IEEE Transactions on Power Delivery, Vol. PWRD-1, No. 1, January 1986

POWER TRANSFORMERS AND SHUNT REACTORS FOR ARCTIC REGIONS

W. Lampe, Senior Member IEEEASEA TransformersLudvika, Sweden

Abstract

Power transformers and shunt reactors in arcticregions require special attention regarding:

- Cold start- Flow of coolant- Lubrication- Embrittlement- Shrinkage- Iceload on bushings

An analysis carried out reveals that a coldstart - the apparatus is then at ambient temperature -represents an onerous condition, since no majorcooling oil flow can take place and dangerous localhot spots may occur.

While these cold starts for power transformersfrom extremely low temperatures (. -300C) can usuallybe avoided, they might be necessary for the stabilityof the system in the case of shunt reactors.

The adequacy of the design of a modern55 MVAR 735/f3 kV shunt reactor has therefore beendemonstrated in a cold start from -50 C with allessential temperatures monitored,

Once certain necessary precautions have been taken,the service experience from cold regions confirms thatsatisfactory performance of power transformers andshunt reactors can be achieved.

GENERAL

The cold regions of the Northern hemisphereoffer a considerable number of attractive riversfor the generation of electrical power. Reliable trans-mission of this hydro power - which represents the formof electrical energy with the least environmentalimpact - to the load centers, frequently over aconsiderable distance, determines in the long run thepay back of the heavy investments often needed.

In this context the availability of the substa-tion equipment and in particular that of powertransformers and shunt reactors play an important role.

85 SM 374-4 A paper recommended and approvedby the IEEE Transformers Committee of the IEEEPower Engineering Society for presentation at theIEEE/PES 1935 Summer Meeting, Vancouver, B.C.,Canada, July 14 - 19, 1985. Manuscript submittedJanuary 28, 1985; made available for printingApril 22, 1985.

When substation equipment is exposed to a coldclimate, the temperature dependence of the followingtwo main factors should be considered:

1. Dielectric strength2. Mechanical strength (movement of parts,

handling, shortcircuit)

Or, in greater detail:

- Viscosity of coolant- Viscosity of lubricants- Dielectric strength of insulation (air,

liquid, solid)- Embrittlement, strength, ductility, durability

of metals- Iceload on insulators- Temperature shrinkage, loosening- Tightness of gaskets- Embrittlement of plastics, rubber- Disintegration of tin solders (c -65°C)- Loss of power of storage batteries, electro-

lytic capacitors, dry batteries

Deriving adequate design temperatures requiresmeteorological observations at the site over a numberof years. Since a cooling down process of the powertransformers and shunt reactors with considerablethermal time constants is often involved, hot only theextreme low temperatures but also mean values overlonger periods of time should be considered.

Sufficiently accurate temperature data are usuallyalready available, at least for Canada and Scandinavia,the areas covered by the experience of the author.The mean values of the lowest temperatures in Swedenover a period of 30 years (1931-1960) reveal that-35 C will not generally be exceeded although

Fig. 1 Mean values of the lowest measured temperaturesin Sweden for the years 1931-1960

0885-8977/86/0001-0217$01.00O©1985 IEEE

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the lowest temperature recorded in Sweden was -53,3 Cat Laxbacken 1948-01-13. Similar temperature conditions

were also obtained in Finland and Norway. In the latter

country the lowest measured temperature recorded was

-51,4°C in 1886.

In Canada the mean values of the daily minimumtemperatures for January - the coldest month showthat temperatures of -20 to -40°C will prevailover a considerable period of time, for example in theJames Bay area (Fig. 2). The corresponding mean valuefor the highest daily temperatures in the same area

are about 15°C higher (-5 to -150C).

