High-voltage power-transformer insulationE. T. Norris, M.I.Mech.E., Member

Summary

The increase in transmission system voltages in recent years to 500 kV, with even higher prospective values,makes the internal insulation a large proportion of the transformer cost and therefore economically moreimportant. The associated increase in MVA rating, and hence in size and weight, has reached beyondtransport limits and demands better insulation of smaller volume.

These factors can both be helped by increasing the working stresses in the insulation. The paper considersmeans of achieving this by improvements in impregnation, more uniform dielectric fields and the eliminationof creepage, voids and local stresses due to winding connections and transpositions.

Since oil-impregnated paper is exclusively used in high-voltage power-transformer windings, the elementspaper and oil are first considered and then the combination. This depends entirely upon the purity of theelements, the completeness of the impregnation and the subsequent prevention of contamination inservice. These are also discussed.

Throughout the paper the possibilities of non-destructive testing are considered.Distinction is drawn between the orthodox ageing of insulation and the practical life of transformers

in service.Technical literature on the subject, though extensive, is notable for its discordance. The paper endeavours

to incorporate a distillation of this practical information with the author's judgment and practical experiencein the hope that discussion will lead to explicit confirmation or amendment of the conclusions reached.

1 Introduction

For many years, high-voltage power-transformerinsulation has been universally and almost exclusively oil-impregnated paper, the term 'paper' including paper, press-board and presspaper made from cotton, wood pulp, linen,jute or other cellulose material. The term 'oil' means a mineralhydrocarbon oil in accordance with B.S. 148: 1951 or similarspecifications. Other materials such as impregnated wood(usually laminated) and synthetic-resin-bonded-paper cylin-ders and boards are used in small quantities in positionswhere mechanical strength rather than electrical strength isparamount.

The paper is divided into four sections on oil, paper,oil-impregnated paper, and transformer insulation. In manyimportant respects these are four separate subjects. Dealingwith the matter in this sequence will simplify the analysis andemphasize the comparative simplicity of the basic elementsand the complexity of the applied combination. Thus, forexample, oil-impregnated paper has characteristics that wouldnot obviously be expected from consideration of oil and paper,much as, in different ways, the characteristics of salt are notobvious from the constituents sodium and chlorine.

Oil-impregnated paper is also the principal insulation inmany high-voltage cables and capacitors as well as trans-formers. Here again, in many important respects, these arethree separate subjects. Technical characteristics and analysestrue for one are misleading, or not relevant, for another.Failure to realize these distinctions has been the cause of someconfusion and misunderstanding in academic studies, andbetween transformer, cable and capacitor engineers.

When one thinks of the recent spectacular developments inplastics and synthetic insulating materials, it might be expectedthat, although oil-impregnated paper has been known and inuse for many years, it would have been superseded by one ofthese more modern insulating materials. This is indeed begin-ning to be true with cables and capacitors, at least for thelower voltages, but there is as yet no sign of it happening inhigh-voltage power transformers. Designers the world over

Paper 4088 P, first received 8th May and in revised form 2nd October1962. It is an 'integrating' paperMr. Norris is consultant to Ferranti Ltd.

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have, of course, kept abreast of modern developments inplastics, and their apparent apathy is due to the uniquecharacteristics of oil-impregnated paper in relation to thepeculiar combination of electrical, thermal and mechanicalstresses in a transformer winding.

This marriage of oil and paper has almost perfect intimacy.They are the two cheapest insulating materials available, buttheir combination is the best yet known and gives an electricstrength much higher than either separately.

Unlike plastics it has no melting-point or even, which is oneof the limiting features of plastics, no softening-point. There isthus no appreciable loss of mechanical strength at relativelylow temperatures. It will also withstand comparatively hightemperatures for very short times—an essential requirementfor resisting short-circuit stresses.

The one defect in this marriage is that both constituents areeasily contaminated. In each material, impurities of a fewparts in 106 are serious. In theory, of course, purity is possible,and in controlled laboratory conditions very high electricstrengths have been achieved.1 The practical application istherefore entirely a matter of knowing the highest purityattainable and maintainable and then choosing appropriatestresses.

From the knowledge that oil-impregnated paper has beenstudied universally for many years, it might be thought thatits characteristics and behaviour were fully known andestablished. This, however, is not so. Some vital factors havebeen realized only recently, and even today there is room formuch more experimental work and analysis.

The studies in the next two Sections of oil and paperseparately are not comprehensive or complete but deal onlywith characteristics of importance in the combination oil-impregnated paper as applied to high-voltage power trans-formers. Methods and processes of achieving this combinationare not considered.

2 Transformer oilThe oil considered here is a mineral hydrocarbon oil as

normally used in transformers. A typical specification is givenin B.S. 148: 1951. The principal characteristics can be dividedinto mechanical, physical and electrical.

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2.1 MechanicalOil, although a liquid, has two important mechanical

properties, namely changes in volume with temperature andpressure.

The former provides the essential convective or thermo-siphon cooling in naturally air-cooled transformers. The latter,or rather the absence of it, is responsible for the transference ofmechanical vibrations from the core to the tank, i.e. broadlyspeaking for transformer noise.

While neither of these characteristics is directly within thescope of the paper, both temperature and pressure haveimportant secondary effects, which are considered later(Sections 2.2.2 and 5.5).

2.2 Physical

The chemical structure of the oil is the province of theoil chemist, and numerous tests have been devised to prescribeits performance fully, e.g. acidity or neutralization number,saponification, interfacial tension, oxidation and steamemulsion. All have their particular significance to the oilchemist.

2.2.1 Acidity and oxidation

Present-day practical tests are for acidity and oxidationor sludge formation. Years ago these two tests were ofengineering importance particularly when highly refined oilsof the class A type were a standard.

Modern oils come easily within the sludge and aciditylimits of B.S. 148 and stay within these limits for an unknownnumber of years in present transformer operating conditions.Where high values of sludge and acidity have been measured,they are not inherent in the oil but are due to unpolymerizedvarnishes, compounds and paints (in particular metallicdryers) formerly used in the transformer construction.2 Verysmall amounts of these impurities produce copious andapparently unlimited quantities of sludge and sometimes highacidity. When this occurs, the oil supplier is often unfairlyblamed.

For the oil-impregnated-paper insulation considered, therewill be no such varnish or other compounds in the wind-ings, but the painting of cores, the insides of tanks andthe varnishing of taped leads must be carefully watched. Ifthis is done it is doubtful, and certainly not proven, if sludgeor acidity will ever reach a serious value in a modern trans-former operating in normal service conditions, even with theoil exposed to air. This question is considered in a laterSection.

2.2.2 Gas solution

Transformer oil will absorb the following quantities ofgas at 25° C and 760mmHg pressure: air, 10%; nitrogen,8-5%; oxygen, 16%; and carbon dioxide, 100%.

The value for carbon dioxide is included to indicate therange of gas absorptions and to show how much moretroublesome the gas problem could be.

Increasing the temperature or pressure increases the quantityof gas the oil can hold in solution. Fig. 1 shows the effect oftemperature3 and Fig. 2 the effect of pressure.4 In many casesthe gas solubility falls with increasing temperature. Trans-former oil is an exception.

For transformers with the oil surface at atmospheric pres-sure, as in the normal conservator tank construction, onlytemperature variations need be considered, and these havesmall effect. Fig. 1 shows, for example, that a 30° C change inoil temperature changes the amount of dissolved air by only0-45%.PROCEEDINGS I.E.E., Vol. 110, No. 2, FEBRUARY 1963

21 P6

40 60 80Temperature, °C

Fig. 1Nitrogen and air content of transformer oil as a function of tem-perature at 760 mm Hg pressure(a) Air(b) Nitrogen

11OO

1000

3 0 08 10 12

Air content , volume °/o14 16

Fig. 2Air content of transformer oil as a function of pressure

If a transformer, however, is sealed to some degree, pressureand temperature are interrelated by the thermal expansion ofthe oil and the sealing characteristics.5 Contemporary methodsof sealing are discussed in Section 5.5.

