Technical Paper - CIGRE

8/12/2019 Technical Paper - CIGRE

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GENERAL REPORT FOR SCA2Transformers

Chairman: P. Boss (CH)

Secretary: C. Rajotte (CA)

Special Reporters: M. Kadowaki (JP) (PS1)

P. Werle (DE) (PS2)

F. Devaux (FR) (PS3)

Discussion summary prepared by J. Aubin (CA), A.C. Hall (UK)

SESSION OPENING

Pierre Boss, SC A2 Chairman, opened the session promptly at 09:00 with approximately 300 delegatesattending. He welcomed attendees and then summarised SCA2 activities since the 2008 session. He

announced four WG have finished their work and been disbanded; two are about to be disbanded forthe same reasons; and four have been created. In total, nine WG's remain active.

Mr Boss advised A2 is in liaison with SC D1, A3, B3 and C4 and also with IEC, IEEE, and ISO.Together with the support of D1, A2 has published seven Brochures since the last A2 session. He

ended his remarks by confirming several international events supported by A2 in Poland, China,Croatia, Israel, India and Brazil.

PREFERENTIAL SUBJECT 1: TRANSFORMER INCIDENTS IN SERVICE

A total of 7 papers describing transformer incidents in service were selected by SC A2 for presentation

on this subject. They can be classified as:

Risk assessment incidents in substations originated by transformers Study of tank rupture caused by transformer incidents Risk mitigation measure taken for transformer design and manufacturing Proposal for substation design to avoid transformer fire and mitigate its effect

PS1 - Keynote presentation by Mr. Marc Foata (CA)Mr. Foata reviewed briefly the work done at Hydro-Quebec to develop a requirement on transformer

tank withstand to internal flashover. The basic principle of fire initiation and propagation werereviewed along with some statistic of tank rupture event. The typical failure mode of HV bushing,responsible for about 50% of all fires, was presented. The limited efficiency of pressure relief deviceswas reviewed with example for some typical cases. Mitigation measures to insure energy containmentwere presented. Field experience of 735 kV single-phase transformers indicate that tank rupture is

likely to occurs when the arc energy reach a value between 4 and 8 MJ. Appreciable increase incontainment capability can be achieved with tank design enhancements

CIGRE 2010

Study Committee A2


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The Special Reporter Mr. Makoto Kadowaki formulated the following questions to structure the

contributions from delegates to the A2 Paris 2010 meeting.

Question 1-1: Transformer fire is mainly introduced in papers in this subject as this is considered themost critical. What are other critical incidents revealing substation operation?

Question 1-2: Transformer incidents have huge impact on reliability and safe operation of substation.Is there any significant experience which effect on the philosophy of substation design?

Question 1-3: What is the most effective measure in tank design which should be considered in orderto prevent transformer tank rupture after internal fault?

Question 1-4: Accessories such as bushings, OLTCs, and cables are higher in rate of transformer fire.

What is the most effective measure to mitigate risks by them?

Question 1-5: In applying insulation medium with high flash point as alternative of mineral oil, whatconsiderations should be taken for sufficient evaluation of characteristics? Is there any data or

experience available?

Question 1-6: Protection algorithm using mechanical and electrical protections can depend uponpractices in each country and organization. What kind of philosophy is applied on reliable harmonicsof transformer protection?

Question 1-7: Which countermeasure is the most effective in substation design for the mitigation offire risks? Is the method standardised?

Question 1-8: Are there any experiences evaluating countermeasure of substation design against fire,from aspect of social and economical impacts?

Question 1.1

Aside from fire damage, the Australian Cigre members have identified a number of other critical

incidents that may affect substation operation: smoke and debris that result from explosive failure ofother substation equipment, fire fighting water being used in the substation, deflagration and oil spills

from oil filled transformers, and porcelain fragments from bushing failures.

A contribution from Japan reviewed a number of transformer failures that did not lead to any fire:

static electrification in aged transformer, winding displacement caused by seismic force, direct lightingstroke to the power line and resonance between power transformer and power cable induced by

switching operation.

