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Maintenance of in-service mineral

insulating oil

Dr B Pahlavanpour, Nynas UK

Subjectmatter, 2 Name, Division 2012-02-26

Functional Requirements for Oil

An insulant

A heat transfer and cooling medium

An arc quenching medium for tapchanger diverter switches and oil

filled switchgear

Must be compatible with the materials used in electrical

equipment

Subjectmatter, 3 Name, Division 2012-02-26

Oil Condition monitoring

Monitoring and maintaining oil quality is essential to ensure the reliable

operation of oil-filled electrical equipment.

Codes of practice for this purpose have been established by electrical

power authorities, power companies and industries in many countries.

4 Subjectmatter, Name, Division 2012-02-26

Transformer Failure

Before 1930

Lack of transformer oil maintenance

many kinds of oil in use

Lack of specification for manufacture of oil

Lack of standard for determining oil quality

Lack of understanding of what transpires inside

a transformer as it ages

Subjectmatter, 5 Name, Division 2012-02-26

What is happening today?

Reduction of Capital Employed

Reduced Investments

Increased Efficiency

Increase in sales/ volumes/ prices

Increase in load of existing equipment

Reduction of operating cost

(e.g. staff, maintenance, supervision,...)

6 Subjectmatter, Name, Division 2012-02-26

So what are the consequences?

A) Shorter life time

B) Increased risk of failure

Subjectmatter, 7 Name, Division 2012-02-26

Transformer Asset Management

- Long term commitment

- Balancing Operation / Maintenance / Capital

- Balancing Cost / Risk / Performance

- Portfolio view System & Equipment

- Get relevant field data

Treat information as an asset for

decisions

Goal

Fit between

Operations

performance

and corporate

financial goals

8 Subjectmatter, Name, Division 2012-02-26

Parameters to consider ….. to achieve

reliable /available transformers

Design / Manufacturing

Maintenance

Operations

e.g. load

A good specification for

new transformer oil is

necessary a long life

Subjectmatter, 9 Name, Division 2012-02-26

Spending - Result graph

Result

None

All

Spending None Little Unlimited resources

Must

10 Subjectmatter, Name, Division 2012-02-26

Asset Management – separation of 3

perspectives

Asset owner

Return of capital, performance objectives

Asset

Manager

Design, load &

maintenance plan

Service

Provider

Operations excellencies

3 perspectives

of responsibility

Subjectmatter, 11 Name, Division 2012-02-26

Transformer reliablility

2 – perspective methodology

- Grid design

- Operation

- Protection equipment

- Transformer quality

- Analysis => information

- Maintenance actions

12 Subjectmatter, Name, Division 2012-02-26

Uncertainty and Risk

Uncertainty is

not risk!

If an Asset Manager does not

test his/her transformers the

RISK does not necessarily

increase

But the uncertainty and control of the RISK

increases

Subjectmatter, 13 Name, Division 2012-02-26

The chemistry behind a transformer is

complicated

Moisture

Acids

Temp./

Load

Sludge

formation

Upgraded

Cellulose

Metals

Age

Oxygen

OIL /

CELLULOSE

Very high

focus on oil

oxidation

stability

14 Subjectmatter, Name, Division 2012-02-26

Why Oil Maintenance?

Insulating oil in-service is subjected to heat and air

Oil oxidation deterioration

If a certain amount of oil deterioration is exceeded, there is inevitably

some erosion of safety margins and the question of the risk of

premature failure has to be considered.

While the quantification of the risk can be very difficult, a first step

involves the identification of potential effects of increased deterioration.

Unused mineral oils are limited resources and should be handled with

this in mind.

Subjectmatter, 15 Name, Division 2012-02-26

Maintenance standard

IEC60422 Mineral insulating oils in electrical equipment – Supervision

and maintenance guidance.

IEEE C57, 106 IEEE Guide for acceptance and maintenance of insulating

oil in equipment.