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If the oil temperature rises owing to increase in load orambient temperature, the oil expands and increases the pres-sure. For example, an increase from this cause of 7-51b/in2

in the air pressure would increase the air solubility from 10 to15%

The danger arises when the pressure falls. The oil then hasmore air than it can hold and becomes supersaturated. Theexcess air will eventually diffuse out of the oil. It will take sometime, maybe days or weeks, for this to happen, depending onthe ratio of oil surface exposed to the gas and the volume of theoil. If, however, the oil becomes supersaturated by 15-20% orthe pressure falls suddenly, air bubbles will form in the oil.These bubbles will also occur in parts of the oil subjectedto high electric stresses.6 It has been reported3 that the electricstrength of oil can be reduced to one-third by this means. Thisproblem is discussed in more detail in Section 5.5.

2.3 ElectricalIn one respect, the electrical characteristics are simple.

The electric strength of oil is virtually infinite, values overlOOOOkV/in having been measured. Practical values neverapproach this limit. For example, the electric-strength accept-ance test of 40 kV for a new oil in B.S. 148 corresponds to amaximum gradient of 300kV/in, or only 3% of the abovevalue.

Many studies have been made of the electric strength of oil.The results of different research workers are usually discordantin spite of earnest discussions, and perplexity ensues. Theexplanation is that it is not, in fact, the electric strength of oilthat is being measured but the effect of minute impurities onthe electric strength.43 These may vary for a given oil fromsample to sample and even from hour to hour. Since noattempt is usually made to control the nature or amountof impurity it is not surprising that wide variations occur.Indeed it would be astonishing otherwise.

The effect of impurities is variable. The most common one,water, has a definite relation in quantity to the humidity ofthe surface air4 (Fig. 3 is typical). Even so, the effect of moisturedepends entirely on the nature and the amount of otherimpurities present. Large quantities of water do not reduceappreciably the electric strength of an otherwise clean oil.Fig. 4 shows the range of values that have been measured.

Another electrical test is the power factor. This is a dimen-sionless value, conveniently measured and much favoured byresearch workers. As a measurement of power loss it is ofcourse informative and valuable. It is therefore an importantmeasure for cable and capacitor work. In transformers dielec-tric losses in the oil are negligible, and the power factor is notnecessarily significant even when high.

There are authentic cases7 of large transformers in satis-factory service over many years with an oil power factor of0-6-0-8 (power factor and not loss tangent). It is not knownwhether these oils, when new, would have passed the tests ofB.S. 148:1951, but sludge and acidity values were still low andelectric strength was high. These high values have beenencountered elsewhere and seem to be unexplained. They are,of course, quite unusual, but they do indicate that power factoras an absolute value is not necessarily significant.

There seems to be no established correlation between powerfactor and either moisture, electric strength or acidity. In somecases,8 but not in others,9 some relation is found betweenresistivity and moisture.

Contradictions of this kind abound in the technical litera-ture. Here again, it is not a matter of correct measurement orof competent laboratory technique in any particular case. Thediscrepancies arise partly because oils from different fields andrefineries vary hydrocarbonically, but ctiiefly, as in the electric

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2 8 0

2 4 0

in o O

t, oO

8 0

4 0

40 50Relative humidity of

Fig. 3Moisture content of transformer oil as a function of humidity of thecontact air at 25° Cid) Oil temperature, 25° Cib) Oil temperature, 40° C(c) Oil temperature, 60° C(d) Oil temperature, 80° C

100,

52 80[

20 30 "40 50Water in oil, parts in

Fig. 4Effect of water in transformer oil on the electric strength

strength, because of the differing nature and degree of otherminute impurities and, of course, the varying circumstances inwhich the tests are carried out. Moreover, cases where specificrelations have been found may well have been justified for theparticular circumstances and conditions. The power-factortest requires very sensitive and critical measuring techniquesand a skilled laboratory staff, and is not considered suitablefor routine measurements in the field.

It is concluded by Clark10 that in the academic sense, thepower factor test may be used, as may other tests, if we merelywish to detect oil change. To be practical, however, the testsmust be selective and reject those oil changes which are notactually harmful. This the power factor test of oil does not do.

In the absence of a selective test, it is still necessary for usto determine the specific oil property in which we are interestedby the use of a specific test. If it is the dielectric strength test

PROCEEDINGS I.E.E, Vol. 110, No. 2, FEBRUARY 1963

which is in question, it is best to run a dielectric strength test.If it is sludge, it is best to run a test for oil sludging. This is thedirect, the practical way to evaluate oil condition.In general, the power factor of the oil is no guide to the

power factor of the oil-impregnated paper.11 An oil of highpower factor can give oil-impregnated paper of very lowpower factor.12

The temperature limit seems to be in the paper rather thanin the oil.12 Clean oil, if sealed from the atmosphere, with-stands temperatures up to 140° C for long periods.

3 PaperCellulose materials employed include kraft, Manila

and rag papers, as well as cotton, jute and linen fibres asdescribed in the appropriate standard specifications.13"16

The relevant characteristics of paper are physical andthermal. The insulation does not deteriorate electrically untilthe material has lost all mechanical strength and has becomecharred and sufficiently weakened and embrittled to crumble.17

3.1 Physical characteristics

It is only necessary to consider here moisture absorp-tion. It is objectionable partly because it lowers the mechanicalstrength of the paper, accelerates ageing and, to a lesserdegree, reduces the electric strength both in the dry state andwhen oil-impregnated.

3.1.1 Absorbed moisturePaper will contain about 15% moisture when saturated

and between 7 and 9% in normal atmospheric conditions, asshown18,11 in Fig. 5. Equilibrium conditions for given tem-peratures and humidities have been established.19 Paper will

40 50Relative humidity, °/o

Fig. 5Moisture content of kraft paper as a function of the relative humidityof the contact air at room temperature

pick up its equilibrium moisture quite quickly, and it can, con-versely, easily be dried down to 0* 1 % moisture by heating inair. Vacuum is unnecessary except to save time, but the savingis so great that in practice it is usually employed, forming partof the impregnation process (Section 4.3).PROCEEDINGS I.E.E., Vol. 110, No. 2, FEBRUARY 1963

3.1.2 Moisture evolution

Water forms a large part of the thermal decompositionproduct, and this decomposition will be serious if the dryingprocess is carried too far.

It is difficult to determine when paper is dry.17

As the drying process progresses, the rate at which watervapour comes off steadily decreases but water vapour continuesto come off however long the drying is continued. Raising thetemperature increases the rate of drying, but it also increasesthe rate of decomposition. Water vapour coming from thethermal decomposition of cellulose is not distinguishable directlyfrom water vapour coming from the adsorbed state. Thus thefact that water vapour continues to come off after protracteddrying at any temperature above about 100° C does not meanthat the paper can be dried further. It is not feasible to reach acondition where no more water vapour is evolved by paper.

An increase in temperature will result in a higher rate ofmoisture evolution which will gradually decrease. If heated,further moisture is again thrown out. Pumping off more watervapour beyond this point while maintaining the paper at ahigh temperature is merely continuing the thermal decomposi-tion; it is not decreasing the content of adsorbed water.

The gases evolved in the thermal decomposition of paperare approximately in the proportions17 water: carbon dioxide:carbon monoxide of 70 : 12 : 18. The last two indicate thatthe cellulose is being pyrolized rather than dried. Laboratorymethods of distinguishing in this and other ways betweendrying and decomposition have been developed.17 They arenot applicable to large masses of inaccessible paper atdifferent temperatures and moisture conditions. In trans-formers, as will be discussed later, it is fortunately notnecessary to carry the drying process so near the danger limitas 0 • 1 % apparent moisture.

