I·Transformer Oil Analysis: An Overview

Parveen Goyal, A.K. Chatterjee and Narender Sharma

ABC Paper, Saila Khurd-144 529, Distt. Hoshiarpur, Punjab

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The present paper discusses the imp,ortance of transformer oil analysis (TOA) in a cost effectiveelectrical maintenance programme. Some of the important tests in transformer oil analysisprogramme such as gas in oil analysis, screen tests like dielectric breakdown, power factor, interfacialtension etc, metals in oil, moisture content, PCB (Polychlorinated biphenyl) content besides othercommon tests including physical properties and their significance, have also been described. Thedescription of various steps and elements of a transformer oil analysis programme and the recentdevelopments have also been touched upon. The application of transformer oil analysis and thebenefits derived in ABC Paper have also been summarized. The complete evaluation &understanding of the relation between the analysis and follow up actions in transformer oil analysis(TOA) may assure a safe & uninterrupted operation with cost effective management of transformingequipments.

INTRODUCTION

Transformer is one of the most important and strategictools in the electrical transmission & distributionmanagement system, the operational reliability of whichin turn, depends to a large extent on the condition ofthe oils used in it as an insulation & heat transfermedium. Transformer oil analysis (TOA) has thereforebecome one of the most critical and important part ofelectrical maintenance programme because monitoringof transformer oils not only provides information aboutthe condition of the oil but at the same time indicatesthe condition of the transformer and enables the earlydetection of other latent faults such as aging ofinsulating paper, contact acing etc. Earlier, thetransformer oil analysis was limited to the monitoringof physical properties of oil but over a period of timeTOA has been evolved to a complete oil analysisprogramme covering other tests such as dissolved gasanalysis (DGA) following physical analysis withcritical elements like equipment audit, monitoring andevaluation. Some petroleum based mineral oils calledas transformers oils are used in the transformers asinsulating, heat transfer and are quenching medium.

Transformers are susceptible to problems such asoverheating, partial discharges, arcing etc which in turnresult in the chemical decomposition of the insulatingoils producing several gases that are totally or partiallydissolved in the oil. Since above defined faults i.eoverheating or low energy faults can not be detectednormally by electrical tests, the analysis of transformeroil has become a valuable means of detecting suchmalfunctions in transformers. The chemistry ofinsulating oil and the contaminations in the oil are thefactors directly affecting the operational life oftransformers. Transformer oil analysis indicates someimportant parameters such as dielectric breakdowrvolmge, presence of conductive contaminants, colourchange, presence of acids, sludge indication and manymore which predict the health of a transformingequipment. Since the transformers are structurally verycomplicated system and hence it is difficult to determinethe cause when a problem arises. Therefore, monitoringof the condition of transformer oil is particularlyimportant for early detection of problems in order toplan the corrective action required to be taken andtherefore minimizes the chances of damages &breakdowns. Keeping in the view the importance of

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Ltransformer oil analysis, some theoretical and practicalaspects of transformer oil analysis have been discussedin the present paper. Some of the critical requirements ofthe transformer oils have been summarised in the table 1.

Significance of Transformer Oil Analysis

The sampling is the first and the most critical step inany oil analysis programme. The sampling requires careto ensure that the sample representative of the oil beingsampled. The sample location should be a point wherea live oil sample can be collected rather than a pointwhere the oil is static. Documentation of the sample

Analytical Test

Table 1: Requirement of Transformer oils

Requirement

Visual apperance

Electrical strength of oil

Water content of oil

IFf (Interfacial Tension)

Acidity

Flash point

Sediment (% by wt)

..

Change of oil color

>50kV

< 20 PPM

> 0.015

< 0.5 mg KOHl g

Nil

• Transformer oil analysis lowers the maintenancecosts by replacing the practice of preventive maintenanceto a more cost effective predictive maintenance.

• TOA prevents premature failure of machinecomponents and therefore extends equipment life

• Maintenance costs and down time are reduced byeliminating unnecessary component change andpremature scheduled maintenance respectively

• Better assessment of equipment performance ispossible with TOA

• Lubricant costs are reduced by optimizing thedrain intervals from TOA data.

EXPERIMENT AL

Transformer oil analysis was earlier limited tomeasuring few physical and chemical properties of oilbut over a period of time, this has been evolved to acomplete programme consisting of a series of stepsincluding equipment audit, monitoring, evaluation etc.besides oil testing.

Important Steps in Transformer Oil Analysis

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alongwith equipment details, operating conditions,environmental variables, type of oil, sampling procedurefor meaningful oil analysis. Availability of samplingkits has now made the sampling procedure simple andreliable.

,Determination of the analytical tests required foranalysis of a particular oil to get the information aboutthe oil and the condition of the equipment is the secondimportant step in the transformer oil analysis. Whilemaking selection of analytical tests, the importance ofthe equipment in the plant operation, cost of repair anddowntirrie should be given due consideration since thereare a large number of tests and many of which measurethe same properties. Therefore, sometimes specializedoil analytical tests may not be cost effective for the lesscritical equipments.

