Quality Confirmation Tests for Power Transformer Insulation Systems (2019)

Unused Mineral Insulating Oil1.1 Introduction The purpose of this chapter is to determine the characteristics and test methods ofunused mineral oil containing additives or without additi

Quality Confirmation Tests for Power

Transformer

Insulation Systems

Behrooz Vahidi

Ashkan Teymouri

Quality Con firmation Tests for Power Transformer Insulation Systems

Quality Con firmation Tests for Power Transformer

Insulation Systems

123

Department of Electrical Engineering

Amirkabir University of Technology

Tehran, Iran

Department of Electrical EngineeringAmirkabir University of TechnologyTehran, Iran

ISBN 978-3-030-19692-9 ISBN 978-3-030-19693-6 (eBook)

https://doi.org/10.1007/978-3-030-19693-6

© Springer Nature Switzerland AG 2019

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Power transformer insulation is an indispensable part of a power transformer Theimportance of insulation was increased over the years due to the increase in thevoltage rating of transformers Within the last decades, although research onthe transformer insulation and diagnosis methods has been improved so much, theinsulation of HV transformers remained more or less unchanged and for EHV andUHV transformers, the oil–paper insulation is dominant The book in hand is thefirst edition and based on the oil–paper insulation.

The contents of this book are divided into five chapters The first and secondchapters explain the oil insulation The third chapter explains the paper insulation.The fourth andfifth chapters deal with the tests

The authors’ special thanks go to all readers in advance who will give us afeedback on the book

v

1 Unused Mineral Insulating Oil 1

1.1 Introduction 1

1.2 Mineral Oil 1

1.3 Classification of Mineral Oil Based on Application 3

1.3.1 Transformers Oil 3

1.3.2 Switchgear Oil in Low Temperatures 3

1.4 Additives 8

1.5 Special Cases 9

1.6 Analysis of Potentially Corrosive Sulphur 9

1.7 Oil Contamination 10

References 11

2 In-Service Mineral Insulating Oil 13

2.1 Introduction 13

2.2 Oil Monitoring and Purification 13

2.3 Oil Ageing and Degradation 14

2.4 Oil Tests 14

2.4.1 Color and Appearance 15

2.4.2 Breakdown Voltage 15

2.4.3 Water Content 15

2.4.4 Water in the Insulation System 17

2.4.5 Acidity 19

2.4.6 Dielectric Dissipation Factor (DDF) and Resistivity 20

2.4.7 Additives and Oxidation Stability 21

2.4.8 Sludge and Sediment 22

2.4.9 Interfacial Tension 22

2.4.10 Particle Content 23

2.4.11 FlashPoint 23

2.4.12 Compatibility of Insulating Oil 23

2.4.13 Pour Point 24

vii

2.4.14 Density 25

2.4.15 Viscosity 25

2.4.16 PCB 26

2.4.17 Corrosive Sulphur 26

2.4.18 Dibenzyl Disulphides (DBDS) 27

2.4.19 Passivators 27

2.5 In-Service Oil Monitoring 27

2.5.1 Uninhibited Oil Monitoring 28

2.5.2 Inhibited Oil Monitoring 28

2.6 Time Schedule of Sampling and Testing In-Service Oil 28

2.7 Available On-Site Tests 29

2.8 Classification of Operating Oil 29

2.9 Corrective Actions 30

2.10 Purification 30

2.10.1 Physical Purification 31

2.10.2 Chemical Purification (Refinement) 32

2.11 Replacing Oil in Electrical Equipment 34

2.12 Adding Passivators 35

2.13 Determining Water Concentration in the Oil 35

References 36

3 Chemical Indicators 37

3.1 Introduction 37

3.2 Insulation Paper Life Determination 37

3.3 Cellulose 39

3.4 Cellulose Molecular Structure 39

3.5 Cellulosic Insulation 40

3.6 Degree of Polymerization 40

3.7 Oil Impregnated Insulation Paper 41

3.8 Ageing of Oil Impregnated Insulation Paper 42

3.9 Ageing Mechanism 43

3.9.1 Pyrolysis 43

3.9.2 Hydrolysis 44

3.9.3 Oxidation 46

3.10 Influence from Acids 46

3.11 Ageing of Oil 47

3.12 Oil Oxidation 47

3.13 Degradation Products in Oil Impregnated Insulation Systems 47

3.14 Degradation Products from Cellulosic Insulation 48

3.14.1 Water 48

3.14.2 Acids 48

3.14.3 Furans 49

3.14.4 Carbon Oxides 49

3.14.5 Hydrocarbons 50

3.15 Degradation Products of Oil 50

3.15.1 Acids 50

3.15.2 Sludge 50

3.16 Chemical Indicators 50

3.17 Furan Compounds 51

3.17.1 Furans Origin 51

3.18 The Relationship Between DP and Furans 52

3.19 Stability 53

3.20 Furans Disadvantages 53

3.21 CO2 and CO 54

3.22 The Combination of CO2/CO Ratio and 2-Furfural 54

3.23 Methanol 55

References 62

4 Dissolved Gas Analysis (DGA) 65

4.1 Introduction 65

4.2 Total Flammable Dissolved Gas in the Transformer 67

4.3 Allowable Concentration of Gases in a Transformer 67

4.4 Gas Ratio Methods 67

4.4.1 Dürrenberg Method 68

4.4.2 Rogers Ratio 69

4.4.3 IEC Ratio Method 69

4.5 Duval Triangle Method 71

4.6 Detection of Partial Discharge Using DGA 72

4.7 Impact of DGA Accuracy on Fault Detection 72

References 73

5 Other Tests 75

5.1 Introduction 75

5.2 Partial Discharge (PD) 76

5.2.1 Corona Discharge 76

5.2.2 Surface Discharge 76

5.2.3 Discharge in Composite Insulation Materials 78

5.2.4 Electric Discharge in Cavities 78

5.2.5 Electric Treeing 78

5.3 Partial Discharge Measurement 78

5.4 Insulation Monitoring by PD Measurement 81

5.5 Comparison of Electrical and Audio Detection Methods 83

5.6 Partial Discharge Formation in Transformers 84

5.7 Dielectric Response Analysis 84

5.8 Polarization 85

5.9 Polarization and Depolarization Current 85

5.10 Insulation Spectroscopy in Time Domain 88

5.11 FDS Test 91

5.12 Returning Voltage Method 93

5.13 Isothermal Relaxation Current 94

5.14 Frequency Response Analysis (FRA) 96

5.15 Frequency Response Analysis Theory 97

5.16 Application of FRA in Power Transformers 97

5.17 FRA Test Features 98

5.18 Frequency Response Measurement Methods 98

5.18.1 Swept Frequency Method (SFM) 98

5.18.2 Low Voltage Impulse Methods (LVI) 99

5.19 Comparison of LVI and SFM Methods 99

5.20 Detectable Defects by FRA 101

References 102

Index 105

Unused Mineral Insulating Oil

1.1 Introduction

The purpose of this chapter is to determine the characteristics and test methods ofunused mineral oil containing additives or without additives used in transformers inwhich oil has been used as the insulator This chapter is written using the standardIEC 60296 [1] and is not applicable to the insulating oil used in cable or capacitor It

