Transformer Oil Guide

TRANSFORMER OIL HANDBOOK

www.nynas.com/naphthenics

TRANSFORMER OIL HANDBOOK

CONTENTS

INTRODUCTION

1 HANDLING AND STORAGE1.1 General1.2 Storage in tanks1.3 Choice of vessels1.3.1 Transport in ships1.3.2 Transport in road tankers, railcars and containers1.3.3 Transport in “Flexi-bags” (rubber bags)1.3.4 Transport in drums1.4 Contaminants – and possible corrective measures1.4.1 Water 1.4.2 Particles 1.4.3 Chemical contamination1.4.4 Base oils/solvents

2 MAINTENANCE OF TRANSFORMER OILS IN SERVICE

2.1 General2.2 Sampling2.3 What to analyse and why2.3.1 Colour and appearance2.3.2 AC-breakdown voltage2.3.3 Water content2.3.4 Neutralization value2.3.5 Dielectric dissipation factor and/or DC-resistivity2.3.6 Interfacial tension2.4 Filling a transformer with new oil2.5 Frequency of oil testing2.6 Other analyses to evaluate transformer status2.6.1 Gas-in-oil analysis and furfuraldehyde content2.6.2 Polychlorinated Biphenyls (PCB )

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3 REQUIREMENTS OF OILS IN SERVICE

3.1 General3.2 Viscosity3.3 Viscosity and flash point versus boiling range3.4 Low temperature properties3.5 Flash point3.6 Density3.7 Water content3.8 Particles3.9 Electrical breakdown (AC)3.10 Dielectric dissipation factor (tan delta/power factor)3.11 Interfacial tension3.12 Neutralization value3.13 Corrosion3.14 Oxidation stability3.15 Gassing tendency3.16 Impulse breakdown3.17 Influence of PAC on gassing properties and

impulse breakdown3.18 Streaming charging3.19 Health and safety

Appendix 1 Basic chemistry of transformer oils

Appendix 2 Refining techniques

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INTRODUCTION

WHY MINERAL OILUsing mineral oil as a coolant and insulating medium in transformers isnot new: indeed it has been in use for over a century. Today, mineral oilis still being used as an insulating medium because not only does it offerthe best compromise between cost and performance, but compatibilitywith other transformer materials is also very good. Other far moreexpensive, fluids such as silicon oils, certain types of esters etc., aretherefore reserved for applications where their specific characteristicscan justify the higher price. They will not be further dealt with in thishandbook.

THE IMPORTANCE OF QUALITYIn their basic chemical structure, the types of mineral oils used today arenot very different from the oils that were used 100 years ago. However,the quality of the oils has been greatly improved due to advances inrefining techniques and because there is a better understanding of whatis required in transformer applications.

But why use better quality oils? To begin answering this question,compare the relative amount of oil used today with the amount used inthe past for a certain installed power. The need for better qualitybecomes obvious.

Year Litres of oil / kVA1930 ca 3.51960 1.01980 0.25

Higher thermal load of the transformer oil requires better oxidationstability. The decision to use better quality transformer oils is also justi-fied by the costs and reliability influence of a transformer failure regard-less if it is caused by bad insulation from sludge formed or deterioratedpaper insulation.

The expected lifetime of a transformer today can be up to 40 years andthe same expectation goes also for a high quality transormer oil.

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It is interesting to note that if low quality oil is chosen, the performanceof the oil will not become apparent for some time – approximately 10years – by which time the transformer’s warranty period will haveexpired (the majority of transformer failures in the early years are dueto mistakes made in manufacturing, transportation and installation). Itis easy to compare the costs incurred by shorter transformer lifetimeand/or shutdown against the relatively low extra cost of high quality oil.Because the lifetime of the paper depends on the stability of the oil(when the oil deteriorates, degradation of the cellulose fibresaccelerates) it is obvious that it is preferable to use a good quality oilfrom the beginning, rather than wait for deterioration of the paper,damage which is irreversible.

A study by US inspection and insurance companies found that:

* 10% of all power transformer failures were due to deterioration of in-sulating material.

* Internal failure overloading in high voltage windings was promoted bydeposits of material (sludge).

For these and other reasons it is important to understand the influenceof different oil parameters when selecting oils for electrical equipment,especially transformers.

The main difference between transformer oil production, past andpresent, is that the industry has come to specialize in core competenceareas: companies that produce transformer oils are specialists, unlikefuel producers. Today, by working with crude oil supplies of consistentquality and consistent production conditions it is possible to produceoils of very high quality.

1. HANDLING AND STORAGE

1.1 GENERALWhen a transformer oil has been properly produced to fulfil specifiedrequirements, the next decisive step is to store and deliver the product,without affecting its properties.

A number of properties crucial to the performance of the oil can beinfluenced during storage and handling. A high level of expertise isrequired to maintain the quality of transformer oils, because they are soeasily contaminated.

Below, the risks that are to be avoided and the precautions that are to betaken in connection with the handling process will be discussed.

Some oils, especially used oils, may be harmful and therefore should behandled with care and necessary protection. For further informationplease see section 3.19 Health and Safety.

The aspects of handling and storage of transformer oils are given tosafeguard the properties of transformer oils. The requirements areexplained in detail in chapter three.

CompatibilityAs a rule of thumb, new transformer oils conforming to one and thesame specification are miscible with each other in all proportions.However, the characteristics of the blend should be tested if there areany doubts. The most important parameters to check are:• Interfacial tension• Dielectric dissipation factor, tan delta (power factor)• Oxidation stability

The blend must of course meet the specification.

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There are materials which can influence the oil properties and/or causeproblems in handling, and this should be considered in advance. Someoils might have higher solubility which can influence the rubber ingaskets, for example. This in turn may influence oil properties such asinterfacial tension, dielectric dissipation factor, colour, and it may evenlead to leakage. It is therefore important that the rubber is oil-resistant.Paints used in tanks, rubber bags and other liner materials should betested for compatibility.

INSTRUCTIONS, ROUTINES AND QUALITY ASSURANCE

All handling actions and routines for Nynas Naphthenics aredocumented in instructions for the guidance of our personnel and sub-contractors at the depots, and, later on, with our customers.

The instructions are part of our quality system, and their development,distribution, implementation and effectiveness meet the requirements ofISO 9001, to which Nynas has been officially certified since 1991. For thehandling of our oils, including transformer oils, the main instructions fordepots are very comprehensive and include all the steps necessary todeliver products to our customers.

Independent inspectors, who have the knowledge and authority tosupervise and safeguard the deliveries, are used for all shipments andmany road/rail transport operations.

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1.2 STORAGE IN TANKS

Tank requirementsStorage tanks used for transformer oils are no different from tanks usedfor other oils or chemicals.

The material is normally mild steel or stainless steel. The mild steeltanks can be coated with oil-resistant paints. Compatibility with thetransformer oil has to be checked or recommended by transformer oroil producers.

The water content of a transformer oil greatly affects its insulatingproperties. In storage tanks this means that contact with humid air mustbe avoided, especially in warm climates with relative high air humidity.The most common way of doing this is to equip the tank with a silicagel breather which extracts the humidity from air entering the tank. Itcan also be achieved by connecting a nitrogen or dry air source to thetank via a pressure valve. With such equipment, the water content in thetransformer oil can also be lowered to approximately 10 ppm bybubbling the dry gas through the oil. This is the preferred technique.

If the product has been contaminated with water, it is important that thetank has a sloped bottom with a low suction point, to drain off freewater before final drying.

It is also important to keep the oil dry in mild steel tanks, to prevent thetank from rusting. For this reason the tank must also have a dischargeconnection some distance above the tank bottom, to prevent particlesfrom being sucked out together with the product. This will alsoinfluence the lifetime and efficiency of the particle filter. The filters inthis case are either those located at the loading station or at thecustomers, in the degassing unit or in a factory system.

In the storage tanks for delivery direct to customers, the transformeroils normally have a water content of 20 ppm or less.

The loading line should be equipped with a particle filter that has anominal pore size of not more than five micrometers.

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Dedicated handlingIt is vital to keep the tank(s) completely separate, dedicated for use withtransformer oil. This applies of course to all piping and lines that areused to transport oil to loading stations and other shore tanks.

If it is not possible to keep the tanks/lines separated from otherproducts, cleaning must be carried out to a degree and with a techniquedictated by the character of the previous product.

There are a number of products which can be very harmful for atransformer oil, and where the risk of contamination is obvious, evenafter cleaning. They can be grouped as:

• Engine oils

• Heat transfer oils

• Used oils

• Other formulated lubricants

• Halogenated hydrocarbons

Inspection routines and samplingThe quality of the transformer oil has to be checked against therequirements according to established routines. The following generalguidelines can be given, but the frequency and the extent will depend onlocal factors such as handling system design and type of quality system:

• Shore-tanks and lines must be inspected for cleanliness before fillingup with new product. If not, the quality of the old product must beknown (analysed).

• After filling the tank, the product should be sampled, analysed andapproved against specification.

• If turnover of the product is low, the product should be analysedregularly or before delivery to a customer. The analysis of these samplesshould only concern parameters influenced by storage, e.g. dielectricdissipation factor, water content, interfacial tension and appearance.

The sampling should generally be done according to IEC 475. Howeveran average sample from a tank could also be a running sample andsupplemented with an absolute bottom sample.

It is essential that the sampling equipment is dedicated for transformeroil, or is at least thoroughly cleaned before use. The sample bottlesshould be new, dark and with suitable seals and labels. If white bottlesare used, they must be kept in a dark storage place.

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1.3 CHOICE OF VESSEL

1.3.1 Transport in shipsShips are normally used only for transportation from a refinery to adepot, for further storage, before distribution to end customers. Theships should be of the category “chemical tankers”, with stainless steelor coated tanks and line/pump systems which are easy to clean anddrain. Ships dedicated to transporting base oils, where the previouscargo is known, can also be used, providing the cargo was of the samenature and refined to the same degree as the transformer oils. Note thateven in the case of contamination by well-refined, paraffinic base oilsmight influence the low temperature properties of a transformer oil. Noformulated oil products such as hydraulic oils can be tolerated as aprevious cargo without thorough cleaning. Anything within thecategory of engine oils cannot be accepted at all as a previous cargo.

CleanlinessKeeping the product dry is the most common problem whentransporting transformer oils in ships’ tanks.

In some ships it is possible to pass a flow of nitrogen through the oil orput a nitrogen blanket on the top of the tank. Silica gel breathers are notsuitable for ships’ tanks, due to water and high humidity. Tanks, linesand pumps are normally very easy to drain, clean and inspect forsuitability.

Information about the sensitivity of the product must be given to allpeople involved in loading, discharge and transportation. It is essentialto inspect the ships’ equipment for product handling before use and tocheck the product very carefully after loading and discharge. Even withperfectly functioning quality systems, one would imagine thatinspections are not needed, but there are still too many uncontrolledfactors which can influence product quality.

Although water is the most common problem, the most costly andtime-consuming problems are caused by chemical contamination that isthe result of poor cleanliness and/or an incompatible previous cargo.

See also Section 1.4.3

SamplingRunning samples, or an average of level samples should be taken fromeach loaded tank and supplemented with an absolute bottom sample.These samples should be kept for an appropriate time. For analysis andrelease of product, a composite sample per product is prepared. Therequirements for sampling equipment are the same as those describedon page six.

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1.3.2 Transport in road tankers, railcars and containersDeliveries by road are by far the most common means oftransportation. Over the years, road tankers and containers havebecome safe methods of transportation.

Special road tankers equipped with silica gel breathers are used whenthe requirements for dryness are extra high. When deliveries have to bemade direct to a site where no degassing unit is available, thisrequirement is understandable, even if this procedure is not to berecommended, since all transformers should be filled via a degassingfilter. For other deliveries where the oil is degassed at site, these tankersmake for extra costs with minimal benefit.

Rail tankcars are considered very safe in terms of contamination risksand can be equipped in the same way as road tankers.

CleanlinessRoad tankers and rail tankcars are usually made of stainless steel,sometimes aluminium, and are easy to clean. The loading/dischargelines are short and easy to inspect, and there are no pumps involved ifnot specifically requested by the customer. Pumps, due to their designare difficult to clean and therefore involve a risk of contamination. Theymust be either dedicated for transformer oils, or very thoroughlycleaned, if possible by flushing with the product itself or a low viscositynaphthenic base oil.