No contribution was received to Question 1.2

Question 1.3

A contribution from France reported the occurrence 6 serious fire failures of 600MVA 400/225kVauto-transformers in 30 years. This represents a 0.015% failure rate due to this cause. Studies in

partnership with the manufacturers involved have been made to understand the reason for the tankrupture that resulted from each of these events, in particular to determine the correlation between faultarc energy and tank internal pressure withstand capability, using the Hydro-Quebec model formula(ref. CIGRE - A2_102_2010 - CIGRE 2010 M. Foata; JB Dastous Hydro Quebec). It is shownthat existing tanks have a 1-2 bar withstand capability with a maximum 3 bar withstand at somelocation around the tank e.g. in the vicinity of the main tank flange. The Hydro-Quebec model is


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considered to be a realistic tool for use by both users and manufacturers. The Hydro-Quebec model

calculations are now a requirement in transformer technical specifications and are used also toevaluate existing tanks.

Transformer internal pressure withstands capability was the subject of another report from France. Thefinite-element-method (FEM) was used for example to study the tank design for a 735 kV 110 Mvar

shunt reactor. Again the internal arc energy was calculated using Hydro-Quebec Specification (SN14.1h, Part 1, Annex A, page 61), which also estimates the effects of tank volumetric flexibility.

Volumetric flexibility is the ratio of tank volume to applied pressure and is expressed as m/kPa. Thereport contains several interesting FEM derived images of pressure and vacuum deformations studied.

Question 1.4

A contribution from India reports on most effective measure to mitigate risks: use of RIP bushing in

place of OIP bushing can mitigate the risk of transformer fire; several steps can be taken to reduce riskof failure of tap changers: regular monitoring of BDV and moisture content, use of DGA and

localization of tap changer in a separate compartment. Other mitigation strategy such as nitrogen

injection and shaped thank construction are also deployed.A further report from an Indian utility report the following actions that were deployed successfully to

mitigate risk of failure of accessories: measurement of capacitance and tan delta on an annual basisand monitoring of trend with reference to the base value recorded at the time of installation;measurement of BDV value of oil in the OLTC chamber on half yearly basis and replacement of oil, if

required; introduction of scheme to restrict the number of OLTC operations to five times in a day tominimize wear and probability of failure; and modification to cable box of the transformer to provide

proper ventilation so that condensation of moisture does not take place inside the cable box.

A contribution from Poland indicates that most bushing insulation failures are caused by ingress ofmoisture into the bushing core and would recommend that utilities operating older units should replacebushings that exceeded approximately 20 years of service. Diagnostic instruments based on dielectric

polarization method provide information on the average moisture content in bushing insulation, butcannot detect local wet pockets in the bushing core. These can be detected by recording of partial

discharges at regular time intervals, by continuous monitoring or by thermal analysis. A thermal modelis proposed for this purpose.

A contribution from Germany reports on measures that can be deployed to mitigate risk of fire on tapchangers: first and preferred measure is to reduce the possibility for the occurrence of such a fault

condition, moisture content in insulation continues to be a major cause of problems and should beeliminated, pressure-relief devices can also be applied and finally the use of less flammable liquids can

be considered.

A contribution from Croatia reports that mechanically rigid bushing connection to the switchyard cancause bushing failure. Visual inspection of burst bushings debris has often shown that the failure

mechanism is connected with moisture ingress and/or test tap deterioration. A fast developing faultcannot be effectively prevented by periodic diagnostic testing. In that case a bushing monitoringsystem should be used.

A contribution from USA reports on the behaviour off pole-type transformers under internal faultcondition. An oil filled unit would explode while the unit filled with natural ester would not. One of

the main dielectric characteristics of ester fluids is the different impulse breakdown mechanism.Therefore, a different set of dielectric rules is required in design of esterfilled power transformers. Acontribution from France also supports these findings. Dielectric properties and streamer propagation

of ester are shown to compare favourably with mineral oil.

