REPORT Activities of Cigre SC A2...

This paper summarizes the activities of CIGRE Study Committee A2“Trans-

formers” (SC A2). It is an update to the paper presented in Electra in 2009 [1].

The paper introduces the scope, strategic direction and key domains of SC

A2 activities. Those activities are then discussed with a focus on the last decade

and present day. The workshops prepared by SC A2 are listed and an overview

is given of possible future activities.

CIGRE, Transformers, Specifications, Design, Reliability,

Maintenance, Monitoring

The first CIGRE Study Committee (SC) dealing with transformers, then des-

ignated “SC 12”, was formed in 1949, more than 60 years after the invention of

the transformer around 1885.After the reorganization of CIGRE in 2002, SC 12

became Study Committee A2, one of the three SCs dealing with equipment

(SC A1, SC A2 and SC A3). Under this new organization, instrument trans-

formers were transferred to SC A3 (HV Equipment).

Even if the theme of SC A2 is “Transformers”, the scope of this SC covers:

– Power transformers (including industrial, DC converter and phase-shift-

ing transformers)

– Reactors (shunt, series, saturated and smoothing)

– Transformer components (bushings, tap changers, accessories, etc.)

In the past, the activities of SC A2 “Transformers” (and its predecessor SC

12) focused on design problems related to the rapid increase of rated voltage and

power; whereas now, the application of new materials, service conditions and

their impact on transformer performance have become of central importance.

The two strategic directions of SC A2 are currently:

• Services to customers (reliability, life management, economics,workshops,

etc.)

• Technology issues (safety, new technologies and concepts, electrical envi-

ronment, pre-standardisation work, etc.)

EELLEECCTTRRAA N° 255 - Avril 201126

SC ANNUALREPORT

2011

SC A2

Activities of Cigre SC A2“Transformers”

ClaudClaude e RRAAJOTTE, JOTTE, HyHydro-Québec dro-Québec TTransransÉÉnergie,nergie, Montréal Montréal (C(Canada)anada)[email protected]@hydro.qc.cadro.qc.caChairman Chairman CCIGRE IGRE SC SC AA2 2 “T“Transransformers”formers”

1102-141 Electra255.qxp:0507-012Mep221 30/03/11 10:34 Page 26

There are four key domains for SC A2: (1) Specifications, procurement and

economics, (2) Design, manufacturing and testing, (3) Operation, reliability,

safety and environment and (4) Maintenance, diagnostics,monitoring and repair.

Key domains (1) and (2) are associated with transformer technology, while key

domains (3) and (4) are associated with transformer utilization.

As shown in Figure 1, these key domains cover the whole transformer life

cycle:

Good technical specifications are vitally important to ensure that the trans-

former ultimately procured is suitable for the purpose intended.The transformer

user must specify an impressive number of parameters to ensure that the trans-

former will suit the application. These parameters are of several kinds: primar-

ily electrical (insulation level, voltage and voltage regulation, load and over-

load, impedance, etc.), physical (weight, dimensions, quantity of insulating fluid,

position of the electrical connections, etc.), environmental (noise,material used

as oil, etc.), and functional (type of cooling, accessories,maintainability, etc.), in

addition to numerous other details to be specified.

In 2000, SC A2 prepared a“Guide for customer specifications for transform-

ers 100 MVA and 123 kV and above” [2]. This document has been used inten-

sively by transformer users to prepare thorough specifications as it covers all

aspects of performance, design, factory testing, delivery to site and erection.

In 2004, SC A2 prepared a brochure entitled “Economics of Transformer

Management” [3, 4]. This document addresses the impact of the requirements

set out in specifications, and also economic aspects of transformers over their

entire life time.

In 2006, SC A2 prepared a brochure to discuss “Condition Monitoring and

Condition Assessment Facilities for Transformers” [5, 6]. It considers poten-

tial benefits of a standard interface between monitoring systems and transform-

ers in order to provide guidance regarding specifications for a transformer

intended to be “condition monitoring ready”.