All hoses should be dedicated for use with transformer oils, since theyare very difficult to inspect directly. An assessment of cleanliness mustbe based on a sample obtained by flushing the hose.

Road tankers should be cleaned and inspected, unless the previous cargowas a transformer oil or base oil that can be identified and accepted. Avisual inspection of tanks, lines and valves should always be performedto check for cleanliness.

SamplingSamples to verify the quality of a transformer oil load should be takenat the following points:

– at the end of the loading line, to verify that the right product is loadedand is up to specification.

– from the different compartments, as running samples, to yield asatisfactory average. After approved analysis the different compartmentsamples may be mixed into an average sample.

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– if the transport vessel is filled from the top, underline samples shouldbe taken, and analysed, from each line, to verify that no contaminantsare present in the discharge system.

The samples must be taken with clean equipment or equipment that isdedicated for transformer oil use. The bottles/containers used forstorage should be of glass or aluminium. Glass bottles for long-termstorage have to be dark, or kept in the dark. If other materials are to beused, they should be tested for suitability.

1.3.3 Transport in “flexi-bags” (rubber bags)Together with euro-containers, “flexi-bags” (rubber bags) aresometimes used for transformer oils. The advantage is of course that therubber bag can be returned as a small package after use while thecontainer is used for other types of cargoes. There are, however,disadvantages and risks connected with these rubber bags.

First of all, there is the difficulty of inspecting them before they arereused. Either they have to be dedicated or to have been cleanedthoroughly.

The material in some of the bags is also questionable and must bechecked for compatibility with the transformer oil, unless it has beenapproved by the oil supplier or user.

CleanlinessThe requirement for cleanliness is the same as for other bulks.Complications arise because of the difficulty of inspecting, or ofkeeping track of the dedicated rubber bags. This adds to the importanceof sampling and analysing the loaded product.

SamplingThe same requirements for line samples should be applied as for otherbulk deliveries. The rubber bag has to be equipped with some type ofsampling device to make it possible to take samples of the insulating oilafter loading without spillage of oil. The problem is that it is notpossible to take an average sample (i.e. a running sample: a mix of thetop, middle and bottom samples) from a rubber bag. This results in lesscertainty about product status after loading.

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1.3.4 Transport in drums

Choice of drumSteel drums of approximately 210 litres are the second most commonmethod for delivering transformer oils. The drums are made of mildsteel and have two openings sealed with screw caps, of which tri-sure isthe most common type. The thickness of the steel varies: for sitedeliveries of transformer oils, 1.0 mm steel is normally used.

Due to the difficulties and costs of cleaning and inspecting used drums,new drums offer a high degree of security and are manageable at areasonable price.

However, the empty drums should be randomly checked by visualinspection for cleanliness, especially if they have been stored empty forsome time. The drums should be kept with the caps on during storagebefore filling. The main thing to check for is rust formed by airhumidity or free water. Some drum suppliers offer drums filled withnitrogen, which reduces the risk of formation of rust inside the drumsbefore filling.

Cleanliness & storage of drumsDue to the risk of contamination from foregoing products, a cleaningprocedure for the drum filling system is very important. These systemsare not easily inspected, and even if they are well drained, pumps andfilters might still trap the previous product. The drums are filledthrough a pipe inserted down to the drum bottom, via one of theopenings. The drum is normally placed on scales to obtain the correctweight. After filling, the drums are systematically sampled and sealed.It is important to keep a certain free volume in the drum to allow forvolume expansion of the oil caused by changing temperature. It is mostimportant to sample and test the oil from the first drums. This is wherecontamination is most likely to be identified. In some cases the drumsare filled with nitrogen or dry air before filling with oil to avoid waterpickup from humid air. This is relevant for weather conditions withhigh humidity and relatively high temperature (>60% relative humidityand >20°C). These precautions, however, do not replace the recommen-dation to fill oil into transformers via a degassing filter.

It is also important to check the seals and to tighten the screw caps withthe prescribed torque. Even if a sealed drum seems to be properlyclosed, leakage of water through the caps can occur, and therefore thedrums should be transported and stored upside down or horizontal untiluse.

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At storage sites the drums should be stored on pallets and not placeddirectly on the ground (e.g. on planks or boards) and protected bysuitable shelter from rain and sunlight. The drums should be placedupside down or on their side until used. Shelf life will be affected bytemperature changes so the drums must be kept as cool as possibleduring storage. If there is some oil remaining in a partly used drum itshould be stored on its side, protected as above, with the oil level abovethe bung. Special attention should be given to the rubber gasket aroundthe bung when closing it, as tightening it too much will cause damage. Itis not advisable to store half-emptied drums for long periods due to therisk of contamination.

SamplingThe same requirements apply for line samples as for bulks. For thedrums themselves, the number of samples and how to take them isdefined in IEC 475. In brief, the procedure can be described in thefollowing way:

A bottom sample should be taken from drum numbers 1, 2, 5, 10, 100,200, etc... until the last drum, and each sample should be visuallyexamined for particles and haze. A composite sample from all drumsamples should then be analysed against specification. If a contaminantis detected, the check must be extended until an approved drum in theseries is found. The contaminated drums must be segregated and an oilexpert should decide how the oil is to be used (disposed of). Drumnumber 1 should also be checked for contaminants, by analysing, forexample, water content and dielectric dissipation factor.

1.4. CONTAMINANTS –AND POSSIBLE CORRECTIVE MEASURES

1.4.1 WaterWater is the most common contaminant in transformer oils duringhandling and storage. This is of course due to the presence of water inalmost any environment and the fact that water is also widely used forcleaning transport vehicles and handling equipment.

There are three main ways of getting rid of the water. These do notinclude heating, because water solubility in oil increases with increasingtemperature. (Please see page 29, figure 5).

Large amounts of free water must be drained off when it hasprecipitated to the bottom of the tank/container.

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Dissolved water can be picked up by dry air or dry nitrogen bubbledthrough the oil. A slightly elevated temperature will speed up the dryingprocess by increasing the water solubility of the air/nitrogen.

The final method is degassing, which is a very common technique andwhich is also the only suitable one in some cases. The equipment can beeither stationary or mobile. The technique is to heat, vacuum treat andfilter the oil through a particle filter. The heated oil is exposed to avacuum, which causes the water to evaporate to a high degree. There arenumerous manufacturers of this kind of equipment.

However, the equipment needs a lot of electricity to function and isquite expensive to buy. Relatively inexpensive equipment and lowrunning costs make air drying the cheapest method .

1.4.2 ParticlesParticles will, together with water, lower the breakdown voltage.Particles are also present in the oil environment when it is stored,transported or filled into a transformer. Particles are removed by simplefiltration through particle filters, which are also a part of degassingfilters.

When loading transformer oils for delivery to customers, a five micro-meter filter, or smaller, should be used.

The lifetime of these filters depends upon the amount of particles andfibres they have to remove from the oil. Due to collected particles, thepressure drop over the filter increases up to the maximal pre-givenvalue, which indicates when it is time to change the filter cartridge.

1.4.3 Chemical contaminationSmall amounts of chemical contaminants can enter the oil duringtransport, handling or filling of the transformer. The contaminants comefrom other products that have been handled in the same equipment.Their nature varies, but they often share the common characteristics ofbeing strongly polar and surface active. In these categories of productsthe most common types are engine oils, metal-cutting oils, rust-prevention products, detergents, vegetable oils and their derivatives, etc.The list is long and knowledge about a specific contaminant has to besought from a transformer oil supplier or a transformer producer.

Chemical contaminants of these types will influence the values fordielectric dissipation factor and/or interfacial tension. The ageingproperties might also be affected. These polar contaminants can beremoved by treatment with activated clay.

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Even after clay treatment, however, there is no guarantee that the oilwill be unaffected and restored to its original condition. Added ornatural inhibitors in the oil might be removed by clay treatment andnon-polar parts of the contamination will be left in the oil, possiblyleading to shorter product lifetime. On the positive side, some low-refined oils can gain in oxidation stability after clay treatment. See alsopage 56 in the refining section, where clay treatment is used as the lastpolishing step in refining. Please note that used clay has to be handled asenvironmentally hazardous waste.

After clay treatment, the oil must always be tested for oxidationstability among other analyses.

1.4.4 Base oils/SolventsBase oils/solvents will influence the viscosity and flash point propertiesof an oil; by how much will depend on the amount introduced into theoil.

Small amounts of base oils will normally not have an excessivelynegative effect on the transformer oils, as they are mostly clean andwell-refined nowadays. Depending on the viscosity of the base oil,however, it will influence cooling properties of the transformer oil; inthis case, the viscosity of the contaminated transformer oil should beanalysed. Even in relatively small amounts, a low-refined base oil mightdecrease oxidation stability. If it is a paraffinic oil, the low temperatureproperties might be damaged.

Solvent contamination will almost certainly have an adverse effect onthe flash point. The problem with these types of contaminants is thatlittle can be done to correct them. Apart from dilution, when smallvolumes are involved, re-refining or downgrading to fuel are the onlypossible measures.

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2. MAINTENANCE OFTRANSFORMER OILS INSERVICE

2.1 GENERALThe cost of a transformer is high, but monitoring the performance ofthe transformer system via the oil is inexpensive compared to the costsof a failure in a transformer and the costs of an interruption in powersupplies. This is absolutely true for power transformers, but with smalldistribution transformers, the cost has to be justified case by case, by forexample a “What If” study.

In combination with a computer-based expert system for interpretationof analysis results, and for storage of previous transformer operationdata, the monitoring of transformers in service is even more to berecommended.

For further guidelines and follow-up recommendations, it is advisableto contact the producer of the transformers and/or power distribu-tors/independent laboratories.

Choice of oilTo ensure the long service lifetime of a transformer oil (i.e. costperformance considerations), the most important step is to select an oilthat has the properties required for the equipment in question: differentequipment needs different oil grades. For example, a high-voltage, high-loaded transformer demands a better oil than a low-voltage, light-loaded transformer. Normally, the producer of the equipmentrecommends the type of oil to be used, since the oil should beconsidered an essential part of the equipment and not just an undefinedextra poured into the transformer. This is the modern approach toquality thinking. (Compare this with the automotive industry, whichprovide detailed specifications for the types of oil to be used in thedifferent parts of a vehicle.)

Transformer diagnosisA transformer oil carries information about the condition of thetransformer. Analysing the oil in service can therefore give earlywarnings about paper degradation, hot spots, electrical faults andproblems with moving parts such as pumps. To avoid serious problems,these data can be used as a guideline for corrective measures in thetransformer.

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2.2 SAMPLINGTo ensure that the sample is representative of the oil in the equipmentthat is to be tested, strict routines have to be followed when performingthe sampling. If not, the analysis results might lead to false conclusionsconcerning the status and entail time wasting and expense in obtainingtransport and testing the sample.

Detailed sampling requirements are given in the IEC 475 standard. Thefollowing, in particular, should be noted:

• Ensure that sampling is done by an experienced person, i.e. someonewho is aware of the sensitivity of transformer oils and who will only useequipment that is clean, dry and in other respects suitable. There havebeen cases of samples being taken in engine oil bottles or alcoholbottles. Doing so will affect the electrical dissipation factor, flash pointor water content. Just a few ppm of engine oil will destroy the dielectricdissipation factor.

• Normally, oil samples should be taken from “living oil”, i.e. from oilin circulation. Only use the bottom valve of the tank, when a test forfree water and sediment has to be performed.• Start the analysis by draining a sufficient volume of oil from thesampling line.• Cleaning of the sampling container is especially important, if the oil isto be tested for particle content (described in IEC 970).• Rinse the container with the liquid being sampled according to thefollowing steps:

– Let the oil flow down the side of the container. This is to preventair from becoming mixed and trapped in the oil. This is extremelyimportant when samples are taken under humid conditions wherewater saturation can be very rapid. Alternatively, fill the samplingcontainer with a clean tube leading from the transformer to thebottom of the container, and let the oil fill the container from thebottom, until it overflows.– Because the container should be filled to 100% of its capacity, glassbottles are less suitable since a small air volume has to be left forexpansion. – After sampling, ensure that the cap is clean, undamaged andcorrectly attached to the container. Some rubber seals might affectthe oil.– Label the sample so that it can be easily identified.– Store in a dark place or in a suitable box if glass or light plasticbottles are used. Mineral oil is usually very sensitive to UV light andmay deteriorate if exposed to light. This will first be seen in areduction of the interfacial tension.– For gas-in oil-analyses, special sampling equipment has to be used;the procedure is detailed in IEC 567.