is important to know that insulating oil is made from refining, reforming or mixingpetroleum products and other hydrocarbons It needs to be explained that this chapter

is applicable only to the unused mineral insulating oil and it is not applicable to therefined or used oil The characteristics and test methods of the used mineral insulatingoil will be presented in Chap.2as well

1.2 Mineral Oil

Transformer oil is a mineral insulating oil used in transformers and similar electricalappliances There are also some other mineral oil types that are designed for the lowtemperatures which are used in the oil filled switchgears in the very cold climate.Mineral insulating oil is obtained through refining, reforming or mixing petroleumproducts with other hydrocarbons and additives It should be noted that these additives

do not include esters, silicone fluids and synthetic aromatic chemicals Additives arethe chemicals which are added to the mineral oil to improve its specifications Forexample, antioxidants, metal passivators, electrostatic charging tendency depressant,gas absorbers, pour point depressants, anti-foam compounds and refining processesimprover are some additives [1] The definition of these chemicals will be presented

in the following:

• Antioxidant additives: Compounds added to mineral insulators to improve

oxi-dation stability, including inhibitors, peroxide decomposers and metal passivators[1]

© Springer Nature Switzerland AG 2019

B Vahidi and A Teymouri, Quality Confirmation Tests for Power

Transformer Insulation Systems,https://doi.org/10.1007/978-3-030-19693-6_1

1

• Inhibitors: An antioxidant additive containing a variety of phenolic compounds

including DBPC1and DBP2as described in IEC 60666

• Other antioxidants: Includes other antioxidant additives including sulphur or

phosphorus compounds [1]

• Metal passivators: Metal passivator agents are essentially added as electrostatic

charging reducers, but may also improve oxidation stability [1]

Therefore, mineral insulation oil used in the equipment is classified according tothe presence of additives into three types including oil with inhibitors, oil withoutinhibitors and oil with very low levels of inhibitors The total inhibitor content is lessthan 0.01% in oil without inhibitors according to the standard IEC 60666 Oil with avery low amount of inhibitor contains inhibitors less than 0.08% in accordance withIEC 60666 Finally, the oil containing inhibitors is mineral oil with a minimum of0.08% and a maximum of 0.4% of inhibitors [2]

Mineral insulating oil is stored and transported within the certain containers afterthe production as shown in Fig.1.1 The manufacturer of oil must ensure that nocontamination with PCB3and PCT4compounds, used oil, dechlorinated oil or otherimpurities is achieved [3] Hence, the mixture of unused oil with the used oil isconsidered to be the used oil For more information about recycled oil, refer to thestandard IEC 62701 [3]

Fig 1.1 Transformer oil in special bulks

1 2,6-di-tert-butyl-para-cresol.

2 2,6-di-tert-butyl-phenol.

3 Polychlorinated terphenyl.

4 Polychlorinated biphenyl.

1.3 Classification of Mineral Oil Based on Application

Mineral insulating oil is divided into two groups according to their applications Thefirst group is allocated to oil for transformers and the second group is allocated tooil for switchgears in low temperatures Now, each group will be described indepen-dently

1.3.1 Transformers Oil

These types of oil are also divided into 3 groups based on the antioxidant additives.The first group contains no antioxidants, indicated by the letter “U”.5The secondgroup contains a very low amount of antioxidant additives and is indicated by theletter “T”.6The third group also contains antioxidants which is represented by theletter “I”.7This category is important because the lowest starting temperature of thetransformer (LCSET8) varies with this classification The LCSET value in the IEC

60296 is−30 °C [1] More information is given in Table1.1

1.3.2 Switchgear Oil in Low Temperatures

Insulating and cooling are two main characteristics of this oil Other tics include viscosity, density, pour point, water content, breakdown voltage and thedielectric dissipation factor [1] The stability of the oil is affected by the oil quality,oil refinement type and additives which determine oil characteristics such as appear-ance, interfacial tension (IFT), sulphur content, acidity, corrosive sulphur content,2-Furfural content, other similar compounds and gases in the oil Performance is also

characteris-a fecharacteris-ature which relcharacteris-ates to the chcharacteris-archaracteris-acteristics of the long-term behcharacteris-avior of oil in the

Table 1.1 Maximum

viscosity and pour point of

transformer oil at the lowest

cold start energizing

5 Uninhibited transformer oil.

6 Trace inhibited transformer oil.

7 Inhibited transformer oil.

8 Lowest cold start energizing temperature.

operation or its response to high electrical shock and high temperatures includingoxidation stability, the gassing tendency and electrostatic charging tendency (ECT)[1].

Likewise, the characteristics of oil that are related to the oil safety issues duringstorage, operation and environment protection are called HSE, such as the flashpoint, density, polycyclic aromatics (PCA), PCB (polychlorinated biphenyls) andPCT (polychlorinated terphenyls) [1]

1.3.2.1 Features

The characteristics of transformer oil and switchgears must be in accordance withthe specifications in IEC 60296 Definitions of each feature of transformer oil andswitches are defined below [1]

• Viscosity: The viscosity of the oil affects the heat transfer and hence increases the

temperature in the device Lower viscosity facilitates oil circulation and improvesheat transfer Low temperatures lead to an increase in the viscosity of the oil which

is the cause of the crisis at the moment of starting the transformer In this situation,oil has a low flow or no circulation which can lead to an increase in the temperature

in hot spots, and it has a negative effect on the speed of some parts movement such

as switches, pumps, regulators, power circuit breaker and tap changer mechanism.The viscosity in the LCSET should not be greater than 1800 mm2/s The LCSETtemperature for the transformer is−30 °C in IEC 60296 Other LCSET tempera-tures in Table1.1can be determined by an agreement between the manufacturerand the customer Switchgear oil should also have a lower viscosity at LCSETtemperature no more than 400 mm2/s [1] LCSET temperature for switchgear is