Question 1.5


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A contribution from Germany reports that the use of less-inflammable liquids for tap-changers

minimizes the hazard of fire and explosion damage in the event of an electrical failure inside theequipment. Extensive tests on various tap-changer models have been carried out and it has been found

that tap-changers with vacuum switching technology are more suitable for these purposes thanconventional arc-breaking-in-oil tap-changers. The different arc-quenching behaviour impedes theunrestricted use of ester liquids in arc-breaking-in-oil tap-changers.

A contribution from Japan reports on the combustion properties of natural ester oil and synthetic esteroil along with silicon oil. Results indicate that in term of heat release and smoke production, estersperform better than mineral oil but not as well as silicone liquid.

A contribution from UK also addresses the issue of dielectric strength of ester as compared withmineral oil. The impulse strength was investigated to consider the mechanism leading to streamerinitiation, propagation and breakdown. It was found that ester liquids have much lower breakdownvoltage than mineral oil due to their low tolerance to fast streamer. Therefore a linkage between results

under impulse testing and previously published empirical formula was developed to predict thebreakdown voltage of esters at very large gaps.

A contribution from France addresses the issue of gas-absorbing additives for transformer oils, as an

economical solution to mitigate transformer incidents in service. Highly refined oils contain lessaromatic compounds due to the hydrogenation process and as a consequence, the gassing tendency oftoday mineral oils becomes more and more positive. Using M/DBT as complement to mineral oils will

restore the original performances of these insulating liquids. In addition, M/DBT will improve thedielectric strength of the insulating liquid.

Question 1.6

A contribution from Sweden referred to aspects of current harmonics in relation to HVDCtransformers and their significance for converter transformer protection, especially differentialprotection. It was reported this form of protection has to be evaluated for the effects of inrush and

excitation current harmonic content. The evaluation must ensure the protection will be stable despitethe presence of the harmonic frequencies and magnitude and take account of the effect of the extended

tap-range normally adopted for HVDC transformers. Experience of existing HVDC transformerprotection systems is good apparently and no changes are foreseen. The report also refers to

differential protection fault clearance times. Here, studies indicate a 3-cycle fault clearance time wouldpresent a potential tank interruption risk. The report emphasizes there is the additional protection ofthe customary earth fault protection system as a backup.

A contribution from Japan referred to work by the Japanese Electric Technology Research Association(ETRA) on behalf of utilities, manufacturers and academic organisations to study fault incidentconditions, methods to improve tank strength, tank internal pressure rise and related protectionsystems. Results from these studies have been applied to the design of new transformers and to

increase existing tank strengths. Factors such fault locations, arc current and arc voltage magnitudesand durations, tank flexibility and mechanical strength were considered. The rate rise of pressure as

well as the attained peak pressure and duration needed to be evaluated and the tank intrinsic strengthdetermined. Experimental and simulation derived data has been used to increase tank strength by asmuch as 60% without the need for tank reinforcement and has provided more assured safety margins.

Question 1.7

A contribution from Spain said a major national utility, in its document MT 2.60.1, has introduceddefined safety requirements against fire risk which apply now to all new substations and for upgrading

modifications to existing ones. Both active and passive aspects are being instituted depending on thesubstation type and its location, whether rural, urban or inside or outside other buildings. Active fire


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extinction methods adopted for each of these locations includes: gas, high/medium/or low expansion

foams, and detection sensors such as, thermal/thermal kinetic, straight thermal or optical sensors.Passive protection methods include fire resistive materials for walls and doors, GIS insulated

equipments, adequate and effective safety exit precautions, better protected cables and fire resistivepaints etc.

Discussion

Several spontaneous contributions were raised from the floor on the topic of transformer incidents inservices. Opinions were expressed on the limited efficiency of pressure relief devices in someapplications. The specific problem of GSU directly connected to alternator was raised since in a caseof internal failure, opening of the HV breaker do not prevent the circulation of a heavy short circuitcurrent into the fault until the alternator voltage has decayed.