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2.3 WHAT TO ANALYSE AND WHY

2.3.1 Colour and appearanceColour and appearance give rapid and useful information that can beobtained on site. An experienced person observes immediately ifsomething is abnormal. Combined with the smell, much informationcan be acquired. Dark colour may indicate that the oil has started todeteriorate, which is also the first step to free sludge. The appearance ofan oil can indicate if free water is present and if the oil contains anyimpurities such as fibres or cellulose particles. A bad smell may indicatearcing, which causes cracking of the oil. However, systematicdocumentation of these observations is necessary to ensure continuityin the event of personnel changes.

2.3.2 AC-breakdown voltageThe AC-breakdown voltage is of importance as a measure of the abilityof an oil to withstand electrical stress.

The breakdown voltage depends on the water and particle content ofthe oil. It is particularly important to check this before starting-up anew transformer and also when the transformer oil and paper insulationstart to deteriorate, because the deterioration process generates waterand particles.

2.3.3 Water contentThe water content in the transformer oil gives an indication of the watercontent in the paper material. Too high a level of water in the oilindicates that the paper also contains a lot of water, and this will affectthe ageing of the paper, i.e. trigger decomposition of the fibres in thepaper, which leads to irreversible damage that might cause an electricalbreakdown in the transformer. Two more things are worth noting: oldtransformer oil that has started to oxidise has a higher saturation level forwater than a new oil; water is also produced during oxidation both fromoil and paper, which will further accelerate the breakdown of the paper.

2.3.4 Neutralization valueThe neutralization value indicates if the oil contains any acidic material.A high or increasing value indicates that the oil has started to oxidise. Ahigh value might cause problems with corrosion and the acid can formsoaps with metal ions in the oil and affect electrical properties. Theseacids will also increase the solubility of water in the paper because oftheir polar structure. These acids also promote the degradation of thepaper (catalytic action).

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2.3.5 Dielectric dissipation factor and/or DC-resistivity These characteristics are both very sensitive to contaminants and ageingproducts. In some oxidation stability tests, the dissipation factor ismeasured in the oil after the ageing test to indicate the ability of the oilto withstand oxidation.

These tests normally give similar indications of impurities, so it is notnecessary to do both tests. However, DC-resistivity is more affected bythe water content of the oil.

2.3.6 Interfacial tensionThis is a very sensitive analysis and can, in combination with dielectricdissipation factor, give an early warning signal when the deterioration ofthe oil starts (see section 3.14 Oxidation stability).

2.4 FILLING A TRANSFORMER WITH NEW OILWhen a new oil is pumped into the transformer, it loses some of itsproperties, such as dielectric dissipation factor and interfacial tension.These parameters are very sensitive to contaminants from thetransformer, and may be introduced during handling and filling.

The recommended limits from IEC 422 of setting values for unusedmineral transformer oils filled into new power transformers, arepresented in the table on the facing page.

The figures given are compared with the requirements in IEC 296 forthe setting values of the same oils before they are filled into thetransformers.

The water content in the oil to be filled into 72.5 kV-type transformersshould be agreed upon between supplier and user, depending on localcircumstances.

The table below comes from IEC 422, but different transformerproducers might have stricter requirements on water content andelectrical breakdown. For transformers larger than 170 kV, a maximumof only 5 ppm water is required by some producers/users, compared tothe IEC 422 maximum recommendation of 10 ppm. It is very difficultfor oils to pass the requirements in the table, if they have not beendegassed, indicating that it is absolutely necessary to fill the transformerwith a degassed and filtered oil. It is very important to check that thedegassing unit is clean and is not contaminated with used oil. If the unitcontains used oil, the dielectric dissipation factor and resistivity mightbe affected and fall outside the levels specified in the table, and this

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might also lead to a shorter lifetime for the oil in service. It is alsonecessary to ensure that the unit has not been used for PCBcontaminated oils. We also know that silicon oils handled by the samesystem might contaminate mineral oils and cause excess foaming.

A portion of the oil will evaporate together with the water. The filteringtemperature should therefore not be too high and it should be related tothe vacuum. For example, ASTM D 3487 recommends a maximum of80 °C, if a pressure of 1mmHg in the vacuum equipment can be reached.This is especially important for inhibited oils where the phenolic typesof inhibitor have a higher vapour pressure than the oil and the lifetimeof the oil might be shortened.

If intermediate storage is used, make sure that it is suitable fortransformer oils. Please see under heading “Handling and Storage”,section 1.

2.5 FREQUENCY OF OIL TESTINGIt is very difficult to give a general recommendation about how often atransformer oil in service should be tested, and how far it can bepermitted to deteriorate. This has to be done on a case-by-case basis,depending on the circumstances. For example, owners of large powertransformers will probably check the transformer regularly, but forsmall distribution transformers, owners tend to accept a higher risk. Therisk assessment, however, should be based not only on the size of thetransformer, but more on the effect of a failure. A “What If” studycould be carried out to evaluate costs and other consequences of afailure.

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Transformer New oil requirementrange <72.5kV 72.5-170kV >170kV according to IEC 296

Properties

Colour max 2.0 max 2.0 max 2.0

Water content mg/kg max 15 max 10 max 30 in bulk

Interfacial tension 44 for new oil (mN/m) min 35 min 35 min 35 as typical value

Dielectric dissipation factor90°C max 0.015 max 0.015 max 0.010 max 0.005

Resistivity 90°C G ohm m min 60 min 60 min 60(G=giga=109)

Breakdown min 30 before/min 50 voltage (kV) min 40 min 50 min 60 after treatment

19

In a risk failure analysis, the trend in the results must be considered: e.g.increase in tan delta versus the time, metal content versus time,neutralization value versus furfural content, etc.

In IEC 422 electrical equipment is divided into eight different classes,with different recommendations for follow-up frequency. In thefollowing table only two of them are included: one high-loadedcategory O power transformer with a voltage above 420 kV and onelow-loaded of up to 72.5 kV, category C.

In addition to these tests, flash point can be measured and the decreasefrom initial value should not be more than 15 °C. Flash point should bechecked if an unusual smell is noted. This could indicate that the oil hasbeen exposed to high arcing and has thus cracked into low boilingcomponents.

The inhibitor content can be measured to evaluate if the inhibitorcontent is drastically reduced.

The metal content of the oil, as particles, can be measured to evaluate ifany mechanical problem exists in the transformer, e.g. moving partswith wear problems.

The frequency guidelines facing side are from IEC 422. In practice, thedifferent power distributors have longer or shorter follow-up periods.For the more important equipment most power suppliers have a regularfollow-up put into a computer system that decides when a sample shallbe taken. Trend analysis can be carried out in these systems andexperience from all types of equipment under different workingconditions can be gathered.

2.6 OTHER ANALYSES TO EVALUATETRANSFORMER STATUS

2.6.1 Gas-in-oil analysis and furfuraldehyde contentThese tests are carried out in order to evaluate the physical condition ofthe transformer with regard to factors such as arcing, hot spots andpaper deterioration. The importance of these tests is increasing, due tothe development of sophisticated analysis equipment: HighPerformance Liquid Chromatography (HPLC), Gas Chromatography(GC), etc.

Gas-in-oil analysis and interpretation thereof is specified in IEC 567“Guide for sampling of gases...” and IEC 599 “Guide for theinterpretation of the analysis of gases...”.

Gases will form in oil-filled transformers due to normal ageing but also,to a much greater extent, as a result of failures. The reasons for thefailures can be interpreted from the types of gases that are formed. Themanufacturers and the power utilities have built up their own

20

Property accto IEC methods

Appearance,sediment and sludge

Breakdown voltage

Water content

Neutralization value

Dissipation factor90°C

Interfacial tension

Frequency

Not a routine test. To be done in combi-nation with other tests

After filling, refilling orreconditioning prior toenergizing. Then after12 months, for Cat. Oevery two years and forCat. C every six years.

Cat O: After filling orrefilling. Then after 3and 12 months and afterthat in connection withdissolved gas analysisCat. C not a routineanalysis

Every six years

Cat. O: After 12 monthsand then every six yearsCat. C not a routine test

Every six years

Recommended action limits

Sediment or free sludge

Cat. O 50kVCat. C 30kV

Cat. O 20mg/kg

Cat.C Free water atroom temperature.

max 0.5 mgKOH/g

Cat. O max 0.2Cat. C max 1.0

min 15mN/m

Action

Recondition the oil or change to new

Recondition or, if more economical,replace the oil.

Check the reason and consider recon-ditioning.If free water, drain off beforereconditioning starts.

Replace or recondition

Check with manu-facturer’s instruction.

Investigate

knowledge concerning evaluation. The detection of the gases isperformed by gas chromatographic methods using different ways ofextracting the gases from the oil. IEC 567 has been revised (1992) andincludes more effective methods.

The following gases can be analysed: hydrogen, oxygen, nitrogen,methane, ethane, ethylene, acetylene, carbon monoxide and carbondioxide. The amounts and ratios between the gases are used for theinterpretation of probable failures.

In the table below there are some examples of interpretations.

Carbon monoxide and carbon dioxide levels in the gas-in-oil analysisgive an indication of the paper deterioration. But a more precise andearlier warning signal about the status of the paper is given bymeasuring furfuraldehyde content of the oil. This is done by an HPLCmethod, see IEC 1198 (1993), where the detection level forfurfuraldehydes is deliberately low, in order to give an earlier warningthan the gas-in-oil analysis. It is a relatively simple analysis, thoughsomewhat expensive equipment is needed, but it gives very usefulinformation as a routine test.

2.6.2 Polychlorinated Biphenyls (PCB) New mineral oils, produced from crude oil, do not contain any PCBs.PCBs originate from synthetic insulating liquids and they wereoriginally used as insulating liquids because of their good electricalproperties and low flammability. After their negative environmentalimpact was discovered, their use was banned in many countries. Thereis always a danger that PCBs may be introduced into new transformer

21

Nitrogen + 5% or less oxygen. Normal operation of sealed transformer.

Nitrogen + more than 5% of oxygen. Check for tightness of sealed transformers.

Nitrogen, carbon dioxide, Transformer overloaded giving some cellu-carbon monoxide. lose breakdown. Check operating conditions.

Nitrogen and hydrogen. Corona discharge causing electrolysis of water or corrosion.

Nitrogen, hydrogen, methane with Sparking or other minor faults causing some small amount of ethane and ethylene. breakdown of the oil.

Nitrogen with high hydrogen cont- High energy arcing causing rapid ent and other hydrocarbons including deterioration of the oil.acetylene.

oils by mixing with re-refined oils, or as contamination from used oils.Therefore PCB content has to be measured in any mixture of old andnew oils. It might also be necessary to measure the PCB content of oilsthat are sent for disposal. In many countries, PCB-containing oils areclassified as hazardous waste and require special methods of disposal.

References:

1) IEC 296 Specification for unused mineral transformer oils for transformers and switch gears.2) IEC 970.3) IEC 1198.

4) IEC 422 Maintenance and supervision guide for transformer oils in service.

5) A guide to transformer maintenance. S.D. Myers, J.J. Kelly, and R.H. Parrish. TMI, 1981.

6) IEC 567 Guide for sampling of gases and of oil from oil-filled electrical equipment and for the analysis of free and dissolved gases.

7) IEC 599 Interpretation of the analysis of gases in transformers and other oil-filled electrical equipment in service.

22

3. REQUIREMENTS OF OILS IN SERVICE

3.1 GENERAL

The oil in a transformer has several functions, the most important ofwhich are of course insulation and cooling. Another function is to carryinformation about the condition of the active parts of the transformer.

SpecificationsThe main requirements for a transformer oil are listed in variousnational and international specifications and standards. However, thesestandards only state the minimum requirement for transformer oils, andmany transformer producers and electricity companies have their own,stricter requirements, based on their own particular needs. Basicstandards are IEC 296, BS 148, VDE 0370 and ASTM D 3487. Seeexplanation below.

The physical requirements are fairly similar, but with regard tooxidation stability, there are big differences between the commonmethods – IEC 1125 A, B and C, DIN 51554 (Baader test), ASTM D2440. All of them try to simulate long-term oxidation behaviour in ashort time. See also table on page 35–36.