Transformer oil

The oil property that changes most dramaticallywith temperature is its flow resistance - itsviscosity (Fig. 3). Standard mineral transformeroil (IEC class 2 corresponding to ASTM D41040)increases its flow resistance by nearly 10 whenthe temperature decreases from normal workingtemperature (about 70 C) to -500C. In Sweden a

transformer oil with lower viscosity has beenstandardized, where the viscosity shall not exceed800 cSt at -300C. Similarly, an "arctic" oilwith:'1000 cSt at -500C is mentioned by Vasilchenko /I/.

cSt

lo,5104

a3';0I

I .... I I :".

-50 0 50 100 °C 150t

Fig. 2 Mean value of the daily minimum temperaturefor January in Canada(National Atlas of Canada, 1971)

While design temperatures of -350C seem to besufficient for Scandinavia, Canadian utilitiesusually specify -50 C ambient temperature, at whichthe equipment should be capable of starting orcontinueing service.

The following chapters will discuss the necessaryprecautions for power transformers and shunt reactors,the available tests and service experience at lowambient temperatures.

THEORETICAL CONSIDERATIONS

The active part of conventional power transfor-mers and shunt reactors consists of three main compo-nents:

1. Magnetic steel to carry the magnetic flux2. Copper conductor to carry the electric

current3. Insulation material to withstand the dielect-

ric stress

For a 10 C temperature drop the losses in themagnetic circuit increase by approximately 0.8%and the copper losses decrease by 4%, whilethe mechanical properties of both components remainmore or less unchanged. Thus, lower temperaturesscarcely influence the first two components. However,transformer oil which together with cellulose repre-sents the insulation and also takes care of the cool-ing of the transformer, depends strongly on temperature.

Fig. 3 Viscosity of different insulating liquidsversus temperature

I Transformer oil according to IECII "III "-

class 1class 2class 3

S Transformer oil according to Swedish standardB Silicon oil Baysilone M50EL

The slope of the viscosity as a function oftemperature follows approximately the same law forall mineral oils consisting of different paraffinic,naphtenic and aromatic hydrocarbon components. Lowviscosity, achievable with a blend of lighter hydro-carbons, is usually accompanied by a low flame point,which thus limits the lowest allowable viscosity inpractice.

The viscosity of silicon oils, on the other hand,increases much less than that of mineral oil at lowtemperatures. From normal operating temperatures downto -50°C their viscosity becomes only 20 times higher.Nevertheless, silicon oils have not been used to anyextent in cold regions because of two drawbacks:

1. Nearly 10 times higher viscosity at normaloperating temperature compared with mineraloil and consequently requiring a more spaciouswinding design and increased cooling capacity.

2. Silicon oil considerably more expensivethan mineral oil.

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To secure sufficient cooling especially duringa cold start seems therefore to be the most importanttask.

Another equally important question is how lowtemperatures influence the dielectric withstandstrength of the insulation system. First, the tempera-ture influence itself on the breakdown mechanism ofoil must be discussed. In all technical designs oilbreakdown values doubtless refer more or less to theimpurities found in the oil. In this context water andfibers play a decisive role. In oils with very lowwater and fiber content the dielectric strength remainsabout the same over the whole range of temperaturesunder consideration here. Theoretically, a slightincrease in dielectric strength with decreased tempera-ture can be expected because of the lower mobility ofthe impurities and charge carriers. Fibers are prevent-ed from entering the stressed area /2/. Anotherconsequence of the high viscosity at low temperaturesis that only the first breakdown in an oil gap can becounted. The gel-like oil "remembers" the breakdowntrace and consequently already the second breakdownwill occur at a lower voltage level.

In practice the dielectric strength of the oilwhich loses its solubility for water with decreasingtemperature, drops significantly at the temperaturewhere the oil becomes saturated with water andfree water falls out.

Secondly, and probably of greater importanceis the effectiveness of the cooling itself on thedielectric strength. If hot spots arise due to the non-existing oil flow the oil temperature in their neigh-bourhood can become so high that it creates free gases.The dielectric strength will then decrease drastically.If above all, the water content in the cellulose ishigh (a few percent per weight may occur in oldertransformers), water vapor will be released attemperatures far below the boiling point ofthe most volatile oil components. The evolutionof gas bubbles from cellulose with 3% water contenthas been observed down to 1500C. In a dry transformerinsulation (0.5% moisture content) such gas developmentshould not be found up to 2000C.

