulating Oil in Electrical Equipment – Supervision and Maintenance Guide

IEC 60529

Degrees of Protection provided by Enclosures (IP Code)

IEC 60567

Oil-filled electrical equipment - Sampling of gases and analysis of free and dissolved gases - Guidance

ISO 8501-1

Preparation of steel substrates before application of paints and related products – visual assessment of surface cleanliness

ISO 9001

Quality management systems – requirements

ISO 12944-2

Paints and varnishes – corrosion protection of steel structure by protective paint systems – classification of environments

ISO 14001

Environmental systems – requirements, with guidance for use

ISO 19011

Guidelines for quality and/or environmental management systems auditing

The standard should be the most up-to-date published version available when a tender is submitted. The latest version of IEC standards can be obtained from the IEC website (www.iec.ch). Purchasers should also consider looking at other websites, e.g. national standards, ISO, etc. In the event of conflict between the requirements in a specification and those of the specified standards, the usual practice is that the specification should prevail. The following order is recommended, but the purchaser needs to re-confirm their own order:     2

Particular technical specification/technical data sheet General technical specification International standards (e.g. IEC) National standards (e.g. NBN, BS EN, DIN EN etc.)

DEFINITIONS

For the purpose of this guide specification, the definitions listed in IEC 60050(IEV) and other relevant IEC standards have been used. Additional more specific transformer terms, or terms used by purchasers that may not be generally understood, or are not already covered by existing standards, should be defined in this section.

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3

Guide for preparation of specifications for power transformers

EXTENT OF SUPPLY

Preface In this section, the Purchaser should give a brief but a clear statement about the equipment, the components to be supplied and the limits of contractual responsibility that are to exist between the manufacturer and purchaser, in order that no misunderstanding about respective liabilities can occur. 3.1

General

The supply of a transformer comprises the design, manufacture, quality assurance and testing at the manufacturers works and depending on the contract, transport, complete erection, commissioning, and setting to work at a site. The basic parts of the transformer will comprise the main tank, active part, terminations, cooling facilities, fittings and the systems and equipment for oil preservation, tap changing, protection, control and monitoring, and any other component necessary for the proper operation of the power transformer. 3.2

Additional Requirements

The extent of supply may also include other additional requirements such as,              3.3

making route surveys and obtaining any permits, licences and statutory authority approval alterations to existing routes to permit passage of the transformer any additional auxiliary equipment or facilities transport to site site commissioning tests and inspections site civil works, in particular transformer foundations, oil containment, fire and blast containment noise reduction measures the supply and commissioning of secondary equipment. warranties and service agreements tools, spare components and maintenance procedures documents, operating instructions and maintenance procedures safety training of personnel to work on site working language of all documents provided under the contract Exclusions

In an enquiry, the purchaser should state any aspects of transformer engineering, installation or operation that will not be acceptable. Equally, the manufacturer should state any aspects of the purchaser’s specification that will not be complied with. 3.4

Alternative offers

The purchaser may request offers based upon alternative means of obtaining the required extent of supply, or describe the extent of supply required in a manner that will

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Guide for preparation of specifications for power transformers

permit a manufacturer to interpret what is required and submit alternative solutions and offers. Alternatively, the manufacturer may tender alternative offers, which may only be accepted in writing by the purchaser. 3.5

Transfer of liability/ownership

The purchaser should consider special issues of transformer ownership, condition verification, and liability during transformer transportation between the factory and the site. Refer to Section 9 - Transport of this document. 4

PURPOSE OF THE EQUIPMENT

Preface It is of utmost importance for the purchaser to state the purpose of the equipment and how it is to be used (see 4.1) A ‘Purpose of the Equipment’ statement is particularly important if the transformer is to be used for an unconventional or select application such as for example, for rectification, arc furnace, railway supply, heavy duty motor applications, use with solid state power electronics equipment, or installed within a sound mitigation house or not. The purchaser or manufacturer should resolve any doubts about the purpose of the transformer preferably before the enquiry is issued but certainly before a contract. 4.1

5 5.1

Some Examples 

“The autotransformers referred to in this specification will be installed at XXX transmission substation”, placed within a sound mitigation enclosure, and rated for both step-up and step-down operations”



“The transformers referred to in this specification will be installed at XXX wind farm for connection to 3.6MW wind turbine generators and used wind turbine step-up transformers ”



“The transformers referred to in this specification will be installed at XXX railway trackside substation to supply directly the YYY railway track overhead (or underground as appropriate) electrical power supply system.”



“The transformers referred to in this specification will be installed at the XXX foundry and used to supply individually and directly open hearth electric arc furnaces.”



“The transformers referred to in this specification will be installed in the indoor substation at the offices of XXX Co Ltd, address details, and used for mixed non-industrial loads including a 100% secure demand supply to a 3MVA mainframe computer installation.”

SYSTEM OPERATING CONDITIONS General Description of Network

It is often helpful for the purchaser to give some background information concerning system operating conditions where the transformer will be installed. Purchasers should try to strike the correct balance between omitting potentially important 7

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Guide for preparation of specifications for power transformers

information and providing large amounts of information of little interest or use to the manufacturer, which cause confusion as to their actual requirements. A single-line diagram of the installation is often particularly helpful in giving general information. It can also be referred to in providing other useful system information. 5.2

Insulation Co-ordination

Procedures for insulation co-ordination are described in detail in IEC standards 600711 and 60071-2. Use of these procedures to determine suitable insulation levels for the transformer is usually considered to be the responsibility of the purchaser. In practice, insulation levels are usually pre-determined by an Industry or National Standard. Purchasers may wish to refer to the network operator(s) for guidance. Determination of suitable insulation levels should also be made with reference to IEC standard 60076-3, which describes which tests are applicable to transformers of which voltage class. Purchasers have a number of choices with regard to the applicability of testing, notably whether a chopped wave lightning impulse test is required and whether a switching impulse test or a short-duration ac induced voltage is required. Any such requirements should be clearly stated in the specification. For certain tests, there are a number of different methods allowed by IEC standard 60076-3. Where users have a requirement for a test to be performed in a certain way, e.g. long-duration ac induced voltage test with three-phase excitation and the neutral earthed, this should be stated in the specification. 5.2.1 Method of System Earthing Purchasers should provide information in the enquiry document as to the method of system earthing on the power system to which the transformer will be connected, e.g. star point earthed, earthing transformer for unearthed systems etc. In addition the purchaser should state whether the earth point is solidly connected to earth, or via some form of earthing resistance or inductance. Where impedance earthing is used the purchaser should state the value of the impedance to be installed to restrict fault current. 5.2.2

Unusual Operating Conditions

Energisation imposes both electrical and mechanical transients on large transformers. Certain applications involve frequent energisation, e.g. arc furnace supply, generator transformers in pumped-stored schemes, and shunt reactors. This may require special design considerations, especially if the transformer is directly connected to gasinsulated switchgear or if it is to be energised from the LV side. The specification should give information on the expected frequency of energisations, and on the method of energisation. 5.3

Short-Circuit Withstand

Clause 3.1 of IEC standard 60076-5 requires that: Transformers together with all equipment and accessories shall be designed and constructed to withstand without damage the thermal and dynamic effects of external short circuits ... External short circuits are not restricted to three-phase short circuits; they include lineto-line, double-earth and line-to-earth faults. 8

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Guide for preparation of specifications for power transformers

To meet the requirements of this standard, purchasers must provide sufficient information about the power system parameters to enable a transformer designer to calculate the steady-state short-circuit currents using the method of symmetrical components. In practice this means specifying, for each system to which the transformer is to be connected, the positive-, negative-, and zero-sequence impedances. The positive sequence impedance is usually given as the short-circuit apparent power of the system (in MVA). It is equally possible to specify a maximum short-circuit current (in kA), which is often the breaking capacity of the switchgear to which the transformer will be connected. The positive and negative sequence impedances of transmission networks are usually assumed to be equal. The positive and zero sequence impedances of transmission networks are not equal, and the zero sequence impedance is usually given as the ratio of zero sequence impedance to positive sequence impedance. For overhead transmission lines the ratio of zero sequence impedance to positive sequence impedance is typically 3-4, although is often assumed to be rather less. Some users consider it best to assume the effect of the system impedance in limiting short-circuit currents is negligible, i.e. that the transformer is connected to an infinite busbar. To meet the requirements of the standard, purchasers must also include sufficient information to calculate dynamic short-circuit currents. In practice, this means specifying the ratio of reactance to resistance for each system to which the transformer is to be connected. Note that the ratio of reactance to resistance for large transformers is typically very high, and the effect of the system is usually to reduce the ratio of reactance to resistance. Certain purchasers consider it best to make the pessimistic assumptions that the effect of the system on the ratio of reactance to resistance is negligible. Other purchasers consider it best to make the more pessimistic assumption that the ratio of reactance to resistance is infinite. From this it follows that the ratio of dynamic to static short-circuit current is 2√2. Further guidance may be found in power systems text books, e.g. “Short-Circuit Duty of Power Transformers”, by Giorgio Bertagnolli and published by ABB. Unusual Operating Conditions Certain specialised applications involve unusually frequent or unusually severe shortcircuits, e.g. arc furnace supply (directly or indirectly). Transformers operating on certain networks may also be exposed to unusually frequent or unusually severe shortcircuits. These applications may require special design considerations. The specification should give information on the expected frequency of short-circuits and if necessary their expected duration and severity. 5.4

Over-Excitation

Over-excitation under steady-state or transient conditions can cause damage to transformer cores and the associated insulation and supporting structures. Transformers specified in accordance with IEC standard 60076-1 have only a limited tolerance of over-excitation (10% at no load, 5% at full load). Large transformers are frequently subject to more severe over-excitation. Over-excitation can arise owing to operation at: 

Below rated frequency 9

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Guide for preparation of specifications for power transformers

Above rated secondary voltage, either as a result of a high network voltage or a low load power factor

Note that certain applications impose severe over-excitation requirements, e.g. generator transformers which are also used for synchronous compensation and SVC transformers. A typical requirement for large transformers is as follows (taken from IEEE standard C57.12.00-2010). Transformers shall be capable of: a) Operating continuously above rated voltage or below rated frequency, at maximum rated kVA for any tap, without exceeding the limits of observable temperature rise [...] when all of the following conditions prevail: 1) Secondary voltage and volts per hertz do not exceed 105% of rated values. 2) Load power factor is 80% or higher. 3) Frequency is at least 95% of rated value. b) Operating continuously above rated voltage or below rated frequency, on any tap at no load, without exceeding limits of observable temperature rise [...] when neither the voltage nor volts per hertz exceed 110% of rated values. 5.5

DC Magnetisation

Under certain operating conditions, dc currents may flow through transformer windings causing asymmetrical magnetisation of the core and possible saturation. Rectifier transformers of all kinds, including HVDC transformers, may be exposed to dc currents. Trackside transformers may also be exposed, especially during severe winter weather conditions. Quasi-dc currents may flow in power systems owing to solar activity (geo-magnetically induced currents), exposing generator and network transformers to the risk of dc magnetisation. According to IEC standard 60076-1, any requirement for tolerance of dc currents is unusual and the requirement should be stated in the specification. The specification should state the maximum expected dc current and duration, and any requirements concerning noise or reactive power absorption whilst exposed to the maximum expected dc current. In practice, it may be difficult to assess conformity with any such requirements through design review or test. 5.6

Harmonics

Transformers specified in accordance with IEC standard 60076-1 have only a limited tolerance of harmonics in the load current (5% total harmonic content, including 1% even harmonic content). Where these requirements are exceeded, transformers should instead be specified in accordance with IEC standards 61378-1 and -3 (industrial applications) or IEC standards 61378-2 and -3 (HVDC applications). 5.7

Loading

Transformers specified in accordance with IEC standard 60076-1 are usually also specified as being capable of loading in accordance with IEC standard 60076-7. This may not be appropriate in all cases, e.g. transformers directly connected to semiconductor convertors which have a lower over-load capacity than a transformer specified in accordance with IEC standard 60076-7. Note that IEC standard 60076-7 10

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Guide for preparation of specifications for power transformers

does not apply to arc furnace transformers. In the case of generator transformers, the active power the prime mover is capable of supplying may vary ambient temperature. Certain users consider it best to include the capability curve of the prime mover in the specification. 5.8

Neutral Point Loading

Unbalanced loading or neutral point loading is unusual for large transformers. If there is any requirement for unbalanced loading or neutral point loading, then this should be included in the specification. Note that certain winding arrangements are incompatible with neutral point loading owing to high zero-sequence impedance. For further guidance, see IEC standard 60076-8. 6