Contributors were invited to provide a written version of their comments for inclusion in the sessionproceeding.

PREFERENTIAL SUBJECT 2: TRANSFORMER LIFE

This subject has raised much interest and 12 reports were selected for publication. Discussion of thissubject was lead by the Special Reporter Peter Werle who had introduced 8 questions addressing

specific aspects of transformer life duration. 24 prepared contributions were presented and aresummarized below in sequence with the specific question laid down in the Special Report.

PS2 - Keynote presentation by Paul Jarman (UK)

Mr. Jarman reviewed the various transformer components that can determine the transformer lifeduration. Assessment of remaining life is important since regulatory and financial planning regimesdemand replacement planning on a 2-20 year timescale. Moreover, there is a need to maintain networkreliability, which means maintaining component reliability. There are many excellent conditionassessment and diagnostic techniques for transformers, but they usually predict failure only a few

months ahead. End of life is either a failure or a condition requiring replacement. All factorsinfluencing life expectancy were reviewed. Transformers are failing randomly up to age 45 but more

data is needed. Evidence from scrapped transformers indicates a very wide range of thermal lifetimes.Thermal modelling shows the main determinant of lifetime prediction is design. We still need a goodindicator for ageing. Good standards and specifications are needed.

Activities of IEC TC-14 were also reviewed as Mr. Jarman is the Chairman of TC-14.

The Special Reporter Mr. Peter Werle, formulated the following questions to structure the

contributions from delegates to the A2 Paris 2010 meeting.

Ageing diagnosis based chemical indicators and DGA analysis

Question 2-1: What is the best way to increase the efficiency of interpretation algorithms?

Question 2-2: How important is a fast implementation of the standardized procedures in case of theapplication of chemical indicators like methanol?

New methods and approaches for diagnosis, testing and fleet screening

Question 2-3: What kind of the diagnostic measurements, tests and available interpretation tools has

higher impact for the utilities and can help to improve service reliability?


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Question 2-4: Which methods of evaluation are most efficient to provide the required feedback for the

introduction of new solutions?

Experiences with on-line monitoring and off-line diagnostic methods

Question 2-5: Considering the wide range of the available condition assessment methods, what are thepossibilities to optimise the maintenance strategy with focus on their efficiency and costs?

Question 2-6: What is the best way to combine on-line and off-line diagnostic methods?

Transformer life time estimation, failure rates and replacement issues

Question 2-7: What is the most promising methodology for the estimation of the remaining life?

Question 2-8: What kind of models can be helpful to improve the accuracy in the determination of the

residual lifetime?

Question 2.1

A contribution from Canada commented that in the case of a chemical indicator to evaluate solid

insulation ageing, the important points to consider are: Laboratory validation of the sensitivity to paperaging, validation of the influence of the oil condition on the chemical indicator, validation of the long-term stability of the chemical indicator, validation of the repeatability of the test and field calibration

of the relation between level of the chemical marker and degree of polymerization of paper samples.

A contribution from Germany stressed the fact that, to this day, all practical DGA diagnoses have beenmade without considering any differences between seal type and free breathing transformers. A

method is proposed, using the rate of absorption of nitrogen to determine a "transformer opennessnumber" (TON). This would allow the oxygen consumption rate to be calculated from the oxygenconcentration.

A contribution from Japan reported on the persistent difficulty in the diagnosing of internal condition

when gas level exceeds criteria. It was found that the gas concentration pattern differs significantlywith the type and location of the faults. The gas generation pattern might also change as the internalabnormality progresses. The method of continuous recording of gas level is also supported by (UK)with regular observation of the gas signature.

Question 2.2

A contribution from Sweden recalled the difficulty of relating the furan level to a DP level andidentifies a new source of error: the introduction of Irgamet 39 passivator could actually reduce by a

factor of 10, the amount of furans dissolved in oil. The methanol method is promising but the sameproblem remains: how can we distinguish a mild degradation of bulk cellulose from a severedegradation of conductor insulation in local hot spots or local hot areas.