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No. 255 - April 2011 EELLEECCTTRRAA 27

SC A2

Figure 1: Key domains for SC A2


Current activities

WG A2.33 – Transformer Fire Safety Practices was created in 2007 after the

publication of a paper published in Electra on Fire Avoidance [7]. IEC TC 14 has

shown an interest in including recommendations on transformer fires in the

relevant standards. The aim is to prepare recommendations for transformer

fire safety practices that will help transformer designers and users define and

apply best practices in that domain. The scope encompasses primarily: a) Avoid-

ing tank rupture, b) Protecting fire victims and c) Preventing fire outbreak.

WGA2.36 – Guide for Transformer Procurement Process was created in 2008.

Its purpose is to carry out a full review of existing CIGREA2 documents on pro-

curement [2, 11] and to update them, taking into account current market

conditions and new commercial pressures under which customers operate. It also

has the mandate to prepare a new guide for assessing the capability of transformer

manufacturing plants, which evaluates technical competence and experience,

and can be integrated into and be supported by process control functions under

existing quality assurance procedures.

Up to the middle of the twentieth century, system voltage rose step by step

to a maximum level of roughly 300 kV. Around 1965, rated voltage climbed to

800 kV and design and test problems for this new generation of transformers

became the focus of SC A2 activities in the late 1960s and early 1970s. Research

and development concentrated on the design of windings able to withstand

impulse stress and on related test procedures.

Transients in large power transformers sparked extensive studies into resonance

phenomena inwindings.When gas-insulated systems (GISs) for EHVapplications

with direct SF6-connection to the transformer were introduced, there appeared a

new kind of overvoltage impacting transformers: very fast transients (VFTs).

The increase in rated voltagewas also accompanied by a sharp increase in trans-

former power rating (more than 1,000MVA for a three-phase transformer).As the

short-circuit power of systems increased, short-circuit withstand capability of such

transformers became amajor concern and SC A2 discussed problems extensively.

In 1995, SC A2 published a brochure entitled “Thermal aspects of trans-

formers” [8] addressing a key issue for loadability. It discusses all aspects of

thermal performance, including determination and direct measurement of


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SC A2

Figure 2: Power transformer fire


allowable hot-spot temperature, heat run test procedures, application of dissolved

gas analysis to evaluate test results, overload practices and their impact on

projected service life. More than 15 years after its publication, this document is

still an authoritative reference on the subject.

In 2002, a brochure entitled “The short circuit performance of power trans-

formers” [9, 10] was published. The document discussed design and force

calculations, manufacturing issues, monitoring and winding movement

detection techniques.

It also became of prime concern that transformer purchasers and manu-

facturers come to a common understanding of the requirements. In 2002, SC A2

thus also published a brochure devoted to design review [11, 12].More recently,

in 2010, SC A2 published a similar brochure but specifically focused on design

reviews of HVDC transformers [13].

About 40 years after the introduction of 800 kV transformers, the recent

development of UHV transformers in the range of 1,100 kV for AC applications

and converter transformers of 800 kV for DC applications will probably require

further investigation, in which SC A2 may play a major role.

Current activities

WG A2.38 – Transformer Thermal Modeling was created in 2008. Its

purpose is to describe state-of-the-art techniques in transformer thermal

modeling to evaluate winding hottest-spot temperature as well as hot spots on

other metal parts (outside the windings). It will study advanced transformer

modeling tools and application of direct measurements to obtain relevant param-

eters for modeling. Any applicable recommendations will be made to improve

standards. The applicability of thermal modeling to revise existing transformer

thermal performance will also be discussed with examples. Transformer

loadability in service will be explored through dynamic modeling.

JWGA2/C4.39 – Electrical Transient Interaction between Transformers and

the Power System was created in 2008. The objective of the JWG is to assess

and discuss the different types of electrical transient interaction between

transformers and other components of the T&D power system. It is motivated

by a general increase in transformer dielectric failures in the system, some with

no specific causes apparent. Technical articles have been written and recently

IEEE and CIGRE working groups have summarized the relevant findings. It is

recognized that fast transient overvoltages do exist and can damage transformer

windings. However, many unknowns remain regarding this issue related

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Figure 3: Transformer thermal modeling


to transformer design and testing (particularly of its insulation), transformer

protection, and interactions between transformers and fast transient system

“sources”, such as circuit breakers, capacitor banks and power electronics. In this

context, the focus of the JWG is to pursue improvement of transformer

reliability based on recommendations regarding equipment specifications, design

review, system planning and operation.

