Transformer Oil Analysis: An Overview

I·

Parveen Goyal, A.K. Chatterjee and Narender Sharma

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

.•

The present paper discusses the imp,ortance of transformer oil analysis (TOA) in a cost effective

electrical maintenance programme. Some of the important tests in transformer oil analysis

programme such as gas in oil analysis, screen tests like dielectric breakdown, power factor, interfacial

tension etc, metals in oil, moisture content, PCB (Polychlorinated biphenyl) content besides other

common tests including physical properties and their significance, have also been described. The

description of various steps and elements of a transformer oil analysis programme and the recent

developments have also been touched upon. The application of transformer oil analysis and the

benefits 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 transforming

equipments.

INTRODUCTION

Transformer is one of the most important and strategic

tools in the electrical transmission

distribution

management system, the operational reliability of which

in turn, depends to a large extent on the condition of

the oils used in it as an insulation

heat transfer

medium. Transformer oil analysis (TOA) has therefore

become one of the most critical and important part of

electrical maintenance programme because monitoring

of transformer oils not only provides information about

the condition of the oil but at the same time indicates

the condition of the transformer and enables the early

detection of other latent faults such as aging of

insulating paper, contact acing etc. Earlier, the

transformer oil analysis was limited to the monitoring

of physical properties of oil but over a period of time

TOA has been evolved to a complete oil analysis

programme covering other tests such as dissolved gas

analysis (DGA) following physical analysis with

critical elements like equipment audit, monitoring and

evaluation. Some petroleum based mineral oils called

as transformers oils are used in the transformers as

insulating, heat transfer and are quenching medium.

Transformers are susceptible to problems such as

overheating, partial discharges, arcing etc which in turn

result in the chemical decomposition of the insulating

oils producing several gases that are totally or partially

dissolved in the oil. Since above defined faults i.e

overheating or low energy faults can not be detected

normally by electrical tests, the analysis of transformer

oil has become a valuable means of detecting such

malfunctions in transformers. The chemistry of

insulating oil and the contaminations in the oil are the

factors directly affecting the operational life of

transformers. Transformer oil analysis indicates some

important parameters such as dielectric breakdowr

volmge, presence of conductive contaminants,

colour

change, presence of acids, sludge indication and many

more which predict the health of a transforming

equipment. Since the transformers are structurally very

complicated system and hence it is difficult to determine

the cause when a problem arises. Therefore, monitoring

of the condition of transformer oil is particularly

important for early detection of problems in order to

plan the corrective action required to be taken and

therefore minimizes the chances of damages

breakdowns. Keeping in the view the importance of

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transformer oil analysis, some theoretical and practical

aspects of transformer oil analysis have been discussed

in the present paper. Some of the critical requirements of

the transformer oils have been summarised in the table 1.

Significance of Transformer Oil Analysis

The sampling is the first and the most critical step in

any oil analysis programme. The sampling requires care

to ensure that the sample representative of the oil being

sampled. The sample location should be a point where

a live oil sample can be collected rather than a point

where 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

PPM

> 0.015

< 0.5

KOHl

Nil

• Transformer oil analysis lowers the maintenance

costs by replacing the practice of preventive maintenance

to a more cost effective predictive maintenance.

• TOA prevents premature failure of machine

components and therefore extends equipment life

• Maintenance costs and down time are reduced by

eliminating unnecessary component change and

premature scheduled maintenance respectively

• Better assessment of equipment performance is

possible with TOA

• Lubricant costs are reduced by optimizing the

drain intervals from TOA data.

EXPERIMENT AL

Transformer oil analysis was earlier limited to

measuring few physical and chemical properties of oil

but over a period of time, this has been evolved to a

complete programme consisting of a series of steps

including 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 procedure

for meaningful oil analysis. Availability of sampling

kits has now made the sampling procedure simple and

reliable.

Determination of the analytical tests required for

analysis of a particular oil to get the information about

the oil and the condition of the equipment is the second

important step in the transformer oil analysis. While

making selection of analytical tests, the importance of

the equipment in the plant operation, cost of repair and

downtirrie should be given due consideration since there

are a large number of tests and many of which measure

the same properties. Therefore, sometimes specialized

oil analytical tests may not be cost effective for the less

critical equipments.

Once the data is generated from the oil analysis, the

evaluation and the interpretation of the information

gathered about the condition of the oil

the equipment

as well are required. Evaluation of analytical data in

some cases identifies a problem whereas in most of the

cases, the analytical data alone is not sufficient enough

to make accurate judgments about oil or equipment

condition. Therefore, comparison of current sample data

to previous sample and similar operations is desirable

I·

to improve the r~liability of recommendation based on

analytical tests. Comparison can be made on the basis

of engine types, operating environment, frequency of

use, equipment type, severity of service etc.