A contribution from Canada submitted that development of interpretation algorithms requires a

significant number of samples to compare the level of the chemical indicator and the degree ofpolymerization, the procedures should be standardized in the earliest stages.


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Question 2.3

A contribution from Sweden commented that no single diagnostic method or test can tell the complete

picture of a unit. Conditions assessment is like a puzzle, one needs several pieces to see the wholepicture. Dependent on the task, different methods will be needed: fleet screening, advanced survey ona selected group, assessment on individual transformer.

Question 2.4

A contribution from Germany comments that a lot of methods for condition assessment are available(conventional and modern) and the art is to use them in combination and having evaluated newmethods go back and review older ones. For instance recent data from on-line monitoring of DGA

suggest that the CO2/CO ratio should not be related blindly to solid insulation aging.

Question 2.5

A contribution from Germany described a tap changer monitoring system that is intended to reduce

maintenance costs. It relies on temperature and motor torque monitoring along with contact wearcalculation.

A contribution from Mexico describes a transformer monitoring system that is expected to reducemaintenance cost and improve reliability in a nuclear plant.

Question 2.6

A contribution from Australia reports on examples of application provided by Australian Cigre

members. In one case, replacement of two old 200 MVA autotransformers was postponed by 5 years

in spite of degraded HV winding insulation and HV bushings. Bushing monitoring was provided alongwith hydrogen and moisture monitoring of transformer oil. In another case, two 1125 MVAautotransformers with intermittent partial discharges were fitted with RF detectors, but the PD sourcehas not been localised yet.

A contribution from Mexico reported a case where an on-line monitoring malfunction prompted off-line investigation to localise the source of the problem.

Question 2.7

A contribution from Canada indicates that two methods are available. A statistical method as

described in the recent report from Cigre WG C1.16. This report mentions that it is critically important

to group as much as possible assets of the same type and also that care should be exercise withdefinitions of what constitutes effective end-of-life. A second method would be to use models asdescribes in IEEE and IEC loading guides.

A contribution from Spain describes the philosophy applied at Iberdrola Distribucin Elctrica todevelop an Health Index that is converted into a probability of failure and a Consequence Index related

to energy not supplied. The product of these two components gives the Risk Index that allows aranking on individual transformers. Health Index is based on 13 indicators mainly related to periodic

tests (electrical and chemical), family risk and operating conditions on transformer and mainaccessories.

A contribution from Japan reports on the method developed at TEPCO to optimise the life expectancyof transformers. Approximately 4500 transformers with 66-550 kV primary side voltages are in

service. In order to maintain a very low failure rate in spite of equipment aging, the expected lifeduration and maintenance strategy has been studied in detail. As a part of these studies, twenty units


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removed from service were examined to investigate transformers deterioration process by checking

reduction of insulation capability, polymerization degree, and fastening strength of coils. The result ofthe study shows that the remaining life of some designs of transformers, all of which suffering from

insufficient drying process and consequently containing more water in the insulating papers, lead to adecrease of the degree of polymerization. On the other hand, the expected lifetime of other units isdetermined by a decrease of the fastening strength of windings, and can be estimated to 65-75 years.

A contribution from Zambia reports on the application of the furans test to estimate the degree ofpolymerization. Although the correlation, between furans and DP is not very good, it was adequate foridentifying the units that required closer attention and trigger an end-of-life strategy that would include

the following: Increased vigilance in tracking deterioration, detection of incipient faults, mitigation ofthe impact of a failure and decision relative to renewal or refurbishment.

Question 2.8

A contribution from France reports on the EDF methodology for the development of a transformerThermal Ageing Signature. This method considers mainly design and manufacturing, qualification

tests, thermal modelling, fleet screening, chemical indicators and on site monitoring. Methanol

production has now been measured on 294 transformers. This promising young ageing tracer has to beconsidered as a new indicator to enhance the use of the traditional indicators (2FAL and CO/CO2).

Discussion

Several spontaneous contributions were raised from the floor on the topic of transformer life.