In old transformers free water is sometimesfound on the bottom of the tank. This water cantheoretically freeze into ice needles, which dependenton the density 'specific gravity)of the oil can flow todielectrically stressed areas and cause breakdownsthere (Fig. 4). This danger is corpletelg eliminatedif oil with a density : .87 ton/m at 20 C is chosen.IEC 296 /12/ recommends a density of .895 ton/mias an upper limit and often oil densities i.87 ton/m3are obtained without special measures.

-50 0 SO OC 100T

Fig. 4 Oil and water density versus temperatureTransformer oil acc. to IEC Publ. 296 (limit)Ice-needle-safe oilWater

TESTS

To secure the cooling function remains the mostcritical point during the cold start of large powertransformers and shunt reactors. Our efforts havetherefore been concentrated on deepening the under-standing of the physics of oil flow dependent on thegeometry and material chosen and on verifying in afull-scale test a design based on the investigationsin an oil-flow model.

Oil-flow model

A flexible model (Fig. 5), which represents onespacer pitch of a winding, was built in order to studythe influence of all design parameters on the tempera-ture rises in windings /9/. The oil flow throughthe common spacer ring, the collar system and alongthe winding conductors was made visible and thusdetailed knowledge of oil viscosities, flow pattern andmass flow was gained.

The temperature of the model could be loweredto -25°C to permit simulation of cold starts. Highviscosity oils were also used to simulate extremecold conditions.

All essential temperatures in the windingsand the oil were automatically scanned by a micro-processor, which allowed to run the model also duringnight without personnel. Up to 120 temperature sensorswere used in the different winding arrangementsunder investigation. Thanks to the high degree ofautomation, the model allowed to check easily thereproducability of the measurements and to comparein great detail measured and calculated results.A typical reproducibility defined as the differencein thermocouple readings between two experimentswith the same set of parameters, is 0.5 K with astandard deviation of 0.7 K.

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Five years ago, it was therefore decided to per-form a full scale test on a 55 MVAr shunt reactor735/i3 kV for James Bay built according to the findingsfrom the oil-flow model.

The experiments were carried out in IREO's coldchamber. The dimensions of this probably largestavailable cold chamber (8.2 x 4.9 x 6.6 m) and theloadability of the floor did not allow the completereactor to enter. The original windings with thecomplete insulation system (all barriers included)were therefore mounted on a steel cylinder insteadof the heavy core and put into a smaller tank. Thenumber of radiators was reduced from the original8 to 4.

Fig. 5 Oil-flow model

1. Heat exchanger for low bottom-oiltemperature

2. Cooler3. "Tank" for transient experiments4. "Tank" with thermocouples5. Winding6. Automatic data acquisition system

A considerable number of different combina-tions of oil-flow conditions, bottom-oil temperatures,spacer height, number of discs, number of oil-guidingbarriers, collar systems and vertical duct widthshas been investigated.

In particular, the great importance during a coldstart of the design of the barrier system at thelower end of the winding and the flow restrictionsat places where the oil-flow has to change directionwere confirmed. The cold start of distribution trans-formers with their simpler design and insulationsystem does not therefore usually represent anydifficulty /14/ (provided that a certain oil level iskept) as has also been demonstrated in full-scale Fig. 6 Shunt reactor, 55 MVAr 735/' kV, ONAN,experiments. in IREQ's cold chamber

Since the results from this oil-flow modelare in good agreement with the full-scale test tobe described later, detailed results have been omitted.