−40 °C in IEC 60296 In this case, other LCSET temperatures can be selectedbased on the agreement between the manufacturer and the customer

• Pour point: Pour point in mineral insulating oil is the lowest temperature in which

the oil still has a flow It is recommended that pour point should be 10 K belowthe LCSET [1]

• Water content: A small amount of water is required in mineral insulating oil

in order to achieve the suitable breakdown voltage and low electrical losses Toprevent the separation of water from insulating oil, the amount of water should belimited [1] Before filling the equipment with oil, the oil must be tested according

to IEC 60422 with a special device as could be seen in Fig.1.2

• Breakdown voltage: The breakdown voltage of the insulation oil indicates its

resistance to the electric shock in electrical devices The breakdown voltage must

be measured according to IEC 60156 by using a special device as shown in Fig.1.3.The breakdown voltage should be greater than 70 kV

• Dielectric Dissipation Factor (DDF): This is a measure of the dielectric losses in

oil DDF values higher than those in the requirements of the standard IEC 60296can indicate the oil pollution with polar contaminants or the inappropriate quality

of the purification process The DDF should be measured in accordance with IEC

Fig 1.2 Fully automatic

water content measurement

equipment

Fig 1.3 Oil breakdown

voltage tester

60247 and IEC 61620 at 90 °C [1] It should be noted that, in the case of anagreement between the manufacturer and the customer, DDF could be measured

in other temperatures but the temperature measurement must be stated in the report

• Appearance: By examining an oil sample at ambient temperature, the presence

of visible contaminants, free and suspended water will be revealed [1]

• Acidity: Mineral insulating oil must not contain any acid compounds Acidity

should be measured according to IEC 62021-1 or IEC 62021-2

• Interfacial tension: The IFT indicator sometimes indicates the presence of polar

compounds The IFT must be measured according to EN 14210

• Sulphur content: There are various sulphur compounds in mineral oil based on

the type of oil source and the degree and type of refining process The refiningprocess reduces the amount of sulphur and aromatic hydrocarbons Some sulphurcompounds, that are naturally available in oil, have a tendency to metals [4] Thesecompounds could act as the oxidative inhibitors or corrosion accelerator inhibitors.The amount of sulphur should be measured according to the standard IP373 orISO14396

• Corrosive sulphur: Some sulphur compounds such as mercaptans on the surfaces

of steel, copper and silver have a very corrosive effect and should not be present inthe unused oil This type of sulphur should be specified according to DIN51353.Other types of sulphur compounds such as DBDS (dibenzyldisulphide) may cause

Cu2S in the insulating paper As a result, electrical insulation properties will bereduced This phenomenon causes problems in various devices especially trans-formers when they are in service [4]

• Oxidation stability: Oil oxidation increases the acidity and sludge formation In

oil with the high oxidation stability, less sludge is formed and oil has a longerlife and hence it could be used as the insulating oil for longer periods Oxidationstability is measured according to IEC 61125 In the case of oil that contains themetal passivators, the oxidation stability measurement test should be done beforeincreasing the metal passivators

• The oil tendency to absorb gas: The tendency to absorb the gas in mineral

insulat-ing oil when oil is exposed to a slight corona is called gas absorption characteristic.This feature is only important for certain devices such as high voltage transformersand bushings This property is a measure of the rate of absorption or release ofgas within the oil in experimental conditions [1] The tendency to absorb the gasdepends on the amount of aromatic compounds in the oil according to the standardIEC 60628

• ECT: The electrostatic charging tendency is an important feature in designing

some HV and EHV transformers in which the speed of the oil pump is highenough to increase the electrostatic charge This charge can cause serious damage

to the electrodes [5] It should be noted that ECT could be reduced by adding somemetal passivators

• Flashpoint: The safe operation of an electric appliance is based on the use of oil

that has a high flash point The flash point is measured by the Pensky-Martensclosed cup method

• Density: Under the cold weather conditions, the density of the oil should be low

enough to prevent the formation of ice from freezing water [1] The occurrence ofsuch a phenomenon can cause problems such as surface discharge on the conduc-tors

• Polycyclic aromatic content (PCA): Some polycyclic aromatic compounds are

in the carcinogenic group and hence controlling their amount inside the mineralinsulating oil is essential The total amount of PCA compounds could be measured

by extraction with dimethyl sulfoxide (DMSO) [6]

• Polychlorinated biphenyl content (PCB): Mineral insulating oil should be free

from PCB More information about this compound is available in IEC 61619

• 2-Furfural content: 2-furfural and related compounds in non-corrosive mineral

oil could be produced by inappropriate re-distillation after the solvent extractionduring purification or by contamination in used oil [1] Mineral insulating oil mustcontain a small amount of 2-furfural and its corresponding compounds Thesecompounds are measured according to IEC 61198 Other furans include [1]:(1) 5-hydroxymethyl-2-furfural (5HMP)

(2) 2-fufurylalcohol (2FOL)

(3) 2-acetylfuran (2ACF)

(4) 5-methyl-2-furfural(5MEF)

• Particle content: Particles in the mineral insulating oil may be present during the

preparation, maintenance or transportation of oil These particles can affect thebreakdown voltage Particle measurement is carried out in accordance with IEC60970

• DBDS content: This compound causes corrosion at the normal operating

temper-ature of the transformers and can produce Cu2S Therefore, this compound shouldnot be present in unused oil DBDS is measured according to IEC 62697-1 [4]

• Gases in the oil: Some oil could produce gases such as hydrogen, hydrocarbons

and carbon oxides without any thermal or electrical fault in the transformer or evenwithout any shock to the performance of the transformer at temperatures below

120 °C This phenomenon can lead to a large amount of gas production and canmake a mistake in interpreting the DGA’s results In addition, the oil containinginhibitor substances produces less than the uninhibited oil To measure the solublegasses in the oil and performing DGA test, special devices as shown in Fig.1.4

are used

Fig 1.4 Mobil GC—portable gas chromatograph

1.4 Additives

The general type of additives must be presented in the technical specifications of thedevice Concentrations should also be stated for antioxidant and passivators

• Antioxidant additives: Antioxidant compounds reduce the oil oxidation and

hence reduce the formation of degradation products such as sludge and acid pounds [2] Awareness of the type and amount of the added antioxidants will

com-be helpful in monitoring the reduction of these products while using the device.Additives that slow down the oxidation of mineral oil include [2]:

1 Inhibitors compounds such as phenol and amines, especially DBPS and DBP.Determination and measurement of DBPS and DBP should be done according