Utility, organisation standards TGN158

16 Subjectmatter, Name, Division 2012-02-26

Aim

Prevent forced outage

Eliminate unnecessary maintenance

Provide the appropriate amount of maintenance at right time

Prevent premature ageing

Subjectmatter, 17 Name, Division 2012-02-26

Basic oil characteristics required for reliable performance

High dielectric strength to withstand the electric stresses imposed in

service

Sufficiently low viscosity so that its ability to circulate and transfer heat

is not impaired

Adequate low-temperature properties down to the lowest temperature

expected at the installation site

Resistance to oxidation to maximise service life

18 Subjectmatter, Name, Division 2012-02-26

IEC60422 recommendation

Three group of tests are recommended:

Routine tests (Group 1)

Complementary tests (Group 2)

Special Investigative tests (Group 3)

Subjectmatter, 19 Name, Division 2012-02-26

Routine tests (Group 1)

Minimum tests required to monitor the oil and to ensure that it is

suitable for continued service. If the results obtained from these tests

do not exceed recommended action limits usually no further tests are

considered necessary until the next regular period for inspection but,

under certain perceived conditions, complementary tests may be

deemed prudent.

20 Subjectmatter, Name, Division 2012-02-26

Complementary tests (Group 2)

These are additional tests, which may be used to obtain further

specific information about the quality of the oil, and may be used to

assist in the evaluation of the oil for continued use in service.

Subjectmatter, 21 Name, Division 2012-02-26

Special Investigative tests (Group 3)

These tests are used mainly to determine the suitability of the oil for

the type of equipment in use and to ensure compliance with

environmental and operational considerations.

22 Subjectmatter, Name, Division 2012-02-26

Group one tests

Inhibitor content 1 IEC 60666

Dielectric dissipation factor (DDF) 1 IEC 60247

and resistivity

IEC 62021-1 or

IEC 62021-2

Acidity (neutralization value) 1

Water content 1 IEC 60814

Breakdown voltage 1 IEC 60156

Colour and appearance 1 ISO 2049

Property Group Method

Subjectmatter, 23 Name, Division 2012-02-26

Group 2 tests

Particles (counting and sizing) 2 IEC 60970

ASTM D971

EN 14210

Interfacial tension (IFT) 2

Annex C of

IEC 60422

Sediment 2

Sludge

24 Subjectmatter, Name, Division 2012-02-26

Group 3 tests

Annex B of IEC

60666

Passivator content 3

Dibenzyl disulfide (DBDS) content 3 IEC 62697-1

IEC 62535

ASTM D1275B

DIN 51353

Corrosive sulphur 3

Polychlorinated biphenyls (PCBs) 3 IEC 61619

Viscosity 3 ISO 3104

Density 3 ISO3675

Pour point 3 ISO 3016

Compatibility 3 IEC 61125

Flash point 3 ISO 2719

Oxidation stability 3 IEC 61125

Subjectmatter, 25 Name, Division 2012-02-26

Oil sampling

It is essential that every effort be made to ensure that samples are

representative of the insulating oil in equipment

Sampling should be performed by an experienced person, who has

received adequate training, in accordance with IEC 60475.

Where available, manufacturer’s instructions should be followed.

26 Subjectmatter, Name, Division 2012-02-26

Important required information

Temperature of the oil sample

WTI reading(s)

Proper labelling of sample

Equipment type and number

Tests requested

Reason for sampling

Ambient air condition

…………..

Name of person taking the sample.

Subjectmatter, 27 Name, Division 2012-02-26

Handling and analysis of oil sample

The sample begins to deteriorate as soon as it has been taken

Sunlight

Air

Heat

Analysing the sample in the laboratory as quickly as is reasonably

practical

28 Subjectmatter, Name, Division 2012-02-26

Categories of equipment

In order to take account of different user requirements, equipment has

been placed in various categories as O, A, B, C, D, E, F, G

O, 400 KV and above

A, 170 to 400 KV

B, 72.5 to 170 KV

C, transformers <72.5 KV. OCB, switchgear

D, Instrument transformers >170 KV

E, Instrument transformers <170 KV

F, Diverter tanks of on-load tap-changers

G, Circuit breakers <72.5 KKV

Subjectmatter, 29 Name, Division 2012-02-26

Evaluation of mineral insulating oil in new equipment

Particles - - See Table 9

Total PCB content (mg/kg) Not detectable (< 2 mg/kg total)