SITE INFORMATION

IEC 60076-1 contains details of the general site conditions under which transformers are expected to operate. For the purposes of a contract, purchasers are advised to provide additional information detailing the conditions at the site where a transformer is to be installed or at other sites where it may be subsequently installed, including for example, details of transport routes, site access, the layout of the site and any site limitations or restrictions. In addition, the purchaser shall specify any unusual site conditions, such as high altitude, environmental contamination, high or low ambient temperatures. 7

CONTRACT WORKS

Preface The purchaser should state all the general or specific details that the manufacturer is contractually required to comply with, in this part of the specification. 7.1

Installation Details

This should include details of:  

7.2

arrangement of the contract works at site or method of supply the environmental conditions and requirements for the installation and it’s equipment Documentation Requirements

The documentation requirements differ among purchasers. The purchaser should be well aware of any internal needs for documents. In case of confidential documents, the manufacturer will probably refuse to make these available to the purchaser. In such a case, it can be agreed to have these documents readily available for (re)viewing at the premises of the manufacturer. Below is a list of documents a purchaser can request, which may at times be considered confidential: 

drawings detailing overall and individual major component dimensions and 11

WG A2-36

weights drawings, photographs and other records detailing the transformer internal arrangements and parts permits and licences verifying approval and details of a transport route to site, including identification and description of the most difficult parts of the route calculations (refer to design review guide)

 

 7.3

Details of Additional Requirements   

7.4

power supply available for auxiliary equipment transport gauge profiles method of line, neutral and earthing connections

Details of Requirements for Interfacing with Other Equipment, or to Achieve Interchangeability or Permit Parallel Operation   

bushing electrical connection points bushing securing flange diameter bushing flange details: number and size of securing bolt holes, bolt hole diameter and pitch circle diameter insulation levels (BIL) connection design, cable bushing size of winding connections to accommodate the different lengths of standardised bushing fittings and maintain a minimum gap to earth details of any current transformers mounted within or external to the bushings or bushing turrets, including tolerances and definition of tolerances to facilitate work by other manufacturers other fittings, e.g., gas and oil actuated relays, cooling pumps, valves, etc. for parallel operation, details of the existing transformers including no-load voltage per tap step, number of tapping steps and percentage step, the tapping step numbers, impedance voltages per tap position, vector group details; please refer IEC 60076-1 for additional details.

   

 

8

Guide for preparation of specifications for power transformers

ENVIRONMENTAL CONSIDERATIONS

Preface The requirements specified in this section relate to the care for the environment and include any special conditions and precautions that have to be incorporated in the design, manufacture and operation of the transformer to comply with such environmental considerations. Some of these considerations may be of legal nature. 8.1

General

The principal considerations are:   

audible sound losses oil pollution 12

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   

Guide for preparation of specifications for power transformers

fire hazard and risk of producing contaminants or other unacceptable byproducts blast hazards visual impact seismic risk

Control or avoidance of oil pollution, fire and blast hazards are largely a civil engineering matter but the effects can be mitigated by attention to transformer design. These considerations apply to both new and existing installations and are largely site specific, but other purchaser or statutory requirements may also apply. Consequently, there are responsibilities on both purchaser and manufacturer to ensure that the site conditions are properly assessed and the requirements and responsibilities clearly defined and fulfilled. 8.2

Audible Sound

Low sound power levels in the vicinity of substations are an increasing environmental requirement. The purchaser should state in the specification the maximum permissible sound power and/or pressure level allowed for the transformer and the method of measurement (e.g. IEC 60076-10). The specification should also state any contractual liabilities that will ensue in the event of exceeding the specified sound power and/or pressure/intensity level during the factory acceptance tests. Sound panels and sound houses have been useful in helping to reduce sound pressure levels. It is important that the transformer design considers these sound mitigating options to ensure that they will not adversely affect the thermal and dielectric performance of the transformer. 8.3

Losses

The provision of the financial value per kW (unit cost / kW) of no-load and load losses forms an essential part of a transformer specification. Without this information and without any guidance from a purchaser of the transformer loading regime or the system operating costs, a tenderer will very likely offer a tender which has the lowest purchase price, but may not necessarily provide for the lowest total lifetime cost of ownership. The better alternative is for the purchaser to provide information that will permit a tenderer to optimise his designs. This is done by the purchaser calculating the cost of the no-load and load losses that will arise under the intended transformer operating regimes, together with projections of service life, load growth, interest rates etc., and stating them in the specification. In most cases, by providing the cost of losses in this manner, a purchaser can avoid the risk of disclosure of confidential information and the submission of a multiplicity of offers. It also allows each tenderer to focus on submitting the most efficient design possible within the limits of those economic loss values and on reducing his tendering costs. There is more specific information relating to loss evaluation, penalties, bonuses, and rejection provided within Appendix A in this specification. Loss guarantees Typically, the purchaser requests the manufacturer to provide the following loss guarantees in the tender: 

no-load losses measured on the principal tapping at 100% rated voltage, or 13

WG A2-36

  8.4

Guide for preparation of specifications for power transformers

as specified by the purchaser load losses at reference temperature at principal tap position and rated power between windings, or as specified by the purchaser cooling equipment losses (kW) Oil pollution

Joints between oil containing parts of the equipment should be leak tight under all circumstances. Gasket joints should be designed to avoid deterioration of the gasket due to oil or excess strain. The probability of an oil spill occurring within a substation is typically very low. However some substations, for example due their proximity to waterways, have a higher risk for discharging harmful quantities of oil into the environment. The purchaser may consider the application of natural or synthetic ester fluids, since they are less harmful to the environment. 8.5

Explosion or blast hazards

The probability of a transformer exploding is even smaller than a transformer catching fire. In the unlikely case, however, the explosion or blast hazard can at best be mitigated to some extent by fitting adequate pressure relief devices to the principal oil containing components at greatest risk, such as the main tank and tap changer diverter, together with means to control the diffusion of oil when they operate. However, in some cases, the rate-of-rise of pressure can be so rapid that relief devices cannot prevent tank rupture. Oil impregnated paper (OIP) bushings fitted with porcelain insulation may fail explosively creating a potential for damage to persons or property. Modern equivalent bushings of the epoxy resin impregnated paper type are now widely used, particularly with silicon rubber or other composite type insulators. The application of these alternative bushing designs greatly reduce the risk of explosion due to bushing failure. If the risk of explosion is a major issue on the specific site, the application of solutions such as ester fluids does not relieve the problem. Possible solutions that need to be considered include installation of the transformer in underground bunkers or in blast proof above ground housings. If neither of these solutions is possible then the purchaser may need to consider the use of SF6 gas insulated transformers. Other additional means may need to be provided to mitigate the consequences. 8.6

Fire hazards

Routine monitoring, maintenance, testing, and good asset health assessment programs can greatly help to prevent internal transformer faults, which in turn helps to prevent fires from occurring. The consequences of a transformer fire can be reduced by using fire protective walls and/or water deluge or gas blanketing protection systems, but these facilities are not always readily available or feasible at all sites. Fire risk might be reduced by considering natural or synthetic ester fluids, since they have higher flash points as compared to typical petroleum based fluids, as long as the transformer designs can accept their use. Ester fluids also self extinguish when the ignition source is removed whereas petroleum based fluids do not. Considerable quantities of water may be required as well as oil-water separators and storage facilities for the effluent. 14

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Guide for preparation of specifications for power transformers

Consideration should be given during the transformer and site design stages to provide the best means to cope with a transformer fire or explosion. In the case of fire, it is very important that the fire is contained and quenched as quickly as possible. In the absence of deluge techniques, this is often best achieved by electrical isolation of the transformer and, if conditions allow, applying water directly through a remote entry point in the oil system, or by gas blanketing. Reference is also given for further information in a new CIGRÉ technical brochure “Transformer Fire Safety Practices”, which is under preparation by CIGRÉ WG A2-33 with planned publication during 2013. 8.7

Visual impact

The visual impact of a transformer installation may be reduced by screening, height reduction, choice of finishing colour and in general, by attaining tidy surfaces and clean aesthetic lines. The purchaser should state any requirements in the enquiry. One should pay attention that any screening does not interfere with the cooling of the transformer. 8.8

Seismic impact

Where the transformer is intended for operation in an area of known seismic volatility, the specification should state this and indicate the degree of severity of any seismic shock which needs to be contained. 9

TRANSPORT

Preface The available means of transport between a manufacturer’s works and site should be pre-planned and stated by the party who will be responsible for the transport (either purchaser or transformer manufacturer) and agreed upon between the parties involved before the contract is signed. In any case the local requirements near to and at the site of assembly should be specified and stated by the purchaser. Additional provisions may also have to be included to permit subsequent transportation to other sites. Any statutory requirements governing transformer movement, and that of any associated loose equipment including oil, should also be clearly stated in the enquiry. In some cases, transport may be more effectively achieved by adopting a particular transformer design and construction, e.g. in the case of three phase transformers, for instance by adopting a 3 x single-phase type of construction. Alternatively, built-on-site design and construction techniques may be used. It is advisable to define the costs, risks, and responsibilities for transport of transformers between purchaser and manufacturer by using international regulations like the Incoterms. The current version of this is Incoterms 2010. The codes used are listed below. Incoterms 2010 rules for any mode or modes of transport:    

EXW FCA CPT CIP

Ex Works Free Carrier Carriage Paid To Carriage and Insurance Paid to 15

WG A2-36

  

Guide for preparation of specifications for power transformers

DAT DAP DDP

Delivered At Terminal Delivered At Place Delivered Duty Paid

If both parties want the manufacturer to deliver the transformer at the purchaser site of assembly, loaded or unloaded, without or with customs activities performed and duties paid, DAP or DDP should be used respectively. (Note that unloading on site and placing in the cell of assembly is not included in either DAP or DDP and must be specified or agreed upon in the contract) Incoterms 2010 rules for marine and waterway transport:    

FAS FOB CFR CIF

Free Alongside Ship Free On Board Cost and Freight Cost Insurance and Freight

For full details see Incoterms 2010 published by “International Chamber of Commerce” (ICC). Reference is also given for further information in a new CIGRÉ Technical Brochure “Guide on Transformer Transportation”, which is under preparation by CIGRÉ WG A242, autumn 2012, and with planned publication during 2013. 9.1

General

In the enquiry, the purchaser should state the maximum transport dimensions and weights permissible by road, rail or water, in addition to the requirements at the site of assembly. Responsibility for the load profile for each transport mode, transport route to site, the means of transport, any necessary route alteration, modification or refurbishment, statutory approval or licences, shipping reservations and documentation or any other requirement concerning the delivery of the contract works to site should be agreed between purchaser and manufacturer before a contract, if not specified in the enquiry by the purchaser. 9.2

Design for transport

The Transformer with its active part (windings, core, framework, clamping arrangements and general structure) and tap changer must be of robust design and safeguarded for transportation, capable of withstanding any shock (and duration of shock) to which it may be subjected to during transport, taking all planned transport modes into consideration. Necessary mechanical withstand capability has to be built into the design without any in-tank temporary bracings / supports / reinforcements. It is required to give information about the peak impacts which will be used in the design of the transformer to withstand transport impacts. These values are requested to be filled in the table below.