• Metal passivators: Some of these additives form a layer on the copper and prevent

the catalytic effects of copper in the oil and formation of the harmful copper sulfidedeposits in layers of the paper resulting from the reaction of corrosive sulphurcompounds in oil [1] Some of these additives reduce the oil oxidation speed.Thus, the passivators slow down the oxidation process and deactivate the activesurfaces of copper wires against catalytic reactions and hence lead to the desirable

results in oxidation stability (IEC 61125) Other passivators are used to reducethe electrostatic charging tendency of oil Three major categories of benzotriazolederivatives are used as the passivators as follows [1]:

1 N-bis (2-Ethylhexyl)-aminomethyl-tolutiazole (TTAA)

2 Benzotriazole (BTA)

3 5-methyl-1H-benzotriazole (TTA)

Determination and measurement of these additives are in accordance with IEC

60666 Other compounds that are used as metal passivators include [1]:

1 N,N-bis(2-ethylhexyl)-1H-1,2,4-triazole-1methanamine (TAA)

2 Diamino-diphenyldisulphide

3 Nicotinic acid

4 Hydroquinoline

5 Other sulphur-based compounds

It should be noted that for the above compounds, IEC test methods are not available

• Pour point reducer: These additives are used to improve the viscosity in the

cold climate and pour point of oil Two important categories of these additives arepolynaphthalenes and polymethacrylates

1.5 Special Cases

For high-temperature transformers designed for long lifetimes, the acceptable limits

of oxidation stability may be more limited In such situations, inhibited oil is oftenused The limits are as follows [1]:

• Total acidity: Max 0.3 mg KOH/g

1.6 Analysis of Potentially Corrosive Sulphur

The Cu2S sedimentation mechanism is still not completely clear, but it is possiblethat sulphur compounds with copper form a complex and then converted into Cu2Safter being absorbed into insulating cellulose layers Increasing the temperature and

the presence of oxygen greatly affect this mechanism Cu2S is observed especially

in devices where corrosive sulphur compounds are present in oil and their copper isnot covered and the operating temperature is also high and the amount of oxygen islimited [1] It seems that if the amount of oxygen is relatively low, the process offorming and transferring the complexes is better whereas if the amount is greater,the process of complex destruction and conversion to Cu2S is desirable [1].However, a large number of sulphur compounds are corrosive to copper, but only

a few of them are found in the mineral insulating oil The only compound thathas ever been found to have the potential for Cu2S formation and seemed to beremarkably found in transformer oil is dibenzyl disulfide (DBDS) In most of theoil which contains Cu2S, DBDS is observed However, if the purification process iscarried out, this reactive compound is easily removed from the oil Other compoundsincluding disulphides, oxidized sulphur compounds and elemental sulphur can form

Cu2S This phenomenon is stated in IEC 62535 when the oil is tested before andafter adding these compounds

In order to determine the corrosive sulphur compounds in oil containing metalpassivators, the protective layer of the passivator is formed on the surface of copper[1] This layer prevents the metal reaction with sulphur compounds in the oil and theformation of harmful Cu2S in the insulating paper of the device Therefore, the testmethod in IEC 62535 cannot determine the sulphur compounds in the oil containingmetal passivators In order to determine corrosive sulphur compounds in oil withmetal passivators, these additive materials must first be removed from the oil Thefollowing two methods are used for this purpose It is to be explained that bothmethods are for the new unused oil and not for the used oil [1]

Mineral insulating oil that is likely to be contaminated with silicone oil, phthalates

or other chemical compounds should not be used in transformers because thesecompounds create the foam when transformer degassing is performed, and it maymake the process hard or impossible

trans-4 IEC 62697-1, Insulating liquids—quantitative determination of corrosive sulfur compounds

in used and unused insulating liquids—part 1: test method for quantitative determination of dibenzyldisulfide (DBDS)

5 B Vahidi, G.H Rassuly, Transformer oil static charge analysis using open circuit system Iran.

J Electr Comput Eng (IJECE), 6(2), 144–149, 2008 (in Persian)

6 IEC 60867, Insulating liquids—specifications for unused liquids based on synthetic aromatic hydrocarbons

In-Service Mineral Insulating Oil

2.1 Introduction

Mineral insulating oil is used in electrical equipment of power plants, sion system and distribution systems The quality of oil monitoring and maintainingare vital for oil-filled electrical equipment reliable operation Therefore, operationalsolutions have been developed by the power generation and transmission companies

transmis-If the degradation of the oil (due to ageing or contamination) exceeds a certain limit,the reliability is eliminated and the risk of a premature fault increases Although therisk analysis is very difficult but the first step in this way is to identify the effects of thefaults It is also important to note that leakage of oil can have negative environmentaleffects, especially if it is contaminated with the materials such as PCB

This chapter, which is developed using IEC 60422 [1], provides tips on monitoring,servicing and maintaining the quality of electrical insulating oil It is necessary toexplain that IEC 60296 is used to check the mineral insulating oil before enteringthe transformer but after the oil has been filled into the transformer tank, IEC 60422should be used to check the oil The contents of this chapter are relevant to the oilthat is manufactured according to IEC 60296 [2] and also applicable to transformers,switchgears and other electrical equipment where oil sampling is feasible in normalworking conditions

2.2 Oil Monitoring and Purification

Before proceeding with the discussion, it is necessary to introduce the tests thatare used to monitor the oil in the equipment Routine tests (group 1) include theminimum requirements for monitoring the oil that are required to ensure the continuedoperation Additional tests (group 2) are also the tests which are carried out for moreinformation on the quality of the oil and may be used to evaluate the oil for thecontinued operation At the end, special tests (group 3) are the tests which are often

© Springer Nature Switzerland AG 2019

B Vahidi and A Teymouri, Quality Confirmation Tests for Power

Transformer Insulation Systems,https://doi.org/10.1007/978-3-030-19693-6_2

13

performed to determine the suitability of the oil for using in a particular equipment and

to ensure that the oil is adapted to the environmental or operational conditions Oncethe oil has been monitored and its status are informed, corrective actions should betaken depending on the circumstances These include physical purification, chemicalpurification and in some cases, PCB cleaning Physical purification is a process thatreduces gases, water, solids and contaminants by simply using the physical methods

of removing oil or reducing them in oil Chemical purification (regeneration) is

a process that reduces the soluble or insoluble pollutants by using chemical andphysical methods to remove them from oil or reduce their amount in oil PCB cleaning

is also a process that reduces the amount of PCB in the mineral oil or removes it