3.2 Thermal decompositionThe mechanical strength of paper is affected as an

ageing process by time and temperature. There has beenextensive study of this phenomenon over many years. Origin-ally mechanical strengths, tensile or tearing, were used20 ascriteria. These were not found to have any close relation to theessential characteristics of insulation and have been supersededby chemical methods.21 The most recent of these by Fabre22

is based on the degree of molecular polymerization termedDP, the DP number being the relative value compared withunheated paper. There seems to be little relation between theageing measured in any of these terms and the practical life ofthe insulation. This aspect is considered further in Section 5.4.

The more recent chemical rate or Arrhenius formula differslittle from the tensile-strength formula for the more usual over-load temperatures, i.e. up to say 130°C. For the highertemperatures up to 250° C reached in short-circuit conditionsthe new formula gives greatly increased lives. Thus for thesame life at 105° C and a usual half-life of 6° C, the life at 250°Cby the Arrhenius formula is 100 times the old value.

In some ways both formulae are unfortunate. They enablenumerical 'life' values to be calculated to degrees quite outsidethe exactness of the supporting data. Thus (again with the samehalf-life), the calculated 'life' is decreased 40% by an increasein temperature of only 3° C. It is impossible for an operatingengineer to estimate or determine the mean winding tempera-ture over years in service within this order of magnitude, andso such numerical calculations have little significance orrealism.

Ageing is greatly affected by moisture and oxygen, but therelations between the various parameters and the performanceof paper in service are very complex and are still the subject ofmuch study. Thermal decomposition occurs at all tempera-tures, though the rate becomes very small for temperaturesbelow, say, 100° C. This is obvious also from operating

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100

10

1

0-1

0-01

>

/

/

/

y

50 100 150 200 250Temperature , °C

300 350

Fig. 6Relation between rate of gas evolution and the temperature ofkraft paper heated in the absence of air

experience over the last 50 years. Fig. 6 (taken from Reference16) shows the evolution of gas due to the decomposition, andFig. 7 (taken from Reference 22) shows the formation of

10

200 400 600 800Degree of polymerization

1000

Fig. 7Formation of water by the thermal decomposition of kraft paper

great for various reasons and moreover can be obtained inother ways using ordinary paper.23'24 The choice therefore iseconomic and must include the increase in the cost of thepaper. If, however, the increase in temperature rating is10°C, the paper need only be used for that part of the insula-tion which operates within 10° C of the hottest-spot tempera-ture. Cost differentials are generally small, vary as the losslevels, and decrease as the kVA size increases.45

Since the modifications vary in nature it is not possible togeneralize, but some criticisms report that the tendency toacidity in the oil is increased and that, although the deteriora-tion in mechanical strength with time may be less, the initialstrength is lower.23 There are also potential difficulties withcopper migration from the conductors if working tempera-tures are increased. These and other possible 'side effects'must be sought out and resolved, so that, in due course, thepractical applications for these papers will be established.Their effect on transformer life involves other factors discussedin Section 5.4.

4 Oil-impregnated paperSince oil-impregnated paper is physically a mixture and

not chemically a compound, one would expect, prima facie,that its characteristics and performance should follow fromthe component values. There are, however, important excep-tions.

From the two previous Sections it follows that the thermallimit of this combination is determined by the paper and notby the oil. It is established12 that oil impregnation does notaffect the ageing or thermal decomposition of the paper.

The effect of the paper is to split the oil into minute gaps—for an individual sheet the thickness of the interwoven fibresand for a cable or taped conductor in a transformer thethickness of an individual sheet. One theory of electric strengthis that it represents the strength of these small oil spaces,partly because the strength of the oil is probably lower thanthat of the fibre and partly because its permittivity is lowerso that it takes a greater share of the electric stress.

The electric strength of the combination will depend on hownearly ideal conditions of impregnation and elimination ofimpurities can be achieved in practice. The nearest approach isfor capacitors. There are no mechanical stresses to be con-sidered, thermal stresses are calculable and the electrostaticfield is comparatively simple. Moreover the units can bepermanently hermetically sealed in manufacture, and process-ing can be controlled in almost ideal conditions.

Cables are a little more difficult. The electrodes are notideal, and thermal conditions are complicated by I2R loss inthe conductor. Mechanical stresses are caused by bending,laying and jointing. Consequently typical breakdown gradientsmay be 3000kV/in for capacitors and 1500kV/in for cables.Conditions in transformers are discussed in the next Section,but the corresponding stress will be well below lOOOkV/in.

water. Ageing is increased markedly by both temperature andmoisture. There is, however, some neutralization here, asincrease in temperature tends to drive off moisture. Theresultant will depend upon the balance of the effects in eachparticular case.

In recent years a number of methods of treating the paperin the processing stage have been developed to give reducednominal ageing for a given temperature.* Alternatively thetemperature for the same nominal ageing can be raised.Commercially and superficially this seems an importantadvance, but, in fact, the net benefit that can be utilized is not* Acetylation, cyanoethylation, amine and other additives are typical.

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4.1 Contamination by moistureThe physical equilibrium between water and oil has

been noted. There is a similar equilibrium between paper andwater. Typical relations are shown in Fig. 8, taken fromReference 22. In practice, paper has a greater affinity for waterthan for oil and hence will, in general, absorb water out of theoil. This explains the effectiveness of the blotter filter press.The amount of water in the paper is thus always very muchgreater than the amount in the oil, both relatively and abso-lutely. Water in the paper affects electric strength, powerfactor, ageing, losses and mechanical strength. It is thus themost serious problem.

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10 20 " 30 40 50 '" 60Water in oil, parts in 106

70 80

Fig. 8Humidity equilibria of the combination of air-oil-paper

i[a) Oil temperature, 20° C[b) Oil temperature, 30° C<c) Oil temperature, 50° C(d) Oil temperature, 90° C

4.2 Contamination by airUnless the paper is perfectly oil-impregnated there will

be voids of air or other gas. These lower the breakdown valueof the paper because their intrinsic electric strength is lowerand (since the permittivity is also less) the stress in the voidswill be much greater than in the paper. Breakdowns in thevoids do not immediately mean failure of the dielectric, andthese partial discharges are known as corona.

There has been much recent study of corona in oil-impregnated paper,26'25 and various means of detection andlocation have been suggested.27'28 Corona is assumed tocause progressive damage leading to eventual prematurefailure (this is by no means inevitable) at relatively lowstresses. The obvious conclusion is that corona should beeliminated. There would then presumably be no progressivedeterioration and working stresses could be based directlyon the puncture breakdown value of the insulation as deter-mined by standard short-time tests. This thought gave riseto the expression 'corona free' as an aim for a standardguarantee.

A recent international study of this subject by a C.I.G.R.fi.study group25 has made it clear that the matter is not nearlyso simple and clear cut. The term 'corona' covers a numberof phenomena, some harmful, some obviously not. Moreover,the expression 'corona free' is not absolutely attainable—evenin capacitors and even at working voltage. The expression cantherefore only have a relative meaning depending on thenature and sensitivity of the measuring equipment.26 Thematter is discussed in more detail in Section 5.1.2.

4.3 ImpregnationThe impregnation of dry paper with clean dry trans-

former oil is basically easy. It occurs naturally in time bysimple immersion. The time, however, is too long for practicalthicknesses and volumes, and so heat, vacuum and pressureare all employed in industrial impregnation processes. Theseare frequently combined with the preliminary treatments forachieving the basic elements of dry paper and clean dry oil.Many methods are in use but are not considered further here.

The electric strength of oil-impregnated paper is muchgreater than that of the oil and paper separately, roughly inthe ratio 1 : 10 (Section 2.3).

Moisture absorption from either air or surrounding oil isas much in ultimate amount as for dry paper, but the timePROCEEDINGS I.E.E., Vol. 110, No. 2, FEBRUARY 1963

2 3 4 5 6Moisture in paper, %

Fig. 9Effect of moisture absorption on the electric strength of oil-impreg-nated paper

element is much greater—a matter of weeks compared withhours. It is correspondingly much more difficult to extractmoisture out of oil-impregnated paper if it has been onceallowed to penetrate.