16

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Guide for preparation of specifications for power transformers

Outside tank Inside tank Max g-forces used for the design

Axis

Duration (cont. or msec)

Comments:

Longitudinal Vertical Lateral

9.3

Impact recorders

During transport, at least one 3D - accelerometer impact recorder with measurements in X, Y and Z axis (both plus and minus directions) should be used. The impact recorder(s) should have the possibility to measure acceleration events with 3D curve in the range of 1024ms or more. The number of such stored events must be sufficient for the transport. Acceleration range should be adjustable up to 10g with a frequency range of 1-100Hz. In addition the impact recorder should also be able to register both dynamic inclination curves and time-synchronous inclination events. In case of transportation without oil one impact recorder should also be fixed to the active part during the transportation and one outside the tank. A master-slave connected system could be of value as events inside and outside the tank will be time synchronised. Mounting:  the impact recorders should be bolted rigidly to the transformer tank (or active part).  impact recorders on the transformer tank should be mounted at or near the location where the transformer is supported by the transport vehicle. a) in most cases: the bottom plate of the transformer. b) girder trailers: near or at the supporting brackets of the transformer.  the mounting location inside the tank is dictated by accessibility, i.e. near a manhole or cover of bushing turret. Fixing to the core and coil clamping system is a good option.  the mounting orientation is irrelevant as long as the axis of the impact recorder align with the axis of the transformer.  the mounting location should be rigid, preferably near the corner of three intersecting surfaces, i.e. bottom plate near a stiffener and the tank wall. 9.4

Transportation with or without oil

If the transformer is transported without oil, provision should be made to prevent the ingress of moisture and to maintain the internal insulation in first-class condition. In addition the transformer should be filled with breathable dry air and maintained at a continuous positive pressure. The use of nitrogen is possible but is not preferred on the grounds of safety. The air pressure and dew-point should be monitored continuously throughout the period immediately after the oil is removed until the transformer is refilled with oil at site. At all times alternative standby means should be provided to restore any loss of air pressure immediately. The dew point of the dry air should be measured and recorded to ensure it is below -40 17

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Guide for preparation of specifications for power transformers

°C. The dew point should be checked again within 24 hours of the oil having been removed and the transformer dry air filled, the measurements being recorded in the test report and on the shipping tag. If the dew point readings indicate an average moisture level at the surface of the transformer insulation is higher than 0.5%, the manufacturer must dry the transformer. The maximum acceptable dew point shall also be indicated on the shipping tag. The dew point of the shipping gas shall be recorded along with the pressure and temperature of the shipping gas at the time of the dew point measurement. This information shall be recorded on the test report, the shipping documents and on a tag at the location of the dew point measurement. All tubing, valves, cable connections, and fittings attached to the payload should be adequately protected to minimise risk of damage during loading, transport and unloading. 9.5

Loose equipment

Other transformer equipment such as bushings, cooling equipment, tap changers, power, and control and regulation cabinets may be shipped disassembled, and transported and secured in accordance with the manufacturer’s shipment and storage guidelines and precautions. Where necessary, airtight seals or covers should be used. Parts such as instruments, cabinets, terminal boxes that are mounted on the main tank should be attached by means of anti-vibration mountings, and be protected with covering/enclosure to prevent physical damage during transport. 9.6

Inventory

Each individual component or part should be properly prepared for despatch, itemised and labelled. Each item should be named, coded and identified by make or manufacture, size, type, drawing number or part number and recorded in a transport inventory that should form part of the contract documentation. 9.7

Handling and markings

To facilitate handling, the longitudinal and transverse axes and centre of gravity for transport of the main indivisible unit and any other parts as may be required to conform to regulations, should be clearly marked. The axes of the transformer main unit should also be permanently marked on the four sides. The transformer must also be equipped with necessary and well functional bracing locations and brackets for securing the transformer to the transporter (rail-car, truck flat-bed, girder-trailer, ship, barge and airplane). In addition to lifting brackets, jacking pads and haulage points for use at load breaks during the shipment and final assembly should be provided. All these appliances must be dimensioned and labelled for the intended purpose. In addition, these should be shown on the transport layout drawing for the transformer with allowable load and minimum sling angle for lifting purposes. All other items arranged for transport must also have their lifting and haulage points and a safe working load (SWL) clearly marked at all stages of delivery and erection. 9.8

Documentation

The manufacturer should provide for approval and in advance of delivery, documentation which fully describes the transport arrangements and specifies all the 18

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Guide for preparation of specifications for power transformers

instructions and requirements necessary to ensure that the transformer and all its equipment will be delivered to site properly and with any necessary and statutory approval. A transport outline drawing must include:   

       

9.9

dimensions, weight and centre of gravity for transport of the transformer. the shipping centre of gravity should be shown on all four sides. details of transformer base showing acceptable jacking, blocking and skidding/sliding locations, as well as locations that are not acceptable for these purposes. These acceptable locations must be marked on the transformer with symbols or notification as to their purpose. the location of lifting and pulling eyes, jacking steps and bearing surfaces for moving the transformer by sliding or use of rollers. acceptable means of securing the transformer to the carrier for all modes of expected shipment. size, position and height above foundation of the jacking steps. maximum sling angle from vertical when lifting main tank. any special lifting requirements including the use of spreaders or special slinging equipment shall be shown on the drawings. special precautions when moving the transformer. if transported with oil, the oil level suitable for shipping the transformer. the transport outline drawing must accompany the transformer during shipment and be readily available to those handling the transformer during shipment. Storage

Transformers may require to be stored for short and long periods of time, but as a rule of thumb they should not be stored for more than 6 months without oil. It is not necessary for the radiators and bushings to be installed on the transformer during the storage period, but they need to be properly stored as recommended by the manufacturers and transformer manufacturer. Special bushings installed within the blank-out plates of the bushings are recommended which will accommodate low voltage testing during the storage period to confirm the condition of the transformer. The transformer bushings (if installed) or special test bushings should be properly grounded. It is important that all blanking plates and covers are designed for long-term outdoor storage and have a surface treatment that prevents corrosion during the storage period. Heaters within all control cubicles should remain energised during the storage period to help prevent condensation from corroding electrical components. 9.10

Storage without oil

Storage without oil should, as a general rule, only be used for shorter term storage (up to 6 months), i.e. during breaks in transport operations or shorter storage pending a site of assembly is finished. Longer storage (more than 6 months) should be avoided because of the danger of cavities in the solid insulation developing, which might be difficult and even impossible to remove with vacuum treatment before oil filling. 19

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Guide for preparation of specifications for power transformers

When stored without oil, provisions should be made to prevent the ingress of moisture and to maintain the internal insulation systems within the transformer in first-class condition. In addition the transformer should be filled with breathable dry air and maintained at a continuous positive pressure of 14 kPa for instance and with a dew point below -40 °C. The air pressure and dew-point must be monitored continuously throughout the period of storage. In addition the unit should be subject to regular inspection during the storage period. At all times an alternative standby means of restoring any loss of air pressure immediately should be provided. Following an extended storage period with air, it is recommended that an extended vacuum process be performed prior to the final oil fill. 9.11

Storage with oil

Storage with oil is suitable for long term storage of transformers. The storage site must be equipped with an oil retention system to handle a volume large enough to pick-up the full content of oil in the stored transformer. The pressure equalisation system for the main tank and tap changer(s) (i.e. for instance a conservator) should be installed and filled with a sufficient amount of oil to allow for normal oil level variation caused by the changes in ambient temperature. In addition, any dehydrating breather(s) must be installed. The oil level within the main tank and tap changer should be routinely inspected to ensure proper filling. The best practice is to remotely monitor the oil levels by activating gauges. 10

SAFE WORKING SYSTEMS

Purchaser specifications may include details of health and safety requirements at the installation. Possible requirements might include:    

languages to be used on site limitations on working hours compulsory safety training for workers and supervisors details of local safety regulations likely to affect the installation and commissioning of the transformer. These may be consolidated into an appendix to the specification, or a separate document where they would apply equally to the installation and commissioning of other types of equipment.

Typically the specification will include the requirement for the manufacturer to provide a draft work plan (“method statement and risk assessment”) for the installation of the transformer. This then allows the user to check which activities may be subject to local safety regulations. This also allows the user to check which activities may affect normal operation or other work at site. Less typically, purchaser specifications may include details of health and safety requirements at the manufacturer’s works. One possible approach would be to ask for certification to OHSAS 18000. Another would be to ask for documents on health and safety, e.g. health and safety policy, independent health and safety adviser’s report, details of accidents reported to the lawful authorities. A more conventional approach would be to make an assessment of the manufacturer’s works, in accordance with the guidelines in CIGRÉ TB 530, Guide for Conducting Factory Capability Assessments for Power Transformers.

WG A2-36

11

Guide for preparation of specifications for power transformers

TECHNICAL REQUIREMENTS

Preface The IEC standards include detailed requirements that affect the design and manufacture of transformers depending upon their rating, voltage and application. Often, however, there are other additional local or regional technical requirements that need to be included in a specification, as well as requirements which arise from the purchaser's previous experience. Therefore, in this section, the purchaser should state any technical requirements different from or not contained in the IEC standards or other listed standards. Any additional technical information that will assist a manufacturer to optimise the design and manufacture of the transformer should also be provided by the purchaser. It is important to remember that the purpose of a specification is not solely to describe what is wanted but also, to state what is not wanted. The latter often result from the purchasers’ previous experience. Equally, the manufacturers' experience can also complement the purchaser’s specification. Therefore the opportunity exists during the tender stage for exchanges of further information between the purchaser and the manufacturer by means of formalised design reviews and consultations. 11.1

General

Transformers should conform to the standards listed in the specification. Please see 1.2 above for recommended list of standards. Where the purchaser has a distinct preference for either a core type or shell form transformer this must be clearly stated in the specification. 11.2

Normal and Abnormal Operating Conditions

The following should be specified: 11.2.1 Gas and Oil Actuated relays Gas and oil actuated relays, used to indicate presence of accumulated gas or sudden oil movements, should not operate inadvertently when any combination of pumps start up and run, or in the event of loss or restoration of the auxiliary supply. 11.2.2 Overloads It is only necessary to specify overload requirements in detail where they are in excess of what is listed in IEC standard 60076-7. It would be as well to state this directly. Where more onerous requirements are specified, the following information should be included as a minimum:     

Preload (and duration) Overload (and duration) Ambient temperature Maximum allowable temperatures during overload Method of test or verification

In case of partial loss of cooling equipment, similar considerations will apply.

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Guide for preparation of specifications for power transformers

Note that restrictions may apply to the use of tap changers during overloads. 11.2.3 Geomagnetic Induced Current (GIC) Effects Solar activity can cause Geomagnetic Induced Currents to flow in the earth and these currents can find their way onto the power system usually via the earthed neutral points of transformers. The occurrence of GIC’s in electrical grids is linked to position on the earths’ surface and to the orientation and length of overhead line circuits connected. Higher latitudes are generally more affected, being closer to the magnetic poles. Purchasers should determine whether the transformer being specified will be located at a site which may be subjected to GIC events from time to time. GIC’s are quasi-dc currents (they are not true dc but have a frequency of around 1 Hz) that will flow through the transformer neutral into the windings, creating an effective dc component on the transformer magnetising flux. When a GIC flows in the transformer, the core may “half cycle saturate” and this can cause a significant increase in stray flux, increase in VAR consumption and generate harmonics. The stray flux can heat up windings, clamping, structural parts, flux shields and the transformer tank. The temperature rise experienced in any object is depending on:     

details of the design constructional details intensity of the GIC in duration and magnitude loading condition of the transformer heat transfer capacity of the affected structures

Purchasers should note that certain transformer types are more susceptible to GIC type events, including the use of five limb cores, single phase units and shell type transformers. Where GIC’s are a potential risk the purchaser may state this and any preference in transformer design for avoiding GIC effects. Additionally the purchaser may specify the maximum magnitude of the GIC to be considered in the design and the time period that this current must be carried by the transformer. 11.3

Design Requirements

11.3.1 Flux density The flux density in any part of the magnetic circuit including shunts should not attain a value that causes saturation. This should apply under the specified voltage, frequency and tap positions, including transitory effects of combined system voltage and frequency fluctuations. An adequate safety margin should be included. The purchaser should state the over-excitation capability of continuous operation above rated voltage and at frequencies above and below rated frequency. A minimum acceptable V/Hz ratio could be specified for unloaded and fully loaded conditions. For GSU’s (and unit auxiliary transformers) the purchaser should specify the short time over-excitation vs. time due to load-rejection. 11.3.2 Voltage regulation The purchaser shall state any requirements for de-energised tap changer (DETC) or on-load tap changer (OLTC), and specify the voltage and impedance variations for all tap positions. 22

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Guide for preparation of specifications for power transformers

11.3.3 Cooling The purchaser should specify the internal and external cooling mediums and the circulating mechanisms that are required, by referring to the cooling method identification symbols in an appropriate standard, such as IEC 60076. Any different cooling method should be clearly stated in the specification. The purchaser should state the percentage of any spare cooling capacity if required. In the absence of such a requirement being specified, the manufacturer should state the minimum percentage cooling capacity that can be removed for maintenance or replacement. If any forced internal or external cooling medium is specified, the specification should also state the minimum amount of inherent natural cooling required. If no amount of natural cooling is specified, the manufacturer should state the maximum natural cooling capability of the cooling equipment offered in the tender. 11.3.4 Control detail The cooling equipment and its control scheme shall be designed to ensure that the desired transformer ratings can be obtained. Pumps and fans are usually initiated by thermostatic control derived from winding temperature hottest-spot indicators or other temperature monitors. The appropriate sequence in which forced internal and external cooling mediums are required to operate should be specified, for instance ONAN / OFAN / OFAF or ONAN / ONAF / OFAF. It may be preferable to start or stop pumps sequentially, and/or with soft start capability to avoid sudden excessive oil velocity changes. To accommodate sudden load increase, the cooler control system should incorporate means to initiate pumps and fans immediately upon sudden load increases above a threshold value agreed between purchaser and manufacturer. Outdoor mounted cooling pump and fan control equipment should be housed in a weatherproof cabinet designed, for instance, for protection grade IP53. 11.3.5 System Earthing The purchaser should state the method of earthing any transformer neutral terminals in the specification. In the absence of such information a manufacturer may design the transformers for use with solidly earthed neutral connections, and shall state on the name plate that the neutral shall be directly earthed. The purchaser must state the type and ohmic impedance of the alternative earth connection if solidly earthed neutrals are not to be used. 11.4

Core

The temperature of any part of the core or its support structure in contact with oil is not to exceed what is specified in IEC standard 60076-2. Refer to CIGRÉ Technical Brochure of WG A2.38 regarding thermal modelling and direct temperature measurements of the core and its structural parts. The purchaser or manufacturer may prefer to test these parts of the transformer to higher levels of voltage than specified in some standards.