It is also worth noting that no real chemical requirement is listed, suchas the nitrogen, sulphur or PolyAromatic Compound (PAC) content ofthe oil. These chemical components cause differences between oilswhich are not specified by the given parameters and will influenceimpulse breakdown, gassing tendency, streaming charging and oxidationstability.

As can be seen from the properties listed facing page, specifications canbe divided into four types.

In the table, facing page, you will find different specifications,international and national, listed hierarchically. Individual transformermanufacturers and different power distributors also have their differentspecifications and requirements. We will now turn to consider some ofthe physical and chemical parameters listed in the specifications, as wellas some unspecified parameters.

23

IEC 296 International standard

BS 148, ASTM 03487, VDE 0370 , etc National specifications

ABB, GEC-Alsthom, etc Transformer producer specifications

RWE, EDF, etc Power distributor specifications

Summary of property requirements

Chemical Electrical propertiesOxidation stability Breakdown voltageOxidation inhibitor content Dissipation factorCorrosive sulphurWater contentNeutralization number

Physical Additional requirementsViscosity Impulse breakdownAppearance Streaming chargingDensity Gassing propertiesPour Point Aromatic structureInterfacial tension Polyaromatic structureFlash point Solubility properties

Please also see table on page 45.

CompatibilityThe compatibility of different oils has always been a matter ofdiscussion. It is however safe to assume that oils which meet the sameIEC 296 class can be mixed together.

In the following, we take a closer look at these requirements and whatinfluence they have on transformer oils in service.

24

3.2 VISCOSITYThe viscosity of an oil is important for the cooling of the transformer:the lower the viscosity, the better the cooling. Increasing temperaturereduces viscosity, and a small change in viscosity with temperaturemeans a high viscosity index (VI), while a big change indicates a lowindex.

In lubrication applications, oils with high VI give better performance. Incooling applications a low VI is preferred, because lower viscosity atoperating temperature means better cooling. Below we have comparedtwo oils, one naphthenic and one paraffinic. Both have the sameviscosity at 40 °C, but as can be seen there is a relatively big differencebetween the two oils at a normal transformer working temperature.

IEC 296 Class II oil High VI Low VIParaffinic Naphthenic

Viscosity at 70 °C mm2/s 4.2 3.4

It is never an advantage to use high viscosity oils, because the higher theviscosity the worse the cooling properties. This gives higher workingtemperatures, higher losses, resulting in faster deterioration of the oiland the paper. A naphthenic oil is therefore preferable.

IEC 296 is currently divided into three viscosity classes, I, II and III.Oils with viscosities of around 9-10 mm2/s are those which are mainlyused today.

Maximum viscosity values for the different classes are given belowand, as will be seen, an oil satisfying Class II also meets therequirements for Class I. The difference between them is the flash pointrequirement

I II III

Viscosity at 40 °C (mm2/s) <16.5 <11 <3.5

3.3 VISCOSITY AND FLASH POINT VERSUS BOILING RANGEAn oil is a mixture of hundreds of different molecules and is thereforesaid to have a boiling range, rather than a boiling point. When theboiling range is higher, the viscosity of the oils increase. The inherentdifference between naphthenic and paraffinic molecules gives aparaffinic molecule a higher boiling range for a given viscosity. Seefigure below.

25

Figure 1 shows two boiling point curves, one for a naphthenic oil andone for a paraffinic. Both oils have a viscosity of approximately 8 mm2/sat 40 °C. The third curve is for a typical switch gear type of oil with aviscosity of about 4 mm2/s at 40°C and a flash point of 100 °C.

Transformer oils have roughly the same flash point, 140 °C, whichmeans that the boiling curves start at the same temperature. Theparaffinic oil will have a much higher boiling temperature at the 50 %point and at the end as well. This higher boiling point means that an oilrefined to the same degree contains more PACs of three rings or more.This higher PAC content will affect properties like impulse breakdownand streaming charging. The three-ring molecule anthracene, forexample, has a boiling point of 350 °C. This temperature corresponds towhat is nearly the end point for the naphthenic oil, whereas more than25 % of the paraffinic oil has a boiling point above 350 °C. Moreinformation on this subject will be found in the part covering extractionon page 55.

The curves in figure 1 were produced with ASTM D 2887, a GCmethod used in refineries for optimizing and checking production. Thecolumn used for the method is of a non-polar type, calibrated with N-alkane.

If a capillary column is used, more individual peaks can be identified,and so this latter method can be used as a tool for fingerprintingdifferent oils for traceability.

3.4 LOW TEMPERATURE PROPERTIESLow temperature properties are important in a cold climate, and themajor specifications include both pour point and viscosity at lowtemperatures. In some countries, Sweden and Canada among them, it isbeing discussed whether to increase the requirement, e.g. to specify thatthe cloud point should be the same as, or lower than, the pour point andto measure viscosity at a low shear rate, or at -40 °C. All thesemeasurements are made to simulate the flow in a transformer duringcold conditions.

26

Figure 1

Viscosity and flash point vsboiling range

500

450

400

350

300

250

2000 20 40 60 80 100

°C

%

Viscosity FlashpointNaphthenic oil 7.7 144Paraffinic oil 7.5 136Switch gear oil 3.5 100 Paraffinic

Naphthenic

Switch gear

Paraffinic oils contain N-alkanes which, when they are cooled down,can crystallise and impede the free flow of the oil; the actual amount ofN-alkanes can be measured with a Differential Scanning Calorimeter,DSC. When a cloud point occurs in the oil, it is no longer a Newtonianfluid (i.e. a fluid that does not change viscosity at different shear rates)and is in fact a two-phase system.

When an oil is cooled, the N-alkanes start to crystallise, release heat andshow up in the curve as a difference between the sample and a reference.The area between the curve and a straight line then corresponds to theamount of N-alkanes. The crystallisation point correlates with the cloudpoint of the oil. In the upper curve, no crystallisation is visible for thenaphthenic oil.

Naphthenic oils contain few, if any N-alkanes. This means that no shearstress is needed to get the oil moving at low temperature. For aparaffinic oil, if used in a self-circulating transformer, the oil might besolid, even though it is possible to measure a viscosity with high shearrate methods, such as a standard viscosity test. The two curves in Figures3 and 4 were plotted at -30 °C and -40 °C respectively. The values aretaken from the IEC WG5 (Working Group 5) and are measured asdynamic viscosity at different shear rates. It is clear that the paraffinicoil at -30 °C has a higher viscosity at a low shear rate, compared to thenaphthenic oil that does not change viscosity at different shear rates.The paraffinic oils fulfilled the viscosity at -30 °C with the normalkinematic viscosity measurements.

27

Figure 3

Brookfieldviscosityat -30 °C

Figure 2

The two DSC curves in Figure 2, one fora paraffinic and the other for a naphthenic oil, illustrate the formation of wax crystals.

±0

N-base

P-Base

Cooling ∆T

Heating

Temperature Temp-100

3000

1300

2000

1 3 5 7 9 11

Paraffinic oil

Paraffinic oil

Naphthenic oilSpindel rpm

3.5 FLASH POINTThe flash point of an oil is specified for safety reasons. In the three IEC296 grades, the following flash points are stipulated.

I II IIIFlash point PM° C ≥140 ° C ≥130 ° C ≥95 ° C

IEC specifies the PM (Pensky Marten) closed cup method. In USA theCOC (Cleveland Open Cup) is used, which gives a 5-10 ˚C higher flashpoint value. The flash point depends on the light part of the oil and isextremely sensitive to contaminants from lighter oils such as gas oil orgasoline. Even though both methods yield relatively poor repro-ducibility, the closed cup method is preferred because it provides betterrepeatability.

3.6 DENSITYIn cold climates it is important to specify oil density to avoid theoccurrence of ice floating in the oil at low temperatures. This can occurwhen there is free water present in not energized transformers andwhich can cause failure during start up. Oils with high aromatic contenthave higher density than oils with more naphthenic and paraffinicmolecules. The density decreases with increasing temperature and thestandard coefficient 0.00065/°C is used for calculating the density atother temperatures than those measured. This coefficient will varysomewhat with the different oils, depending on the structure and degreeof refining.

Nynas Naphthenics Nytro 10BN, Nytro 10X and Nytro 10GBN allhave a flash point exceeding 140 ˚C and a viscosity less than 11 mm2/s at40 ˚C, thus meeting the requirements of both IEC Class I and II.

28

Figure 4

Brookfieldviscosityat -40 °C

Paraffinic oil

Paraffinic oil

Viscosity Cp

Naphthenic oil

6000

8000

4000

2000

6 10 14 18 22 26 30Spindel rpm

3.7 WATER CONTENTThe water absorption of the oil depends on the temperature and theamount of polar molecules. In figure 5 we can see that it is difficult tomaintain a low water content in oil that is stored in areas wherehumidity and temperature are high. It is also obvious that just heatingthe oil will not reduce the total water content if there is free water in the system, the reason being that water solubility increases withtemperature.

If free water is present, however, the lower viscosity obtained whenheating the oil gives faster separation but will increase the amount ofwater dissolved. From the same diagram we can also see that higheraromatic content (polarity) gives a higher saturation level for the water.

During oxidation of the oil, water is formed as an oxidation product, inwhich case it is an advantage for the oil to have high solubility, so thatthere will be no free water.

Oils with a high water content may foam when they are degassed. Thisis not real foam. Real, stable foam can be found in contaminatedsystems – for example oils contaminated with particles or other liquidsincompatible with the oil, e. g. silicon oils. As a general rule, clean liq-uids do not foam.

3.8 PARTICLESOil treated with modern refining techniques has a low particle content,but as soon as it is transported and stored this content increases. Bypassing the oil through a degassing unit, which contains a particle filter,the particle content will be reduced to an acceptable level. If the oil iscirculated through the transformer and a degassing unit, it cleans thetransformer from dust and loose cellulose particles.

Figure 6 shows the results of a study on particle content in oil handledfrom the process plant down to storage and degassing. The upper curveis an oil taken after transportation and storage, the lower one afterdegassing and filtering.

29

Figure 5

Solubilityof waterin oil

Temperature °C

Transformer oil

Liquid paraffin

Depends on:• Polarity of the oil• Temperature

Water content ppm

XyleneX

200

175

150

125

100

75

50

25

00 20 40 503010

3.9 ELECTRICAL BREAKDOWN (AC)This property is very complex and the measured value depends onparticle content, type of particles, water content and the test methodused. The normal method for specifying AC breakdown is IEC 156. Inthis method the electrodes are spherical or hemispherical at a distance of2.5 mm and the voltage is increased by 2 kV/s until breakdown occurs.The result is stated as an average of six tests, due to the low repeatabilityof each test.

Even a low-refined oil may have a high breakdown voltage, so thismethod tells us nothing about the refining of the oil. The removal ofwater and particles can give a breakdown voltage of more than 70 kV toany oil. The IEC specification has over 30 kV as a minimum level, and ifthe result is lower than this, all it requires is a simple degassing treatmentto raise breakdown to more than 50 kV (normal value > 70 kV).

Methods other than the IEC 156 can be used for electrical breakdown,e.g. ASTM D 1816 and D 877 which are included in the ASTM D 3487specification. ASTM D 1816 uses the same type of electrodes as IEC 156and the results can therefore be compared if the electrode distance is thesame or the result is calculated to the same distance. The other method,ASTM D 877, does not give comparable results due to different types ofelectrodes.

30

Figure 6

Particulatecontaminationafter transport and storage and after filter

Figure 7

Electricalbreakdown AC

90

80

70

60

50

40

30

20

10

00 20 40 60 80 100

Depends on:. Water content. Particle content. Temperature

Water content, ppm

Contaminated oil

Breakdown kV

Clean oil

Particle size,micrometers

After transport+storage

Number of particles/100 mlGreater than indicated

After filter

100000

10000

1000

100

10

0 20 40

It is also interesting to note that high solubility in an oil keeps some ofthe sludge in solution that is produced when the oil starts to oxidise,thereby reducing the amount of free particles that can lower electricalbreakdown.

The amount of particles might also be influenced by the carbonisationof oil molecules that occurs in partial electric discharges. The amount ofparticles formed is related to the result of the carbon residue analysis ofthe base oil, and here the naphthenic oils have an advantage over theparaffinic oils.