4.4 Quality testsThe principal electrical characteristics are insulation

resistance, power factor and dispersion. The last two of thesehave the advantage of being dimensionless and easilymeasured.

The measurement most in favour is power factor. Technicalliterature abounds in studies in which it is tacitly assumedthat power factor is the main criterion of a good insulation.This is perhaps plausible for capacitors and cables where thepower loss is important both as a loss and as setting athermal limit to the rating. As a means of determiningmoisture—its chief use in transformer insulation—it can bemisleading. It represents the ratio, with suitable dimensionalconstants, of power loss to capacitance. Both of these areaffected by moisture, the latter because the permittivity ofwater (about 80) increases the capacitance. In one instancemoisture increased the power loss by 27 % and the capacitanceby 31 % making the power factor 3% better, quite obscuringthe increase in loss and implying that the material wasactually better when wet. This is an extreme case butillustrates the possibility of delusion. It is sometimes possibleto correct this error by using the product of power factor andpermittivity or loss factor, giving, in effect, for a fixed gradientE (volts per mil) the power loss per unit volume P (watts percubic inch) of insulation. Thus for a uniform field at 50 c/s,

P = 0-7E2 cos <f>k x 10~4 . . . . (1)

where cos (j> is the power factor and k the permittivity.Some correlation between loss angle, dispersion and

moisture content has been found for oil-impregnated paperinsulation29 where conditions of moisture and temperatureare uniform throughout the material. It will be shown laterthat these conditions are never attained in a transformerwinding.

5 Transformer insulationThis Section covers the application of oil and oil-

impregnated paper to high-voltage power transformers. It hasbeen explained that transformer windings are more complexthan cables or capacitors. The electrodes are of more irregularshapes and the dielectric fields are far from uniform. There

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are high internal temperatures due to the losses, which arenot only variable with the load but are many times thepossible dielectric losses in the insulation. There are alsomechanical stresses introduced deliberately as part of thedesign and, to a much greater degree, in short-circuitconditions.30

5.1 Application

Transformer insulation can be divided into threecategories: conductor insulation, coil insulation and themajor insulation, the first two forming the interturn andintercoil insulation.

Until comparatively recently the conductor had to with-stand merely the voltage between turns and the voltagebetween adjacent coils with the assistance of a washer oran oil duct inserted for cooling purposes. The latter stresswas usually the maximum or controlling one. In modern disccoil windings the coils and turns are frequently interleavedor transposed to improve the surge voltage distribution andhence the impulse strength. It may well happen then thatphysically adjacent conductors are many turns apart involtage, and subjected to stresses correspondingly higher thanthe voltage per turn. This increase in voltage now means thatthe power-frequency stress between conductors may becomemore important than the surge stress.

Owing to this interleaving of turns and to transpositionsfor reducing eddy-current losses in the strands, the dielectricfield may be distorted and the maximum stresses increased.

In disc windings, spacers of pressboard15 are used toprovide radial oil ducts required for cooling. The criticalstress here is by creepage over the surface, which is fortunatelynormal to the grain of the spacer. The puncture value is thenusually greater.

The major insulation, i.e. between high- and low-voltagewindings and between these windings and earth, includeswrappings, cylinders, washers, collars, flanges and barriers.The dielectric field distortions mentioned above for con-ductor insulation apply in equal degree for major insulation.

Where the major insulation is concentrated in a singlevolume the orthodox arrangement is a series of oil spacesand solid barriers or cylinders. In such an assembly the stresson the oil is much greater than in the solid insulation byvirtue of its lower permittivity. The breakdown stress in oilis also lower; hence, as the voltage is raised, one or more ofthe oil spaces will in due course break down. This will throwlarger stress on the barriers, and they, as a general rule, willwithstand it. The function of the barriers is chiefly to breakup the oil path into smaller components, which will havediversity characteristics, and prevent the lining-up of a chainof impurities. This type of insulation is therefore inherentlystatistically reliable. It has been proved by operating experi-ence over many years.

The local failures in an oil space will start as individualspasmodic weak static discharges and will probably notregister as corona or a partial discharge. They are practicallyharmless but should not, of. course, occur at any voltagewithin the working range. As the voltage is increased theywill become a continuous discharge and could ultimatelydamage the barriers sufficiently to start a major breakdown.

Since the oil path contributes so little to the electricstrength and is in fact a handicap, the barrier type of insula-tion is wasteful in space, even though, in many cases, someof the oil spaces can also be utilized for cooling and thusserve a double purpose. A practical illustration of the extentand complication of insulation in modern high-voltage powertransformers is shown in Fig. 10.434

The ideal is obviously a completely solid insulation as in>cables or capacitors. The breakdown stresses are much higher,and consequently the insulation distances can be greatlyreduced giving a smaller and more efficient transformer.Since the distance between primary and secondary windingswill be much less (in the ratio of, say, 3 : 1) the reactancewill be automatically reduced. This is a help where lowreactance is required by the purchaser, but not, of course,otherwise. There are two major difficulties here.

The electric strength of layers of oil-impregnated paper isat its best for a stress normal to the laminations. This canbe practically achieved, for example, in a cable except at theterminations and in a capacitor except at the edges of the foils.In a transformer, however, as previously explained, the dielec-tric field is affected by winding complications, connections andcooling ducts. If the dielectric field is parallel to the lamina-tions the electric strength is greatly reduced, maybe to one-tenth of the normal value, and much of the advantage ofsolid insulation is lost.

The second difficulty is that paper readily bends only intwo dimensions. It is difficult to achieve a completely solidinsulation wall when the field becomes 3-dimensional, such asat the ends of windings, between coils or at any otherirregularity in the geometry. There is then a danger of smallvoids or oil spaces. These reintroduce the stress limitation ofoil and further reduce the advantage of solid insulation.

Nevertheless the attraction of completely solid insulation isso great that efforts have been made to overcome these two-major difficulties.33-34

The aim is to design the insulation, whether oil-impregnatedpaper, pressboard or presspaper, so that it is at all pointsnormal to the dielectric field, so that it fits tightly aroundcorners, coil edges and particularly all points of high stress,and so as to eliminate oil voids. To achieve this it will benecessary to scarf, overlap or interleave all joints in washers,channels and angles. Static shields may be necessary in placesto cover sharp edges and prevent local high gradients. Theseprinciples apply equally to disc-coil, multi-layer and inter-leaved shell windings.

All these requirements are simple in theory and must beshown in the working drawings and instructions. They mustalso be met in every detail in the course of assembly of thewinding and coil insulation—mostly in internal positionsinaccessible for either check or subsequent inspection. Thepossibilities of minute but disastrous imperfections creepingin here are obvious and justify designers in moving cautiouslyin this direction.

Cooling is more restricted with solid insulation, so that localheating due to electric losses at higher voltages may needattention.

For a uniform field the temperature rise 6 in degreesCelsius through a thickness d of insulation cooled in bothdirections will be approximately

6 = 3l-25Pd2 (2>

Combining this with eqn. 1 gives

6 = 2l-9E2kcos<f> x 10-4 . . . . (3>

Assuming a power factor of 0 • 05 and a permittivity of 3 • 5the temperature rise will be

6 = 3-8£2 x 10-4 (4>

As a typical example, the major insulation of a 330 kVtransformer with a working voltage of 33O/-\/3 = 190kVwould then have a temperature rise due to dielectric losses ofabout 14° C if in one piece with no internal cooling. This,temperature does not depend upon the voltage gradient inthe insulation.

PROCEEDINGS I.E.E., Vol. 110, No. 2, FEBRUARY 1963

5BCo

h

8

^^j^^^j^^^j^d^^^^^ji^^^^^^^i^^^x^^^^^^^^^L^^^]^^^^^^

Fig.10Cross-section of the winding of one phase of a 400 MVA 400/275 kV 3-phase transformer• • Solid insulation| | ..Spacers and oil ductsI / / I Copper conductors .

