TECH BULL 36 R2

It is well known

that regular oil

analysis is

extremely

useful in

monitoring the

condition of

engines,

drivetrains,

hydraulics,

turbines and

many other

types of oil

lubricated equipment. The same

can be said for transformer oils

which are used to insulate many

transformers and other electrical

distribution equipment. The

analysis of transformer oils not

only provides information about

the oil, but also enables the

detection of other potential

problems, including contact

arcing, ageing insulating paper

and other latent faults and is an

indispensable part of a cost-

efficient electrical maintenance

programme.

Transformer maintenance has

evolved over the past 20 years

from a necessary item of

expenditure to a strategic tool

in the management of electrical

transmission and distribution

networks. Extreme reliability is

demanded of electric power

distribution and, even though the

failure risk of a transformer and

other oil-filled electrical equipment

is small, when failures occur they

inevitably lead to high repair

costs, long downtime and very

real safety risks. Moreover,

transformers are too expensive

to replace regularly and must be

properly maintained to maximise

their life expectancy.

By accurately monitoring the

condition of the oil, many types

of faults can be discovered

before they become serious

failures and outages can

potentially be avoided.

Furthermore, an efficient

approach to maintenance can

be adopted and the optimum

intervals determined for

replacement. Some of the checks

are relatively simple: on the

operation of the gas relays,

on the operation of the on-load

tap-changer, on oil leaks, etc.

However, breakdown of one of

the most crucial elements, the

oil / paper insulating system, can

only reliably be detected by

routine oil analysis. By measuring

certain physical and chemical

properties of the oil, in addition

to the concentrations of certain

dissolved gases, a number of

problem conditions associated

with either the oil or the

transformer can be determined.

ISSUE 36

A member of the Set Point group

TRANSFORMER

OIL ANALYSIS

By Neil Robinson, B.Sc. Hons.

Neil Robinson

A basic introduction to an essential part of a

cost-efficient maintenance programme

ENSURING

TRANSFORMER

RELIABILITY

WEARCHECK AFRICA IS AN ISO9001 AND ISO14001 REGISTERED COMPANY

the oil. An increase in acid number often

goes hand in hand with a decrease in dielectric

strength and increased moisture content

as shown in Figure 1. Again, just like their

industrial cousins, the acid content of

transformer oils is determined by

potentiometric titration with potassium

hydroxide.

DIELECTRIC STRENGTH

The dielectric strength of a transformer oil

is a measure of the oil’s ability to withstand

electrical stress without failure. Because

transformer oils are designed to provide

electrical insulation under high electrical

potentials, any significant reduction in the

dielectric strength will indicate that the oil is

no longer able to perform this vital function.

Some of the things that can cause a reduction

in dielectric strength include contaminants

such as water, sediment, conducting particles,

oil degradation by-products and cellulose

paper breakdown. The test method for

determining dielectric strength is relatively

simple and involves applying an AC voltage

at a controlled increasing rate to two

electrodes immersed in the transformer oil.

The gap is a specified distance and when the

current arcs across this gap the voltage

recorded is used to determine the dielectric

strength.

POWER OR DISSIPATION FACTOR

The power factor of a transformer oil is the

ratio of true power to apparent power and

is a measure of the current leakage through

the oil, which in turn is a measure of the

contamination or deterioration of the oil. In

a transformer, a high power factor is an

indication of significant power loss in the

The following are some common tests

performed on electrical transformer oils:

MOISTURE CONTENT

One of the most important functions of

transformer oil is to provide electrical

insulation. Any increase in moisture content

can reduce the insulating properties of the

oil, which may result in dielectric breakdown.

Water and oil, due to their differing chemical

properties are not mutually soluble. However,

up to a certain limit, a small amount of water

will dissolve in the oil. This limit is a function

of the temperature of the system and the

solubility increases exponentially with

increasing temperature. This is of particular

importance with fluctuating temperatures

because, as the transformer cools down,

any dissolved water will become free,

resulting in poor insulating power and oil

degradation. A point to note is that as the

oil ages in service, a certain amount of

oxidation occurs which changes the chemical

make up of the oil. This, in turn, allows more

water to dissolve. In addition, many

transformers contain cellulose-based paper,

which is used as insulation in the windings.

Again, excessive moisture content can result

in the breakdown of this paper insulation

with a resultant loss in performance. The

moisture content of the oil is determined

using a coulometric Karl Fischer instrument.