2.3 Oil Ageing and Degradation

The safe performance of the mineral insulating oil in an insulating system depends

on some of the essential features that can affect the overall performance of electricalequipment In order to perform its various tasks, such as insulating, cooling and sparkignition, the oil must have special characteristics including [1]:

• High insulation strength to withstand the electrical stress during the operation

• Low viscosity for the proper circulation and heat transfer

• Proper specifications at low temperatures for the operation at the lowest ture in the installation site

tempera-• Persistence against oxidation to maximize its lifetime

Oil is destroyed during the operation due to the working conditions In manycases, the oil is in contact with air and exposed to oxidation Degradation is faster

in higher temperatures and metals, organic metal compounds or both act as a lyst in the oxidation process Oxidation changes the color, forms acidic compoundsand in advanced stages, the formation of sludge happens Ultimately, the insulationproperties are affected In addition to oxidation products, many of the unwantedcontaminants such as water, solids and polar oil soluble contaminants are producedduring the service of the oil which affect the electrical properties These contam-inants and any of the degradation products cause changes in one or more of thecharacteristics of the oil

cata-2.4 Oil Tests

Many tests can be considered for the mineral insulating oil, but the best and the mostaccurate tests and also their permissible values are presented in IEC 60422 for theused oil to determine the suitability of the oil to continue the operation Each of thesetests will be described but to access for their values, see the standard IEC 60422 [1]

2.4.1 Color and Appearance

The color of the insulating oil is determined in light and expressed in terms of thenumber which is determined by the reference color in the standard This characteristic

is not very important, although it is useful for the comparison A rapid increase in thecolor indicating number may clarify the degradation or contamination of the oil Inaddition to the color, the appearance of the oil may indicate insoluble water, insolublesludge, carbon particles, fiber or other contaminating materials

The breakdown voltage is the measurement of the oil strength against the electricalstresses and absolutely, it is crucial for the safe operation of electrical equipment [1].This parameter is highly dependent on the temperature of the oil during sampling.Dry and clean oil has a high breakdown voltage Insoluble water and in particularsuspended particles in combination with high levels of soluble water are driveninto regions with more intense electric fields and greatly reduce the breakdownvoltage [1]

Hence, the measurement of breakdown voltage indicates the presence of inants such as water or suspended particles more than anything else Low breakdownvoltage can indicate the presence of one or all of these factors However, the highbreakdown voltage does not necessarily mean the absence of these contaminants [1].The value of the breakdown voltage is only important when the sample is removed

contam-at the opercontam-ating tempercontam-ature of the transformer Samples taken contam-at tempercontam-aturesbelow 20 °C may show the optimistic results when tested at the laboratory tem-peratures [1]

• Ageing process for liquid and paper insulation

Therefore, the water containment in the liquid and paper insulation has a greateffect on the operation status and life of the transformer The presence of water inthe transformer insulation is due to two reasons:

• The absorption of moisture from the environment

• Insulation destruction

Fig 2.1 Power transformer control box with oil and windings thermometers

Water is fed into oil-filled electrical equipment through the oil The water is soluble

in oil and may also be a hydrate absorbed by bipolar products Water solubility inoil (Ws) in mg/kg depends on the oil condition, temperature and type of the oil Theabsolute value of the soluble water (Wabs) is independent of the temperature, type andstatus of the oil which is calculated on the basis of mg/kg The absolute amount ofsoluble water can be measured according to IEC 60814 Relative humidity (Wrel) is,

in fact, the ratio of absolute value to water solubility in oil or Wabs/Ws, expressed as

a percentage Relative humidity can be measured by using a water-in-oil dissolutionmethod or via online capacitive sensors [3]

If the absolute water content is greater than the water-solubility in the oil (Wabs

> Ws or Wrel > 100%), the excessive water is insoluble and free which may beseen as fog or drop Usually, the temperature is measured directly from the oil flowduring sampling If the temperature of the oil is measured using the oil thermometer

or temperature corrections for the OFAF and ONAN cooling as shown in Fig.2.1,this should be explicitly mentioned in the report The oil-soluble water is directlyproportional to the relative humidity or the ability to dissolve water in the oil Withthe high oxidation of the oil and increase in bipolar side products due to ageing, theability to dissolve water in the oil increases which is itself dependent on the type ofoil The ability to dissolve water in very old oil is much higher than the unused oil[1]

Fig 2.2 Dehumidification of power transformer windings, a Vacuum furnace, b Winding

2.4.4 Water in the Insulation System

Transformers are dried during the production to reduce the percentage of waterpresences in the cellulosic insulation to 0.5–1% depending on the requirements ofthe manufacturer or customer as shown in Fig.2.2 After this initial drying, the activepart is constructed and its cellulosic insulation moisture content increases with respect

to the environmental conditions Again, the active part of the transformer, shown inFig.2.3, must be dried in different vacuum furnace as could be seen in Fig.2.4 Itshould be noted that the moisture content inside the transformer will be increaseddue to the operating conditions in its service time

The available water in the insulation system is divided into a transformer betweenthe paper and oil insulation in which most of the water is present in the cellulose.Temperature variations greatly increase the amount of water soluble in the oil, but

it has little effect on the water contained in the paper insulator The thermodynamicequilibrium between the water absorbed by the paper and dissolved water in oiloccurs when the transformer is under the load after a long time at a relatively hightemperature

So this equilibrium is temperature dependent As could be seen in Fig.2.5, athigher temperatures, much water is transferred from the cellulose to the oil However,

if the temperature of the oil is not high enough this balance will not be achieved due

to the reduced water transfer from cellulose to oil Different methods are provided bymeasuring soluble water in the oil for determining water in the paper All calculationsrelated to the relationship between soluble water and the water concentration in theinsulating paper depend on the balance between the oil and the paper This balance

is affected by various parameters such as temperature difference between the oil andthe paper [4]

The breakdown voltage and soluble water in the oil are highly interdependent.Both of them are temperature dependent, so their measurements at different operatingtemperatures of the transformer provide good information for the reliable moisture

Fig 2.3 Active part of a power transformer

Fig 2.4 Active part drying vacuum furnace used during the production of transformer

Fig 2.5 Example of variation in saturation water content with oil temperature and acidity for

insulating oil originally conforming to IEC 60296 [ 1 ]

assessment of the paper/oil insulating system The analysis of the soluble waterconcentration in the oil is highly dependent on the temperature of the oil sampleduring sampling by placing the thermometer in the oil flow path