Full-scale cold start tests

In connection with the heat-run tests severalpower transformers were placed outdoors in orderto study their cold starts. The results from theseexperiments are not conclusive because of the following:

1. Temperatures below -12°C were not obtained.

2. Temperature gradients within the transformersstill existed when the cold start wasinitiated.

The temperature and oil-flow condition differmarkedly between service at cold temperatures andstart at cold temperatures after the whole transformerhas reached ambient temperature. A cold startdoubtless represents the more onerous condition andshould therefore be avoided. While this is to someextent possible with power transformers - theycan stay in service during the extreme cold periods(they are then usually also needed) - shunt reactorshave often to be switched in order to guarantee m J

the stability of the network.

Fig. 7 Full-scale reactor winding 55 MVAr withthermocouples

Totally, 60 thermocouples were used for themeasurements (Fig. 7). 26 thermocouples measuredthe conductor temperatures in the neighbourhoodof the upper and lower yoke. 12 thermocouples monitoredthe temperature distribution in the oil betweenthe winding and the tank. Oil temperatures in the commonspacer rings and at the inlet and outlet of theradiators were also monitored. At critical places twothermocouples were installed because of the risk forbreakage during handling.

Fig. 8 explains the principal paths of thethe oilflow. A naphtenic oil (Voltesso 35)was used, thus avoiding wax precipitation.

Fig. 8 Oilflow in the shunt reactor

1. Tank2. Winding3. Core4. Radiator

Main oilflow through radiator- - - Oil flow in tank

After 12 h the air temperature in the coldchamber reached -50°C, closely followed by the bottomoil temperature in the tank (Fig.9). The top oiland the top discs of the winding dropped theirtemperature much more slow and finally reached -45 Cafter 6.5 days of cooling. At the beginning the toppart of the reactor cools down with a time constant of36 h which increases for the last part to more than48 h.

Fig. 9 Cooling down of the shunt reactor

o Ambient+ Bottom oilx Top oilv Winding (top disc)

With an assumed time constant of two days thetemperature differences between the upper part ofthe reactor or transformer and the ambient can becalculated (Fig. 10). After one day of cooling onlyhalf the initial temperature difference exists. Afterfour days the temperature difference between thetransformer and the ambient has decreased to negligiblevalues. In addition, the important temperature at thelower spacer ring will follow the ambient temperaturemuch faster as has been demonstrated by the test.Heat remaining in the shunt reactor or transformerfrom intermittent service will only help the coldstart conditions, if the de-energized period is lessthan half a day.

6 t

Fig. 10 Temperature over ambient (,LT) versustime for different starting temperatures( oT0=Tstart -Tamb) assuming a time

constant of 2d.

The small difference between ambient temperatureand bottom-oil temperature which exists down to - 52 Cmust be interpreted as an oil flow from the radia-tors. When the ambient temperature reaches -54°Cafter 2.5 d, the flow through the radiators stopsand therefore the temperature difference betweenbottom oil at the winding entrance and the ambientincreases (Fig. 9).

Later on the variations in ambient temperature(after 4.5 d) are scarcely reflected by the bottomoil temperature since no oil movement takes place.The bottom-oil temperature also after the switching-on of the load (after 6.5 d) remains below the ambienttemperature and it does not exceed the ambienttemperature until this reaches -150C (Fig. 13).This confirms the expected rather restrictedoil-flow through the radiators during the cold start.

The cooling curves for the oil between the windingand the tank (Fig. 11) reveal an oil flow throughthe radiators and/or down the tank wall at least forthe first 16 h, although the temperature at theentrance of the winding then comes already close to-50 C. The lowest measured points in Fig. 11 stemfrom the thermocouples at the entrance to the commonlower spacer ring of the winding.

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T

Fig. 11 Oil temperatures in the tank during thecooling period after different coolingtimes t

After the cooling period correct windinglosses were injected in the winding by a directcurrent equal to 105% of the rated current. Theaddition takes care of the eddy losses in windingduring AC excitation. These eddy losses are

also in reality distributed quite uniformly alongthe winding height. The DC excitation duringthe test was therefore realistic as to thedistribution of these losses. The current was

kept constant during the 24 h period of heating.Of course, the resistivity of the windingincreased by nearly 40% during this time, sinceits temperature changed from -50°C toabout +50 C.