5.1.1 Design calculations

In practical engineering design the stresses that can beworked to are primarily based on electric strengths measuredin standardized conditions for the materials1'13> 15 and forvarious forms of electric field in assemblies and sub-assemblies, such as conductor sections16 and oil ducts betweenturns and coils.31

For every part of the insulation in a particular design thestresses must be determined separately in each of the follow-ing conditions:

Power frequency: normal workingpressure testsover-voltage tests

Impulse test: full-wave and chopped-wavetransferred surges

Switching surges

The most severe of these stresses will determine the amountof insulation to be used in each part. Since the electric field isgenerally far from uniform and the conditions just outlinedvary relatively in degree in different parts of the winding, it isobviously not possible to lay down any particular stressor thickness of insulation as a design standard. There are,indeed, other qualifications also to be considered in reachingthe final decision.32

5.1.2 Corona

Corona has been considered briefly in Section 4.2 foroil-impregnated-paper insulation. The subject is much morecomplex when transferred to winding insulation, where, insidea single transformer, there will be the variety of stressesjust described.

The term 'corona' is, at present, used to include also ioniza-tion and all forms of partial discharge. In a transformer thesemay occur at sharp points such as terminal nuts in the oil, inoil spaces in the major insulation, or in air or gas pocketswithin the insulation. It is thus obvious that corona covers awide variety of phenomena not all equally dangerous.

It is measured in picocoulombs, microvolts or picowatts,the most usual instruments being the N.E.M.A.-RIV meter25

for the second unit and the E.R.A. discharge detector28 forthe first. The relation between these units is not yet established.

The detection of corona within a transformer winding ismore difficult because it cannot be measured at its seat orplace of occurrence. Its effect must be transmitted to a terminaland will be more or less attenuated in the journey. Even whenso detected the problem of location remains. For large trans-formers involving high-voltage connections, it is not easy toeliminate external interference.

Detection depends entirely on the sensitivity of the methodof measurement. There seems to be no absolute zero ofcorona,26 not even in capacitors at working voltage.25

'Corona-free' is unrealistic and can be defined only as zeroreading on the particular measuring instrument employed.

Since absolute zero is not practicable it becomes essentialto determine some criterion for dangerous or harmful corona,and this criterion is, at present, unknown.

It is known from long experience in service of synthetic-resin-bonded-paper capacitor bushings that pronouncedcorona can occur at working voltage with no harmful effect.Similar experience, though not so widespread, is available foroil-impregnated-paper insulation. The author designed pos-sibly the first oil-impregnated-paper high-voltage capacitorbushings in 1927. Eight 132kV bushings of this type wererecently examined after 30 years' service. All had power-factor/voltage curves better than when originally put intoservice. Three of them had corona below working voltage.26

No bushings of this type had ever failed in service.

436

It is evident therefore that corona in oil-impregnated-paperinsulation is not necessarily damaging, and, indeed, in certainforms can be argued as desirable in relieving stress. Presentmethods of detection give no indication of the dangerousor destroying effect and are in that respect unrealistic.

Fortunately corona measurement is not necessary as aroutine test in transformers as being built and tested today.There is no clear evidence of transformers failing in servicedue to corona. This is probably because designing a trans-former to meet the present standard pressure, over-voltageand impulse tests ensures such a margin of insulation strengththat the transformer has no corona which could becomedangerous at working voltage or in normal workingconditions.25

The study of corona measurement and assessment will, how-ever, continue. It is a promising form of non-destructive test-ing. When its present limitations have been overcome and itcan be relied upon to detect and even locate dangerousstresses within the windings, it may be possible to reduce thepresent standard insulation tests. This aspect is considered inSection 5.3.

5.2 Operating conditions

It has been explained that conditions for the fullutilization of oil-impregnated paper in a transformer are muchfarther from the ideal than in a capacitor or cable. The dielec-tric field cannot be controlled so uniformly, as described in theprevious Section. Impregnation of the paper cannot be soperfect in the first place and cannot be maintained so wellin service—even in sealed units. Consequently working stressesin the insulation are lower in practice than in cables and muchlower than in capacitors.

For example, it is not present practice to use and maintaindegassed oil in a transformer. One must distinguish herebetween 'degassed' and 'de-aerated'. De-aerated is taken tomean the absence of air or gas in suspension and this can andshould be achieved. Degassed means the absence of air orgas in solution and demands, first, the initial degassing of theoil before impregnation. This is practicable. It also meanspreserving the oil from any contact with air or other gas notonly during manufacture but continuously throughout itsservice life. This would require absolute hermetic sealing of thetransformer without an air or gas cushion. The sealing mustbe sustained during service and maintenance operations. Thisis not as yet practicable in large high-voltage power trans-formers. The future possibilities are discussed in Section 5.5.

A large transformer may contain 30 tons of paper andpressboard and 150 tons of oil. It is quite possible to dry thispaper down to 0 • 1 % moisture. To achieve this as an averagefor the whole mass of the paper would, however, invite therisk of thermal decomposition in parts of it (Section 3.1.2).Fortunately indeed this degree of elimination of moisture isnot essential.

It has been explained that power factor as a measure ofpower loss is not so vital in a transformer as in a cable orcapacitor. The capacitance is of minor importance. The totaldielectric losses are quite negligible in comparison with theI2R and core losses in the transformer. Local high values maybe objectionable in causing hot spots in the insulation. Thesewill not, however, be shown in an overall power-factormeasurement. The effect of moisture on these values is not ofimportance, and consideration of moisture can be limited toelectric strength and mechanical ageing.

Measurements in service show moisture values of the orderof 1^4%.22-23'25 In general service conditions, normaloperation will keep the moisture in the oil36'23 down to 10-20parts in 106, and in an oil-impregnated-paper winding down

PROCEEDINGS I.E.E., Vol. 110, No. 2, FEBRUARY 1963

to about 2%. It is not thought practicable to expect moisturecontents less than 1% in a transformer in service, even ifsealed.23

The custom of expressing the amount of water in the oil inparts in 106 and in the insulation in per cent, combined withthe relative quantities of these materials in a power trans-former, obscures the true relations. A practical quantitativeexample is enlightening and sometimes startling.

From the values just quoted, a.typical transformer in goodcondition in service will have, say, 15 parts in 106 of water inthe oil and 2% in the insulation. Assuming again 150 tons ofoil and 30 tons of paper, the total quantity of water in theinsulation will be

30 x 2240 x 210 x 100

= 134 gal

It is not generally realized that a transformer in good work-ing condition will normally have so much water in itsinsulation.

The total quantity of water in the oil will be

150 x 2240 x 158-6 x 106 = 0-58 gal

even though there is five times as much oil as there is paper.It is nevertheless not negligible, because if increased from 15to, say, 25 parts in 106, i.e. from 0-58 to 0-97 gal, the waterin the insulation according to Fig. 8 would be increased from134 to 200 gal.

It seems unlikely, however, that, in a large transformer withvariable temperature and moisture conditions throughout itsinsulation in both space and time, the equilibria postulated inFigs. 3, 5 and 8 are ever reached.11

The reduction in electric strength in service due to moistureis small. Fig. 9 derived from a range of published measure-ments shows that the electric strength is only reduced about10% for 3% moisture.