This is an extremely sensitive test and can

detect water at levels down to a few parts

per million.

ACID NUMBER

Just like lubricating oils, transformer oils

are oxidised under the influence of excessive

temperature and oxygen, particularly in the

presence of small metal particles which can

act as catalysts. Oxidation products are

usually acidic in nature and result in an

increase in acid number. Further reaction

of these acids with the bulk oil can result in

sludge and varnish deposits. In the worst

case scenario, the oil canals become blocked

and the transformer is not cooled adequately,

which further exacerbates oil breakdown.

Furthermore, an increase in the acidity has

a damaging effect on the cellulose paper.

Oil degradation by-products such as acids

and hydroperoxides also generally have the

ability to conduct an electrical charge, which

in turn reduces the insulating properties of

Figure 1

Temperature

Oxygen WaterMetal

catalysts

Temperature

Oil

Paper

Paper degradation

Sludge and

varnish

Paper chain

scissors

+ water

Oil oxidation

Acids

and

hydroperoxides

The cellulose materials are the weakest link

in the insulation system. Since the life of the

transformer is actually the life of the cellulose

insulation, and degradation of the cellulose

is irreversible, the decay products should be

removed before they can do any further

damage to the cellulose. With proper

maintenance the cellulose can virtually have

an indefinite life. To test for furanics, a

sample of the oil is obtained and certain

chemical techniques are used to extract the

furans from the oil. The extract is then

analysed using a process called high

performance liquid chromatography (HPLC).

The results are usually reported in terms of

parts per billion (ppb).

DISSOLVED GAS ANALYSIS (DGA)

The analysis of gases from petroleum

products has been performed for decades

using gas chromatography. However, this

technique was not applied specifically to

transformer mineral oils until the late 1960s

or early 1970s and is now commonly called

dissolved gas-in-oil analysis (DGA). DGA has

become a standard in the electrical

maintenance industry throughout the world

and is considered to be the most important

oil test for transformer oils in electrical

apparatus. More importantly, an oil sample

can be taken at any time from most

equipment without having to take it out of

service, allowing a “window” inside the electrical

apparatus that helps with diagnosing and

troubleshooting potential problems.

As the insulating materials of a transformer

break down from excessive thermal or

electrical stress, gaseous by-products form.

transformer oil, usually as a result of

contaminants such as water, oxidised oil

and cellulose paper degradation. It may also

be any substance in the oil that either resists

or conducts electricity differently to that of

the oil itself, which may include diesel fuel,

lubricating oil and kerosene. The test is not

specific in what it detects and is usually

carried out at elevated temperatures

because contaminants that affect the test

may remain undetected at 25ºC and only

reveal themselves at >90ºC.

INTERFACIAL TENSION (IFT)

The interfacial tension of transformer oil is

related to its deterioration. Transformer oil

is generally a hydrocarbon and thus

hydrophobic. However, when the sample

undergoes oxidative degradation, oxygenated

species such as carboxylic acids are formed,

which are hydrophilic in nature. Interfacial

tension is the surface tension of a sample

of the oil carefully floated on top of a layer

of water. The more hydrophilic the oil

becomes, the lower the value of the surface

tension between the two liquids. Studies

have shown that there is a definite

relationship between acid number and IFT.

An increase in acid number generally shows

a decrease in IFT. However, when there is

a loss in IFT without the corresponding

increase in acid number, it is generally

because of contamination with another

hydrophilic substance not derived from

oxidation of the oil.

FURANICS OR DEGREE

OF POLYMERISATION (DP)

The solid insulation (cellulose-based products)

in transformers degrades with time at

rates which depend on the temperature,

moisture content, oxygen and acids in the

insulation system. Heat and moisture are

the main enemies of the solid paper insulation

with oxidation as the primary culprit.

When degradation occurs, the cellulose

molecular chains (polymers) get shorter.

Chemical products such as furanic derivatives

are produced and dissolve in the

transformer oil. Of the furanic compounds,

2-furaldehyde is the most abundant. Its

concentration in oil has been related to the

degree of polymerisation (DP) and

consequently to the physical strength of the

solid insulation (see Figure 2).

Degree of polymerisation

Furanic concentration

Figure 2

The by-products are characteristic of the

type of incipient fault condition, the materials

involved and the severity of the condition.

Indeed, it is the ability to detect such a

variety of problems that makes this test

such a powerful tool for detecting incipient

fault conditions and for root cause

investigations after failures have occurred.