3.10 DIELECTRIC DISSIPATION FACTOR (tan delta/power factor)This is a parameter that will always be found in a transformer oilspecification. The loss angle depends on the amount of ions in the oil. Anormal degree of refining always gives a low value for this parameter,but it is very sensitive to contaminants, e. g. engine oils. So big is theeffect of molecules of this type, that just a few ppm will destroy the tandelta.

Water itself does not affect this property, but can participate in formingstable complexes with oxidation products, or with other impurities togive high tan delta values.

When an oil starts to deteriorate, an increase in tan delta can be found atthe beginning of the oxidation process, followed, after a time by adecrease. What probably happens is that peroxides form together withmetal complexes, as the first step of oxidation. These complexesproduced have a strong polarity and a high content of ions, and so thiscauses an increase in dielectric loss. This peroxide will be decomposedinto new radicals, forming oxidation products with a lower tan delta.After the initial decomposition stage, oxidation products, such as acidsand esters form, causing the tan delta to increase again.

Normal value for an oil as manufactured is < 0.001 at 90 °C and 50 to 60Hz. In some specifications power factor is used instead of tan delta. Thedifference is little at low measured values.

3.11 INTERFACIAL TENSIONThe interfacial tension test measures the strength of the interfacebetween oil and water.

The interfacial tension depends on the polar groups in the oils, whilsttan delta (90 °C, 50 Hz) tells something about the content of ionizablecontaminants.

31

Here are some examples of our analysis results for different oils.

Tan delta Interfacial tension Water contentmN/m ppm

Oil exposed to daylightstored in clear glass bottles 0.0031 36 50

Before test and reference stored in aluminium bottles 0.0010 44 18

This test tells us three things: oils are sensitive to light exposure,interfacial tension is more sensitive for detecting oxidation products,and tan delta and water content are higher in oils at the beginning of theoxidation process. (Heavily deteriorated oil in service may haveinterfacial tension values less then 18 mN/m).

3.12 NEUTRALIZATION VALUEIn a well-refined oil the neutralization value must be less than 0.01 mgKOH/g oil, but because this method has a repeatability of 0.03, theminimum requirement is less than 0.03. This level, however, is too highto give any guidance on the quality of the oil.

Well-refined, non-contaminated oils must have a target value below0.01.

3.13 CORROSIONIn IEC 296, this requirement is based on a method where a copper stripis immersed in the oil at 140 °C. Its sensitivity is low, and manycompanies use alternative methods such as the silver strip test orpotentiometric titration of mercapto sulphur in the oil. Corrosion is ofcourse an important parameter of the oil, and for switchgears silvercorrosion is one of the most important things to consider.

3.14 OXIDATION STABILITYThere are two types of oil on the market, inhibited and non-inhibited.In fact all oils are inhibited – the inhibited ones with hindered phenoladded (radical destroying) and the non-inhibited with natural inhibitors(peroxide destroying). The majority of all oils used in the world todayare inhibited with phenolic inhibitors at different levels.

Oxidation is influenced by the two main parameters oxygen andtemperature. Without oxygen, the oil will not oxidise; however all oilscontain a small amount of air even after degassing (in a sealed dried unit0.05 to 0.25% oxygen by volume will still remain) which will participatein the oxidation. Heat accelerates this deterioration, for example: a

32

temperature that is 10 °C higher, reduces lifetime by half. This is a ruleof thumb, but it is not true for all oils, because other reactions may startat high temperatures (compare with an egg stored at 40 °C and 100 °Crespectively).

What happens when an oil starts to oxidise?

The first reaction is the creation of a free radical (1). This reaction istriggered by heat, UV light, mechanical shear and - also important in atransformer – high electrical fields.

This step is important, and it happens in all oils. If nothing stops thereaction, the next step is the creation of peroxide through oxygenradicals (2) (3).

This peroxide is not stable (4), and the next reaction may be for theperoxide to give two new radicals which can continue the oxidation(5).There are two basic types of anti-oxidants, radical- and peroxide-destroying. Radical-destroying stabilises free radicals by donating ahydrogen atom in reaction (1). Phenols and amines are well-knowntypes. Peroxide-stopping prevents the formation of additional freeradicals by decomposing the peroxide to a more stable compounds.

The formation of radicals is important for stability. It is not fullyunderstood, but the metal complexes we have already discussed (seepage 31) are important. These metal complexes act as a catalyst foroxidation. Some complexes, though, are also known to be inhibitors.

RH ➝ R˚ (1)

R˚ + O2 ➝ RO2˚ (2)

RO2 + RH ➝ RO2H+R˚ (3)

RO2H ➝ RO˚ + OH˚ (4)

RO˚ + RH ➝ ROH + R˚ (5)

This is comparable to the vitamins our bodies need. We need a complexmix of different kinds, where C vitamins act as a water soluble andvitamin E as an oil soluble inhibitor radical destroyer. We also needsome traces of peroxide inhibitors. It is well known that smokers needextra vitamins to protect themselves from radicals created in their bodies.

33

The oxidation process of different oilsFigure 8, below, illustrates four types of oxidation behaviour, describingwhat happened to different oils during oxidation.

Curve A is a white oil or some other absolutely clean oil. In this oilthere is nothing to stop the oxidation, and you will find a high contentof acid and yellow sludge as oxidation products, and also an increase inviscosity (ageing without vitamins).

Curve B is an oil that contains natural inhibitors, and at the beginningof oxidation, peroxide-stopping additives (natural) stop the creation ofradicals from peroxides. After some time, various radical-stoppingmolecules have been produced, e.g. polyaromatics, whose oxidationproducts are highly reactive phenols. The sludge produced is black.(The oil contains trace elements of peroxide destroying inhibitors fromthe start and produce radical stopping inhibitors during the ageingprocess).

Curve C contains only phenol in a white oil. After some time theinhibitors have been consumed and oxidation accelerates as in a cleanwhite oil. (Only vitamins without trace elements).

Curve D contains both natural and synthetic inhibitors. The type andamount of the natural inhibitors are less than in a fully optimized oil.Normally an oil produced to give a good response to inhibitors doesnot have the high PAC content necessary in an oil optimized to be non-inhibited. As oxidation products, these PAC-based molecules can act asradical-destroying inhibitors and prevent further oxidation.

Curve E shows the peroxides (see reaction (3), facing side) produced forthe uninhibited oil in curve B. These peroxides also form a complexwith metal ions.

34

Figure 8

Oxidationbehaviour

Top

Time

A

B

C

D

E

Oxidation testsThere are several methods for testing the oxidation stability of an oil.Remember, though, that oxidation is not the same as thermal stability.Thermal stability is the temperature when an oil starts to decomposewithout oxygen. The cracking temperature can be as high as 350 °C.

The oxidation tests most commonly used in Europe are the IECmethods IEC 74, IEC 474 and the new IEC 813, methods which aretoday compiled in IEC 1125 as A, B and C respectively. In Germany,DIN 51 554 is used for testing oils.

IEC 74 or IEC 1125 A has long been in use for testing uninhibited oil.It employs copper as a catalyst and with oxygen bubbling through theoil at 100 °C. Sludge and acid number are measured after the test.

IEC 474 or IEC 1125 B mainly measures the volatile acids occurring asoxidation products. The test temperature is 120 °C. The catalyst iscopper and oxygen is bubbled through the oil.

A high induction time does not necessarily mean that the oil is bettersuited for use in a transformer than another oil with a lower inductiontime. The oil can be black and contain a large amount of non-solublesludge, but only produce a small quantity of volatile acids.

IEC 813 or IEC 1125 C measure both volatile and oil-soluble acids.The sludge is determined in the same way as in IEC 74 and 474. Thetemperature is 120 ˚C and a low flow of air is bubbled through the oilwith copper as a catalyst.

ASTM D 2112 is intended for transformer oils inhibited with phenolictype of inhibitors. The test is made at 140 °C in the presence of copper,water and with an over pressure of oxygen (90 psi). The time for the oilto react with a certain amount of oxygen is reported as the result.

ASTM D 2440 is similar to IEC 74/IEC 1125 A, apart from thetemperature which is slightly higher, 110 °C, and the test durationwhich is 72 and 164 hours.

DIN 51 554 or the Baader test in which there is no air flow throughthe oil and it is only air over the oil which takes part in the oxidationprocess. The temperature is 110 °C and the catalyst is copper. Saponi-fication number, sludge and dissipation factor are measured after thetest. The measurement of saponification number instead of acid numberis a little doubtful, because oxidation products not harmful to thecellulose or the copper will contribute to the result.

The main question when discussing the above methods is which methodgives the best correlation with an oil in service. The only way toestablish this is to correlate results from transformers that have the sameload and construction and that are filled with different oils that havedifferent levels of oxidation stability.

35

Comparison of oxidation resultsLet us now compare three different oils. The first oil, Nytro 10GBN, is an uninhibited oil. The second Nytro 10BN is an uninhibited oil,which has been optimised for high performance in the Baader test andalso given high performance in IEC 813. The third is Nytro 10X, an oil developed for high response to a synthetic inhibitor DiButylParaCresol (DBPC). But the base oil for Nytro 10X also meets the IEC74 limit before the inhibitor is added.

In the table we can see that the inhibitor in the inhibited oil Nytro 10Xreacts and forms a stable ester which gives a high saponification value.This is a very stable molecule and, therefore, will not present anyproblems in the transformer. Note that the acid number of Nytro 10Xafter 28 days of Baader test is still very low – the same as for new oil –and that the same is almost true of the tan delta.

36

Nytro Nytro Nytro IEC 29610GBN 10BN 10X Specification

IEC 1125 A

NV mg KOH/g 0.11 0.08 <0.01 <0.4Sludge wt% 0.02 0.02 <0.01 <0.10

IEC 1125 B

Induction period h <50 >80 >236 ≥120

IEC 1125 C, 164 h

NV total mg KOH/g 0.27 0.17 <0.01Sludge wt% 0.06 0.06 <0.01

IEC1125 C, 500 h

NV mg KOH/g 0.04Sludge wt% <0.01

ASTM D 2440 72 h

Total acid no, mg KOH/g 0.14 0.09 <0.01Sludge wt% 0.03 0.02 <0.01

ASTM D 2440 164 h

Total acid no, mg KOH/g 0.18 0.10 <0.01Sludge wt% 0.05 0.04 <0.01

DIN 51554 Baader test, 140 h

NV mg KOH/g 0.03 <0.02Sludge wt% 0.02 <0.01Saponification no, mg KOH/g >0.6 0.20 <0.05Tan Delta 0.04 <0.005

DIN 51554 Baader test, 28 days

NV total mg KOH/g 0.08 0.02Sludge wt% 0.06 <0.01Saponification no, mg KOH/g 0.30 0.11Tan Delta 0.10 0.005

Interfacialtension

Days at100°C

40

30

20

10

0 2 4 6 8 10 12

0.05

0.045

0.04

0.035

0.03

0.025

0.02

0.015

0.01

0.005

0

Acid value mg KOH/g and tan delta

Nytro 10 BN

Acid value

Interfacial tension

tan delta 90°C

We may also note, in IEC 813 164 h and 500 h, that the Nytro 10BN oilhas flattened out. The radical-destroying inhibitors, formed as oxidationproducts, are still working. Other studies show the behaviour to be astep-by-step increase in oxidation.

Nynas has also tested other methods. In Figures 9 and 10 you will findsome graphs illustrating what happens when oxidation starts in aninhibited oil and in an uninhibited one (Nytro 10X and Nytro 10BNrespectively). An interesting point is that interfacial tension is one of thefirst parameters to change during oxidation.

ConclusionDifferent methods produce different results, and so in optimising theproduct we have to consider several methods and not just one.

The inhibited oil Nytro 10X shows perfect response to the inhibitorand scores better than the uninhibited in all the tests we haveperformed. The long Baader test period shows the same. However, ingeneral the addition of an inhibitor to an oil is no guarantee for superior

37

Figure 9

Open beaker testat 100 °C,Nytro 10X

Figure 10

Open breaker testat 100 °C,Nytro 10BN

The test was performed in an open glass beaker at 100 °C with air above the oil. A copper strip is used as a catalyst. The method is used by Nynas Naphthenics AB for screening.

Nytro 10 x

Acid value

Interfacialtension

Days at100°C

40

30

20

10

0 2 4 6 8 10 12

0.05

0.045

0.04

0.035

0.03

0.025

0.02

0.015

0.01

0.005

0

Interfacial tension

Acid value mg KOH/g and tan delta

tan delta 90°C

performance. The oil has to be refined in advance to a high level to besensitive to the inhibitor intended. A traced content of PAC moleculestogether with the phenols, may produce antagonistic effects.