2.4.5 Acidity

The acidity of the oil shows the amount of acid contamination in the oil The acidity

of the used oil is due to the formation of oxidation products Acids and other tion products, along with water and other solid contaminants, affect the insulationproperties and other oil parameters [1] Acids influence the cellulosic material which

oxida-is the main reason for the corrosion of the metal surfaces of the transformer Thegrowth rate of acidity in the operating oil represents a good indicator of the ageingrate [1] The acidity value is a good criterion for identifying the need for the chemicalpurification or substitution of the oil Usually, the oil with an inhibitor, as long as theinhibitor levels are sufficient, shows no significant increase in the acidity

Fig 2.6 a Oil tan delta and resistivity tester, b closer view

2.4.6 Dielectric Dissipation Factor (DDF) and Resistivity

These specifications are very sensitive to the presence of polar contaminants andageing products in oil Changes in the amount of these contaminants (even if theiramounts are very low) can be monitored by measuring these indicators by a specialdevice shown in Fig.2.6[1] The permissible limits of these indicators are highlydependent on the type of the transformer

There is an inverse relationship between the DDF and the resistivity, so that withdecreasing the resistivity, the DDF value increases Typically, there is no need to doboth tests on the oil and DDF measuring test is common DDF value and resistivitydepend on temperature and humidity Figure2.7shows the variation of resistivitywith respect to the temperature and humidity in oil without any solid contaminant InEHV and UHV transformers, special attention should be paid to the insulation DDFvalue because the high value of this coefficient can lead to the thermal exhaustionand ultimately a transformer failure [1]

Oil, which is known to be appropriate, has characteristics similar to the curves

A and B in Fig 2.7, and at low and high temperatures, the results of their DDFtests are acceptable Oil that is considered inappropriate is similar to the curve Cand the results of the DDF test are appropriate at 90 °C and therefore, the DDF testresults are not suitable at low temperatures [1] This indicates the presence of water

or degradation products at a cold temperature without any chemical contamination.Inappropriate results at both temperatures indicate a higher level of contaminationwhich means that there is no possibility of improving the oil’s condition with thephysical purification [1]

Resistivity measurement is important for monitoring in-service oil The tivity is proportional to acids caused by oxidation and it is affected by unwantedcontaminants such as water Other compounds in used oil that can affect the specific

resis-Fig 2.7 Example of variation of resistivity with temperature for the insulating oil [1]

properties are including aldehydes, ketones and alcohols [1] Increasing the perature as well as the water deposition at low temperatures reduces the resistivitybecause of reaching the saturation point In some measurement transformers, it hasbeen observed that during the very short oxidation process, there has been a signifi-cant change in the DDF value that led to a fault in the equipment For this reason, it isrecommended that the unused oil DDF value will be performed after the exposure toshort-term oxidation according to IEC61125: 1992, Method C in order to determinethe suitability of the oil for the service

tem-2.4.7 Additives and Oxidation Stability

Mineral insulating oil stability is called the oxidation stability under thermal stressesand in the presence of oxygen and copper catalysts This test indicates the life

expectancy of the oil in the operating conditions of electrical equipment This eter determines the oil stability against the formation of acid compounds, sludge andcompounds that affect the insulation DDF value in certain conditions [1] Furtherinformation is available in IEC 61125 This characteristic mostly depends on therefining process Refined mineral oil has a variety of natural compounds that act

param-as an oxidation inhibitor These materials are known param-as natural antioxidants Oilthat contains only natural antioxidants is called the uninhibited oil Synthetic oxi-dation inhibitors can be used to enhance the oxidation stability [1] Typically, aphenolic type is used in transformer oil Most commonly used compounds are asfollows [1,2]:

(1) 2,6-di-tert-butyl-phenol (DBP)

(2) 2,6-di-tert-butyl-paracresol (DBPC)

It is also necessary to explain that the inhibitor effect is different depending onthe chemical composition of basic oil IEC 61125: 1992, Method C can be used todetermine the oxidation stability Since the test is designed for the unused oil, theanalysis of the results for the used oil is difficult This test is usually used to evaluatethe oil status in new equipment

There is a difference between sediment and sludge The sediment is defined as theinsoluble material contained in oil and includes the following:

• Insoluble products caused by the destruction or oxidation of solid or liquid rials

mate-• Solid products due to operating conditions such as metal or carbon particles, metaloxides and sulfides

• Fiber and other materials of foreign origin

Sludge is a polymerized product caused by the destruction of solid or liquidmaterials as shown in Fig.3.7 The sludge is dissolved in oil to some content whichdepends on the temperature and the solubility of oil More than that, the sludge turnsinto solid and turns into the sediment Sediment or sludge can change the electricalcharacteristics of the oil In addition, its accumulation may reduce the heat transfercapacity of the oil and causes the thermal degradation of insulating materials

influence strongly the rapid reduction of IFT The rate of IFT reduction of ited oil is usually greater than those with an inhibitor The rapid reduction of IFTcan indicate the lack of oil compatibility with some materials inside the transformer

uninhib-or accidental contamination when the oil is injected However, oil with an IFT valueless than or close to the values listed in IEC 60422 should be further investigated

In case of overloading the transformer, material deterioration is faster and IFT is acriterion for identifying this deterioration [1]

2.4.10 Particle Content

Possible sources of suspended particles in the insulating oil are different in cal equipment The electrical equipment may be the self-contained particles fromthe manufacturing process or the oil can contain suspended particles in the pro-cess of storage or injection due to the improper filtration Ageing of metal, oil andother materials inside the transformer may create dust particles during the operation.Hot spots with more than 500 °C can also form the carbon particles The carbonparticles generated in the selector switch (a part of tap changer as shown in Fig.2.8)oil may enter into the main transformer oil reservoir due to the leakage of the com-partment [5]

electri-The image of a power transformer oil reservoir is shown in Fig.2.9 The effect ofthe suspended particles on the oil electrical strength depends on the type of particles(metal, fiber, sludge, etc.) and water concentration [5] The breakdown voltage test

is not enough to identify the suspended particles, alone

2.4.11 FlashPoint

Due to the electrical discharges or under very high temperature conditions, oildestruction produces lightweight hydrocarbons which will reduce the flash point.The low flash point indicates the presence of combustible and volatile materials inoil

2.4.12 Compatibility of Insulating Oil

To overflow a transformer or reinject new oil into a transformer, the new oil must

be in accordance with IEC 60296 [2] and it must have the same classification as thein-service oil Practical experience shows that in the case of overflowing in-serviceoil with an unused (new) mineral oil in a small amount (e.g 5%), there are not anyproblems Although, overflowing in-service oil with unused oil in large quantitiescan lead to the sludge formation in the transformer The most important point is