23

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11.5

Guide for preparation of specifications for power transformers

Tank

Transformer tanks are usually constructed from welded steel plate and reinforced to withstand transport, handling or excess pressures during fault conditions without distortion. The purchaser can specify whether or not a cover-type or a bell-type tank is required. The design and positioning of lifting points, stiffeners and underbases on the tank should prevent distortion of the core during lifting and transport. For personnel safety it is recommended to specify the maximum tank surface temperatures, according to local laws and regulations. 11.5.1 Handling facilities Handling facilities may be required to permit movement, assembly and dismantling of the complete oil-filled transformer at site or elsewhere and should be agreed before a contract, unless otherwise specified. Possible facilities include the following:  

    

four jack pads near the corners of the tank, designed to take the weight of the complete transformer lugs for lifting the transformer during transport. The lifting lugs and attachments shall be designed to allow for possible unequal lifting forces, together with an adequate factor of safety allowance lifting eyes for main transformer tank cover, conservator tanks and on-load tap changer if applicable, a suitable reinforced base frame to form a skid assembly for skidding the transformer in any direction using rollers if applicable, permanently mounted or removable wheels, arranged to permit bidirectional movement hauling eyes on all sides of the tank if applicable, riding lugs for transport on a side-beam road trailer or railway car. The riding lugs (removable if necessary) should be capable of taking the weight of the main unit, complete with oil filling if requested

11.5.2 Cover The tank cover may be bolted or welded to the tank. If the purchaser has a preference, this should be stated in the enquiry. In case of a welded cover, it is preferred to weld before final testing. The tank cover should be designed with a sufficient slope to shed water. Fixings should be provided for attachments to ensure a safe working environment when personnel have to work on top of the transformer. All tubes, equipments, etc. on top of the transformer should be located in such a way as to minimise hindering movement of personnel. 11.5.3 Oil-tight Joints Oil leaks from the main tank-cover joint or other joints are unacceptable under any static oil-head or forced oil conditions at any ambient or maximum operating temperatures. Only joints of proven design, capable of preventing deterioration of any seal or gasket materials should be specified or supplied.

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Guide for preparation of specifications for power transformers

All bolted flange joints should be provided with suitable gaskets and made from oil resistant, non-perishable material installed within smoothly machined grooves designed to stabilise the gasket position and to provide suitable compression stop. The thermal performance of the material must exceed the maximum temperature attained by the metal parts in contact with the gaskets under all conditions. For bolted pipe joints or similar, the "O-ring" type of flange seal may be preferred. If cork type gasket materials are used, the metal mating surface shall be thoroughly cleaned to prevent the gasket from sticking. 11.5.4 Vacuum and pressure requirements The assembled transformer, including the tank, coolers or radiators, conservator (if not equipped with a rubber diaphragm), oil pumps, all oil connections, valves, pressure relief devices and other fittings, should be capable of withstanding, with minimum permanent distortion:  

when oil filled, an internal overpressure of 35 kPa without oil, full internal vacuum

The transformer conservator tank, if equipped with a rubber diaphragm, need not be designed for a full vacuum but a vacuum-tight valve should be provided in the connection between tank and conservator. The pressure relief diaphragm should be replaced by a steel plate. Note: It is necessary to ensure that the transformer tank is not accidentally sealed, as it might be the case by a valve between the tank and the conservator, unless a suitable bypass arrangement is specified. It is usual for power transformers to be designed and equipped for vacuum filling and oil treatment in the field, whether or not the transformer is shipped with oil. 11.5.5 Valves The transformer tank should typically be equipped with the following valves and fittings, the positioning of which shall be subject to approval of the purchaser.  





  

at least an oil valve at the top and bottom of the tank for taking oil samples shall be provided, unless an alternative arrangement is proposed a drain connection valve at each end of the tank at the bottom wall of the tank, complete with a blanking plate. The connection should vent the tank as close as possible to the junction of the tank wall and the base, so that no more than a few mm of oil will remain in the tank when empty two elbow valves, complete with a blanking plate for filling connections, should be provided on the tank cover and located at diagonally opposite corners a valve fitted with a blanking plate and located on the tank cover in line with the bottom sampling valve should be provided for attaching a vacuum gauge, a pressure gauge or an oil level indicator when vacuum filling one or more valves for immediate or future connection of on-line monitors for dissolved gas a siphon valve with no return valve for draining the OLTC tank (if applicable) residual oil discharge valves for the expansion tank(s)

25

WG A2-36

Guide for preparation of specifications for power transformers

11.5.6 Circulating and eddy-currents The tank should be designed or incorporate measures to minimise the losses caused by circulating and eddy-currents and avoid onerous temperatures at any part of the tank surface and at flanges between parts of the tank and its components especially at gasket sealed joints. 11.5.7 Access Openings Access openings should be provided as appropriate in the tank cover and walls to permit unhindered access to inspect, repair or remove current transformers, tap-changer components, winding connections and other devices that may require routine or emergency maintenance. An opening that allows personnel access should not be less than 500mm diameter or 500mm x 500mm. Hand holes should be approximately 400mm diameter or 300mm x 600mm. All openings on the cover should have a raised flange to prevent water from entering the openings when individual covers are removed. At least two openings should be provided on the tank cover for access to the interior without lowering the oil below the top of the core. 11.5.8 Conservator Tanks A conservator tank shall be provided of sufficient size to accommodate the change in oil volume that will occur between the specified ambient temperature limits in service with the transformer operating at full load or overload and the cold oil temperature with the transformer out of service. The typical conservator oil volume is approximately 10% of the sum of the oil volumes of the main transformer tank and the coolers. The main tank and on-load tap changer diverter compartment shall have separate conservator tanks. Oil proof rubber diaphragms (bladders) are typically used within the conservator tanks for the transformer main tank to minimise atmosphere contact with the insulating fluid. Each conservator tank should have a suitable oil level gauge mounted on the conservator tank so as to be easily read from ground level. The gauge shall be graduated to indicate the oil level at temperatures of -10°C, +5°C, +15°C and +20°C or other values specified by the purchaser. A float switch shall be provided having a set of low and high oil-level alarm contacts. A dehydrating breather shall be connected to each conservator tank. The inner diameter of the pipe connecting the breather and the conservator tank shall be sized large enough to not inhibit pressure equalisation. 11.5.9 Tank Earthing The following grounding and bonding facilities for earthing purposes should be provided on the tank and other separate free standing parts such as radiator banks:  



at least two suitable earthing terminals on the main tank one earthing terminal should be located, for instance, towards the extreme right hand end of the low voltage side and the other diagonally opposite on the high voltage side one suitable earthing terminal on each cooler bank support structure 26

WG A2-36

 

Guide for preparation of specifications for power transformers

earthing straps, to bond the tank cover to the main tank one earthing terminal on the main tank near each set of surge arresters to allow a “high frequency” earthing connection from the arresters

Other internal and external metal parts of the transformer shall be earthed to the tank or separately and directly earthed. Whichever method is adopted, a uniform earth potential is required throughout the installation. Closed circulating current loops within the earthing systems must be avoided. Flanged joints should be electrically bridged. Internal earthing connections from the core and core clamping structure shall be brought out to bushings mounted in a secure, weatherproof terminal box, mounted on the tank surface and earthed externally in order to facilitate testing of the core earthing system. 11.5.10

Pressure relief

At least one suitable spring operated pressure relief vent should be located on the main tank (preferably on the cover) and on-load tap changer diverter compartment (s) complete with an approved oil deflection collar. In addition, piping can be connected to the oil deflection collar of each pressure relief device in order to direct the oil down near the base of the transformer. The number of pressure relief devices required on a transformer is normally dependent on the total oil volume of the transformer. Suitably rated auxiliary contacts should be provided for these devices. 11.6

Insulating Fluid

The purchaser should state which type of insulating fluid that should be supplied and the specification with which it must comply. The insulating fluid should be free of pcb, copper sulphide, or other chemical having a corrosive sulphur tendency. The purchaser should state whether the fluid should be inhibited or non-inhibited, and if any special additives are required or conversely not permitted. The fluid should comply with the recognised IEC standard and any additional regional or purchaser requirements. Under no circumstances shall any degree of forced-oil circulation create a static electrification hazard in any part of a transformer under any operating condition. 11.7

Bushings

Bushings should comply with a recognised standard such as IEC 60137. The specification of oil/SF6 bushings should be agreed between purchaser and manufacturer before a contract. The interface between the transformer and gas insulated external connections requires special attention to dimensions, limits and tolerances. These design aspects should be agreed between purchaser and manufacturer before a contract and should take into account any purchaser standardisation policies.

27

WG A2-36

11.8

Guide for preparation of specifications for power transformers

Secondary Wiring and Control Cabinets

All conductors, connectors, terminal blocks, wire-ways, terminal markings, etc. shall meet recognised standard and purchaser requirements. Control cabinets should be mounted in a manner to reduce vibration and should be designed to prevent moisture ingress and condensation. Control cabinets should be mounted at a height that enables operational access from ground level. 11.9

Fittings

Preface The number and type of fittings required or provided on a transformer will depend upon several factors including its purpose, construction, rating and voltage. Some other considerations are, the amount of surveillance required or provided by the purchaser, the requirements for automatic control and protection, purchaser policies concerning on-line diagnostic and monitoring requirements. Certain fittings may incorporate protection, control and remote indication facilities. Details of such facilities should be stated explicitly at the time of enquiry or tender, including any requirements for conformity with existing practices. All labels, plates and markings should be manufactured from durable, non fading material. Any instruments or indicators should be capable of being read from ground level. Where any equipment is to operate in parallel or perform in a similar manner with existing equipment, the purchaser should provide complete details of the key parameters of the existing equipment in the enquiry document. Where alarm and trip contacts are required, the purchaser should state the range of operating settings required in the enquiry. Fittings List The following list is representative of the fittings that may be required on each power transformer. In practice purchasers and manufacturers select fittings from such a list as this but may also adopt or recommend additional or alternative fittings for reasons of policy, improved safety, efficiency, security, and maintenance or lifetime costs.  