Nytro 10BN meets all the requirements for an uninhibited oil,including the Baader test.

Nytro 10GBN meets the normal standard requirements and generatesless gas than the other two oils.

3.15 GASSING TENDENCYSome gassing will always occur in a transformer oil when it is exposedto partial discharges. This is because some molecules will reach a higherenergy level and fragments will be detached from these molecules. Thefragments found in oils are H2, CH4.

If gas is produced in large quantities which are trapped because of theconstruction of the transformer, the bubbles formed are dangerous tothe transformer. The reason is that they can cause an electricalbreakdown due to the bad insulating properties of the gas compared tothe oil. This is a known fact in the cable industry, where gas-absorbingoils have been used for many years.

In modern transformers these problems should have been solved in thedesign with a low amount of partial discharges and good oil circulation.However some designs might have to be compensated with gas-absorbing oils.

To find out to what degree an oil will absorb gas, the old Pirelli methodthat was developed for cable oils is still used, though it has beensomewhat altered as specified in IEC 628 A and ASTM D 3484. Todaythere is an alternative method available, that developed in Germany, isnow used across Europe. Known as the Soldner-Muller method, it isincluded in IEC 628 as method B. The gas used in the Pirelli method ishydrogen, and in the Soldner-Muller method nitrogen. Hydrogenabsorption is quite clearly understood, but the nitrogen reaction isunclear.

3.16 IMPULSE BREAKDOWNImpulse breakdown is a property which is not usually described in themajority of specifications. Breakdown behaviour with DC impulse anda heterogeneous gap is very different from the AC strength. It isdesigned to simulate lightning striking a transformer during athunderstorm. The result is independent of the contaminantsinfluencing the normal test, IEC 156.

38

A needle and steel ball are used, as electrodes, at a distance of 2.5 cm.With a negative impulse to the needle, the breakdown has been found todepend on the degree of refining of the oil, with lower aromatic contentgiving a better/higher value. Some manufacturers of heavily loadedpower transformers are interested in having a high impulse breakdown.The methods used, IEC 897 and ASTM D 3300, are quite similar, andthe ASTM specification requires a minimum value of 145 kV negative.This value is obtainable with most oils on the market.

3.17 INFLUENCE OF PAC ON GASSING PROPERTIES AND IMPULSE BREAKDOWNIt is well known, and it has been demonstrated in a number of articles,that aromatic molecules affect gassing properties and impulse break-down. For gas absorption, high aromatic content is desirable. Thearomatics react in a transformer in the same way as in the hydrogenationprocess in essence, absorbing hydrogen by saturating aromatic struc-tures. In impulse breakdown, oils with high aromatic content, particu-larly with high PAC content, show low breakdown values. We also knowthat monoaromatics give a relatively high value.

To test this, we constructed a test programme based on naphthenic oilswith different degrees of refining to examine impulse breakdown andgassing properties. These measurements were carried out in the USA byDoble Engineering Company, please see table below.

We correlated these results with our own HPLC method for determi-nation of PAC-content. This method works on the principle that thepolyaromatics are more polar than mono- and di-aromatics. We devel-oped the method as a quicker and less solvent-consuming alternative tothe IP 346 method. In this method a nitrile-type liquid chromatography

39

Oils tested:Polyaromatics Aromatic Impulse Gassingwt% content % breakdown tendency

kV µl/minHPLC method IEC 590 ASTM D3300 ASTM D230 B

0.01 5 > 300 +32.90.02 7 282 +26.20.07 10 220 +16.30.30 10 196 +11.30.75 10 148 +4.5

column (Alltech 600 CN) is used and the different aromatics areseparated by polarity differences. By reversing the flow when mono-and di-aromatics have passed, the remainder comes out as one peak. Weuse naphthalene as a marker and hexane as a solvent. This methodmeasures not only polyaromatics but also other polar compounds, suchas nitrogen and sulphur-containing molecules. The properties measuredcan be considerably influenced by nitrogen, especially basic nitrogen.

Note that the last three oils have the same aromatic content by IRmeasurement, but differ a great deal in PAC content. The difference inaromaticity also depends on the distillation range, as we saw earlier.These oils, however, have the same boiling range.

Nynas has analysed different oils on the market, and it is clear thatuninhibited oils giving high oxidation stability also have a high PAC, insome oils it is possible to find 6, 7 and 8-ring PACs. As describedearlier, these polyaromatics act as natural inhibitors that are added tothe oil in order to protect the oil from oxidation.

3.18 STREAMING CHARGINGWhen an oil is pumped through a duct, as in a transformer, negativelycharged species from it can be adsorbed by the material on the ductwalls. This means that the oil will be positively charged when leaving

40

Figure 11

Impulse break down

Figure 12

Gassing tendency

Impulsebreakdown,kV

0.010.020.070.300.75

Polyaromatics HPLC

Aromaticcontent

Impulsebreakdown

Gassingtendency

57

101010

>300282220196148

+32.9+26.2+16.3+11.3

+4.5

Gassing tendency, µl/minASTM

Polyaromatics HPLC% b.w.

1

0.1

0.01

120 140 160 180 200 220 240 260 280 300

Polyaromatics HPLC% b.w.

0.010.020.070.300.75

Polyaromatics HPLC

Aromaticcontent

Impulsebreakdown

Gassingtendency

57

101010

>300282220196148

+32.9+26.2+16.3+11.3

+4.5

1

0.1

0.01

0 4 8 12 16 20 24 28 32

the duct. Some authors claim that this is a serious problem intransformers, and a lot of research is in progress to explain thephenomenon. Because different transformers have different coolingsystems, the occurrence may be more serious in some types oftransformers than in others.

Our oils were tested in this respect in the USA at the DobleEngineering Company. The method used was presented in IEEE Trans.PAS-103 No. 7 1984.

The graph makes it clear that streaming charging is low in a clean oilwith a low content of polar molecules.

Once again, the aromatics in themselves are not so very important inthis respect, but the basic nitrogen existing in oils at ppm or even ppblevels are more influential. It is extremely difficult, however, to measuresuch small amounts in a transformer oil that contains several thousandmolecules.

It may be concluded, though, that highly-refined oils are better thanlow-refined ones.

It is shown that well-refined oils give a low value, but what happenswhen an oil starts to deteriorate?

A very mild open-beaker oxidation test was performed at atemperature of just 100 °C. The results clearly show that an inhibited oilgives a low result compared with a non-inhibited oil.

From this we conclude that if a low value of streaming charging iswanted, the oil must be well-refined and inhibited.

41

Figure 14

Streaming charging afteroxidation 48h and 72h

Figure 13

Streaming charging

292434

High refined oil InhibMedium refined UninhLow refinedMedium refinedparaffinic

0.020.52.02.4

Streamingcharging

Streamingcharging, µl/m3

PAC, % b.w.

Streaming Charging uC/m3

Uninhibited

Inhibited oil

Time h

Streaming Charging lower for clean oil compared to oils that are refined to be notinhibited (Hetero and PAC molecules)

Oil type PAC HPLC

2.4

2.2

2

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

2 6 10 14 18 22 26 30 34

250

200

150

100

50

01 48 72

42

3.19 HEALTH AND SAFETYLow refined oils have shown evidence of being carcinogenic by theInternational Agency for Research of Cancer (IARC), a body withinthe World Health Organization (WHO). The finding is based onepidemiological studies made in the past, in for example the textileindustry, where low refined oils were used. Although the correlationexists, it is important to relate the risk to other substances such as Benzalpha Pyrene (B(a)P) which is much more dangerous. Given goodpersonal hygiene when handling transformer oils, the risk can benegligible. Low refined oils have to be labelled in accordance with lawsin different countries.

For highly refined oils, no epidemiological studies show any risk ofcancer.

The main question is how to specify a low refined carcinogenic oiland a high refined oil that is not carcinogenic. The following points willgo some way in answering these questions.

In Vivo test. The only test accepted is skin painting of experimentalanimals. The literature contains much data to show that low refined oilsare carcinogenic while highly refined oils are not.

In Vitro test. The Ames test, in which micro-organisms are used, is themost common method to predict carcinogenicity in oil products. Insome cases it gives a lower threshold value than the skin painting test.

Analytical methods The molecules responsible for giving cancer aremainly the 3-7 ring aromatic in the oil. Analytical methods have beendeveloped to measure the PAC content in the oil, IP 346, GCMS (GasChromatography Mass Spectrometer) and HPLC methods, etc. Theonly method that has been shown to correlate to the skin painting test isIP 346. The European Union has decided to use this method as a markerto predict carcinogenicity (oils with values less than 3% are not to belabelled as carcinogenic as of 1994). In other parts of the world othermethods to predict carcinogenicity are discussed, e.g. the Ames test.

Oils refined with old technology, such as acid clay treatment or onlymild hydrofinishing, or when the oils have been subjected to discharges,the polycyclic content can be high. These kind of oils should thereforebe handled with extra care and the proper precautions should be taken.

Nynas transformer oils are produced by severe hydrotreatment, yieldinglow PAC content. According to IP 346, the PAC content measured asDMSO extractable compounds of Nynas transformer oils is below 3%.The oils are labelled in conformity with EU regulations.

43

Oils for the futureToday, Nynas Naphthenics have in their product portfolio a number of differentoils with various properties and performances. These oils are used in differentelectrical equipment such as HVDC installations, power transformers, switchgears and distribution transformers, as well as cable oils.

In the future, our opinion is that the basic line of transformer oils will consist of three grades:

High grade, inhibited (Nytro 10X) Inhibited, with synthetic inhibitors having excellent electrical properties.

High grade, uninhibited (Nytro 10BN) Uninhibited, with high resistance to oxidation.

Standard grade (Nytro 10 GBN) Higher aromatic content and therefore gas-absorbing.

As a supplier, we ourselves cannot develop or produce an oil for the future. Whatwe have to do is to build up a knowledge of the chemical structure and of itseffects on different properties. We also have to adapt our refining technology to amore efficient way of producing the oils. However, deciding what properties areimportant for the transformer is not our main task. That is something which you,as a customer, user or producer of transformers, have to decide. We will then beable to develop and produce tailor-made products with, hopefully, an acceptablecost/performance level.

44

Characteristics Designation 1 2 3Density ISO 3675 ASTM D 1298 IP 160Kinematic viscosity ISO 3104 ASTM D 445 IP 71

Flash point (Closed cup) ISO 2719 ASTM D 93 IP 34/304Pour point ISO 3016 ASTM D 97 IP 15Interfacial tension ISO 6295 ASTM D 971Colour ASTM D 1500 IP 196

Corrosive sulphur ISO 5662 ASTM D 1275 BS 5680Carbon type composition IEC 590 *ASTM D 2140Carbon residue ASTM D 189 IP 13 *ASTM D 524Water content IEC 814 ASTM D 1533 BS 6470Neutralization value IEC 296/82 *ASTM D 974 *IP 139

Breakdown voltage AC IEC 156 BS 5874 *ASTM D 1816Dielectric dissipation factor IEC 247 BS 5737 *ASTM D 924DC-resistivity IEC 247Impulse breakdown Voltage IEC 897 ASTM D 3300Gassing tendency IEC 628 A BS 5797 *ASTM D

2300 B

Oxidation stability: IEC 1125 A IEC 74IEC 1125 B IEC 474IEC 1125 C IEC 813DIN 51554ASTM D 2112ASTM D 2440 *Comparable

methods

REFERENCE LIST OF METHODSListed below are the methods used in this handbook to characterise transformer oils.Some methods have several designations, the most common of which are mentionedhere. In some cases comparable methods are listed, which are marked with an asterix.Please note that a comparable method might give a different value for the samecharacteristic.

45

COMPARISON OF SOME REQUIREMENTS FOR CERTAIN SPECIFICATIONS.