TESTS IN OIL ANALYSIS

Various tests in transformer analysis are designed

normally 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 with

respect to serviceability is the outcome of collective.

consideration of the following tests.

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 result

of oxidation of transformer oil under the influence of

excessive temperature and oxygen, whereas the reaction

is catalyzed by the: presence of small metal particles ..

The increase in the acid value of oil decreases the

insulating quality of the oil. Properly refined new

transformer oils contain no acids. On the other hand

the oxidation products of oil undergo polymerization

to form sludge that precipitates out. Acidity is measured

as acid number, which is equivalent to the mass of KOH

in milligrams required to neutralize the acids in one gm

of oil. This test is generally performed with Gerin Oil

Acidity Test Kit.

Dielectric Test.

The dielectric test is performed to ascertain the dielectric

strength of the transformer oil. In other words it is the

measurement of the voltage at which the oil breaks

down. This test serves as an indicator of the presence of

contaminants such as water, dirt, moist cellulose fibres

etc. Dielectric tests are conducted in accordance with

AST M D 877 or ASTM D 1816 in the temperature range

between 20-30

"C,

The test equipments used for making

dielectric tests are usually Portable Oil Dielectric Testers.

Power Factor Test

Transformer Oil power factor testing is usually carried

out with the Doble type MH or M2H tester. This test is

used to measure the dielectric losses and hence the

dielectric heating i.e energy dissipated as heat. The

comparison of the test results with the standard values

reveal the quality and integrity of the insulating oil.

This test is performed as an acceptance and preventive

maintenance test for insulating oil..

Interfacial Tension (1FT) Test

This test is used for measuring interfacial tension of

transformer oils of petroleum origin against water and

gives an indication of the sludging characteristics of

the transformer insulating oil. The attraction between

the water molecules at the interface is influenced by the

presence of polar molecules in the oil influence the

attraction between water molecules at the interface. The

more the polar molecules, the lower will be the 1FT.Since

the 1FT Test measures the concentration of polar

molecules in suspension and in solution in the oil .a

measurement of dissolved sludge precursors in the oil

can be predicted long before any sludge is precipitated.

The 1FTtest is performed by following the Drop-Weight

Method (ASTM D 2285) in which 1FT is determined by

measuring the volume of a drop of water supported by

oil.

Tests for Moisture Content and oxidation inhibitors are

performed due to operating environment and age of the

equipment.

Moisture Content

Moisture content in transformer oils affects the

insulating properties of oil to a great extent and may

reduce the dielectric strength of oil.. The fluctuating

temperature further magnifies the effect of moisture. The

moisture content also causes breakdown of cellulose

insulation in the windings of some transformers as the

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moisture is absorbed by the cellulose paper. This test is

performed in accordance with ASTM 0 1533.

Oxidation Inhibitor

The test for oxidation inhibitors is performed according

to ASTM 0 2668 procedure. Generally new refined

transformer oils contains some amount of naturally

occurring chemical compounds that retard oil oxidation.

Once these compounds are exhausted, the deterioration

of the oil is increased and to overcome this condition

synthetic oxidation inhibitors such as 2,6-ditertiary-

butyl para-cresol (DBPC) are added. Therefore this test

is of significance to know the level of oxidation inhibitor

present in the oil. Required DBPC content in the oil is

between 0.15 percent and 0.30 percent for new

equipment.

Other test used in the oil analysis are the visual

appearance, color and sediment.

PCB Analysis

Polychlorinated Biphenyl (PCB) analysis of transformer

oils is a part of environmental Protection Agency

regulations in some countries. Transformers with oils

containing more than 50 ppm PCB is considered a PCB-

contaminated unit. The PCB in transformer oils are

measured according to ASTM 0-4059 method.

Table 2: Dissolved Gas Analysis of Transformer Oil

Gas

Normal Concentration(ppm)

Hydrogen

200

Methane

Ethylene

Ethane

Acetylene

Carbon dioxide

2000

Carbon monoxide

300

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NEW DEVELOPMENTS IN TRANSFORMER OIL

ANALYSIS

Earlier the transformer oil testing was limited to the

above tests but now some more advanced tests Dissolved

gas analysis (DGA) following the above tests has been

also included.