Contributors were invited to provide a written version of their comments for inclusion in the sessionproceeding.

PREFERENTIAL SUBJECT 3: TRANSFORMER MODELINGThis session was introduced by the Special Reporter; Francois Devaux (FR) who informed theaudience that A2 selected 10 papers from the 18 that were offered in answer to the questions raised in

the Special Report firstly transients which included the new situations that have arisen because oftransformer-system interactions namely inrush current and switching effects and their protectionmeasures. The selected papers contain references to the relevant new tools and techniques especiallythose for modelling transformers and include comparisons between the results obtained directly fromtransformers to those obtained by modelling.

PS3 - Keynote presentation by Stefan Tenbohlen (DE)

Mr Tenbohlen identified three active areas of transformer modelling developments: windings; 3D

electromagnetic fields and thermal-hydraulic analysis using computer finite dynamics (CFD). Thepresentation was impressively supported for example, with computer generated video-like images ofoil flows in modelled windings and winding sections with illustrations of the effects of inadequate orblocked oil channels in windings. Other techniques described included the application of finiteintegration techniques (FIT). The scope for applying all these new analytical tools was evident.Examples were given of calculated winding frequency response characteristics and internal voltagedistributions; algorithms derived to optimise winding lumped parameters; determining winding oilflow quantities, oil velocities and oil thermal distributions using comprehensive thermal-hydraulic

analysis tools. It was emphasised time and again that validation of these modelling tools is an absoluterequirement. However, these various analytical tool developments clearly have extended transformerknowledge and insight and will not only provide better designs but have the potential to provide more

accurate prediction of expected test results e.g. the thermal performances expected on temperature rise

tests.


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This Keynote Address was followed by contributed presentations in answer to the 8 questions posed in

the Special Report. No contributions were submitted in reply to Questions 3.3 and 3.6. The SpecialReporter Mr. Franois Devaux formulated the following questions to structure the contributions from

delegates to the A2 Paris 2010 meeting.

Transient modelling

Question 3.1 - What are the limits of frequency for different modelling tools, what are the most

acceptable tools to model high frequency phenomenon (from 500 kHz to several MHz) and how toinclude non-standard type of impulse including VFTO surges?

Question 3-2: What can be reported on new failure experiences with transients, in GIS or airsubstation, and on their subsequent modelling investigations?

Question 3-3: What are the experiences of HF transients using on-line measurement systems for field

applications? Are there new examples of measurements of field HF transients and their comparisonswith calculations?

Question 3-4: Based on modelling analysis data, what should be the new test requirements andprotection measures to address transient phenomena?

Thermal modelling

Question 3-5: What is the experience with applications (modelling and direct measurements) on otheroil flow paths (without washers) and/or other type of cooling (ON)?

Question 3-6: Can we get additional guidelines on how to use data obtained by direct measurement formodelling purposes and their application for operation and life management?

Question 3-7: A lot of information is available on internal winding temperature analysis. Are thereexperiences and information on global hydro-thermal modelling or on modelling other parts of thetransformer (other than the internal winding) and on their associated direct measurements?

Question 3-8: Is there any information on the oil flow distribution between windings, associated withrepresentative thermal conditions (modelling and direct measurements). Are there measurementsensors available for characteristics other than temperature measurements (oil flow, pressure) and

what is their association with direct measurements on transformer?

Question 3.1

A contribution from Japan answered to the question with presenting a simple electric model tosimulate the interleaved windings to perform an analysis easily. Its applicability was evaluated bycomparison with measurements of the winding model

A contribution from France presented different numerical approaches and their frequency applications.The Lumped parameter model is well suited for the analysis of transients in power transformers, frompower frequency to lightning or switching surge. It can account for frequency contain up to 500 kHz to1MHz or more depending on the size of the transformer: The larger the transformer or electric element

physical size, the lower the frequency. For the higher frequency contain transient over-voltages asthose switching of GIS disconnectors would generate, distributed parameters model as MTL isadvisable. Calculations in frequency domain allows to account for frequency dependency of

parameters and can also handle both standard and non standard input waves when applying Lumpedparameters or MTL models : Using FFT to find the spectrum of these waves, calculating frequencyresponse of these waves and using reverse FFT to bring back these response to time domain


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A contribution, submitted by Sweden, explained the frequency ranges of transformer models.Transformer models are often lumped, e.g. disc pair models, and cover frequencies up to 200-300 kHz.