The most dangerous effect of moisture arises where atransformer is idle. Moisture absorption is then entirelydependent on the oil maintenance. Periodic measurement ofthe electric strength of a sample of oil is not enough because,as stated in Section 2.3, this can be high in an otherwise cleanoil even with serious amounts of water in the oil. Apart from adirect measurement of moisture the most valuable field methodis a crackle test (B.S. 148,1951). This test is liable to be treatedunjustifiably with some contempt because of its cheapness andsimplicity, but should always be carried out unless resistivitymeasurements are possible.8*37

The difficulty of determining the state of the insulation in atransformer in service (Section 3.1.1) is due to the variabletemperature conditions throughout a large mass of insulation.The amount of water in transformer insulation in servicecannot be measured by insulation resistance, power factor ordispersion.11 One method in some use is to install samples ofthe paper or pressboard insulation in an accessible positioninside the tank before the impregnation process is begun.These can be abstracted and the moisture content measuredwhenever it is desired to check satisfactory treatment onarrival on site or indeed periodically in subsequent service.More recently an accessible probe to serve the same purposehas been suggested.11 If the insulation of the heating coil of thewinding temperature indicator is utilized, conditions corres-ponding to the winding hottest-spot temperature will beattained.

5.2.1 Gas absorption

The gas absorption qualities of transformer oil havebeen described in Section 2.2.2. The effect of supersaturationPROCEEDINGS I.E.E., Vol. 110, No. 2, FEBRUARY 1963

and the formation of air or gas bubbles on the electric strengthof oil and the probability of damage can readily be imaginedfrom the previous Section on corona.

Serious failures of large high-voltage transformers havebeen attributed to this bubble formation. It is a trouble thathas only arisen in recent years with the development of varioussystems of sealing the oil in the transformer against theoxygen in the atmosphere and the ingress of moisture.6-3

These systems are discussed in Section 5.5

5.3 Factor of safety

The orthodox method of ensuring reliability and longlife in service has been to consider normal working stressesand to multiply these by a factor of safety intended to cover allabnormal or emergency service conditions. Test values areestablished based on these factors of safety and becomeacceptance tests. Values of 3-5 originated in mechanicalengineering, e.g. for bridges and boilers. The standard pressureand over-voltage tests for transformers have been derived inthis way. These relatively severe tests are in some waysinefficient because, although they may serve their purpose inensuring the safety margin specified, they are not onlyexpensive in testing plant, but frequently create other stressesnot required by the test but which nevertheless have to beincluded in the design.

The more modern approach is to consider not only normalworking conditions but also all abnormal and emergencystresses and to base the tests on the worst values thus obtained.In meeting these tests, the designer must then allow somemargin to cover design and manufacturing variations.

This method becomes practicable only as these extra-ordinary conditions become realized and understood both innature and in degree. In the ideal case, no so-called factor ofsafety would then be necessary for the various acceptancetests. In practice, the test values can be reduced.

In power-transformer engineering the introduction ofimpulse tests, of categories of system earthing conditions andlightning protection have tended in this direction, and reduc-tions in the standard scale of pressure and over-voltage testsare being suggested.

This trend would be strengthened if non-destructive testssuch as corona, moisture detection (Section 5.2) and decom-position (Section 3.2) can be developed into practical routinesafeguards.

5.4 Probable lifeThe nominal ageing of insulation in accordance with

Montsinger or Arrhenius rules has been discussed in Sec-tion 3.2. By supposing an ultimate life to this ageing processvarious National Loading Guides38 or Codes of Practice39

have been standardized, and detailed tables and charts havebeen developed giving overload values and durations for awide range of loading conditions.

However, the theoretical and arbitrary life due to this'ageing' of the insulation is not synonymous with the actuallife of the transformer in service. The latter is in fact muchlonger—so much so as to be not yet known. There is needfor a statistical analysis by supply system engineers of therate of failure with age. An explanation for the absence ofsuch operation records may be that in a supply system witha typical growth in transformer capacity of 10% per annumonly roughly 2 % of the transformers will be over 40 years' old.

It is explained in Section 3.2 that the orthodox ageing isdetermined by the loss of some function of mechanicalstrength (the electric strength being little affected even forextreme ageing), and it is therefore concluded that thetransformer will then be vulnerable to short-circuit forces.

437

This vulnerability, though plausible, has not been establishedin practice. Experimental tests have shown40 that trans-formers 'aged to the point where practically no tensilestrength remained in the insulation, yet withstood standardshort-circuit tests followed by standard insulation tests'.

Moreover, even if these facts are ignored, and it is assumedthat the short-circuit strength is reduced far below its originalvalue and, indeed, below the stresses occurring in service, sothat the transformer is now vulnerable to a short-circuit, itwill still remain in operation until such a short-circuit occurs.

Short-circuits in service are rare. Those of a severityrecorded as such by the average supply authority occur fromonce a year to once in ten years or more,30 and one nationalestimate41 suggests one maximum short-circuit (i.e. a deadshort-circuit under fully asymmetrical conditions) once in atransformer's life-time.

Quantitative calculations are not possible, but taking intoaccount the progressive nature of short-circuit stresses andthe cumulative effect on the strains,30 together with a suggestedshort-circuit distribution,41 and assuming the ageing is suchthat the short-circuit strength is reduced to one-sixth of theoriginal value, it may be judged that, even in this extremecondition, the transformer still has a probable life of eightyears.

To complete the picture (though outside the scope of thepaper) the margin is much greater for distribution trans-formers because the short-circuit stresses are much lower.

It seems, therefore, that, even if the margin on the orthodoxageing calculations just explained are ignored, the estimatedlife of a transformer could be extended by say an additionaleight years for a power transformer and, say 12 years for adistribution transformer.

It is general practice, at present, to retain transformers inservice until they fail, so that this additional life is, in fact,utilized. Even that is not the end, as repair will usually bepossible and give them a further lease of life.

5.5 PreservationThe dominant weakness of oil-impregnated-paper

insulation, stressed in Section 1, of being easily contaminatedparticularly by moisture, emphasizes the need for preservationof the original purity throughout its service life if its initialstrength is to be maintained.

This problem simplifies to that of preventing moisture fromentering the oil-impregnated paper via the oil.

The orthodox method of oil preservation is by the con-servator vessel with dehydrating breather plus an oil sealbetween the breather and the atmosphere to prevent thedrying agent from attempting to dry the world. This has beenstandard practice for large high-voltage transformers.

The ideal method, preventing both water and air fromgetting into the transformer, would be to seal the transformertank hermetically with flexible metallic bellows to take careof oil expansion, as is done in oil-filled cables. If the bellowswere so efficient that pressure variations were negligible thiswould also prevent the supersaturation of the oil and con-sequent formation of air bubbles as discussed in Section 2.2.2.

This construction is not yet attainable in high-voltagetransformers owing to the relatively large quantity of oil, itshigh expansion coefficient and the practical limitation ofmetallic bellows.

The same principle is approached by using a flexibleplastic diaphragm as an expansion joint.35 Such membranesmust be both oil-tight and impermeable to gas and vapourover long periods and must, of course, withstand continuouscontact with hot oil. The practical application of this methoddepends upon the successful search for a suitable material.

If, however, a form of sealing can be attained that is free438

of these pressure variations and so perfect and reliable inservice that the use of degassed, as distinct from de-aerated(Section 5.2) oil is possible, the insulation stress level couldapproach the standard reached in cables or even capacitors(Section 4). The reduction in both size and cost of the completetransformer would then be remarkable.

For this standard to be possible, not only must the initialdegassing, processing and sealing be practically perfect but,and this is the more difficult matter, the construction must besuch that the seal is maintained continuously in service—aseal proof against moisture and air occlusions and alsoagainst overhauls and maintenance in operation. This mustalso apply to the oil circulating in radiators and heatexchangers.

At present, however, there is some doubt as to whethercomplete sealing, even if it can be achieved, is desirable. Itis shown in Section 3.2 that paper and other cellulosematerials generate water in the natural course of ageing(Fig. 7). It is also established42 that, at sufficiently highvoltage gradients, gas is evolved if moisture is present.

A construction that prevents moisture entering the trans-former will equally prevent it escaping, so that this generatedwater will be trapped, reducing the electric strength andincreasing the ageing of the paper. The actual degree of theseeffects is, however, not yet known.

Various methods of semi-sealing, often with nitrogen, toeliminate possible oxidation effects as well as moisture (see,however, Section 2.2.2) have been suggested. These have beencomprehensively reviewed in Reference 35.