Table 3: Various faults in Transformer due to high

levels of gases

••

Gas

Fault

Ethylene

Severe Oil Overheating/Hot Spots

Ethane

Severe Oil Overheating/Hot Spots

Methane

Hydrogen

Severe Oil Overheating/Hot Spots

Partial Discharge, Carona

Acetylene Arcing

Dissolved Gas Analysis in Transformer Oil

Various gases are generated in the transformers during

operation. DGA is performed to determine the

concentrations of certain gases in the transformer oils

the relative ratios of which may be used to diagnose

certain operational problems with the transformers. The

type and concentration of each gas and the rate of

generation can help determine operating quality of

transformer as well as the type of malfunction. The

normal concentration of various gases and the faults

arising as a result of high levels of some major gases

relative to the other gases in the transformers are

summarized in Table

and 3 respectively.

•

APPLICATION OF TRANSFORMER OIL ANALYSIS

IN ABC PAPER: A CASE STUDY

We at M/s ABC Paper have 11 Nos. of transformers

including one power transformers with rating ranging

from 1 MV A to 8 MV A.

Earlier, only dielectric strength and colour testing were

being'performed at regular intervals to assess the

condition of the oil and the transformer as well. Since

no other critical tests such as acidity, DGA etc were being

I·

carried out. As a result of non-implementation of

complete transformer oil analysis programme, the

following problems were observed.

• Deposition of excessive carbon resulted in the

flashing of OLTCof power transformer.

• Burning of 1.5 MV A Transformer due to excessive

sludge and ageing of cellulose paper.

Keeping in view the recurring nature of above problems

resulting in breakdowns ABC paper decided to start

complete oil analysis. To start with, the transformer oil

ofallthe transformers were dehydrated to remove sludge

moisture. Afterwards oil.of three transformers was

completely replaced and the transformer windings were

also cleaned during annual shutdown with hot

pressurized oil to remove sludge and other foreign

particles. Sincethen the transformer oil analysis is being

regularly carried out and approximately Rs. 50000/-

are being spent annually for implementation of oil

analysis programme. Some of the advantages offered by

transformer oil analysis in ABC paper are as follows:

• Smooth running of transformers without

overheating

• Dielectricstrength has gone above 50 KVat 2.5

mm gap in comparison to earlier 20-25KV

•

Improvement in IR values

DISCUSSION

Sincethe lifeof a transformer is dependent on the life of

the insulating medium used, therefore proper analysis

of the oil can help to identify the deterioration problem"

beforeit becomes too criticaland severe. Thebreakdown

voltage is indicative of the amount of contaminant in

the oil. The minimum accepted dielectric strength is 30

kV and 25 kV for transformers with a high-voltage

rating 287.5kV

above and transformers with a ~gh-

voltage rating below 287.5 kV respectively. No single

test determines the actual condition ofthe oilforexample

oil is not necessarily in good condition even when the

dielectricstrength is adequate because this tells nothing

about the presence of acids and sludge: Therefore a

outcomeofallthe test should be considered whilemaking

a recommendation

Acid number value of 0.4is considered to be the normal

service limit and sludging has been found to start when

acid number exceeds 9.4. However different types of

transformers would take different period of time for

appearance of sludge ranging from 10 years for

transformers with free air access to 67 years for

transformers with nitrogen over oil.

The limit for power factor ofnew oilis 0.05%at 25

"C,

high power factor in used oil indicates deterioration or

contamination or both. Operational hazard is expected

from transformer oil with a power factor in excessof 2.0

percent and therefore requires reconditioning or

replacement.

Since,the acidity test alone determines only conditions

under which sludge may form but does not necessarily

indicate that actual sludging conditions exist for which

Iff test is required to be performed. Transformer oils

with Iff values in the range of 0.015to 0.022N/m (15to

22dyn/ cm) are required to be scheduled for reclaiming

irrespective of acidity values. The equipment operating

for 10 years or older should be tested for oxidation

inhibitor concentration when the physical tests are

performed. If the level of DBPC drops below 0.08%,

oxidation inhibitor should be added

The integration of the results of all the tests determines

the actual condition of the transformer oil and gives an

indication of discarding and replacement of oil.

CONCLUSION

It may be concluded that transformer oil analysis is an

important indicator of the transformer problems much

before they actually arise and cause breakdown.

Therefore, proper understanding, designing, training

and implementation of transformer oil analysis

programme in industrial operations is an essential

requirement for improved plant reliability.

ACKNOWLEDGEMENT

Authors are extremely grateful to the Management of

ABCPaper for granting their permission to present this

paper in the IPPTA Zonal Seminar.

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