JWG A2/D1.41 – HVDC Transformer Insulation – Oil Conductivity was

created in 2010. Former JWG A2/B4.28 showed how conductivity variations in

components of oil/solid insulation systems impact the design and reliability of

HVDC converter transformers. It reviewed the effectiveness of existing standard

dielectric DC and polarity reversal tests in respect of oil conductivity and polar-

ization time.Oil conductivity was found to be the dominant factor and, since no

standard procedure is consistently applied to measure oil conductivity, this could

result in dielectric stress during testing lower than in service. JWGA2/B4.28 thus

recommended that priority be given to measuring oil conductivity throughout

the transformer life cycle and evaluating the effect of oil conductivity during the

design and design review stages.

Transformers are among the most important and more costly items of equip-

ment in an electrical network. As transformers are not uniform with respect to

voltage ratio, power, impedance, dimensions, etc., they are not easy to replace

quickly if a major failure occurs. Thus, information about transformer reliabil-

ity is of a great importance, for example to establish a strategy for redundancy and

spare transformers.

In 1983 CIGRE Working Group 12.05 published a report [14] summarizing

the results of an analysis of data collected on the failure of large transformers.

Failure rates were calculated and the influences of various factors (e.g. applica-

tion, voltage and presence of an OLTC) were investigated. The two main

conclusions were that major failure rates were about 2% per annum and that

the primary cause attributed in three-quarters of the cases was design, manu-

facture or materials, although some doubt was expressed about whether survey

responses reflected the real cause.The report also defined parameters required for

the quantitative evaluation of reliability.

In subsequent years, attemptsweremade toupdate and improve the 1983 report

without any real success.Recent reorganization of the electricity industry also seems

to affect the availability of information concerning equipment failures. More

recently, an Electra report published by SC A2 [15] recommended the creation of

a new WG to look specifically into this subject (see current activities below).

SC A2 has also contributed to the study and resolution of reliability prob-

lems. For example, in the early 1980s, an increasing number of faults were reported,

which were clearly related to static charging in large oil-filled transformers. In

collaboration with EPRI and other scientific institutions, SCA2 started a joint

working group with SC D1 (formerly SC 15) to investigate the basic physics of the

phenomenon and to determine the influencing parameters and counter-mea-

sures. In 2000, the results were presented in the brochure “Static electrification

in large power transformers” [16, 17]; however, open questions remain. In the

same year, SC A2 published a report entitled “Effect of particles on transformer

dielectric strength” [18, 19]. The report revealed the difficulty in measuring

particles in transformer oil, though this is crucial to avoid dielectric failures.


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Another example is the creation, in 2005, of WG A2.32 to deal with the

problem of copper sulphide formation in transformer insulation. The

phenomenon has caused numerous failures in transformers and reactors since

sulphide deposits attack transformer solid insulation. The vast majority of

reported failures to date are in shunt reactors, HVDC transformers and gener-

ator step-up transformers. A common factor for such transformers is their rel-

atively high load; otherwise, the connection is less obvious. There are very few

reported failures in open, breathing equipment. The occurrence of large and/or

very frequent transients seems to have had a great influence. In 2009, the brochure

“Copper Sulphide in Transformer Insulation” [20, 21, 22] was published. This

document describes possible mechanisms of copper sulphide formation and

its consequences, influencing factors, mitigation techniques and recommenda-

tions for transformer users. The same year, SC A2 published a report on the

thermal performance of transformers [23, 24] addressing thermally induced

ageing mechanisms, diagnostics, testing and transformer thermal design.

In 2010, a brochure was published by the SC A2/B4.28 JWG entitled“HVDC

Converter Transformers – Design review, test procedures, ageing evaluation and

reliability in service” [25, 26]. The JWG was created to evaluate HVDC trans-

former reliability.One of its recommendations was to create a new activity on oil

conductivity, as discussed in 3.2.

In 2010, a brochure was published “Experiences in Service with New Insu-

lating Liquids” [27, 28]. It covers synthetic esters, natural esters and silicone

fluids that have been in widespread use for several years in various applications

and especially at higher power ratings. This brochure discusses differences in the

physical, chemical and electrical properties of the new fluids and mineral oil,

applications, maintenance and end user experiences with these fluids.