Characteristics IEC 296 (Class II) ASTM D 3487 BS 148/84

Density at 20 oC ≤0.895 ≤0.91 *5 ≤0.895Viscosity at 40 oC mm2/s ≤11.0 ≤12.0 ≤11.0Viscosity at -30 oC mm2/s ≤1800 ≤1800Pour point oC ≤-45 ≤-40 ≤-45Flash point PM oC ≥130 ≥130Flash point COC oC ≥145Neutralization value mg KOH/g ≤0.03 ≤0.03 ≤0.03Gassing properties µl/min – ≤30 *1 ≤5 *1

Antioxidant content for uninhibited oils % not detectable ≤0.08 not detectableWater content ppm ≤30-≤40 *2 ≤35 ≤30-≤40 *2

Interfacial tension mN/m ≥40 -Breakdown voltageas delivered kV ≥30 – ≥30Treated kV ≥50 ≥70 *3 –Dielectric dissipation factor at 90 oC 0.005 0.003 *4 0.005Oxidation stability No comparable requirement

*1 Not comparable figures. Given rather to indicate that there are requirements.*2 The lower level for bulk deliveries and the higher for drum deliveries.*3 Calculated to comparable figure.*4 At 100 °C*5 At 15°C

APPENDIX 1.BASIC CHEMISTRY OFTRANSFORMER OILS

INTRODUCTION

Before starting to discuss the refining of transformer oils, a basic under-standing of the chemistry of the oil is needed.

A mineral transformer oil consists mainly of carbon and hydrogen inmolecules with different structures.

1. THE THREE LETTERS WHICH GIVE THE BASIC STRUCTURE OF A MINERAL TRANSFORMER OIL

for paraffinic structure. This group of molecules can either be straightor branched. The straight type Normal-alkanes (N-alkanes) are knownas waxes. If oils containing N-alkanes are cooled down, their free flow isimpeded. Below the cloud point, these oils are non-Newtonian, and theirN-alkane content has to be reduced before they can be used in a coldclimate. Molecules of this type also have low solubility for water andoxidation products. This may cause problems in the form of precipitatedsludge in the ducts of the transformer. Paraffinic molecules have lowerthermal stability than naphthenic and aromatic molecules.

for naphthenic structure. The molecules in this group are also knownas cycloalkanes. Characteristics: they have excellent low-temperatureproperties and better solvency power than N-alkanes. There are either 5,6 or 7 carbons in the ring structure, but the 6 ring predominates.

46

N

P

Figure 1

Basichydrocarbonstructures inmineral oil

Paraffin Isoparaffin

Naphthenes

Aromatic Polyaromatic

for aromatic structure. All transformer oils contain aromaticmolecules, and this is probably the most important group to discuss.

The aromatic molecules contain at least one ring of six carbon atomswith alternating double and single bonds as their characteristic parts.They are totally different from paraffinic and naphthenic molecules,both chemically and physically.

2. INFLUENCE OF AROMATIC MOLECULES ON THE TRANSFORMEROIL PROPERTIES.Monoaromatics in transformer oils are always alkylated and generallyhave good electrical properties, as well as being gas-absorbents. Theyare relatively stable in oxidation.

Polyaromatics, PACs, are either produced in the hydrogenation processor exist naturally in the oil. With increasing boiling range the PACcontent is normally increased too.

These groups have the following properties:

DESIRABLE* Oxidation inhibiting: during the oxidation process, phenols are pro-

duced which act as a radical destruction inhibitor. In old transformeroils, phenols that have been produced as oxidation products can be measured. Compare this with DiButyl Para Cresol (DBPC), which is added to oils as an inhibitor.

* High gas absorption, superior to that of monoaromatics.

UNDESIRABLE* In an electrical field the aromatic molecules have a negative effect

on such electrical properties as impulse breakdown and streaming charging.

* Some of these molecules are known to be carcinogenic.

3. HYPOTHETICAL OIL MOLECULEOnly polyaromatics and N-alkanes occur in oil as single molecules. Allthe others are combined into molecular structures of various kinds, thecombination of the three letters give words of different meaning.

The figure facing page illustrates a typical oil molecule. One way ofcharacterising oils is by carbon type analysis. There are several methodsfor measuring carbon type, one of them being IR to measure the carbonbonded to the aromatic structure and the carbon bonded to the paraf-finic structure.

47

A

4. HETEROATOMS IN THE OILAll oils contain a small percentage (by number) of hydrocarbonmolecules, including in their structure other elements like nitrogen,sulphur and oxygen. Nearly all the heteroatoms in an oil are bonded toaromatic structures, and if all aromatics are removed by physicalseparation, all sulphur, nitrogen and oxygen are removed as well.

4.1 NitrogenNitrogen-containing molecules can either be basic (e.g. quinolines,pyridines) or non-basic (e.g. carbazoles, pyrolles). The nitrogen contentof transformer oils is relatively small (in the ppm range) but it makes abig difference to their characteristics.

* Some of the nitrogen-containing molecules are charge carriers in an electrical field.

* Some of them act as initiators of the oxidation process, and adding just a few ppm of a basic nitrogen containing molecules to an oil destroyed its oxidation stability.

* Some of them act as passivators of copper or other metals.

* Some act as inhibitors.

48

Figure 2

Figure 3

Heteroatoms in mineral oil

= C in paraffinic structure CP = 32%

= C in naphthenic structure CN = 44%

= C in aromatic structure CA = 24%

X

NOH

X=S, N, O

Carbazol

Pyridin Phenol

NH

N NNH

Pyrolles Carbazoles Quinolines PyridineNon-basic nitrogen Basic nitrogen

4.2 SulphurThe sulphur containing molecules in the oils can give both negative andpositive characteristics to the oil. The sulphur containing molecules ofsome types can cause corrosion of copper and silver. They can also actas peroxide-destroying inhibitors in the oxidation process. One studywhich Nynas carried out showed that the more effective they were asinhibitors, the more reactive they were in copper corrosion. Hundredsof different types exist, but thiofens, carbazoles and sulphides can befound in mineral oils. Mercapto sulphur exists in unrefined oils but mayalso exist, as an intermediate oxidation product, in used transformeroils. Slightly oxidised transformer oils, therefore, may be corrosive tocopper.

As a rule, the sulphur content in a distillate (unrefined oil) increaseswith increasing boiling point.

4.3 OxygenIn new transformer oils the content of oxygen bound to hydrocarbonsas heteromolecules is small. In used oils the bonded oxygen content ishigher, due to oxidation in which acids, ketones, phenols and otheroxygen containing molecules are formed. As stated earlier, the phenolsmay act as radically destroying inhibitors. Water is also an oxidationproduct. It is particularly destructive in oils and may cause the paper todeteriorate with excessive rapidity.

Some of the above molecules are strongly polar and will be oriented inan electrical field giving field losses. Some of them may act as dispersingagents for water.

The chemically bonded oxygen must not be confused with thephysically dissolved oxygen gas. The latter can be removed by degassing.

5. VARIATION OF HYDROCARBON TYPESThe content of the different types of hydrocarbons in an oil varies fromone crude to another, and the resulting amount after refining variesfrom one process to another.

49

Figure 4R R R R

** Normally the level is so high that the feedstock has to be considered ascarcinogenic. A higher boiling point gives a higher value. The HPLCmethod is described in the section on requirements for transformer oils.

The difference between transformer oils and crude oils is that theformer are categorised as »naphthenic oils« and the latter as »paraffinicoils«. However, there is no sharp distinction between the two types ofoil, but rather a sliding scale that ranges from the very paraffinic to thevery naphthenic. This characterisation is based on the IR measurementof the paraffinic content which is usually grouped as shown below.

Oils with;

* CP 42-50% are considered as naphthenic oils

* CP 50-56% intermediate oils* CP 56-65% paraffinic oils

The above levels are not exactly defined, but more intended as guide-lines. Some hydrocracked oils have more than 65% in CP (not naturallyoccurring).

Variation of hydrocarbon type composition in transformer oils dependson the feedstock, the processing type and degree, and also on theintended use of the oil: non-inhibited versus inhibited etc. Non-inhibited oils normally have a higher PAC level as do gas-absorbingoils.

50

Type Amount

N-alkanes wt% <0.05-15%CP (Carbon bonded paraffinic) by IR-method 42-65% *

CN (Carbon bonded naphthenic)by IR-method difference to 100%

CA (Carbon bonded aromatic) by IR-method 14 - 25%

PAC HPLC% >2 **

Sulphur wt% 1.0-2.0

Nitrogen ppm 70-600

Oxygen as hydrocarbon acids, expressed inmgKOH/g 0.05-2.0

Variation in distillates, the raw material for production of

transformer oil.

Below is a broad overview of the chemical composition of productsexisting on the market today.

Note that oils with these variations meet the normal standardsexisting today, but in service some of them will behave quite differ-ently from what is expected.

51

Type Amount

N-alkanes wt% <0.1-10CP % by IR-method 42-65CN % “ difference to 100 %CA % “ 5-20PAC HPLC % 0.02-2.5Sulphur % 0.01-1.0Nitrogen ppm 1-300Acid number <0.01-0.03

APPENDIX 2.REFINING TECHNIQUES

Refining is the collective term for the processes, the refining steps, usedto change the properties of mineral oils to desired ones. Basically theseprocesses can be divided into physical and chemical ones, and fullrefining is normally a mixture of the two types of processes.

Crude oil is the raw material from which transformer oil is produced.Crude oils are extracted in several different parts of the world, the bestknown being the North Sea, the Middle East and Venezuela. Crude oilscan initially be divided into light and heavy types, but also intonaphthenic and paraffinic types. The naphthenic crudes are normallyrich in bitumen and heavy distillate, which also puts them in the heavycategory.

The paraffinic crudes, on the other hand, are often rich in gas oil,gasoline and gases, which puts them in the light category. The crude oilreserves for wax-free naphthenic oils are enormous, and new fields ofnaphthenic oil are still being found in various parts of the world.

The selection of crude oil will depend on whether the refinery is aspeciality producer or a fuel oil refinery. For most of the refineries, thelubricating oil sector is only a minor part of their activities. Only 1% ofall products from crude oil are used as lubricating oils, which includesoils such as motor oils, process oils and transformer oils.

52

Figure 1

Heavy crude oil1%

8%

19%

72%

15%

25%

25%

35%

32%

37%

13%

18%

Bitumen Light gas oil

Heavy gas oil Gasoline

Venezuelan Heavy Northcrude oil Arabian Sea

1. REFININGTo produce transformer oils from crude oil a number of sequences/stepsare used, please see figure 3 below. In a typical “refining train” some ofthe steps described may be excluded due to type of oil and techniqueused.

2. DISTILLATIONIn a “refining train”, the first step is always distillation. In this processthe crude oil is separated into distillates with different boiling pointranges by fractionation. For light crudes this is done under normalpressure, but for heavy crudes, or for further fractionation of heavyresidues, the fractionation is done under vacuum conditions. Vacuumtechniques will lower the boiling point for the hydrocarbons and allowfractionation of heavier molecules. The maximum temperature forfractionation is around +350 °C. Above this temperature thermaldecomposition (cracking) of the oil will start. This process takes place ina fractionating tower and several distillates and a residue, normallybitumen, are produced at the same time and continuously.

Different viscosities are obtained for different oils with a given boilingrange, depending on their chemistry. Paraffinic oils have a higher boilingrange for a given viscosity compared with a naphthenic. This is due tothe higher mobility of a paraffinic molecule compared with a naphthe-

53

Figure 2

Products from crude oil

Figure 3

Segment process oil/base oil· 90% Paraffinic process oil/base oil· 10% Naphthenic process oil/base oil

Fuel 96%

Process oil/base oil1%

Bitumen 3%

➚➚Distillation Dewaxing Extraction Hydrogenation

nic. For distillates from the same type of crude oil, there exists a roughcorrelation between the 50% point on the distillation curve and theviscosity. The flashpoint correlates with the beginning of the distillationcurve, namely the 5% point. This applies to well-fractionated oils.

3. DEWAXINGNaphthenic crude contains hardly any N-alkanes and does not require adewaxing step, but for paraffinic crudes dewaxing is necessary in orderto achieve reasonable low temperature properties.

Traditional dewaxing cannot remove all waxes from an oil. There willalways be waxes below the dewaxing point chosen. The principle forthis process is to blend the oil with a solvent and then cool it down. TheN-alkanes are then allowed to crystallise into long needles and the waxis removed by filtration. After filtration, the solvent is removed fromthe oil by distillation. Some refiners are equipped for “deep dewaxing”,giving -30 °C as pour point, but below this point the oil will still besolid. Other refiners use higher temperatures in their process and usepour point depressants to compensate the higher temperature and toreduce the pour point. These pour point depressants can either be of theesther type or the hydrocarbon type.