Fig 2.8 Selector switch (tap selector) of an on-load tap-changers (OLTCs)

that both unused oil and in-service oil must be the same to overflow If there is anambiguity about the compatibility of oil with each other, you should refer to theoil manufacturer’s instructions The compatibility test is essential, especially for theadditives containing oil

2.4.13 Pour Point

Pour point is the identification of the oil ability to circulate at low temperatures.There is no evidence to suggest that this characteristic changes in the normal ageing

Fig 2.9 Oil reservoir of a transformer with a thermometer

process The change in the flash point is usually due to overflowing the in-serviceoil with different oil [1]

In cold climate, the density is an important factor in determining the suitability andefficiency of oil For example, ice crystals may be floated in high-density oil andlead to the electrical discharges However, density is not an important factor in thequalitative comparison of different oil There is no indication that the density isaffected by the oil degradation

on new sites and manufactured products Unfortunately, the common use of theservice equipment and maintenance of this polluted oil with other mineral oil havecaused extensive contamination of the other mineral oil.

The amount of PCB in the oil must be measured to ensure that oil is not taminated to the PCB Similarly, when there is a risk of contamination (such as oilrefinement, repair of the transformer, etc.), the oil must be tested Then, if there is aPCB, the required measurements must be taken

There are three standard methods for measuring the corrosive sulphur in the oil TheIEC standard method is more obligatory than the ASTM method and all oil must meetthe IEC method requirements The implementation of the ASTM standard method

is simpler and can be used as a preliminary test The standard DIN method is acomplementary method and should be performed in addition to the ASTM and IECmethods The amount of sulphur in the oil depends on the refining process and the type

of oil The reason for the presence of reactive compounds that cause corrosion at theoperating temperature of the transformer is poor refining or available contamination

in the oil [1] Sulphur-containing oil molecules may be decomposed at relativelyhigh temperatures, react with the metal and form the metal sulfides These reactionscan occur in the switchgears and affect the conductivity of the contacts

Some of the sulphur-containing molecules produce Cu2S which precipitates inthe paper insulation This phenomenon weakens the insulation quality and has so farcaused a number of incidents Cu2S sediment is usually found in electrical equipmentwith paper insulators with the corrosive sulphur compounds in its oil or unprotectedcopper in which the operating temperature or ambient temperature is high A group

of substances in the oil causing this problem are disulphides such as DBDS [6].The standard IEC 60296 describes the characteristics of unused oil to prevent thedeposition of Cu2S in paper during the operation Tests for this purpose (IEC62535and ASTM D1275: 2006, Method B) apply to oil that does not contain metal passiva-tors In the case of corrosive sulphur compounds in oil, the results of the tests will bepositive Aged oil (e.g., high-acidity oil), or oil with the low oxidation stability mayhave ambiguous test results as a result of the sludge Combining different factors,

not just the corrosive sulphur, may result in an electrical fault In this case, the riskassessment should be done taking into account the design and operation status.

2.4.18 Dibenzyl Disulphides (DBDS)

DBDS causes corrosion of copper surfaces at the operating temperature of the former and under certain conditions, it can form Cu2S DBDS plays a major role inthe corrosion problem among various corrosive sulphur compounds This compoundhas been produced in most insulating oil since 1988 and 1989 (although this oilhas successfully passed their corrosive tests) Since 2006, the oil produced with themeasurable DBDS amounts is very small It should be noted that in some electricalequipment, the oil is corrosive despite the absence of DBDS

2.5 In-Service Oil Monitoring

After the construction of a transformer or a switchgear, it is filled with mineralinsulating oil In this case, the oil is in contact with the insulation and other materialsinside the equipment and can no longer be treated as oil in accordance with theIEC 60296 Therefore, even if the electrical equipment has not been started, thespecifications of this oil must be compared with the requirements of the in-serviceoil according to IEC 60422 Due to the use of different materials and different ratios

of liquid-to-solid insulation, oil specifications may vary depending on the type ofequipment

During the lifetime of the equipment, its insulating oil is exposed to heat, oxygen,water and other catalysts which affect the oil characteristics [1] In order to maintainthe quality of the in-service oil, it is necessary that the oil is regularly sampledand tested Often, the first indication of oil degradation could be detected by itsappearance Likewise, if the oil is contaminated with PCB, paying attention to theenvironmental factors is very important Where there is a potential for contamination

of the oil to the PCB, it is mandatory to test the oil samples and use the results of therisk assessment to avoid the contamination of the environment.

2.5.1 Uninhibited Oil Monitoring

Oxidation of uninhibited oil is usually monitored by measuring acidic componentsand soluble and insoluble sludge in the oil An increase in DDF value and a decrease

in IFT value usually indicate the oxidation of the insulating oil

2.5.2 Inhibited Oil Monitoring

The oxidation pattern of oil containing inhibitor substances is different from theuninhibited oil The added inhibitor into the oil produces little oxidation products inearly stages After the complete use of the inhibitor, the oxidation stability of the baseoil determines the oxidation rate of oil Reducing the IFT value in inhibited oil usuallyindicates the formation of oxidation products The easiest and the most commonway to monitor the inhibitor consumption is to measure the amount of inhibitor

in accordance with IEC 60666 [1] The amount of inhibitor must be monitored

at appropriate intervals depending on the operating temperature and load level of thetransformer

2.6 Time Schedule of Sampling and Testing In-Service Oil

Actually, it is impossible to provide a comprehensive timetable for the sampling andtesting of operating oil The best scheduling depends on the type, performance, volt-age level, power and conditions of the construction and operation of equipment aswell as the oil status which is derived from the previous tests It is usually necessary

to make a compromise between economic factors and reliability requirements Themain problems are determining the sampling time schedule, and determining the level

of acceptable oil degradation for all insulating oil users, taking into account the ences in operating policies, reliability requirements, and the type of electrical systemused For example, large distribution companies may consider the recommendations

differ-in the standard IEC 60422 to be unprofitable for the distribution transformers.Conversely, some industries, whose activities depend on the continuous and reli-able supplying of electricity, may require more stringent oil quality controls InIEC 60422, four sampling-time schedules and suitable tests for different types ofelectrical equipment are proposed If this equipment is in good condition, samplingschedules and oil testing will increase Usually, tests and measurements should bemade according to the following criteria:

(A) Oil specifications should be tested according to IEC 60422.