Transformer operation and reliability assessment requires extensive data. In

2006, SC A2 published a “Guide on transformer lifetime data management” [29,

30].Datamanagement is important for cost-efficient, risk-minimized and efficient

asset management and maintenance process in utilities. Based on a transformer

engineering view, the brochure describes a generic data model and related aspects

of information technology. The guide should provide support to interested pro-

fessionals and facilitate utility decision-making regarding a lifetime data strategy.

Current Activities

WG A2.37 – Transformer Reliability Survey was created in 2008 to prepare a

brochure describing international transformer reliability survey practices. Data

and information already in the public domain and usually available only locally

will be presented in a comprehensive manner. Differences in such areas as

failure definition, transformer usage and transformer specifications,which poten-

tially influence survey results, will be discussed and best practices identified. The

brochure will include any recommendations to improve the compatibility of data

compiled in different countries and propose a uniform way of collecting,

compiling and presenting data. Specifically, the WG will: a) Review all existing

national surveys and study different practices (data collection, compilation, etc.),

b) Discuss the differences and identify best practices, c) Compile and present the

information available in these national survey reports and d) Make recom-

mendations to improve the situation.

WG A2.43 – Transformer bushing reliability was created in 2010. Bushings

are among the most frequent causes of transformer failure. Based on data from

various research projects and electric power utilities, bushings cause, on

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average, one-quarter of all transformer failures and are the most common cause

of transformer fires.HV bushings are a thin, fragile structure, sensitive to mechan-

ical forces. A bushing explosion may throw fragments at an enormous speed at

over distance of 100 metres or more, or bushings may explode internally and

damage the active part. Among the deliverables are: a definition of bushing

failure, bushing failure rates, failure mechanisms for different bushing technolo-

gies (e.g., resin-bonded paper, oil-impregnated paper, resin-impregnated paper),

on-line and off-line condition monitoring, projected service life, maintenance,

decision criteria (especially for C and tan δ), standardization of test taps and

oil sampling (if appropriate).

Transformers are usually robust, very reliable apparatus requiring relatively

little maintenance.Nevertheless,maintenance programs must be constantly opti-

mized as the transformer population changes, new problems and solutions

emerge, and monitoring and diagnostic technologies develop. During the life

of a transformer, the users must establish a maintenance strategy that will ensure

the appropriate level of reliability and optimized service life.

SC A2 dedicates intensive efforts to these matters. In 2003, SC A2 published

a brochure called“Life management techniques for power transformers”[31, 32].

It covers general knowledge and theoretical issues, basic failure concepts, and

recommendations on failure identification. It includes a catalogue of defects and

faults, a summary of dangerous effects of degradation factors, and diagnostic and

monitoring techniques.

Moisture in insulation represents a risk to transformers in three ways. It accel-

erates depolymerisation of paper, reduces the breakdown voltage of oil, and

increases the risk of bubbling during sudden overload or thermal stress. In 2008,

SC A2 published a brochure, “Moisture Equilibrium and Moisture Migration

within Transformer Insulation Systems” [33, 34], describing these phenomena

in oil-paper insulated power transformers. It gives in-depth knowledge of themain

sources of moisture contamination, solubility of water in oils, adsorption in

cellulose, the physical background formoisture equilibrium and bubble evolution.

Practical advice is provided on moisture determination using equilibrium

diagrams, and onmigration time constants andmoisturemeasurement techniques.

Significant advancements have been made in the past decade to improve tools

to evaluate the mechanical condition of transformers. In 2008, SC A2 prepared

a brochure, “Mechanical Condition Assessment of Transformer Windings using

Frequency Response Analysis (FRA)” [35, 36, 37] providing an introduction to

the FRA test for detecting winding movement. It presents recommendations for

standardization of good measurement practices. It also gives a description of

typical responses and controlling factors,with cases illustrating how various types

of winding movements are detected by FRA. This report formed the basis for an

IEC standard currently under development (PT 60076-18).