The corresponding oil temperatures in theoil space between winding and tank during the heatingperiod (Fig. 12) also confirm the slow temperaturerise at the winding entrance. After 8 h of heating-40°C still prevails there.

t 0 1 2 3 4 6 9 12 17 24).. .. .. . .i . ..o

)3

*50 -25 25 S0 *C75

Fig. 12 Oil temperatures in the tank during theheating period at different timesreckoned from the cold start.

During the first 1.5 h after the cold startthe temperature rises in the winding (Fig. 13) exactlyfollow the values which can be calculated from thethermal capacity of the conductor material, thecellulose insulation and the surrounding oil. Forthe tested shunt reactor winding this rise is 330C/h.Compared with a normal design of power transformers,this value is rather low. Values of 100-200°C/h shouldbe applicable there.

After 2 h of loading local oil flows start andresult in a marked decrease in winding temperatures(Fig. 13). Smaller oscillations of the temperaturesindicate that more and more local oil flows takeplace. The hourly rise of winding temperatures decreasesafter the start of the local oil flow to about 6°Cand after approximately 11 h the steady-state oilflow in the reactor and the radiators are establishedand the winding temperatures increase at the same

rate as the ambient temperature.

8 12 16 20 t h 24

Fig. 13 Cold start of a 55 MVAr reactor winding fromabout -50 C.

x

0

Av

Top disc14th disc from topOil at upper spacer ringTop oil tankBottom oilAmbient

The rise in ambient temperature takes placebecause the cooling capacity of the chamber is lessthan half of the energy dissipated by the reactor duringthis test. A negligible change in ambient temperaturespredominates during the vital first part of the

h, cold start, where all energy is absorbed in the winding.

The measured winding temperatures in the differentdiscs along the winding clearly illustrate (Fig. 14):

- Negligible oil flow during the first 1.5 h- Erratic oil flow after 3 h- Relatively cold lower part of the winding

still after 13 h- Normal winding temperatures established

after 24 h

The full-scale test proved that the chosen discwinding design which is representative for ASEA'sreactor and transformers for EHV systems can copewith cold starts. The highest measured hot-spottemperature before the internal oil flow startedwas only 220C.

PRACTICAL EXPERIENCE AND PRECAUTIONS

All utilities operating in areas subject to coldclimates are certainly aware of the possible problemsthat may arise in power transformers and shunt reactors.One of the major points in their specificationsis therefore the choics of the oil. Viscosity valuesbelow 6000 cSt at -40 1 are usually required /3/and pour point of X -45 C /5/.

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The composition of the oil also plays an import-ant role. Especially, the content of linear paraffinsis critical. Pour-point depressants added to lowerthe cloud point can become ineffective at low tempera-tures /1/. It seems therefore advisable to stay withthe proven naphtenic oils in cold climate regions. Thecrystallization of paraffins and the possibleoverheating have already been investigated 1922 /6/.

Theoretically, the flow properties can changedramatically when the pour-point is reached sincea two-phase system must then be taken into considera-tion. This discontinuity has not been found experiment-ally /3/. One utility changed old oil to the new, lowviscosity oil in shunt reactors in cold areas. Oneutility requires provisions for the insertion ofa heating device in the cooling circuit at site.

to as .5 3 7 13

-40 0 40 IC 80T

Fig. 14 Disc temperatures at different timesafter cold start.

Several utilities lower the risk for transfor-mers in cold areas by not allowing OD cooling. O0 standsfor directed oil flow and means a pump forced oil flowthrough the windings. Under cold conditions thenecessary oil flow would require too high a pressure.

In addition the thermal time constant of the wholetransformer is then only of the order of 10 minutes.

In OF transformers (oil forced by pump throughthe coolers but natural oil flow in the windings) a

by-pass for the coolers is sometimes required /10/.To block the fans at low ambient temperatures forONAF-transformers (natural oil flow with fans blowingon radiators) has been considered.