54

Figure 4

Figure 5

Separation of N-paraffins from the oil by use of a solvent and then cooling the blend and filtering ofthe wax.

FilterPour point

Feed +20°CProduct -15°C

Solvent

Oil

Oil

Blending Cooling and forma-tion of wax crystals

Wax

Feed

Solvent

Oil

Separation

5 % %50 100 5 50 100

Temp. Temp.The 5% point correlateswith the flashpoint

The 50% point correlateswith the viscosity

Dist. curve Bitumen

Crude

?O2@e?O20M?e

?O20M?f?O20M?g

?O20M?hO2@@@@@0M?he

?O2@@@@@@@0M?O2@@@@0M?

?O2@@0M??O20M?

?O20M??O20M?

?O20M? ?O2@@0M? O2@0M?

O20Mf?O2@0Mg

O2@@@0M?hO2@@@@0Mhg

?O2@@@0M?O2@@@@0M?

?O2@@0M??O20M?

?O20M??O20M?

?O20M? ?O2@@0M? ?O20M?

?O20M?e?O20M?f

?O20M?g?O2@@@@@@0M?h

?O2@@@@@@0M??O2@@@@0M?

?O2@@0M??O20M?

?O20M??O20M?

?O20M?@0M? O2@?

O20MeO20Mf

O20MgO20Mh

?O2@@@@@0MheO2@@@@@@@0M?

O2@@@@0MO2@@0M

O20MO20M

O20MW20M.M

W.g??O.Yg?

?W20Y?g?W.M?h?

?W.Yhe?W.Y?he?

?O.Yhf??W20Y?hf?O.M?hg?

O20Y ?O2@@0M ?

?O2@0M ??O2@@0M? ?

O2@0M? ??O2@0M ?

O2@0M? ??O2@0M ?

O2@0M? ?W20M ?

?W.M ?O.Y? ?

W20Y ??W.M ?W.Y? ?

?W.Y ??7H? ?J5 ?

?W.Y ?W.Y? ?7H ?

?J5? ?W.Y? ?.Y ?

??????????

4. EXTRACTIONThe extraction step is one of the oldest methods of removing unstablemolecules from the distillates, and it is still in use. In this process the oilis blended with a solvent (e.g. SO2 or furfural). The mix separates into araffinate phase and an extract phase rich in aromatic and heteroaromaticmolecules.

The amount of aromatics in the raffinate phase is between 5 and 11%. Itis hard to achieve less or more than this, it is depending on the equili-brium between the two phases.

Most of the polyaromatic molecules (PAC) are found in the extractphase. The following table shows individual PACs in two differentextracts. Both these extracts are now considered carcinogenic and haveto be labelled with the skull and crossbones.

Individual PAC ppm

The above figures provide some indication of the types of PACsoccurring in low-refined oils.

Extract A is a normal extract from a naphthenic distillate. Extract B isfrom a higher boiling fraction. These figures also illustrates that whenthe boiling point increases, the number of rings in the PACs alsoincreases.

Due to labelling requirements and high sulphur content, the alternativecommercial value for these streams is lower today than it was someyears ago.

55

Type OIL A OIL B

Phenanthrene 2300 250Anthracene 89 29Benzo(b) fluorene 29 6.3Benzo(b,j,k) fluoranthene 0.5 8.1Perylene <0.5 6.4

Figure 6

Extraction.Aromaticcompounds areremoved by the useof a polar solvent:sulphur dioxide,furfural etc.

CA%

D = Distillate 20-25R = Raffinate 5-12E = Extract >40

R

E

Solvent

Solvent

Oil

Mixing

Raffinate DistD

Extract

Dist

5. ACID CLAY TREATMENTSulphuric acid is the most versatile refining agent known. This acid actsboth as an extraction medium and as a reactive agent, depending ontemperature and concentration. Oleum is still used in some countriesfor producing white oils, but it is steadily being superseded byhydrogenation. White oils are totally free from aromatics andheteroatoms. After the process, the excessive acid has to be neutralised,normally with lime or soda.

Acid clay treatment is an obsolete process today, due mainly to theenvironmental impact of its waste products.

Activated clay is still used as a final finishing to remove trace impuritieswhich are adsorbed on the clay surface. The amount of clay used forthis finishing is low.

6. HYDROGENATIONThe above techniques (extraction and dewaxing) are based on physicalmethods: i.e. separating off unwanted molecules. A more modernmethod, hydrogenation, is based on the chemical conversion of thesemolecules into desired ones. This is done with the help of a catalyst,hydrogen and high pressure/temperature.

In the hydrogenation process, polar compounds, aromatics, hetero-atom-ic molecules are adsorbed on a catalyst surface, where they react withthe hydrogen “dissolved” in the catalyst.

The catalyst itself can be described as a porous inert mineralimpregnated with catalytically active metals which adsorb polarmolecules and favour their reaction with absorbed hydrogen. Thecatalytically active surface area is very large (up to 200 m2/gramcatalyst).

At low severity, low pressure/low temperature and high space velocity,only sulphur, oxygen and nitrogen will be removed as H2S, H2O andNH3.

56

Figure 7

Hydrogenationprocess. X = S, N, O.

Light products, H2S, NH3, H2O

HIGH SEVERITY

MEDIUM SEVERITY

X

CA 25%feed

H2

H2

CA %20-25

CA %2-17

H . H . H . H .

H .H . H . H .

H . H . H .H .

H . H . H .

H . H .H .

XX

X

CATALYST

CATALYST

When the severity is increased, the aromatic rings are opened andsaturated; exactly how much depends on the degree of severity. Thereaction sequence depends on polarity, i.e. polyaromatics are reducedfirst, leaving monoaromatics in the oil.

If a more acidic type of catalyst is used, more cracking will take place,giving more light products, e.g. gas oils and gasoline. This type ofcatalyst is not used by the lube producers.

The light ends and other products produced in the hydrogenationprocess are removed from the oil afterwards by stripping/distillation.

The H2S from the process is converted into pure sulphur in a Clausunit. This makes the hydrogenation process an environment-friendlyprocess with a high yield of product and a small amount of wasteproducts. Its disadvantage is the high investment cost.

57

Nynas development of refining technologyNynas has been producing transformer oils for more than 50years. By refining from low-wax crude oils, no dewaxing step isrequired in our refining train. The company began as atraditional oil company producing products of all kinds – fuel,lubes and bitumen. In 1982, it was decided that Nynas shouldconcentrate on two products – bitumen and speciality oils (ofwhich transformer oils is one).

The first refining process used was SO2 extraction, using acidclay as the final step. The clay treatment was discontinued in1975, for several reasons: the cost/yield ratio and, above all,environmental considerations: the acid/clay resulting from theprocess is considered as toxic waste.

In 1975 Nynas invested in the first hydrogenation unit, andin 1988 in a second one operating at high pressure. Todayhydrogenation as final stop accounts for 100% of totalproduction.

A small clay unit is still used for polishing specialityproducts.

58

Figure 8

This handbook is for information and reference purposes only.

Nynäs Naphthenics AB extends no guarantees, warranties or representation of any kindexpressed or implied with respect to quality or to fitness or suitability for any use of anyproducts/methods mentioned in this handbook.

The terms and conditions for the suitability and quality of a specific product boughtfrom Nynäs Naphthenics AB by a customer will be exclusively as stated in the separatesales agreement for such product.

Acidrefined

Solventrefined

Vacuum- Extraction Hydrogenationdistillation

Vacuum- Hydrogenationdistillation

Highly refined

Vacuum- Extraction Acid/Claydistillation

Medium/Highlyrefined

Solvent refined oil

➞ 1975

1975 ➞

1988 ➞

Development of Nynas refining technology

Responsible CareNynas is a signatory to the international Responsible Care programme of theCEFIC (European Chemical Industry Federation).

The programme is the chemical industry’s commitment to continuousimprovement in all aspects of health, safety and environmental protection.Responsible Care is a voluntary initiative, fundamental to the industry’s presentand future performance and a key to regaining public confidence and maintainingacceptability.

The signatories pledge that their companies will make health, safety andenvironmental performance an integral part of overall business policy on all levelswithin their organizations.

SALES OFFICESAustralia & New Zealand

Nynas (Australia) Pty Ltd. One Park Road, Milton, QLD 4064, Brisbane, AustraliaTel: +61 7 387 66 944. Fax: +61 7 387 66 480

BelgiumNynas N.V., Haven 281, Beliweg 22, BE-2030 Antwerp

Tel: +32 3 545 68 11. Fax: +32 3 541 36 01

BrazilNynas Do Brasil LTDA, Rua Jesuíno Arruda 676, 9th Floor cj. 91, Itaim Bibi, São Paulo, SP 04532-082

Tel: +55 11 3078-1399. Fax: +55 11 3167-5537

CanadaNynas Canada Inc., Suite 610, 201 City Centre Drive, Mississauga, Ontario, Canada L5B 2T4

Tel: +1 905 804-8540. Fax: +1 905 804-8543

ChinaNynas (Hong Kong) Ltd. Beijing, Room 703C, Huapu International Plaza

No. 19 Chaoyangmenwai Street, Beijing, 100020Tel: +86-10-6599 26 95. Fax: +86-10-6599 26 94

ColombiaNynas Naphthenics, Planta Algranel, Antiguo Puente de Bazurto, Manga, Cartagena

Tel.: +57 5660 7850. Fax: +57 5660 8755

France Nynas S.A., Le Windows, 19 Rue d’Estienne d’Orves, F-93500 Pantin

Tel: +33 1 48 91 69 38. Fax: +33 1 48 91 66 93

Germany Nynas GmbH, Berliner Allee 26, D-40212 Düsseldorf

Tel: +49 211 828 999 0. Fax: +49 211 828 999 99

Great BritainNynas Naphthenics Ltd, Wallis House, 76 North Street, Guildford, Surrey, GU1 4AW

Tel: +44 1483 50 69 53. Fax: +44 1483 50 69 54

Hong KongNynas (Hong Kong) Ltd, 1301 Chinachem Johnston Plaza, 178-186 Johnston Road, Wanchai

Tel: +852 2591 99 86. Fax: +852 2591 49 19.

ItalyNynas S.r.l., Via Teglio 9, I-20158 Milan

Tel: +39 02 607 01 87. Fax: +39 02 688 48 20

MalaysiaNynas (Hong Kong) Ltd Rep Office, No. 302, Block A, Kelana Center Point

No. 3, Jalan SS7/19, Kelana Jaya, 47301 Petaling Jaya, SelangorTel: +603 7880 9336. Fax: +603 7880 9366

MexicoNynas Mexico S.A. Emerson 150, 801 & 802, Col. Polanco D.F. 11560, Mexico City

Tel: +52 5 545 38 70. Fax: +52 5 25 00 930

Middle EastNynas Naphthenics, Al Jumeira Road, Arenco 34 Jumeira, 14th Street, Villa No. 27, Dubai, U.A.E.

Tel: 97 150 551 76 88 . Fax: +97 1 43 49 32 81

PolandNynas Sp. z.o.o., Ul. Toszecka 101, 44-100 Gliwice

Tel: +48 32 232 74 10. Fax: +48 32 279 28 50

ScandinaviaNynäs Naphthenics AB Norden, P.O. Box 10701, S-121 29 Stockholm

Tel: +46 8 602 12 00. Fax: +46 8 81 20 12

South AfricaNynas South Africa (PTY) Ltd, Gatesview House A3, Constantia Park

Cnr 14th Avenue and Hendrik Potgieter Street Weltevreden ParkTel: +27 11 675 1774. Fax: + 27 11 675 1778

Spain Nynas Petróleo S.A., Garcia de Paredes 86 1°A, ES-28010 Madrid

Tel: +34 91 702 18 75. Fax: +34 91 702 18 81

TurkeyNynas Naphthenics Yaglari Tic. Ltd. Sti. Kantaciriza Sokak 15/3, 81070 Erenköy, Istanbul

Tel: +90 216 368 38 42. Fax: +90 216 368 37 48

Central- and Eastern Europe / Latin AmericaNynäs Naphthenics AB, Box 10701, S-121 29 Stockholm, Sweden

Tel: + 46 8 602 12 00. Fax: + 46 8 508 665 10

RESEARCH & DEVELOPMENTNynäs Naphthenics AB, S-149 82 Nynäshamn, Sweden

Tel: +46 8 520 65 000. Fax: +46 8 520 20 743

www.nynas.com/naphthenics

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