It is not, however, possible to prevent temperature varia-tions in a transformer, but there does not seem to be anyneed actually to introduce moisture traps and pressurechanges and thus run the serious risk of premature ageing,supersaturation of gas in the oil and the consequent formationof gas bubbles.

The orthodox conservator-breather system has provedsatisfactory in service in all parts of the world and overmany years.43 A recent survey36 of large transformers withlong service records and various oil preservation systems,including the standard conservator-breather, showed thatelectric strength and all other oil properties were unaffectedby age up to at least 17 years. It has been shown already(Section 5.2) that the insulation itself is automatically main-tained in good condition by normal service operation. Thismay be partly because the temperature which, according toMontsinger and Fabre, ages the paper acts also to increaseits life by reducing the moisture content.

It has been deduced in Section 2.2.1 that, in modern high-voltage power transformers, neither acidity nor oxidation isserious and that moisture (and gas absorption if pressuresealing is used) is the only service danger. Moisture measure-ments in both oil and insulation are possible (Section 5.2)in laboratory conditions. In the field, electric-strength crackleand resistivity tests on the oil are practical.

Nevertheless the uncertainty and doubt at present of theseand the laboratory tests such as power factor are such that,in addition and as a last resort after the electric strength hasbeen checked, the best guide in service is to look at somepart of the transformer. If the top of the core and clampsthat are in the hottest oil and most accessible are clean orhave only a thin film, and if the oil looks clean and feelsoily and smells of oil and not of burnt feathers, the trans-former may well be assumed to be in satisfactory workingcondition so far as its insulation is concerned.

6 ConclusionsThe principal conclusions are epitomized below. They

should be interpreted in detail by reference to their contexts.PROCEEDINGS I.E.E., Vol. 110, No. 2, FEBRUARY 1963

(a) Oxidation, sludge formation, and acidity in oil arenegligible for high-voltage power transformers working innormal operating conditions (Section 2.2.1).

(b) The electric strength of oil is virtually infinite. Measuredvalues are due to impurities and are correspondingly variable(Section 2.3).

(c) The power factor of oil is not, in itself, a significantguide to oil quality unless circumscribed by particular con-ditions (Section 2.3).

(d) The thermal drying of paper merges into its decom-position due to the generation and removal of moisture. Thedistinction is, of course, vital and delimits impregnationprocesses and heavy overloading in service (Section 3.1.2).

(e) Moisture in winding insulation is normally quite appre-ciable but need not seriously affect the electric strength(Section 5.4).

(/) The practical application of high-temperature papersto increase rating is a matter of economics with very smallcost differentials (Section 3.2).

(g) 'Corona-free' insulation is not absolutely attainable.The expression merely denotes zero reading on the particularmeasuring apparatus employed (Section 4.2).

(h) It is not yet possible to segregate harmful or destructivecorona. Limiting values cannot therefore be set. Nevertheless,progressive and intensive studies are being made of coronaand corona measurement (Section 5.1.2).

(/) The power factor of oil-impregnated paper is, in itself,not a reliable guide to insulation quality. The power factorof transformer windings is still less significant (Sections 4.4and 5.2).

(j) The ideal of solid-oil-impregnated paper, as distinctfrom the usual oil-barrier insulation, depends upon the com-plete elimination of oil voids. Minute imperfections can bedisastrous (Section 5.1).

(k) The orthodox thermal ageing formulae, on whichstandard loading guides are based, are relative and thereforeonly applicable to comparative studies (Section 3.2).

(/) The actual life of transformer insulation in service isnot indicated by the orthodox ageing calculations. It is somuch longer as to be not yet known (Section 5.4).

(m) The conservator-breather-dryer system in general usefor many years all over the world has given excellent service.It has been found to have the unexpected attribute that, withit, the insulation is automatically maintained in good conditionby normal service operation (Section 5.5).

(n) Sealed or semi-sealed constructions permitting pressurevariations in the oil are undesirable (Sections 2.2.2, 5.2.1and 5.5).

(/>) The ideal of degassed oil in service will be possibleonly when permanent hermetic sealing can be attained andmaintained in normal operation. Remarkable improvementin transformer insulation will then develop (Section 5.5).

7 AcknowledgmentsAttention is particularly directed to the reports of the

C.I.G.R.fi. study group meetings given in References 23, 25and 35. These papers are necessarily written by an individualauthor. They are, however, both national and internationalin character as they summarize the conclusions and views,at the time, of most of the world's transformer experts,representing both the manufacturing and operation sides.

The Bibliography gives a short pertinent selection of theliterature on this subject. Many of the references include, intheir turn, their own bibliographies, so that most of therelevant literature on this problem is accessible.

This literature is probably so extensive because of thepractical, as distinct from the academic, complexity of thePROCEEDINGS I.E.E., Vol. 110, No. 2, FEBRUARY 1963

subject. It is certainly contradictory and discordant in vitalrespects. Some of the relations, though disputable whenapplied in general, are justified when suitably bounded byspace, time and circumstance.

The author has endeavoured to incorporate a distillation ofall this published information with his own judgment andexperience in the hope that discussion will lead to explicitconfirmation or amendment of the conclusions reached.

Acknowledgments are due to the authors mentioned in theBibliography and to Ferranti, Ltd., for permission to publishthe paper.

8 Bibliography1 HALL, H. c , and KELK, E. : 'Physical properties and impulse strength

of paper', Proceedings I.E.E., 1956, 103 A, p. 5642 NORRIS, E. T.: 'General discussion on transformer oils', / . Inst.

Petrol., 1946, 32, p. 4343 KAUFMAN, R. B., PIERCE, J. L., and UHLIG, E. R.: The effect of

transformer oil preservation methods on the dielectric strength ofoil', Trans Amer. Inst. Elect. Engrs, 1957, 76, Part III, p. 1315

4 KAUFMAN, R. B., SHIMANSKI, E. J., a n d MACFADYEN, K. W.: ' G a sand moisture equilibria in transformer oil', ibid., 1955, 74, Part I,p. 312

5 DICKSON, M. R. : The effects of dissolved gases in the design andoperation of oil-immersed transformers' (E.R.A. Report Ref.Q/T139)

6 CHADWICK, A. T., RYDER, D. H., and BRiERLEY, A. E. : 'Oil preserva-tion systems; factors affecting ionization in large transformers',Trans Amer. Inst. Elect. Engrs, 1960, 79, Part III, p. 92

7 MAURER, L., and WOYNER, T. : 'Unterschuchen an Betriebsoelenaus Wandertransformatoren fur 220 kV Elektrotechn', Elektrotech.Z., 1956, 77, p. 885

8 STANNETT, A. w.: The resistivity test for insulating oils', Elect.Times, 12th January 1956, p. 43

9 ROHLFS, A. F., and TURNER, F. j . : 'Correlation between the break-down strength of large oil gaps and oil quality gauges', TransAmer. Inst. Elect. Engrs, 1956, 75, Part III, p. 1439

10 CLARK, F. M.: 'Are- new types of transformer oil needed?', Gen.Elect. Rev., May 1948, p. 9

11 STANNETT, A. w.: The measurement of water in power trans-formers', Proceedings I.E.E., 1962, 109 A, Suppl. 3, p. 80

12 GAZZANA-PRIAROGGIA, P., PALANDRI, G. L., a n d PELAGATTI, U. A . :'Influence of ageing characteristics of oil-filled cable dielectric',ibid., 1961, 108 A, p. 467

13 'Papers for electrical purposes': B.S. 698: 195614 'Presspaper for electrical purposes': B.S. 255: 196015 'Pressboard for electrical purposes': B.S. 231: 195016 'Paper covered rectangular copper conductors for transformer

windings', B.S. 2776: 195617 MURPHY, E. J. : 'Gases evolved by the thermal decomposition of

paper', Trans Electrochem. Soc, 1943, 83, p. 16118 CLARK, F. M.: 'Chemical changes affecting the stability of cellulose

insulation', ibid., 1943, 83, p. 14319 CLARK, F. M. : 'Moisture in oil-treated transformer insulation',