Designs with remote cooler banks are not to berecommended, since the connecting tubes represent

an extremely high flow resistance at low temperatures.

The operations of the Buchholz relay may

become unreliable for two reasons. First, gases

generated in the transformer cannot reach the relaybecause of the high viscosity of the oil in theconnecting tube. Secondly, Buchholz relays withthe widely used mercurx contacts will not work, sincemercury freezes at -36 C. ASEA uses thereforesince 1976 magnetically activated metalliccontacts. In connection with cold starts oil surges

in the Buchholz relay can be theoretically expected,because a small overpressure might build up in

the tank until the oil flow to the conservatorstarts /11/. To minimize the cooling down ofthe pipe from the transformer to the conserva-

tor a central location above the cover of the transfor-mer has sometimes been discussed. However, this isnot always possible because of the necessary airclearances.

The operating conditions for tap-changers becomemore difficult with increasing cold. Increased vis-cosity of the oil in the disconnector compartmentincreases the switching times of the diverter switchand thus prolongs the burning time of the contacts.These longer switching times have been investigatedin the cold chamber of ASEA's laboratory. To avoidthese difficulties, tap-changer operations at oiltemperatures below -25OC are therefore often blocked,but tapchangers with guaranteed performance at-400C and lower are available.

Theoretically, a higher degree of embrittlementshould also be expected for the paper covering of thewinding conductors as well as diminished clampingforces for the windings. This could then lower the short-circuit strength of the cold transformers.

For safe shipment and handling of transformersand shunt reactors materials in the tanks and especiallyfor the lifting lugs which maintain their mechanicalstrength down to -300C are required.

Control cabinets are usually heated and theirfunctioning in a cold environment is not questioned.Since snow penetrates more easily than drifting sand,cold climate operation requires tight components.

In spite of all the possibilities mentionedabove, fortunately none of the five utilities wehave consulted reported any serious trouble whichcould be attributed to low temperatures. One of theutilities recorded up to 1980 the temperatures of theshunt reactors and switched in preheating,when the temperature fell below -300C. This precautionhas been omitted during the last four years and nodifficulties have arisen.

Utilities do not usually see0any need to startup from temperatures below -30 C for power transformers.It is nearly always possible to have them connectedto the network before the extreme cold period comes.Cold starts with reduced load can be used for powertransformers.

The only trouble reported refers to the coldstart of pumps. If the viscosity of the oil is toohigh, a dangerous overcurrent can be drawn by thepump motor. Therefore, one utility leaves the pumprunning during the winter. Another utility startsthe pumps before it connects the transformer to thenetwork. Also preheating of pumps has beendiscussed.

In polluted areas snow may cause flashoversat the air-side insulation of bushings only whenit melts.

CONCLUSIONS

The practical service experience of powertransformers and shunt reactors in cold regionsconfirms that with the designs available today andwith certain precautions being taken a satisfactoryreliability exists.

A cold start from -50 C in the laboratory ofa 55 MVAr, 735/t3 kV shunt reactor of modern designwith a detailed analysis of the temperatures andthe oil flow demonstrated its capabilities.

ph

The following points should be considered foroperation in arctic regions:

1. Avoid cold starts ( -30°C)Switch on power transformers before theextreme cold comes

Start with reduced load

2. Leave shunt reactors which must be switchedon and off on the system as long as possible

3. Keep to approved naphtenic oils with lowviscosity

4. Choose a suitable kind of coolingMinimize pipe lengths

5. Avoid tap-changer operations below -25 C or

use tap changers with guaranteedperformance at lower temperatures

ACKNOWLEDGEMENT

The author wants to thank his colleagues inthe ASEA Transformer Design Department and in theCanadian and Norwegian subsidiaries for their valuablecontributions and discussions.

My thanks are also addressed to the IREQ person-

nel who carried out the cold start of the shunt reactorin a competent way. I am particularly grateful tomy colleague T. Carlsson, who witnessed the testsand who was responsible for the test program.

References

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