Once the data is generated from the oil analysis, theevaluation and the interpretation of the informationgathered about the condition of the oil & the equipmentas well are required. Evaluation of analytical data insome cases identifies a problem whereas in most of thecases, the analytical data alone is not sufficient enoughto make accurate judgments about oil or equipmentcondition. Therefore, comparison of current sample datato previous sample and similar operations is desirable

I·to improve the r~liability of recommendation based onanalytical tests. Comparison can be made on the basisof engine types, operating environment, frequency ofuse, equipment type, severity of service etc.

TESTS IN OIL ANALYSIS

Various tests in transformer analysis are designednormally on the basis of two factors i.e

i) Serviceability of the oil and ii) Operating environment& equipment age.

A reasonably accurate diagnosis of a transfer oil withrespect to serviceability is the outcome of collective.consideration of the following tests.

1. Acidity Test

2. Dielectric Test

3. Power Factor]

4. Interfacial Tension (1FT)

Acidity test

•The Acidity test is a measure of acids formed as a resultof oxidation of transformer oil under the influence ofexcessive temperature and oxygen, whereas the reactionis catalyzed by the: presence of small metal particles ..The increase in the acid value of oil decreases theinsulating quality of the oil. Properly refined newtransformer oils contain no acids. On the other handthe oxidation products of oil undergo polymerizationto form sludge that precipitates out. Acidity is measuredas acid number, which is equivalent to the mass of KOHin milligrams required to neutralize the acids in one gmof oil. This test is generally performed with Gerin OilAcidity Test Kit.

Dielectric Test.

The dielectric test is performed to ascertain the dielectricstrength of the transformer oil. In other words it is themeasurement of the voltage at which the oil breaksdown. This test serves as an indicator of the presence of

contaminants such as water, dirt, moist cellulose fibresetc. Dielectric tests are conducted in accordance withAST M D 877 or ASTM D 1816 in the temperature rangebetween 20-30 "C, The test equipments used for makingdielectric tests are usually Portable Oil Dielectric Testers.

Power Factor Test

Transformer Oil power factor testing is usually carriedout with the Doble type MH or M2H tester. This test isused to measure the dielectric losses and hence thedielectric heating i.e energy dissipated as heat. Thecomparison of the test results with the standard valuesreveal the quality and integrity of the insulating oil.This test is performed as an acceptance and preventivemaintenance test for insulating oil ..

Interfacial Tension (1FT) Test

This test is used for measuring interfacial tension oftransformer oils of petroleum origin against water andgives an indication of the sludging characteristics ofthe transformer insulating oil. The attraction betweenthe water molecules at the interface is influenced by thepresence of polar molecules in the oil influence theattraction between water molecules at the interface. Themore the polar molecules, the lower will be the 1FT.Sincethe 1FT Test measures the concentration of polarmolecules in suspension and in solution in the oil .ameasurement of dissolved sludge precursors in the oilcan be predicted long before any sludge is precipitated.The 1FTtest is performed by following the Drop-WeightMethod (ASTM D 2285) in which 1FT is determined bymeasuring the volume of a drop of water supported byoil.

Tests for Moisture Content and oxidation inhibitors areperformed due to operating environment and age of theequipment.

Moisture Content

Moisture content in transformer oils affects theinsulating properties of oil to a great extent and mayreduce the dielectric strength of oil.. The fluctuatingtemperature further magnifies the effect of moisture. Themoisture content also causes breakdown of celluloseinsulation in the windings of some transformers as the

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moisture is absorbed by the cellulose paper. This test isperformed in accordance with ASTM 0 1533.

Oxidation Inhibitor

The test for oxidation inhibitors is performed accordingto ASTM 0 2668 procedure. Generally new refinedtransformer oils contains some amount of naturallyoccurring chemical compounds that retard oil oxidation.Once these compounds are exhausted, the deteriorationof the oil is increased and to overcome this conditionsynthetic oxidation inhibitors such as 2,6-ditertiary-butyl para-cresol (DBPC) are added. Therefore this testis of significance to know the level of oxidation inhibitorpresent in the oil. Required DBPC content in the oil isbetween 0.15 percent and 0.30 percent for newequipment.

Other test used in the oil analysis are the visualappearance, color and sediment.

PCB Analysis

Polychlorinated Biphenyl (PCB) analysis of transformeroils is a part of environmental Protection Agencyregulations in some countries. Transformers with oilscontaining more than 50 ppm PCB is considered a PCB-contaminated unit. The PCB in transformer oils aremeasured according to ASTM 0-4059 method.