  

thermometer pockets in each top and bottom oil pipe adjacent to the transformer gas and oil actuated relays for main conservator/tank oil pipes and externally mounted tap changer selector compartments, as appropriate. Sampling and test stopcocks may be required and mounted for operation at ground working height conservators for the main tank and tap changer diverter compartments where required. Conservators should be provided with an oil gauge, drain valve, oil filling facility, lifting lugs, oil sumps and removable end covers dehydrating breathers air release and drain plugs or valves for pipe work, oil expansion bellows, pumps etc separate drain and filter valves at the top and bottom of the main tank, externally mounted tap changer selector compartments and cooler headers 28

WG A2-36

                                

Guide for preparation of specifications for power transformers

isolating valves complete with an open/shut indicator and locking facility valve location plate. The position of each valve in normal service should be shown, i.e. Normally Open (N.O.) or Normally Closed (N.C.) fans pumps oil-surge relays and pressure relief devices for tap changer diverter compartments oil sampling devices easily accessed by personnel anti-vibration pads provision for blanking plate storage earthing lugs on the main tank and each separate cooler structure main tank jacking lugs main tank transport lugs name plate diagram and rating plate owner’s serial number plate vacuum capabilities plate main haulage points main tank haulage rope guides transport anchoring lugs co-ordinating rod stands with co-ordinating gaps or fixings for mounting surge arresters and supporting brackets current transformer test loop(s) for HV and/or LV bushing turrets terminal box for LV current transformer test loop terminal box for HV current transformer test loop terminal box for core and core structure earthing current transformer terminal boxes lifting lugs for tap-changer, cooler structures, main tank cover and other components as necessary winding temperature indicator pockets with protective covers on-line combustible gas and moisture monitors protective covers to protect projections, such as valves, from damage during transport pressure relief devices and associated ducting tank attached fixings for mounting external neutral current transformers cover mounted safety lugs to permit fixing of toe boards / safety fences for safe working purposes winding and oil temperature indicating instruments

11.10 Tap Changers If the transformer application requires that variation of the voltage ratio is required in order to make corrections for changes either in the supply-side or demand-side voltages, de-energized tap changers (DETC’s) and/or on-load tap changers (OLTC’s) should be specified. 29

WG A2-36

Guide for preparation of specifications for power transformers

The choice of which type of tap changer to be used will be dictated primarily by its function and economics, e.g.:   

the size of the tapping range needed to match the expected system voltage variation the step voltages required and number of steps whether or not the transformer can be electrically disconnected from the network in order to change taps

De-energised tap changers (DETC’s) are only used when the need for voltage correction is infrequent because all tap changing by this means has to be undertaken when the transformer is off-line, which requires an outage. In practice, DETC’s are mainly fitted to high voltage windings and when a low number of tap steps are required, e.g., 4, 6 or 8 steps. Usually, each tap step will vary the high voltage winding turns between 1% and 2.5%. The number of tap steps and percentage voltage variation per step should be specified by the purchaser and determined by the purpose of the tappings, i.e., to maintain the low voltage on load at rated value when the high voltage changes or alternatively to maintain the low voltage network voltage at some value as the load varies. In most other respects the electrical, mechanical and thermal design requirements of DETC’s, e.g., current rating of contacts and voltage withstand considerations, are similar to those of on-load tap changers. On-load tap changers (OLTC’s) are designed for connection to line-end or neutral-end of high voltage or low voltage windings. The size of the tapping range is usually specified by the purchaser and is a compromise between the network high and low voltage ranges. Similarly, the number of tap steps will be determined by the range of voltage variation expected in service and the size of voltage change per step required. In practice the number of steps can vary between 10 and 40, depending on voltage, application and current rating of the tap changer. The range of voltage that can be accommodated is determined by the ac power frequency and impulse voltage withstand strength between adjacent taps. OLTC’s are specialised precision electro-mechanical devices. They are invariably purchased by the transformer manufacturer under a sub-supplier contract from an original equipment manufacturer (OEM) and selected from a type tested and proven product range. The choice of OLTC suppliers is often specified by the transformer purchaser. Several types of OLTC’s are commonly available and may be categorised as:    

line-end neutral-end in-tank externally mounted

“Line-end” and “neutral-end” describe the electrical position of the tap changer within the configuration of windings and connections. Line-end OLTC’s are usually positioned at the line end of lower voltage windings. This position is chosen for autotransformers, for instance, when the voltage ratio is low, e.g., of the order of 2:1. Designs employing neutral-end tap changers are usually more economic when the voltage ratio is greater than 2:1. Where delta connected high voltage windings are required, line end tap changers may be specified on the high voltage winding. “In-tank” and “externally mounted” OLTC’s refer to the physical position of the tap changer in or on the transformer. 30

WG A2-36

Guide for preparation of specifications for power transformers

It should be noted that tap changers, particularly OLTC types, are still a frequent source of incipient or major transformer faults and a cause of unplanned transformer outages. It is important therefore that proven equipment is used wherever possible and that a transformer design is adopted which best meets the needs of the purchaser’s operating regime, supply responsibilities and long-term maintenance requirements. 11.11 Monitoring Purchasers of power transformers are routinely selecting equipment for on-line monitoring of the transformer operating status to minimise forced outages; planning maintenance activities; increased personnel safety; and as well for maximising the performance of their transformers. IEC standard 60076-1 and CIGRÉ Technical Brochure 343 provide monitoring recommendations. On-line monitoring equipment is available for DGA, moisture in oil, oil temperatures, oil pressure, load current and voltage measurements, pump/fan operation, conservator membrane condition, tank vibration, winding hot spot calculations and/or direct hot spot measurements, bushing condition, partial discharges and OLTC condition. Purchasing monitoring equipment in the original specification is an efficient means to the option of adding monitors at a later date. 11.12 Interchangeability Interchangeability refers principally to transformers of similar rating, voltage and other operating characteristics that are or have been purchased under different contracts and sometimes from different suppliers, but are all designed to have common dimensions and layout, in order to allow them to be physically interchangeable with each other with a minimum of adaptation, if any. It is possible for transformers purchased earlier to be replaced later by more modern designs, having larger ratings but designed to be installed and occupy the same “space”. Utilities purchase transformers to meet requirements of this kind in order to increase the availability of electricity supply and reduce costs by minimising the outage time in the event of a transformer having to be removed from service and replaced by a spare, stored strategically for that purpose. Interchangeability is of special importance where transformers are required for installation and connection to gas insulated busbars. In these instances the concept of interchangeability extends beyond the transformers to include also the busbars, especially at the interface between the two systems. When interchangeability is required, the purchaser should undertake to specify and detail the key features, dimensions and interfaces that are to be repeated on each transformer and provide all necessary reference drawings. The arrangement and physical dimensions of the high, low and possibly other voltage bushing connection points, (sometimes referred to as cover layout), is a vital part of this information. When specified, the transformer including fittings and other major interfacing components shall be interchangeable with other transformers and related equipment defined by the purchaser. 11.13 Standardisation The concept of standardisation is not to be confused with that of interchangeability. There is some similarity between the two requirements, especially when transformer components such as tap changers, valves and other interfacing fittings are required to 31

WG A2-36

Guide for preparation of specifications for power transformers

be replaceable. Standardisation refers to a policy to limit the variation of transformer types, ratings, voltage ratios, impedances, tapping ranges and other principal electrical, mechanical and thermal characteristics of a purchaser’s transformers. The policy reduces the complexity of the purchaser’s stock of transformers, bushings, fittings, tap changer components and other items and tends to minimise maintenance practices and costs. The aims of standardisation are:     

minimise system design, operating and capital costs simplify maintenance procedures and requirements, and system planning reduce stock held items optimise spares reduce purchasing and other front-end costs

Standardisation need not be confined to utilities and other organisations that have large and varied stocks of transformers. Smaller purchasers can purchase transformers by reference to national and international standards that include recommended transformer ratings and other features, including in some cases, losses and dimensions, for which manufacturers have proven up-to-date designs offering economic savings that are not obtainable from custom made nonstandard alternatives. This will also give the possibility to replace the transformer with one from another company in case of an emergency. 11.14 Exclusions In most cases, every opportunity is taken by a manufacturer to comply with the requirements of the specification or to propose alternatives that permit improvements that best fulfil the purchaser’s needs or meet the manufacturer’s capabilities. Deviations of this kind from a purchaser’s specification may be raised in pre-tender discussions between a purchaser and potential manufacturer but ultimately, any tender submission must either comply with the specification or, if this not possible, a manufacturer should categorically state the non-compliance and exclusions. All exceptions should be discussed between the purchaser and tenderer and a resolution made prior to an award of order. The purpose is to avoid misunderstandings. If no exclusions are stated, the contract works have to be treated as fully compliant with the specification. The tenderer should state any non-compliance with the specification in the tender submission and any alternative offers should be submitted in full and separately from the main offer. 12

MANAGING QUALITY

Preface Quality assurance certification indicates the manufacturer’s general ability to design and consistently manufacture transformers to a purchaser’s specification, IEC standards and other requirements. It also establishes evidence to some degree of the capability with respect to transformer engineering, design and manufacture attainable by the manufacturer. The purpose is to ensure that the final product will fulfil its service function and be comparable to other similar products previously proven by type and routine tests and service experience. However, the possession of the requisite quality assurance certification does not by itself guarantee that the transformer manufactured 32

WG A2-36

Guide for preparation of specifications for power transformers

will be of a suitably high “quality”. It is important for the customer to ascertain competence and capability in addition to the acceptance of the manufacturer’s quality assurance documentation. It is likely that in some situations a potential manufacturer who has ISO 9001 certification may be deemed unsuitable as being not capable of producing the transformer required as a consequence of the assessment of capability procedure. Further information relating to the proper management of quality is provided in CIGRÉ Technical Brochure 530; Guide for Conducting Factory Capability Assessment for Power Transformers”. A Quality Inspection and Test Plan is a major component of the quality assurance concept. It is prepared by the manufacturer and submitted with each tender for approval by the purchaser. As with other quality assurance requirements, it forms a vital and essential part of a transformer contract. 12.1

Quality Inspection and Test Plan (QITP)

A Quality Inspection and Test Plan should be submitted with each tender and agreed with the purchaser before contract. Any subsequent alteration to and deviation from the agreed QITP should be submitted to the purchaser in advance for approval in writing. No changes to the QITP ought to be permitted without the prior written approval of the purchaser. The manufacturer should establish a QITP for each transformer, containing a summary of all the inspections and tests needed during manufacture, factory acceptance tests, site erection and commissioning tests. It should be clear from the QITP where inspection activities will be performed, the parties to be present, inspection plans in force and the distribution of testing and inspection documents. The main QITP should be approved by the purchaser before manufacture commences. The purchaser or his representative should also have the right at any time, without advance notice, to witness any inspection, manufacturing procedure or test at the manufacturers or the sub-suppliers plant and to be informed of the result. Inspections and tests performed in the presence of the purchaser or his representative will not imply any limitation of the manufacturer’s responsibility. 12.2

Quality Inspection and Test Plan (Outline)

The following description of the elements contained in a quality plan, is for a transformer manufactured to this type of specification and may form the basis of a document to be agreed between purchaser and manufacturer before a contract. A quality plan describes: 





lists of activities, identifying all the activities involved in the design, manufacture and supply of the transformer by the manufacturers’ internal procedure document reference number. references to all the mechanical, electrical and test requirements needed to ensure that the design and manufacture of the transformer will be at least to the minimum standard, necessary to ensure that it will be in accordance with the specification and be fit for service. all the activities and responsibilities of the manufacturer, sub-supplier and 33

WG A2-36

 



  

Guide for preparation of specifications for power transformers

purchaser, that are required to execute the design, manufacture and testing of the transformer at the factory, preparation for despatch, delivery, installation, commissioning and setting to work. the identification reference of all documentation, standards, procedures, works instructions, drawings, test methods, acceptance criteria etc. the controls which each activity must succeed in passing, for example: a) identifiers - serial number, model number, type number (usually used to substantiate purchased items) b) performance (usually statements accrediting the electrical, mechanical, thermal and chemical performances, against which manufacturing activities and specification requirements can be checked) c) status (usually confirmed by visual examination) d) dimensions (usually approved by reference to drawings and other similar reference documentation) details of the means of recording the progress of the design and manufacture of the transformer, in particular, inspection and approval points by labelling, inspection cards, certification etc. the identity and authority of persons with assigned responsibility for approving the satisfactory completion of each activity. the location of each activity, inspection and approval each deviation, failure to comply, modification to the plan or to any supporting documentation, e.g., drawings, works instructions, scheduled information, design changes.