The same type of models resolving individual turns cover frequencies that in theory range to around 1MHz. Frequencies above one MHz would require highly resolved models. Some problems:numerically heavy, frequency dependent conductor impedance, proximity effect, skin depth,

verification by experiment problematic. It was pointed out that risk for transformer decreases withfrequency.

Question 3.2

A contribution from Brazil suggested more accurate evaluation of switching transient phenomena ontransformers and power systems requires wideband frequency modelling studies. Using examples ofvoltage transient studies it was concluded guidelines are required for modelling transmission lines,

busbars, switching equipments and transformers.

A second presentation from Brazil described the circumstances in which 3 single-phase 525kV150MVA COPEL transformers failed in service between November 2007 and February 2008.Research suggested the existence of VFT overvoltages possibly having rise times of tens ofnanoseconds and amplitude of 2.5 p.u. It was claimed detecting these faults by classical monitoringtechniques is not effective. It was suggested improved transformer design and testing standards are

required.

As a result of this experience, it was said COPEL have increased the transformer voltage withstandrequirements to include for example 2 p.u. peak to peak test voltage levels having wavefronts of 10 50 nanoseconds.


Question 3.4

A contribution from France reported EdF is actively pursuing core saturation, overvoltage and inrushcurrent studies. The transformer air-core reactance characteristic is believed important in the study of

temporary overvoltages at transformer terminals when energised. Data from manufacturers supportingthis study is required to improve understanding of the phenomena involved and to ensure betteroperating conditions for transformers.

A second contribution from France concerned the harmonic temporary overvoltages that can arise

under inrush current effects. It was said overvoltages of up to 2.5 p.u can occur for between 0.1 to 10

seconds duration and that the present IEC recommended AC voltage tests do not adequately coverthese transient overvoltage (TOV) occurrences.

A contribution from Sweden reported VFT can result in voltage stresses in bushings increasing as

much as 2.4 p.u. when test circuit resonances coincide with bushing internal resonances. Theyconsider the frequency of bushing failure due to VFT in GIS installations is low. It was recommendedCigre should collect and analyse information to substantiate claims that the interaction betweenbushings and GIS circuits is an important factor.

This concluded the contributions on transients and transformers interaction considerations. Thefollowing contributions all relate to the thermal modelling aspects of transformers.


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Question 3.5

A contribution from France referred to the findings obtained from thermal studies involving an ONAN

disc winding without oil guidance internal washers. The numerical analysis of the winding thermaland hydraulic aspects was performed using computational fluids dynamics (CFD) methods and tookinto account radial oil channel height, additional pressure loss and additional heat loss up the winding.

Account was also taken of the winding and external cooling system gravity centre effect. Four caseswere studied: varying the radial oil flows by between 20% - 200% of the typical value; simulating

reduction of the radial oil duct height from 6mm to 5mm; reducing the thermal gravity centre by0.5m; confirming mass oil flow increases as temperature increases. The conclusions were that windingtemperature is directly mass oil flow dependent and local winding temperatures are influenced thenatural convection induced radial oil flow.


Question 3.7

A contribution from Finland described the results of studies and measurements of the thermalconditions outside the winding i.e. in the metal components influenced by the winding induced

magnetic field. Representative 3D-models of these external winding parts were said to yield very goodinformation However, the point was made some of the gained knowledge was not equally and readilymeasurable, despite details being shown of at least one attempt using thermocouples to validate the

model derived data with the actual thermal values occurring in these metal parts. Simpler studies using2D-models of different core and winding cross-sections were considered useful and were to berecommended.