Jndustr. engng Chem., 1952, p. 88720 MONTSINGER, v.: 'Loading transformers by temperature', Trans

Amer. Inst. Elect. Engrs, 1930, 49, p. 77621 DAKIN, T. w.: 'Electrical insulation deterioration treated as a

chemical rate phenomena', ibid., 1948, 67, Part I, p. 11322 FABRE, J., and PICHON, A.: 'Deteriorating processes and products

of paper in oil. Application to transformer', C.I.G.R.fi., Paris,1960, Report No. 137

23 LANGLOIS BERTHELOT, R. : 'Factors affecting the thermal performanceof oil immersed transformer windings', C.I.G.R.£., Paris, 1962,Report No. 135, 1962

24 POPOV, i.: Temperature rise and length of life of transformers',C.I.G.R.E\, Paris, 1962, Report No. 101

25 AIGNER, v.: 'Corona detection in transformers', C.I.G.R.fi., Paris,1962, Report No. 145

2 6 HARTILL, E. R., RYDER, D. H., JAMES, R. E., SMITH, L., a n d TAYLOR,F. w.: 'Some aspects of internal corona discharges in transformers',C.I.G.R.fi., Paris, 1962, Report No. 102

27 MEADOR, J. R., and DILLOW, N. E. : 'Dielectric tests on transformers-as influenced by further BIL research', Trans Amer. Inst. Elect.Engrs, 1960, 79, Part III, p. 99

28 MOLE, c : 'Design and performance of portable AC dischargedetector' (E.R.A. Report Ref. V/T115)

29 SMITH, D. c. G. : The relation between dispersion and moisturecontent in paper insulation' (E.R.A. Report Ref. V/T128)

30 NORRIS, E. T. : 'Mechanical strength of power transformers inservice', Proceedings I.E.E., 1957, 104 A, p. 289

31 STANDRING, w. c , and HUGHES, R. c.: 'Impulse breakdown charac-teristics of solid and liquid dielectrics in combination', ibid., 1962,.109 A, p. 473

32 NORRIS, E. T.: 'Design', Journal I.E.E., 1947, 94, Part 1, p. 9133 ALBRIGHT, w. D., and MOORE, H. R. : 'Inner-cooled shell-form

power transformers', Trans Amer. Inst. Elect. Engrs, 1959, 78,Part III, p. 46

439

34 HARTMANN, HANS: 'Progress in the design of transformers', 40C.I.G.R.k, Paris, 1946, Report No. 108

35 LUTZ, H.: 'Transformer oil preservation systems and associatedproblems', C.I.G.R.E., Paris, 1960, Report No. 134 41

36 DEGNAN, w. j . , and SHIMANSKI, E. J. : 'A field survey of transformeroil quality', Trans Amer. Inst. Elect. Engrs, 1956, 75, Part I, p. 575 42

37 FORREST, J. s.: 'An electrical resistance test for insulating oils',Journal I.E.E., 1948, 95, Part II, p. 337 43

38 'Guides for loading oil-immersed distribution and power trans- 44formers', American Standards Association, Appendix C.57 92

39 'Guide to loading of transformers', British Standard Code of 45Practice, C.P.I010, 1959

SUMNER, w. A., STEIN, G. M., and LOCKIE, A. M. : 'Life expectancy ofoil-immersed insulation structure', Trans Amer. Inst. Elect. Engrs,1953, 72, Part III. p. 924A.I.E.E. Committee Report: 'Thermal limits of transformers forshort-circuit conditions', ibid., I960, 79, Part III, p. 1083KRASUCKI, z.: 'Processes leading to discharges in oil-impregnatedpaper' (E.R.A. Report Ref. L/T410)NORRIS, E. T.: 'Transformer oil', / . Inst. Petrol., 1958, 44, p. 367RUSHALL, R. T. : 'Dielectric properties of oil-soaked pressboard asaffected by water', Proceedings I.E.E., 1953, 100, Part IIA, p. 81RODGERS, w. M. : 'Economic implications of higher temperaturesfor power transformers', A.I.E.E. Paper DP62-623

Centre, Sub-Centre and Section Chairmen's AddressesNorth-Eastern Centre: Chairman's Address

THE GROWTH OF POWER TRANSFORMERS

R. Bruce, M.Sc, Member

Introduction.—The development of transformers overthe years has been a gradual process of evolution, theirprinciple having remained unchanged. Great strides havebeen made, however, by more balanced designs and newideas in design, aided by improvements in materials andmanufacturing techniques, in raising the limits of voltageand rating to the present levels, with higher efficiencies andproven reliability.

An important factor in recent progress, resulting in greatlyincreased rating for a given weight of transformer, has beenthe development of cold-reduced grain-oriented core plate,which is capable of low-loss operation at higher flux densitythan the now almost obsolete hot-rolled plate.

The properties of the new material have prompted muchthought in adjustments of the conventional methods of coremanufacture, in order to utilize its characteristics in the mostbeneficial way, in particular, by the use of mitred cornerjoints and techniques for minimizing or eliminating 'through'clamping bolts.

Practical problems arising from physical non-uniformitiesof the plate, such as thickness variation and waviness, arebeing gradually reduced. In particular, we look forward toimprovements in uniformity of magnetostriction and per-meability characteristics, which will be beneficial in the furtherreduction of noise levels.

Mechanical design considerations.—With the growth oftransformer ratings and the increase in weights, it is nownecessary to give greater attention to mechanical design.The mechanical problem is divisible into two main parts:

(a) Winding design with respect to short-circuit forces.(6) Lifting and transport, associated with the design of thetank to withstand vacuum and pressure.

The calculation of axial forces has been a subject of earnestpursuit for many years—the application of digital computershas made practicable complex calculations previously notattempted—but such calculated forces can be accepted only

Abstract 4124 of Address delivered at Newcastle upon Tyne 8th October

Mr. Bruce is with C. A. Parsons and Co. Ltd.440

as a design guide and a confirmation of the degree to whichmispositioning of windings on a limb can be tolerated.

It is significant that any slight movement of a windingunder fault conditions results in an increase in force on arepeated short-circuit, thus emphasizing the necessity ofpreventing displacement by effective clamping.

It is generally accepted that the hoop stresses caused byradial forces in outer windings can be withstood by thetensile strength of the copper conductors themselves, but in alarge transformer with windings of soft copper, a severeshort-circuit may cause small permanent elongation. Thiselongation is a safeguard against a repetition of the processdue to its work-hardening effect, but if there is a possibilityof long-term annealing at operating temperatures, subse-quent faults may have some cumulative action. The use ofwork-hardened copper for windings is being considered as ameans of eliminating elongation, the compromise to besought being windability with a sufficiently high proof stress.The use of silver-bearing copper may also have some benefitin ensuring the elimination of long-term annealing.

In extremes of transport conditions, where the saving ofevery ton in weight is imperative, the use of aluminiumtransformer tanks can be attractive. Benefits in transport-weight reductions by such means must be marginal, but maynevertheless be necessary. In the overall design of core andwindings, and tank, the method of lifting should be so devisedthat materials are utilized to the best advantage and thus keptto minimum weight.

Processing.—Oil-impregnated papers and pressboards arestill regarded as the best materials for the insulation withinhigh-voltage transformers. The natural moisture-absorptionproperties of such materials is an accepted disadvantage, butonce dried and oil-impregnated, and maintained free fromsubsequent contamination, wholly satisfactory behaviour isassured.

The shrinkage of absorbent insulating materials resultingfrom the drying process is a matter which demands closeattention, and workshop techniques must be devised whichwill ensure that the shrinkage is fully taken up and the

PROCEEDINGS I.E.E., Vol. 110, No. 2, FEBRUARY 1963