Dissolved gases are detectable in low

concentrations (ppm level), which usually

permits early intervention before failure of

the electrical apparatus occurs, and allows

for planned maintenance. The DGA technique

involves extracting or stripping the gases

from the oil and injecting them into a gas

chromatograph (GC).

Typical gases generated from mineral oil /

cellulose (paper and pressboard) insulated

transformers include:

Hydrogen H

Methane CH

Ethane C

Ethylene C

Acetylene C

Carbon Monoxide CO

Carbon Dioxide CO

Additionally, oxygen and nitrogen are always

present. Their concentrations vary with the

type of preservation system used on the

transformer. Also, gases such as propane,

butane, butene and others can be formed

as well, but their use for diagnostic purposes

is not widespread. The concentration of

the different gases provides information

about the type of incipient fault condition

present as well as the severity. For example,

four broad categories of fault conditions

have been described and characterised in

Table 1.

The severity of an incipient fault condition is

ascertained by the total amount of

combustible gases present (CO, H

, C

, CH

), their rate of generation

and their ratios with one another. Generally,

transformers will retain a large portion of

the gases generated and therefore produce

a cumulative history of the insulating

materials’ degradation. This is an important

tool for detecting and trending incipient

problems. However, it also means that care

is needed in interpreting values for a first

time analysis on service-aged transformers

(several years old), which could contain

residual gases from previous events.

Some gas generation is expected from normal

ageing of the transformer insulation and it

is therefore important to differentiate

between normal and excessive gassing rates.

Normal ageing or gas generation varies with

transformer design, loading and type of

insulating materials. Routinely, general gassing

rates for all transformers are used to define

abnormal behaviour. Specific information for

a family of transformers can be used when

sufficient dissolved gas-in-oil data is available.

Acetylene is considered to be the most

significant gas generated. An enormous

amount of energy is required to produce

acetylene, which is formed from the

breakdown of oil at temperatures in excess

of 700ºC. Excessively high overheating of

the oil will produce the gas in low

concentrations. However, higher

concentrations are typically symptomatic of

sustained arcing, a more serious operational

issue that can cause a transformer failure

if left unchecked.

DGA is used not only as a diagnostic tool

but also to stem apparatus failure. Failure

of a large power transformer not only results

in the loss of very expensive equipment but

it can cause significant collateral damage as

well. Revenue losses due to operational

outages may be the least worrisome

consequence of a failure. Replacement of

that transformer can take up to a year if

the failure is not catastrophic and can result

in tremendous revenue losses. If the failure

KEY GASES

Methane, Ethane,

Ethylene and small

amounts of Acetylene

Hydrogen, Methane

and small amounts of

Acetylene and Ethane

Hydrogen, Acetylene

and Ethylene

Carbon Monoxide and

Carbon Dioxide

Thermal condition

involving the oil

Partial discharge

Sustained arcing

Thermal condition

involving the paper

GENERAL FAULT

CONDITION

Table 1: Categories of key gases and general fault condition

is catastrophic, then additional losses could

occur, such as adjacent transformers,

environmental problems from the release

of oil (which could be as much as 20 000

litres), and the resulting fire that would have

to be contained and smothered. In order to

avoid such a failure, the sample frequency

of most large power transformers is between

one and three years. However, sampling

frequencies will increase as an incipient fault

is detected and monitored. Often sampling

frequencies are dictated by insurance

requirements, which often stipulate that

annual transformer oil analysis must be

conducted to ensure continued coverage.

PCB ANALYSIS

PCBs (polychlorinated biphenyls) are a group

of synthetic oil-like chemicals of the

organochlorine family. Until their toxic nature

was recognised and their use was banned

in the early 1980s, they were widely used

as insulation in electrical equipment,

particularly transformers. Three types of

PCBs are normally used in electrical

transformers: Aroclor 1242, 1254 and

1260, commonly known by various brand

names which include Askarel, Chlorectol,

Elemex, Inerteen and Pyranol.

One of the most important problems with

PCBs is that they concentrate in the fatty

parts of microorganisms. This concentration

factor between the organism and the water

can be as much as a million times.

Concentrations are further amplified as the

microorganisms become food for animals

further up the food chain, ultimately ending

up in humans. PCBs are very stable and

their degradation process is slow, making

for yet greater amplification in organisms.

Although not overly toxic in themselves,

PCBs are poisons, which have been shown

to cause damage to the reproductive,

neurological and immune systems of wildlife

and humans.