Early in 2011, SC A2 published a brochure, “Guide for Transformer Main-

tenance” [38, 39] helping transformer users define and apply best practices. The

brochure covers the following three areas: a) Best practices, list of periodic actions

applied in service or with outage, and checking and testing to evaluate trans-

former condition, b) Advanced maintenance activities, usually referred to as

“condition-based maintenance”, such as oil additives, oil filtering, oil regenera-

tion, and insulation drying, and c) Human and material aspects of transformer

maintenance, with maintenance planning, maintenance task tracking, mainte-


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nance resources, cost references, level of competence required for different tasks,

training, on-site repair, etc.

Current Activities

A2.40 – Copper Sulphide Long-Term Mitigation and Risk Assessment was

created in 2009 after WG A2.32 published their report [19]. Copper sulphide in

transformer insulation still raises issues of concern to the transformer industry.

To date, the number of reported failures due to copper sulphide deposition on the

windings remains quite low. The need to continue work in this field has been

recognized since thousands of transformers have been passivated in recent years

and many units are in service with corrosive oils.Mechanisms of copper sulphide

formation, the influence of oxygen, magnetic and electrical fields, and materials

other than oil are not yet fully understood. Several mitigation techniques for

removal of corrosive sulphur from the oil have been reported and applied with

apparent success. The addition of a metal passivator is today the most widely used

mitigation technique.More field experience must be gathered to improve current

knowledge in risk assessment and the long-term effects of mitigation techniques

applied. The aim of this WG is to describe the state of the art, develop more

precise risk assessment and evaluate long-term effects of mitigation techniques.

WG A2.42 – Guide on Transformer Transportation was created in 2009.

Examples of recently reported events leading to severe damage to and the scrap-

ping of transformers include derailment during rail transport, and the collapse

of cranes and bridges. Guidance on the connection between the transformer’s

mechanical design, the design review, action when an unwanted transporta-

tion event has occurred, measurements during transportation and their inter-

pretation, and internal inspection on the installation site are topics that should

be addressed by the WG. New constraints caused by urbanisation close to

substations will also be addressed.

WG A2.44 – Transformer Intelligent Condition Monitoring was created in

late 2010. Power utilities are making increasing use of off-line and on-line mon-

itoring and diagnostics systems for power transformers. Lack of standardization

and doubts regarding the reliability and potential benefits of such systems are

impeding technology consolidation. The market now offers numerous sensors

and monitoring systems but there is no consensus on how to manage, process

and convert data to derive relevant information. The objective of this WG is to

explore the processing of transformers data, its conversion into relevant

information, the most appropriate diagnostics algorithms and specifications for

“standard and interoperable diagnostics modules”. The final results could be

useful for manufacturers and utilities by helping them align their projects,

processes and specifications toward better practices, improve their asset manage-

ment techniques by large-scale use of integrated information systems, and develop

optimal policies regarding the use of existing and new monitoring systems.

Recently, CIGRE decided to change its approach to disseminating technical

brochures content. Instead of giving tutorials during which a presenter

summarizes the work done by a WG, the new approach will encourage interac-

tion and discussion during and after presentations.

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Several workshops disseminate the work of WGs during local and interna-

tional events, including the following SC A2 workshops:

– Life management techniques for power transformers

– Transformer short circuit performance

– Economics of transformer management

– Transformer lifetime data management

– Thermal performance of power transformer

– Mechanical condition assessment of transformer windings using frequency

response analysis (FRA)

– Recommendations for condition monitoring and condition assessment

facilities for transformers

– HVDC converter transformer – Design review, test procedures, ageing

evaluation and reliability in service

– Moisture equilibrium and moisture migration within transformer

insulation systems

– Copper sulphide in transformer insulation

– Guide for transformer maintenance

– Experience in service with new insulating liquids

Proposals for future activities have been received from various working

bodies as well as from regular and observer members. The following domains are

of interest for future SC A2 activities: failure investigation/post-mortem

analysis, life cycle decision-making (repair/refurbish/replace), on-site testing,

reactors, ecodesign and eco-use of transformers, short-circuit capability in the

context of transformer ageing and improved FRA interpretation.