Each sheet of the quality plan is usually referenced by:     

contract reference sheet number issue number date of issue authorisation reference

A quality plan is usually divided into sections, each section covering a distinct part of the design, manufacture and supply chain. Each activity in this chain is identified by a unique reference number, usually the section and a subsection number. The principal sections in a quality plan include:       

electrical and thermal design, e.g., core, windings, tests mechanical design and thermal design, e.g., core, tank, fittings, coolers, tap changers, controls, oil preservation, protection, monitoring, etc. purchased items manufacturing work, e.g., core, windings, connections, fabrications, processing testing, e.g., instructions, methods, acceptance criteria, responsible persons, acceptance formalities dismantling, preparation for despatch, surveillance during despatch, tests on arrival at a site erection at site, commissioning, setting to work

For example, a quality plan sheet relating to transformer tank manufacture could comprise the elemental activities and requirements illustrated on the following page. 34

WG A2-36

Guide for preparation of specifications for power transformers

Similar sheets are required for all the other activities involved in transformer design, manufacture, supply and tests, including purchasing and sub-supplier activities. A typical quality plan for transformers complying with this specification may comprise fifty sheets or more and cover twenty or more principal activities containing as many as twenty sub-items per activity. 12.3

Quality Assurance Plan

When submitting a tender, a manufacturer should include a description of the quality assurance plan (QAP) that will be used to ensure that the transformer design, materials, workmanship, tests, service capability, maintenance and documentation, will fulfil the requirements stated in the contract documents, standards, specifications and regulations. The quality assurance plan should be based on and include relevant parts to fulfil the requirements of ISO 9001. The manufacturer is responsible for any sub-suppliers setting up and executing their own quality assurance systems. 12.4

Quality Assurance Manuals

A complete quality assurance manual, describing the execution of all the elements of the quality assurance system, should be available from the manufacturer as a reference for the purchaser or his representative. 12.5

Final Quality File

This should comprise:  

completed Inspection Points Schedule certificates for: c) bushings or other line and neutral terminations d) oil e) current transformers (where installed) f) on-load tap-changers or de-energised tap changers g) other transformer accessories h) paint and anti-corrosion protections i) materials

  

reports on deviations and remedial actions final test protocol statement of compliance with specifications, drawings, and purchase requisitions contract drawings



35

WG A2-36

Guide for preparation of specifications for power transformers

QUALITY

PLAN

(For illustration only)

Issue No: Date:

Customer:

Customer Ref:

Manufacturer:

Manufacturer Ref:

Activity No. 17

ACTIVITY Tank and cover

17.1

Steel plate

17.2

Machining

17.3

Welding

17.31

Weld tests

17.4

Location - openings, fixings etc.

17.3

Internal surface - condition

AAA

17.4

- preparation

17.5

- treatment

17.6

External surface - condition

17.7

- preparation

17.8

- treatment

17.9

Activity Doc. Ref. (1)

Site:

(2)

Inspection

C, TC,

Inspector Manufacturer

(3)

BBB

(4)

Inspector Purchaser CCC

(5)

REMARKS

DDDD

(6)

Paint treatment (3)

(1)

Issued by:

Acceptance Criteria S, D,Q

Sheet No.

eg. Works instruction, IEC standard (2) eg. S = Status D = dimensions Q = quality or performance

C = inspection card TC = test certificate (4) Manufacturers inspector’s reference

36

(5)

(6)

Purchaser’s inspector’s reference.

includes any additional requirements, references or other documentation, instructions or advice

WG A2-36

13

Guide for preparation of specifications for power transformers

FACTORY ACCEPTANCE TESTS AND FINAL INSPECTIONS

Preface The importance of the final factory inspections and acceptance tests cannot be over emphasised in terms of ensuring the transformer manufactured is fit for the intended purpose. The key purpose in carrying out the final tests is to ensure that the design is as intended with regard to type testing and that the quality of manufacture is consistent with the design in terms of type testing. One overriding principle must be considered and that is that any test made on the transformer must add value, either to the Manufacturer or to the Purchaser. The purchase specification should detail all tests that are required to be carried out on the specific transformer. The requirements for testing transformers are described in the international standards. IEC 60076-1 breaks factory acceptance tests into three categories, namely; a) type tests - tests carried out on the first unit of a design and intended to prove the design, b) routine tests – tests carried out on every transformer and c) special tests – additional tests specified by the Purchaser. Purchasers should note that tests defined as type or special tests at some voltages may well be routine tests at another. One important requirement of any purchase specification with regard to testing is to ensure that any non-mandatory tests which may be required are clearly detailed in the purchase specification. Some purchasers may wish a test that is described as a type test or a special test carried out as a routine test and this should be clearly detailed. Where the purchaser wishes to test the transformer not in accordance with the international standards for whatever reason, the required test methodology should be clearly described together with the acceptance criteria. Purchasers who deviate from the published international standards should also be clear as to why this deviation is required and be able to justify this in technical terms. Sometimes the predicted performance of a transformer on test may need to be verified in advance by non-destructive investigatory tests and measurements. For example, recurrent surge oscillographic (RSO) tests may be advisable to confirm the transformer predicted transient voltage behaviour or to determine the most appropriate test connections. If a transformer has been specified for use in a non-conventional or otherwise special application or it is to be subject to unusual operating conditions, then the programme and sequence of tests shall be specified by the purchaser. The operating characteristics of transformers fitted with tappings, dual or multiple voltage ratio connections or alternative vector group connections will alter when these connections are changed. In the absence of supporting evidence from other identical previously tested transformers, additional tests may be necessary to determine the method of test connection that best demonstrates the transformer is fit for purpose. In certain cases, the test environment at the time of test may not conform to the IEC standards. Similarly, at the time of test, the transformer may not be fully prepared as for service and as required by the specification. Unless otherwise agreed, in these 37

WG A2-36

Guide for preparation of specifications for power transformers

circumstances the purchaser may postpone the tests until the circumstances for test are as specified. It is important that the specification document highlights the following issues in such a way as to ensure that agreement with regard to the test procedures is part of the post contract design review process. 13.1

General

The manufacturer should produce a test programme describing all routine and type tests and final inspections. The test programme should be submitted to the purchaser for approval before manufacture commences. The factory acceptance tests may be witnessed by the purchaser or his representative. The purchaser should be notified in writing in a reasonable time period before the start of any test. For the purpose of acceptance tests the transformer should be assembled as for service, i.e. complete with conservator, coolers, auxiliary transformer, supervisory equipment etc. This means that oil-SF6 bushings must not be replaced by corresponding oil-air bushings. Deviations from this requirement should be by agreement between the purchaser and manufacturer. Type test evidence obtained on an identical transformer may be offered to the purchaser for consideration instead of further type tests, providing the evidence is not more than five years old and is submitted at the time of tender. Otherwise, type tests should be made. Routine test and type test evidence for transformer components, for example bushings and tap changers, should be provided by the manufacturer prior to the transformer tests and final inspections. 13.2

Standards and testing specifications

Factory acceptance tests should be performed in accordance with recognised standards. Bushings for instance should be tested in accordance with IEC 60137 and on-load tap changers in accordance with IEC 60076-1, IEC 60214-1, and IEC 60214-2, in the absence of any other standards being specified. 13.3

Testing environment

During the tests at the manufacturer’s works, the test room ambient temperature should conform to the specified standard unless otherwise agreed. The ambient temperature during site tests should not be less than 0°C. 13.4

Measurement accuracy

All measuring equipment should conform to the relevant specified standards or better. The equipment must have a valid calibration certificate, which should be available for inspection at the test location before starting any tests. Indication of calibration status 38

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Guide for preparation of specifications for power transformers

and the calibration certificate reference number should be clearly displayed on the test equipment. Some equipment, such as microphones or partial discharge measurement systems, may require calibration before and after every measurement. In this case the device used to calibrate the test equipment requires a valid certificate of calibration. The latest calibration curves should be available at the test location. Each piece of measurement equipment should have attached a visible record identifying the equipment and showing the last calibration date and calibration reference. 13.5

Tolerances

Purchasers need to consider whether the tolerances detailed in the published standards are consistent with their needs. Where tighter tolerances are required this needs to be specified otherwise tolerances on measured losses shall be in accordance with the specified standards. For example: Purchasers may wish to specify a “no positive tolerance” clause. Special qualifications on tolerances agreed at the pre-contract award stage may apply, for example for the purposes of calculating penalty and bonus payments. Individual tolerances on impedances may be specified but shall be agreed with the manufacturer before contract placement. 13.6

Summary of tests

Unless otherwise agreed, the Test Programme should detail all the tests to be performed on the transformer, in accordance with the specified standards and generally in the order specified by the purchaser or agreed otherwise at the design review. If the test methods are not prescribed in IEC 60076-X or other standards, they should be subject to agreement before contract placement. The following descriptions of the different tests are based on the categorisation in IEC 60076-1. Purchasers may wish to allocate type or special tests to a different category for various reasons and this should be clearly specified. 13.6.1 Routine tests         

measurement of winding resistance measurement of voltage ratio and verification of vector group phase displacements measurement of short circuit impedance and load loss measurement of no-load loss and current switching impulse tests (Um>170 kV) lightning impulse tests (Um>72.5 kV) applied voltage test at power frequency induced over voltage AC tests partial discharge measurements in combination with the induced overvoltage test (Um>72.5 kV) 39

WG A2-36

         13.6.2      

Guide for preparation of specifications for power transformers

functional tests on on-load tap changers at rated voltage and rated current measurement of insulation resistances to earth and between windings measurement of dissolved gasses in dielectric liquid from each separate oil compartment except diverter switch compartment (Um>72.5 kV) SF6 bushing flange isolation check of core and frame insulation for liquid immersed transformers with core or frame insulation. determination of capacitances windings-to-earth and between windings (Um >72.5 kV) measurement of dissipation factor (tan δ) of the insulation system capacitances (Um >72.5 kV) leak testing with pressure for liquid-immersed transformers (tightness test) check of the ratio and polarity of any built-in current transformers Type tests load loss and impedance measurements on every tap position. temperature rise (*) lightning and/or switching impulse (if not a routine test) sound power levels for each cooling mode specified tank vacuum test measurement of fan and oil pump power consumption

(*) Transformers that are to be equipped with sound panels or an enclosure may be temperature rise tested both with and without the sound panels or enclosure, when possible. In either case the transformer guaranteed temperature rises shall not be exceeded. 13.6.3 Special tests        



recurrent surge oscillograph (RSO) measurements dielectric special tests not carried out as routine or type, e.g. partial discharge measurement at Um72.5 kV) c) frequency response analysis (FRA) d) insulation dielectric response (PDC, RVM or FDS etc.) e) low voltage no-load magnetisation current measurement f) low voltage impedance measurement

13.10 Energisation Energisation shall only take place after correct oil treatment and stabilisation time, including any other requests agreed upon between purchaser and manufacturer that may have an impact on the warranty. If possible, the applied voltage should be raised slowly to rated value during the initial energisation. If this is not possible, the transformer is allowed to be directly switched on to the network. During the period after energisation, the transformer should be carefully supervised, especially gas and oil actuated relays, temperature indicators and monitoring equipment. Oil samples for dissolved gas analysis should be taken at frequent intervals to check for diagnostic gases which may indicate a potential problem with the unit. Thermographic measurements should be made of the tank, bushings and connections. Oil leaks should be reported to the manufacturer. 13.11 Trial operation The purpose of a trial operation is to prove the functional capability of the transformer and to show that it will meet its performance target. Where purchasers require a trial operation period this should be clearly detailed in the specification. 13.12 Special tests These are tests such as normal and emergency overload tests, which require special agreement between the manufacturer and the purchaser. Where purchasers require any special tests of this nature these should be clearly detailed in the specification.

44

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13.13 Site test reports The result of the site tests and inspections should be recorded and be compiled in a document, together with the test programme and any non-conformance reports for inclusion in the transformer operation and maintenance manual. 14

LIST OF GUARANTEES AND WARRANTIES

Preface The values of the principal transformer parameters such as voltage ratios, losses and impedances, are critical and must be subject to guarantee. The parameters can be specified by the purchaser. Most guarantees need to be subject to tolerances, but because differences are likely to occur between design targets and what will be achieved in manufacture, realistic tolerances, suitable for transformers for use in normal service conditions, are provided in the IEC standards. Other parameters such as sound power level, overload and overvoltage capability, weights and dimensions and others, may also be the subject of guarantee and tolerances. For whatever purpose, additional guarantees, tolerances and warranties or alternatives, differing from those contained in the IEC standards, may be specified by the purchaser in the enquiry or agreed between purchaser and manufacturer before contract. 14.1

Guaranteed performances

Submitted tenders should guarantee that the equipment offered is capable of the performances required in the purchaser’s specifications. For this purpose the purchaser may establish a list of the characteristics for which the manufacturer must give guaranteed values. It may be impossible to accurately determine all quantities at the time of the tender or they may only be determined during or after manufacture, in which case they may be subject to manufacturing and measuring uncertainty. Therefore, tolerances are necessary on certain values like losses, voltage ratio, short circuit impedances, no-load current. These tolerances are given in the relevant equipment standard but may also be agreed between the purchaser and manufacturer. Temperature rise and sound power level limits are typically maximum permitted values and tolerances are not allowed. When the maximum losses are not specified by the purchaser, a capitalisation formula may be used to compare the tenders and penalties may be applied if the no-load and load losses are higher than the values declared by the manufacturer. Information on capitalisation of the losses is given in Appendix A of this document. Other critical design and performance parameters, such as impedances, may also require special tolerances in certain circumstances. 14.2

Other types of guarantee

The purchaser will require some other guarantees such as: 

financial guarantee from the manufacturer 45

WG A2-36

  15

Guide for preparation of specifications for power transformers

a guaranteed delivery time a warranty period after commissioning

CONTRACT DOCUMENTATION

Preface This refers to the final documentation usually provided by the manufacturer on completion of the contract works. In most cases the documents are a standard package and consequently, may not contain all that is necessary for a particular purchaser’s application. In these circumstances, the purchaser should specify any additional requirements in the enquiry or before a contract. It is also in the purchaser’s interest to provide all the information in order to speed up the submission of tenders and the completion of the contract works. 15.1

An Enquiry document should include the following details, where applicable:     

15.2

A Tender should include:         

15.3

outline and foundation drawings of existing transformers if interchangeability is required proposed drawing of substation showing general position of transformer specified details of primary connections, such as LV phase isolated connections and gas insulated connections list of applicable purchaser’s, statutory or regulatory requirements if parallel operation with existing transformers is required, information in accordance with IEC 60076-1 should be provided

a description of the transformer offered, including tap changers, bushings, oil preservation system, fittings and protective devices. technical data sheets references to other similar transformers previously supplied, including quantities supplied and delivery dates type test reports of similar transformers and components outline, foundation and transport drawings list and details of all non-compliance and departures from the specification details of the Quality Assurance system and a copy of the present Quality Assurance certificate a Quality Plan a Quality Inspection and Test Plan A manufacturer should provide the following documents as part of a Contract: 46

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The documentation required to fulfil a contract will depend on the rating, voltage and type of transformer involved and whether documents relating to identical transformers have been supplied previously. In most cases, most documents will be selected from the typical list below, either as drawings, text or some other agreed form.              