(B) If the results of the tests indicate that one of the main characteristics of the oil is

in an “inappropriate” or “currently acceptable” state or that the rate of change

in one of the characteristics of the oil is high, the retest time will be reduced.(C) Oxidation of oil is accelerated at high temperatures and in the presence ofoxygen and water [1] As a result, the timing of the oil test is shorter in full loadtransformers Some additional tests such as IFT are also recommended in fullload transformers

(D) The timing of the tests is determined according to the benefit/cost analysisbased on life cycle analysis and risk assessment [1]

2.7 Available On-Site Tests

In certain situations, some tests are required to be done on the site instead of the lab.The reasons are as follows:

• Quick assessment of oil condition

• Classification of operating oil

• Remove any oil specification changes due to transferring samples to the laboratory

If the online testing equipment is available on the site, then the oil tests can beperformed on the site but it must be noted that the accuracy of some site tests isless than the laboratory tests Site tests typically include oil appearance, breakdownvoltage, soluble water and acidity (with less precision)

2.8 Classification of Operating Oil

In practice, it is impossible to determine a comprehensive instruction to evaluatethe in-service oil or to propose acceptable limits for tests in all possible cases Theclassification of oil and determination of the necessary corrective actions should bemade according to the results of all tests It is also important to consider the changes inthe test results in a time frame for the final decision making According to the obtainedexperience, operating oil can be divided into three groups: “Currently acceptable”

or “Inappropriate”, “Appropriate” classification, based on the results of the tests andalso the possibility of improving the status of their important characteristics [1]

• Appropriate: Oil is in normal condition Continue sampling.

• Currently Acceptable: Oil destruction is visible Sampling is recommended in

shorter intervals

• Inappropriate: Oil degradation is abnormal It is necessary to plan for the

cor-rective actions

2.9 Corrective Actions

There are generally two types of contamination/degradation of oil: physical andchemical According to IEC 60422, for each of these cases, a different correctiveaction must be taken Similarly, if the process of degradation is accelerated, testsshould be repeated in shorter intervals and appropriate corrective actions should

be performed It is also possible to consult with the manufacturer of the electricalequipment in these cases

2.10 Purification

In-service oil purification must be performed carefully All necessary precautionsshould be considered to avoid the possible risks to staff and the environment aswell Oil purification should be carried out by experienced and familiar experts Anexample of the oil purification system is shown in Fig.2.10

A comprehensive risk assessment is required before the purification operationbegins and also the following statements should be considered

• The necessary measurements should be considered to avoid oil contamination withPCB

• Oil leakage to the environment must be prevented and ensure that pipes and pumpsare tight

Fig 2.10 Oil purification system

• Due to the fact that oil purification is usually performed under the vacuum, itshould prevent any leakage.

• The process of physical purification generates waste such as filter, contaminatedoil and so on Therefore, it is necessary to use the best possible technology toreduce the production of waste

2.10.1 Physical Purification

There are two reports [3,8] that contain information about physical purification.Physical purification is a process that eliminates or reduces the physical contami-nation of oil by using the physical processes (filtration, drying, gasification, etc.).However, the output oil of this process is not always consistent with the specificationsgiven in IEC 60422 The physical purification reduces the suspended particles andsoluble water in oil This process also eliminates some of the soluble gases in the oiland materials such as furans After this operation, the new values should be consid-ered as the basis The physical methods used to remove water and solids from the oilinclude a combination of filtration, centrifugation and vacuum drying techniques

If the vacuum technique is not used in the purification process, it is recommendedthat the oil temperature to be limited to 30 °C, but if the vacuum technique is used, it

is appropriate to change the temperature to the higher values However, while usingthe vacuum process, the temperature should not be exceeded the oil boiling point, asthis could lead to unwanted changes in the oil If the oil boiling point is not available,

it is recommended that the oil temperature must not be increased by 85 °C when thevacuum technique is used [1]

It should be remembered that purification of inhibited oil under vacuum, and athigh temperatures may reduce the antioxidant content Therefore, if it is desirable for

us to reduce the particles or free water, then the physical purification is an appropriatemethod The filters effectively remove impurities from the oil, but only absorb asmall amount of free water If the amount of free water in the oil is high, it should beextracted out of the oil before filtration The equipment used to filter oil contaminatedwith carbon (such as tap changer oil) should not be used for other oil filtration because

of the possibility of the contamination availability The requirements for purificationinhibited oil to prevent the reduction of additives are shown in Table2.1

Generally, centrifuge separators are suitable for the separation of free water fromoil and can also be used to separate the solids from oil If the oil is heated, the viscosity

is reduced and the purification efficiency is higher On the other hand, sludge and freewater in hot oil are more soluble than cold oil Therefore, the cold filtration method

is best suited for the separation of particles and free water and the hot purificationmethod is appropriate for the separation of solvent water and solvent gases If the oilcontains solid particles, it is recommended that it must be passed through a suitablefilter prior to the vacuum purification

Table 2.1 Conditions for

2.10.2 Chemical Purification (Refinement)

Chemical purification is a process that eliminates or reduces the soluble andinsoluble polar contaminants [1] Refinement processes require equipment, expertiseand experience The basic characteristics of the final product must be evaluated inorder to provide the necessary information on the process efficiency and the estimatedlifetime of the remaining oil The output of this process can be the oil that meets thestandard IEC 60296 In the chemical purification process, oxidation stability of theoil with high acidity value is lower than the acidity of the unused oil There are twochemical purification methods, including the percolation and the contact methods[1]

2.10.2.1 Percolation Method

The complete process consists of three continuous steps:

(1) The oil is exhausted from the bottom of the tank, heated to a specific temperatureand passed through the filter to remove the particles and solids suspended andthen enter into the transformer from the top of the reservoir [1]

(2) The oil is again passed through one or more filters or other suitable materials toremove the contaminated polar solvents

(3) Finally, to remove water and gases, a physical purification device (vacuum dryer

or centrifuge) is used The filter is actually an active substance that containsinternal and external polar materials that pass through non-polar components ofthe oil, but absorbs polar contaminants or degraded products that are dissolved

in oil The absorption capacity of the contaminants is usually enhanced by anincrease in temperatures; hence the chemical purification process is carried out

at a temperature between 60 and 80 °C Experience has shown that the totalvolume of oil should be at least three times passed through the absorbing filters.For this reason, it is necessary to use devices with a proper capacity for thispurpose [1]

The number of cycles depends on the amount of contamination and the outputoil expected characteristics In case of severe contamination, transferring the entire