Far more serious are the risks of a fire or

an explosion. At temperatures around

500ºC, extremely toxic compounds -

polychlorinated dibenzofuranes (PCDF) and

polychlorinated dibenzodioxins (PCDD) - are

formed. Small amounts of these compounds

have been found at accidents where

transformers and capacitors have been

exposed to fire or have exploded. Even if

the amounts have been extremely small and

have caused no personal injuries, it has been

necessary to perform very extensive and

costly decontamination work.

PCDDs and PCDFs cause damage and

death in doses as low as 1ppb to

5000ppb. They are some of the most

potent cancer promoters known and can

damage organs such as the liver, kidney

and digestive tract as well as cause

miscarriage and sterility.

Methods of PCB analysis

Current methods of analysis are divided

into two major groups: PCB specific and

PCB non-specific. Non-specific methods

test for PCBs indirectly by detecting

one of the components of the PCB

compound, usually chlorine. In general,

non-specific methods are quicker and

less expensive than the specific methods.

However, these tests are susceptible to

false positive results, since the test does

not detect PCB itself.

Specific methods utilise some type

of chromatography to separate

PCB molecules from each other and

interfering compounds. It is not a case

of simply finding an easily quantifiable

compound, but of quantifying a complex

mixture of compounds. Of the three

major chromatography types, gas

chromatography (GC), thin layer

chromatography (TLC) and liquid

chromatography, GC is the preferred

and most extensively used method.

Terminology associated with PCBs is

defined below

Non PCB

Any fluid, including that in electrical

equipment and any item that has a

measurable PCB concentration of less

than 50ppm of PCB, is considered a

non-PCB item.

PCB contaminated

Any fluid, including that in electrical

equipment, and any item which has a

measurable PCB concentration of 50ppm

or greater but less than 500ppm is

regarded as being PCB contaminated.

Transformer oil that has not been tested

must be classified as PCB contaminated

until shown to be otherwise.

PCB item

Any fluid, including that in electrical

equipment and in any item which has a

measurable PCB concentration equal to or

greater than 500ppm, is regarded as a PCB

item. Once the PCB status is determined,

a sticker is issued and fixed to the item in

question. This allows for quick reference and

ensures that potential cross contamination

is avoided during future sampling,

maintenance and decommissioning if

necessary.

Blending PCB contaminated oil with virgin

or other oil to meet the legal requirements

is obviously an illegal practice that has been

shown to happen from time to time.

This practice simply has the effect of

contaminating virgin oil supplies and ensures

that the PCBs persist in the environment,

leading to further contamination.

Greater detail on PCBs, their management,

disposal and applicable legislative issues

surrounding them, can be viewed on the

Wearcheck web site (www.wearcheck.co.za)

in an article entitled “Guide for PCB

management of insulating oils in South Africa”

by I.A.R. Gray under the Additional Info

section.

As with machinery oil analysis, the ability of

transformer oil analysis to provide an early

warning sign of a problem condition is

Felicity Howden Public Relations 6/2006

Copies of previous Technical Bulletins can be accessed

on Wearcheck’s web site: www.wearcheck.co.za

A member of the Set Point group

Publications are welcome to reproduce articles or extracts from them providing they acknowledge Wearcheck Africa, a member of the Set Point group.

dependent on the quality of the oil sample

that is sent to the lab. A sampling point on

any equipment should be identified and clearly

labelled for the technician. Also, as with

sampling locations in other types of

equipment, the same location should be used

each time a sample is collected to ensure

representative conditions are tested.

This point should be located in a place where

a live oil sample can be collected rather

than in an area where the oil is static.

Just like machinery oil analysis, electrical

transformer oil analysis can play a vital role

in preventing unscheduled outages in electrical

transmission and distribution equipment by

determining the condition of the equipment

itself, as well as other vital components,

including the condition of the oil and the

cellulose paper insulation. Regular routine oil

analysis should be the cornerstone of any

PM programme for all critical oil-filled electrical

equipment, including transformers, circuit

breakers and voltage regulators.

REFERENCES

Anne Reygaerts – Laborelect, courtesy of

Noria Corporation.

NTT WorldWide Technical Bulletins

Lance R. Lewand, Doble Engineering

Company, courtesy of Noria Corporation

I.A.R. Gray - Transformer Chemistry Services

Neil Robinson is managing director of

Wearcheck Africa.

PROPER

TRANSFORMER

SAMPLING