16

Final technical particulars General arrangement drawing Diagram and rating plate Foundation details Terminations Valve plate Protection scheme plate Cooler control scheme Tap changer control scheme Outline arrangement of control cabinets, including any foundation details Internal arrangement of control cabinets Photographs illustrating important features of the transformer assembly Test reports, including commissioning reports Instructions for operation and maintenance, transport, short and long-term storage

EXAMPLES OF TECHNICAL AND OTHER INFORMATION SCHEDULES RELATING TO A TRANSFORMER SPECIFICATION

The following examples of schedules are typical, with goal to summarise the transformer features, performances, guarantees and other details, forming the basis of a specification or tender and ultimately of a contract. The schedules included in this section are for illustration purposes. While they are typical, they neither represent all the schedules that may be included in a specification nor their content. Purchasers and manufacturers must therefore prepare their schedules in such a way that they contain all the information they require or have to offer, for a contract. Also, it must be recognised that, in some cases, it may be necessary that the information exceeds or supersedes that recommended in a standard. The technical schedules relating to the transformer performance and guarantees are probably the most important since, in practice, they are intended to be clear and precise statements about what the purchaser requires, together with similar firm statements of what the manufacturer is offering. The layout and content of these schedules will vary not only with the transformer rating, voltage and application but also with the originality and complexity or otherwise of a purchaser’s requirements. Neither schedules nor the information they contain is intended to be complete, but together with Technical Specification represent purchaser technical requirements and manufacturer offer. Only characteristics marked as guaranteed are subject of penalties or rejection. The Product Source schedule is an example of a summary that records where materials and finished goods are obtained, their identity and other details, like in this case their 47

WG A2-36

Guide for preparation of specifications for power transformers

place of test. These records assure both purchaser and manufacturer that the items listed are acceptable. The List of Trips, Alarms and Analogue outputs is only example. The items or facilities listed will vary, depending on needs of a particular transformer specification, but again, the advantage of such a schedule is to provide complete information both to purchaser and manufacturer of what is required and what is to be supplied.

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SCHEDULE 1A Performance and Guarantees Schedule (To be completed by Purchaser and Manufacturer, as appropriate) Item

Description

Units

1

Type of transformer

2

Applicable standard

3

Rated power at all taps

MVA

4

Number of phases

1 or 3

5

Rated frequency

Hz

6

No-load rated voltage at principal tap (IEC rated voltage) (a)

HV winding

kV

(b)

LV winding

kV

(c)

Tertiary winding

kV

Required

(a) Delta connected Tertiary stabilising winding required?

Yes / No

(b) Tertiary winding required for loading, complete with terminals?

Yes / No

Winding connection, e.g. star/delta/auto (a)

HV winding

(b)

LV winding

(c)

Tertiary winding

(d)

IEC vector group symbol

Type ( graded/non-graded ) of windings (a)

HV winding

(b)

MV winding

(c)

LV winding

Highest voltage for equipment (a)

HV winding

kV

(b)

MV winding

kV

(c)

LV winding

kV

Rated lightning impulse withstand voltage at: (a)

HV terminal

kV

(b)

MV terminal

kV

(c)

Neutral terminal

kV

(d)

LV terminal

kV

Rated switching impulse withstand voltage at: (a) HV terminal

kV

Rated power frequency withstand voltage at: (a)

HV terminal *

kV

(b)

MV terminal *

kV

(c)

Neutral terminal

kV

(d)

LV terminal

kV

*Note: Only 3-phase ACSD testing, if applicable Maximum partial discharge intensity measured during AC induced voltage withstand test

49

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SCHEDULE 1A (Continued) Performance and Guarantees Schedule (To be completed by Purchaser and Manufacturer, as appropriate)

Item

Description

Units

(a)

at 1.1 x system highest voltage Um

(b)

at 1.3 Um

pC

(c)

at 1.5 Um

pC

Required

pC Cooling method Power for different cooling methods

%

Maximum temperature rise at rated power at: (a)

Average windings

K

(b)

Hot spot of windings

K

(c)

Top oil

K

(d)

Core

K

(e)

Tank

K

Overload capabilities according to IEC 60076-7?

Yes / No

Type of tap changing

No tap / On-load / De-energised

On-load tap changer is mounted inside or outside of transformer main tank?

Inside / Outside

Which winding is tapped?

HV / MV / LV

Category of voltage variation

C.F.V.V. / V.F.V.V.

Tapping range (a)

Plus ( maximum tapping )

%

(b)

Minus ( minimum tapping )

%

Tapping step Number of tap steps Arrangement of tapping ( Taps, Linear, Coarse/Fine, Reversing ) HV/MV impedance voltage at rated 100% power and 75ºC and tolerances ( guaranteed value ) : (a)

(b)

on principal tapping (i)

HV/MV

%

(ii)

HV/LV

%

(iii)

MV/LV

%

(iv)

Fault current available at LV terminals

kA

on minimum tapping (i)

HV/MV

%

(ii)

HV/LV

%

(iii)

MV/LV

%

(iv)

Fault current available at LV

50

x

Offered

WG A2-36

Guide for preparation of specifications for power transformers

SCHEDULE 1A (Continued) Performance and Guarantees Schedule (To be completed by Purchaser and Manufacturer, as appropriate)

Item

Description

Units terminals

(c)

kA

Required x

on maximum tapping (i)

HV/MV

(ii)

HV/LV

%

(iii)

MV/LV

%

(iv)

Fault current available at LV terminals

% kA

Zero sequence impedance voltage ( approx. value ) at rated 100% power and 75ºC, on principal tapping, assuming rated voltage (single phase ) applied between line terminals and neutral, LV delta winding ( if exist ) is open circuit : (a) Supply on HV with MV open circuit

%

(b) Supply on HV with MV short circuit

%

(c) Supply on MV with HV open circuit

%

(d) Supply on MV with HV short circuit

%

Evaluation of losses to be used in assessing tender (a)

No-load loss

currency/kW

(b)

Load loss at Rated Powerbmission

currency/kW

(c)

Cooling plant losses

currency/kW

No load losses at rated voltage and rated frequency on principal tap ( guaranteed value )

kW

Load losses at rated power and 75ºC: (a)

(b)

On principal tapping: (i)

HV/MV ( guaranteed value )

kW

(ii)

HV/LV at LV rated power

kW

(iii)

MV/LV at LV rated power

kW

On tapping for maximum loss: (i)

HV/MV

kW

(ii)

HV/LV at LV rated power

kW

(iii)

MV/LV at LV rated power

kW

Tap position number Cooling plant losses

kW

Guaranteed sound power level at 100% rated voltage and frequency:

Guaranteed sound

(a)

at no-load

dB(A)

(b)

at (a) plus cooling equipment

dB(A)

(c)

at (b) plus rated current

dB(A)

(d)

cooler bank only

dB(A)

Terminal connection ( oil/air bushings, oil/SF6 bushings, cable boxes-oil or air insulated, plug-in )

51

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SCHEDULE 1A (Continued) Performance and Guarantees Schedule (To be completed by Purchaser and Manufacturer, as appropriate)

Item

Description

Units

(a)

HV terminals

(b)

MV terminals

(c)

Neutral terminal

(d)

LV terminals

Required

Bushings (a)

(b)

(c)

(d)

Rated current (i)

HV bushing

A

(ii)

MV bushing

A

(iii)

Neutral bushing

A

(iv)

LV bushing

A

(i)

HV bushing

kVp

(ii)

MV bushing

kVp

(iii)

Neutral bushing

kVp

(iv)

LV bushing

kVp

Insulation level

Power frequency test level (i)

HV bushing

kV

(ii)

MV bushing

kV

(iii)

Neutral bushing

kV

(iv)

LV bushing

kV

Creepage distance (i)

HV bushing

mm

(ii)

MV bushing

mm

(iii)

Neutral bushing

mm

(iv)

LV bushing

mm

Bushing current transformers ( BCTs) Test winding is required ?

Yes / No

Compensation winding is required ?

Yes / No

(a)

(b)

(c)

HV BCTs (i)

Number of cores

(ii)

Ratio

(iii)

Class

(iv)

Rated output

A VA

MV BCTs (i)

Number of cores

(ii)

Ratio

(iii)

Class

(iv)

Rated output

A VA

Neutral BCTs (i)

Number of cores

(ii)

Ratio

A

52

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SCHEDULE 1A (Continued) Performance and Guarantees Schedule (To be completed by Purchaser and Manufacturer, as appropriate)

Item

Description

(d)

Units (iii)

Class

(iv)

Rated output

Required

VA

LV BCTs (i)

Number of cores

(ii)

Ratio

(iii)

Class

(iv)

Rated output

A VA Inhibited/ Uninhibited

Transformer oil Number of stand-by: a)

radiators / coolers

b)

fans

c)

pumps Painted / Hot dip galvanised / HDG+painted

Anti-corrosion protection of radiators or coolers

Transformer tank fittings: -

Draining and filtering valves

Yes / No

-

Valves for tank oil sampling

Yes / No

-

Radiator isolation valves

Yes / No

-

Pulling eyes for complete transformer

Yes / No

-

Supports for hydraulic jacks

Yes / No

-

Lifting lugs

Yes / No

-

Tank earth terminals

Yes / No

-

Core earth terminal box

Yes / No

-

Inspection manholes

Yes / No

-

Ladder

Yes / No

-

Skids or wheels adjustable in two directions

Yes / No

Transformer accessories: -

Oil preservation system with or without rubber bag

With / Without

-

Dehydrating breather standard or maintenance free

Standard / Maintenance free

-

Oil level indicator of magnetic type

Yes / No

-

Contact thermometer for the oil temperature

Yes / No on HV/MV/LV side

-

Winding temperature indicator

-

Direct winding temperature measurement using fibre optic sensors

-

Pressure relief device

-

Rapid pressure relay

-

Buchholz relay

Yes / No h Yes / No Yes / No Yes / No

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SCHEDULE 1A (Continued) Performance and Guarantees Schedule (To be completed by Purchaser and Manufacturer, as appropriate)

Item

Description

Units

Required

-

Buchholz relay gas sampling

Yes / No

-

Non-return valve

Yes / No

-

On-line gas monitor

Yes/ No/ Preparation

-

On-line monitoring system

Yes/ No/ Preparation

-

Fire protection system

Yes/ No/ Preparation

Supply voltage for transformer auxiliaries

V

400 / 230 AC

Control / Protection voltage

V

x DC or y AC

Routine tests according IEC 60076-1

Yes / No

Type tests: -

Temperature-rise type test (IEC 60076-2)

Yes / No

-

Dielectric type tests (IEC 60076-3)

Yes / No

-

Determination of sound level (IEC60076-10) for each specified sound level

Yes / No h

-

Measurement of the power taken by the fan and liquid pump motors

Yes / No h

Special tests( depending on voltage level, some of them could be routine tests acc. to IEC 60076-1 ):

H

-

Dielectric special tests (IEC 60076-3)