Table 2: Dissolved Gas Analysis of Transformer Oil

Gas Normal Concentration(ppm)

Hydrogen 200

Methane 50

Ethylene 80

Ethane 35

Acetylene 6

Carbon dioxide 2000

Carbon monoxide 300

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NEW DEVELOPMENTS IN TRANSFORMER OILANALYSIS

Earlier the transformer oil testing was limited to theabove tests but now some more advanced tests Dissolvedgas analysis (DGA) following the above tests has beenalso included.

Table 3: Various faults in Transformer due to highlevels of gases

••Gas Fault

Ethylene Severe Oil Overheating/Hot Spots

Ethane Severe Oil Overheating/Hot Spots

MethaneHydrogen

Severe Oil Overheating/Hot SpotsPartial Discharge, Carona

Acetylene Arcing

Dissolved Gas Analysis in Transformer Oil

Various gases are generated in the transformers duringoperation. DGA is performed to determine theconcentrations of certain gases in the transformer oilsthe relative ratios of which may be used to diagnosecertain operational problems with the transformers. Thetype and concentration of each gas and the rate ofgeneration can help determine operating quality oftransformer as well as the type of malfunction. Thenormal concentration of various gases and the faultsarising as a result of high levels of some major gasesrelative to the other gases in the transformers aresummarized in Table 2 and 3 respectively.

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APPLICATION OF TRANSFORMER OIL ANALYSISIN ABC PAPER: A CASE STUDY

We at M/s ABC Paper have 11 Nos. of transformersincluding one power transformers with rating rangingfrom 1 MV A to 8 MV A.

Earlier, only dielectric strength and colour testing werebeing'performed at regular intervals to assess thecondition of the oil and the transformer as well. Sinceno other critical tests such as acidity, DGA etc were being

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carried out. As a result of non-implementation ofcomplete transformer oil analysis programme, thefollowing problems were observed.

• Deposition of excessive carbon resulted in theflashing of OLTCof power transformer.

• Burning of 1.5MVA Transformer due to excessivesludge and ageing of cellulose paper.

Keeping in view the recurring nature of above problemsresulting in breakdowns ABC paper decided to startcomplete oil analysis. To start with, the transformer oilofall the transformers were dehydrated to remove sludge& moisture. Afterwards oil.of three transformers wascompletely replaced and the transformer windings werealso cleaned during annual shutdown with hotpressurized oil to remove sludge and other foreignparticles. Since then the transformer oil analysis is beingregularly carried out and approximately Rs. 50000/-are being spent annually for implementation of oilanalysis programme. Some of the advantages offered bytransformer oil analysis in ABC paper are as follows:

• Smooth running of transformers withoutoverheating

• Dielectric strength has gone above 50 KVat 2.5mm gap in comparison to earlier 20-25KV

• Improvement in IR values

DISCUSSION

I

Since the life of a transformer is dependent on the life ofthe insulating medium used, therefore proper analysisof the oil can help to identify the deterioration problem"before it becomes too critical and severe. Thebreakdownvoltage is indicative of the amount of contaminant inthe oil. The minimum accepted dielectric strength is 30kV and 25 kV for transformers with a high-voltagerating 287.5kV & above and transformers with a ~gh-voltage rating below 287.5 kV respectively. No singletest determines the actual condition of the oil for exampleoil is not necessarily in good condition even when thedielectric strength is adequate because this tells nothingabout the presence of acids and sludge: Therefore aoutcomeofall the test should be considered whilemaking

a recommendation

Acid number value of 0.4 is considered to be the normalservice limit and sludging has been found to start whenacid number exceeds 9.4. However different types oftransformers would take different period of time forappearance of sludge ranging from 10 years fortransformers with free air access to 67 years fortransformers with nitrogen over oil.

The limit for power factor of new oil is 0.05%at 25 "C, Ahigh power factor in used oil indicates deterioration orcontamination or both. Operational hazard is expectedfrom transformer oil with a power factor in excess of 2.0percent and therefore requires reconditioning orreplacement.

Since, the acidity test alone determines only conditionsunder which sludge may form but does not necessarilyindicate that actual sludging conditions exist for whichIff test is required to be performed. Transformer oilswith Iff values in the range of 0.015to 0.022N/m (15to22 dyn/ cm) are required to be scheduled for reclaimingirrespective of acidity values. The equipment operatingfor 10 years or older should be tested for oxidationinhibitor concentration when the physical tests areperformed. If the level of DBPC drops below 0.08%,oxidation inhibitor should be added

The integration of the results of all the tests determinesthe actual condition of the transformer oil and gives anindication of discarding and replacement of oil.

CONCLUSION

It may be concluded that transformer oil analysis is animportant indicator of the transformer problems muchbefore they actually arise and cause breakdown.Therefore, proper understanding, designing, trainingand implementation of transformer oil analysisprogramme in industrial operations is an essentialrequirement for improved plant reliability.

ACKNOWLEDGEMENT

Authors are extremely grateful to the Management ofABCPaper for granting their permission to present thispaper in the IPPTA Zonal Seminar.

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