The author acknowledges with thanks the sustained support and contribu-

tions of the members of the SC A2 Advisory Group “Strategic Planning”:

A. Petersen, T. Breckenridge, S. Tenbohlen, J. Lapworth, A. Rocha, J. Lukic,

A.Kuechler,A.Mjelvie,A.Mikulecky,C.Dupont, F.Devaux, P. Lorin,Y. Shirasaka

and the Secretary and Webmaster of SC A2, P. Picher.

[1] Presentation of CIGREA2“Transformers – Technical development and inputs

from future activities”, Electra No. 242, February 2009.

[2] Guide for customer specifications for transformers 100 MVA and 123 kV and

above, CIGRE Brochure No. 156, 2000.

[3] Economics of Transformer Management, CIGRE Brochure No. 248, 2004.

[4] Economics of Transformer Management, Electra No. 214, June 2004.

[5] Recommendations for Condition Monitoring and Condition Assessment

Facilities for Transformers, CIGRE Brochure No. 343, 2008.

[6] Recommendations for Condition Monitoring and Condition Assessment

Facilities for Transformers, Electra No. 237, April 2008.

[7] Fire Avoidance in Transformer Substation, Electra No. 231, April 2007.


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[8] Thermal aspects of transformers, CIGRE Brochure No. 96, 1995.

[9] The short circuit performance of power transformers, CIGRE Brochure

No. 209, 2002.

[10] The short circuit performance of power transformers, Electra No. 203,August

2002.

[11] Guidelines for conducting design reviews for transformers 100 MVA and

123 kV and above, CIGRE Brochure No. 204, 2002.

[12] Guidelines for conducting design reviews, Electra No. 202, August 2002.

[13] Guidelines for conducting design review for HVDC converter transformers,

CIGRE Brochure No. 407, 2010.

[14] An International Survey on failures in large Power transformers in service,

Electra No. 88, 1983.

[15] Transformer Reliability Surveys, Electra No. 227, August 2006.

[16] Static electrification in large power transformers,CIGRE Brochure No. 170,

2000.

[17] Static electrification in large power transformers, Electra No. 191, August

2000.

[18] Effect of particles on transformer dielectric strength, CIGRE Brochure

No. 157, 2000.

[19] Effect of particles on transformer dielectric strength, Electra No. 1909, June

2000.

[20] Copper Sulphide in Transformer Insulation,CIGRE Brochure No. 379, 2009.

[21] Copper sulphide in transformer insulation, Electra No. 224, 2006.

[22] Copper sulphide in power transformer insulation, Electra No. 230, 2007.

[23] Thermal Performances of transformers, CIGRE Brochure No. 393, 2009.

[24] Thermal Performances of transformers, Electra No. 246, October 2009.

[25] HVDC Converter Transformers - Design review, test procedures, ageing

evaluation and reliability in service, CIGRE Brochure No. 406, 2010.

[26] HVDC Converter Transformers - Design review, test procedures, ageing

evaluation and reliability in service,, Electra No 248, February 2010.

[27] Experiences in Service with New Insulating Liquids, CIGRE Brochure

No.436, 2010.

[28] Experiences in Service with New Insulating Liquids, Electra No. 252,

October 2010.

[29] Guide on transformer lifetime data management,CIGRE Brochure No. 298,

2006.

[30] Guide on transformer lifetime data management, Electra No. 227, 2006.

[31] Guide for life management techniques for power transformers, CIGRE

Brochure No. 227, 2003.

[32] Guide for life management techniques for power transformers, Electra

No. 208, June 2003.

[33] Moisture Equilibrium and Moisture Migration within Transformer Insula-

tion Systems, CIGRE Brochure No. 349, 2008.

[34] Moisture Equilibrium and Moisture Migration within Transformer Insula-

tion Systems, Electra No. 258, June 2008.

[35] Mechanical-Condition Assessment of Transformer Windings using

Frequency Response Analysis (FRA), CIGRE Brochure No. 342, 2008.


Frequency Response Analysis (FRA), Electra No. 228, October 2006.


Frequency Response Analysis (FRA), Electra No. 237, April 2008.

[38] Guide for Transformer Maintenance, CIGRE Brochure No. 445, 2011.

[39] Guide for Transformer Maintenance, Electra No. 254, February 2011. �

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