Yes / No

-

Determination of capacitances windings-to-earth, and between windings

Yes / No h

-

Measurement of dissipation factor (tan ) of the insulation system capacitances

Yes / No h

-

Determination of transient voltage transfer characteristics (IEC 60076-3 Annex B)

Yes / No h

-

Measurement of zero-sequence impedance(s) on three-phase transformers

Yes / No h

-

Short-circuit withstand test (IEC 60076-5)

Yes / No

-

Measurement of d.c. insulation resistance windings-to-earth, and between windings

Yes / No h

-

Vacuum deflection test on liquid immersed transformers

Yes / No h

-

Pressure deflection test on liquid immersed transformers

Yes / No h

-

Vacuum tightness test on site on liquid immersed transformers

Yes / No h

-

Measurement of Frequency Response (Frequency Response Analysis)

Yes / No h

-

Check of external coating (ISO 2178 and ISO 2409 or as specified)

Yes / No h

-

Measurement of dissolved gasses in dielectric liquid

Yes / No

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SCHEDULE 1A (Continued) Performance and Guarantees Schedule (To be completed by Purchaser and Manufacturer, as appropriate)

Item

Description

Units

Required

Quality Assurance: -

Manufacturer quality assurance acc. to ISO 9001

Yes / No

-

Manufacturer Quality Manual is submitted with offer

Yes / No

-

Manufacturer a sample of Quality Inspection and Test Plan is submitted with offer

A informative transformer general arrangement drawing showing overall dimensions and transport dimensions is submitted with offer

55

Yes / No h Yes / No

Offered

WG A2-36

Guide for preparation of specifications for power transformers

SCHEDULE 1B Performance Schedule (To be completed by Manufacturer)

Item 1

2

3

Description

Unit

Offered

Core construction (a)

Limbs: Banded / Bolted

(b)

Yokes: Banded / Bolted

(c)

Banding / Bolting material

(d)

Core bolt insulation ( if applicable )

(e)

Number of limbs

(f)

Number of limbs wound

No-load current on principal tap (a)

at 100% excitation

% of rated current

(b)

at 110% excitation

% of rated current

Insulation of: (a)

Yoke clamps

(b)

Leg plates

(c)

Core laminations

Whether tank or other flux shields are incorporated Flux density in magnetic circuit (a)

(b)

Maximum value at rated voltage, rated frequency and principal tap (i)

Limb

(ii)

Yoke

(iii)

Shields

Maximum value under any condition of voltage and frequency specified (i)

Limb

(ii)

Yoke

(iii)

Shields

Winding type, e.g. interleaved disc, disc, helical etc. (a)

HV windings

(b)

MV windings

(c)

Tapping windings ( as applicable )

(d)

LV windings

(e)

Windings arrangement, i.e. core/…/…/…/…/

Presence of non-linear resistors?

Yes/ No

56

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SCHEDULE 1B ( Continued ) Performance Schedule (To be completed by Manufacturer)

Item

Description

Unit

Conductor insulation (a)

HV windings

(b)

MV windings

(c)

Tapping windings

(d)

LV windings

Oil circulation ( i.e. natural/partially directed/directed ): (a)

(b)

to windings (i)

HV windings

(ii)

MV windings

(iii)

Tapping windings

(iv)

LV windings

through windings (i)

HV windings

(ii)

MV windings

(iii)

Tapping windings

(iv)

LV windings

(c) maximum oil velocity in windings, all pumps operating

m/s

Maximum current density in any winding at principal tap and rated power (a)

Winding

(b)

Current density

A/mm2

(c)

Current density in HV or MV winding under most onerous earth fault condition

A/mm2

(d)

Current density in LV winding under most onerous earth fault condition

A/mm2

Oil: -

Manufacturer

-

Type designation

-

Applicable standard

OLTC or DETC: -

Manufacturer

-

Type designation

-

Applicable standard

-

Rated current

A

Type of tank: -

cover or bell type

-

cover connection: bolted or welded

57

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SCHEDULE 1B ( Continued ) Performance Schedule (To be completed by Manufacturer)

Item

Description

Unit

Offered

Minimum thickness of transformer tank: -

Sides

mm

-

Bottom

mm

-

Cover

mm

Thickness of radiator plates

mm

Colour of finishing paint

RAL

Overall Dimensions (a)

Maximum height from floor level

mm

(b)

Maximum length of transformer

mm

(c)

Maximum width of transformer

mm

(d)

Minimum height to crane hook for lifting active part out of tank, including allowance for slings

mm

(e)

Minimum height above transformer cover for lifting of OLTC diverter switch insert

mm

(f)

Maximum dimensions for transport (i)

height

(ii)

length

(iii)

width

m

mm mm

Masses (a)

Mass of transformer and coolers, complete with necessary equipment and filled with oil

kg

(b)

Mass of transformer dry active part

kg

(c)

Total mass of copper in windings

kg

(d)

Total mass of cellulose insulation

kg

(e)

Mass of active iron in core

kg

(f)

Mass of tank empty

kg

(g)

Mass of coolers complete with oil

kg

(h)

Maximum mass for transport

kg

(i)

Mass of main tank, erected and filled with oil

(j)

Mass of complete oil

(k)

Maximum mass of one bushing

kg

kg kg

58

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WG A2-36

Guide for preparation of specifications for power transformers

SCHEDULE 2 Product Source information – Manufacturers, Type, Places of Manufacture (To be completed by either the Manufacturer)

Item

Manufacturer

Manufacturer’s type designation

Transformer OLTC equipment DETC tapping switches HV bushings MV bushings LV bushings Neutral bushing Insulating cylinders

N/A

Core plate material Winding conductor

N/A

Transformer tank

N/A

Radiators

N/A

Air cooled oil coolers Water cooled oil coolers Oil Oil valves

N/A

Oil pumps Oil flow indicators Fans Gaskets for oil tight joints

N/A

Pressure relief device Oil level indicators Dehydrating breather Gas and oil actuated relay(s) Control cabinet

N/A

Temperature indicating devices Material for anti-vibration mountings

N/A

Current transformers

N/A

Monitoring devices ( to specify )

59

Place of Manufacture

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SCHEDULE 3 Trips, Alarms and Analogue outputs (To be completed by the Purchaser)

Item / Function

If required

Type

Main gas and oil actuated relay

Yes

Trip/Alarm

Tap changer gas and oil actuated relay

Yes

Trip/Alarm

Tap changer diverter overpressure relay

Yes/No

Trip/Alarm

Pressure relief device operated

Yes/No

Trip/Alarm

Oil temperature

Yes/No

Trip/Alarm/Analogue

Main windings temperature

Yes

Trip/Alarm/Analogue

LV winding temperature

Yes/No

Trip/Alarm/Analogue

Conservator low oil level

Yes/No

Alarm/Analogue

Rubber bag failure

Yes/No

Alarm

Transformer auxiliary voltage fault

Yes/No

Alarm

Power supply fan/pump fault

Yes/No

Alarm

Transformer control cubicle fault

Yes/No

Alarm

Oil flow fault

Yes/No

Alarm

Dissolved gas in oil monitor fail

Yes/No

Alarm

High dissolved gas in oil

Yes/No

Alarm

Dehydrating breather fail

Yes/No

Alarm

Tap change incomplete

Yes/No

Alarm

Tap changer automatic control fault

Yes/No

Alarm

Tap changer lockout relay

Yes/No

Alarm

60

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Guide for preparation of specifications for power transformers

SCHEDULE 4 Clarification / Exclusions Technical Schedule (To be completed by the Manufacturer) Note: Contents of this schedule should be included in commercial part of Purchaser Order

Specification reference

Brief details of clarification / exclusion

61

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SCHEDULE 5 Recommendation for spare parts to be included in the contract (To be completed by the Manufacturer) Item No.

Spare Part Description

Quantity

62

Required (Yes/No)

Offered (Yes/No)

WG A2-36

Guide for preparation of specifications for power transformers

APPENDIX A - Loss evaluation, penalties, bonuses and rejection The total evaluated cost of a transformer is usually taken to be its capital cost plus the lifetime operating costs, particularly the losses. The relative magnitude of transformer no-load and load loss as well as other factors such as the rated power, operating voltage, impedance and thermal performance, has a fundamental effect on the transformer design by affecting the volume, weight, shape and cost. The major determinant of total evaluated cost is the cost of the no-load and load losses, but purchasers are increasingly considering other additional costs to determine the true lifetime ownership costs. For instance, the overall transformer lifetime ownership costs involve:     

transformer capital cost lifetime cost of losses erection and commissioning costs lifetime maintenance, replacement and disposal costs environmental costs

The capitalised cost of losses therefore depends on the accuracy of future estimates of the cost of electricity, interest rates and yearly transformer utilisation rates involving load prediction. These factors tend to become less accurate as the expected future transformer service life is extended. The optimum combination of capital cost and lifetime cost of losses may also be affected by other constraints such as purchasing and system operating policies. For instance:    

adoption of standard ranges of transformer types, ratings and voltage classes where applicable, the ratio of naturally cooled ONAN rating to forced cooled ratings, e.g., OFAF ratio of normal operation rated power to planned or emergency outage throughput power estimated loss ratio to maximise transformer operating efficiency to meet predicted medium or longer term loads

In the absence of the requisite data, resources or methodology for evaluating the cost of losses a purchaser is advised to consult a number of potential manufacturers or other specialists for guidance on how to prepare a suitable statement on loss evaluation for inclusion in an enquiry document. It should be apparent that an estimate of lifetime cost of losses is dependent on the use of accurate forecast information, including the guaranteed losses. To prevent any additional costs arising due to measured losses exceeding guaranteed values, purchasers can impose penalties on manufacturers in order to recover these costs and also seek lower tolerances on losses. A purchaser may also recompense the manufacturer with a bonus payment if the difference between measured and guaranteed losses will result in lower purchaser operating costs. A.1

Loss evaluation formula

There appears to be no limit to the simplicity or complexity of the methods used to 63

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Guide for preparation of specifications for power transformers

determine the cost of the losses but the following general method may be used in the absence of any other. However, no general method can fulfil the function of a customised technique specially devised to meet the needs of a purchaser’s particular application. The factors usually involved are: factor factor factor factor factor factor and Note:

a = intended life of the transformer, years b = availability (the number of hours a transformer is energised expressed in per unit terms, based on 8760 hours in a year). c = financial discount rate, (%). d = cost of power, unit cost (1) / kW at maximum demand, per annum e = cost of energy, unit cost / kWh f = load factor (expressed in per unit terms based on rated power) n = number of a given year of transformer life (n = 1 through “a”) (1)

unit cost means a unit of currency

For generator transformers that are normally intended to operate at rated power throughout the period of their annual availability (i.e., b = 1.0), the formula for determining the evaluated cost of losses is: n=a

∑ { ( e . 8760 . f ) + d } . c

Generator transformer losses =

unit cost/ kW

n=1 This formula is also applicable to shunt reactor losses but with a correction to take account of the lower total time they are connected to the system, i.e., their lower availability: n=a Shunt reactor losses

=

∑

{ ( e . 8760 . f . b ) + d } . c unit cost / kW n=1

Transmission transformers and series reactors normally operate at less than their rated power for a significant part of service life. In addition, two or more transmission transformers are normally connected in parallel to share a common load and to ensure a secure supply. Also, their load usually increases throughout their service years due to load growth up to a prescribed maximum value. The load can also fluctuate from no-load to emergency load conditions. Transmission transformer no-load loss n=a = ∑ { ( e . 8760 . b ) + d } . c unit cost / kW n=1 and the associated load losses n=a = ∑ { ( e . 8760 . f 2 . b ) + d } . c unit cost / kW n=1 64

WG A2-36

Guide for preparation of specifications for power transformers

where in this case, the factor ‘f ’ is the load factor value that is likely to occur in a given year when load demand on the transformer is at a maximum. A.2 Penalties, bonuses and rejections to be applied to losses outside a guarantee. The following clauses are typical of those used sometimes to qualify the contractual value of losses measured on test and are provided here for illustration purposes only. If the individual no-load and load losses or the total losses exceed the guaranteed values by more than 10% the transformer shall be rejected. Where a measured loss exceeds 105% of the guaranteed value, a penalty shall be paid to the purchaser by the manufacturer. Where a measured loss is less than 95% of the guaranteed value, a repayment shall be made to the manufacturer by the purchaser. The difference between the above limits and the measured losses shall be used together with the capitalised value of losses to determine the value of any penalty or bonus to be applied under the contract. The capitalised values of the losses used in these calculations shall be those declared in the enquiry document and contract.

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