A contribution from Korea identified four thermal areas of transformer engineering that requirefurther discussion and debate. They were: existing transformer cooling conceptions; cooling pipeworkarrangements; heat exchangers; plate fin-type designed heat exchangers. Factors such oil flow, oil

flow rate and temperature distributions need more accurate and informed assessment. Unknownunsymmetrical oil distribution in windings can be detrimental therefore informed estimates of oildistribution in windings at the design stage is essential. Winding and oil temperatures were said to begreatly influenced by the hydraulic design aspects. It was recommended oil inlets and outlets arelocated diagonally opposite. Also, oil flow to windings can be regulated by sizing oil duct dimensionsto each winding properly.

Question 3.8

A contribution from Australia confirmed temperatures in distribution transformers were affected by oilflow rates. Studies using prototype transformers confirmed previous contributor's submissions that oil

flow across the active part, end to end and bottom to top for example should a design target. It was

concluded oil flow rate is the most vital factor.

In the second of the three contributions received in answer to Question 3.8, France made the onlycontribution concerning thermal aspects of shell-type transformers. Studies of oil circulations in a

shell-type transformer were made using CFD simulations and Particle Image Velocimetry (PIV)methods. The studies confirmed comparatively similar oil flow and velocity data were obtainedbetween theoretical and measured data. The study was unusual to the extent of the care taken to modelthe porosity of the solid insulations of which there is a much greater amount in the shell-typetransformers compared to conventional core-type units. Apparently very good correlation was

obtained between the measured LV and HV winding average temperature rises and the simulationderived values.

A contribution from Japan closed this PS3 Session with a contribution to Question 3.8detailing the application of an oil flow meter designed and sized to be placed within winding


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bottom end insulation system to measure actual oil flows into the windings. The meter was

said to be capable of measuring oil flows of between a few cm/second up to 100cm/second.

The meter is plastic and the propeller diameter is 10mm. Studies using several of these

meters have been made on a 3-phase OFAF 1500 MVA 500 kV auto-transformer. Measured

results closely correlate to 3D-model derived fluid analysis.

CLOSING REMARKS

In his closing address to A2, Chairman Pierre Boss began by summarising the trends and challengesfacing A2 and the transformer industry, especially those aspects that should engage A2 generally such

as the future power networks and power efficiency but specifically:

Identification of weaknesses Aspects needing further development Better interpretation of diagnostic tools Acoustic emission and migration Better use of available monitors to improve maintenance Recommendations for tank design Studies to evaluate the impact of transient over-voltages on design, ageing of insulation etc Improved asset management Reactors Dynamic loading / improved asset Utilisation / better diagnostic skills More online monitoring / new sensors / better risk management possibly based on a

probabilistic approach / safety / HVDC developments / transient overvoltage considerations,

events frequency and new tests / improved customer specifications.

As regards power efficiency aspects, Mr. Boss emphasized the increasing aspect of carbon footprintconsiderations in the future including transformer replacement versus repair / reduction of shutdowns /improved designs / improved reliability.

Mr. Boss recollected his task when he became A2 chairman in 2004, was envisaged as easy. Such wasnot experienced owing to the succeeding reorganization of Cigre, increased A2 activities worldwide.Mr. Boss thanked his predecessors A2 Chairmen Dennis Allan, Reinhart Baehr and Philippe Guuinicand other colleagues for their assistance and valued support.

Finally he extended best wishes to C. Rajotte, the new A2 Chairman and to the SC A2 Committee. In

reply, Claude Rajotte recounted Pierres involvement in Cigre activities since 1978. He expressedgood wishes on behalf of everyone present to Pierre and presented him with a present.

Mr. Boss drew this A2 2010 Paris Session to a close by confirming the next A2 meeting is theColloquium being held in Kyoto Japan, on 11-16 September 2011. This is a joint meeting with SC D1 and the topics are:

Maintenance monitoring diagnostics New materials Transient phenomena

The session was closed at 17.56.

Technical Paper - CIGRE

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