Interfacial tension (mN/m) Min. 35 Min. 35 Min. 35

DBDS content (mg/kg) <5

Corrosive sulphur Non-corrosive

Resistivity at 90 °C (G.m) Min. 60 Min. 60 Min. 60

Dielectric dissipation factor at 90 °C Max. 0,015 Max. 0,015 Max. 0,010

and 40 Hz to 60 Hz

Acidity (mg KOH/g) Max. 0,03 Max. 0,03 Max. 0,03

Water content (mg/kg) 20 <10 <10

Breakdown voltage (kV) >55 >60 >60

Colour (on scale given in ISO 2049) Max. 2,0 Max 2,0 Max. 2,0

Appearance Clear, free from sediment and suspended matter

<72,5 72,5 to 170 >170

Property Highest voltage for equipment (kV)

Recommended limits for mineral insulating oils after filling in new electrical equipment prior

to energization

30 Subjectmatter, Name, Division 2012-02-26

Evaluation of oil in service

Insulating oil in service is subjected to heat, oxygen, water and other

catalysts,

All of which are detrimental to the properties of the oil.

In order to maintain the quality of the oil in service, regular sampling

and analysis should be performed.

Subjectmatter, 31 Name, Division 2012-02-26

interpretation of results

should be performed by experienced personnel

Characteristic values for the type and family of oil and equipment,

developed by statistical methods

Evaluation of trends and the rate of variation of the values for a given

oil property

Normal, or typical values, for “fair” or “poor” for the appropriate type

and family of equipment

32 Subjectmatter, Name, Division 2012-02-26

Frequency of examination of oils in service

It is impossible to lay down a general rule for the frequency of

examination of oils in service which will be applicable to all possible

situations that might be encountered.

Group 1 (Routine 1 to 2 1 to 3 1 to 4 2 to 6 1 to 2 2 to 6 2 to 6 2 to 6

Tests) – years

O A B C D E F G

Property Equipment category

6 month or less, depending on the rate of

decrease and the absolute value.

Group 3 (passivator content)

Subjectmatter, 33 Name, Division 2012-02-26

Oil tests

Field tests

Only limited number of tests can be performed

Obtain a prompt estimation of oil condition

Establish the classification of service-aged oils

Eliminate any changes to the oil sample’s properties due to

transportation to a laboratory and/or storage of oil samples

Laboratory tests

More accurate

A complete examination scheme of recommended tests can be

performed

34 Subjectmatter, Name, Division 2012-02-26

Classification of the condition of oils in service

oils in service may be classified as:

“Good”

“Fair”

“Poor”

based on the evaluation of significant properties

Subjectmatter, 35 Name, Division 2012-02-26

Good

Oil in normal condition; continue normal sampling

No corrective action is require

36 Subjectmatter, Name, Division 2012-02-26

Fair

Oil deterioration detectable;

more frequent sampling recommended

Subjectmatter, 37 Name, Division 2012-02-26

Poor

Oil deterioration abnormal

schedule effective actions

38 Subjectmatter, Name, Division 2012-02-26

Corrective action

Two types of contamination / deterioration of the oil can be considered

physical

chemical

Each one requires a different remedial action

Subjectmatter, 39 Name, Division 2012-02-26

Important point:

Where a test result is outside the limits recommended value

should be compared with previous results and

fresh sample obtained for confirmation before any other action is

taken.

If rapid deterioration or acceleration in the rate of deterioration is

observed

more frequent tests should be instituted promptly and/or

appropriate remedial action should be taken.

It may be desirable to consult the manufacturer of the equipment

40 Subjectmatter, Name, Division 2012-02-26

Colour and appearance

Dark colour is a symptom

of chemical contamination

or ageing.

Turbidity is a symptom of

high water content.

As dictated by

other tests.

Dark and/or

turbid

Clear and

without

visible

contamination

All

equipment

Subjectmatter, 41 Name, Division 2012-02-26

Breakdown voltage (kV)

G <30

<30 kV for OLTC in star-point

application

<40 kV for OLTC in delta or line-end

application

F

C >40 30 to 40 <30

B, E >50 40 to 50 <40

O, A, D >60 50 to 60 <50

Good: Continue normal sampling.

Fair: More frequent sampling..

Poor: Recondition the oil

42 Subjectmatter, Name, Division 2012-02-26

Water content (mg/kg at transformer operating

temperature)

G Not a routine test.

Action

necessity

>40

F

C, E <30 30 to 40 >40

B, D <20 20 to 30 >30

O, A <15 15 to 20 >20

Good: Continue normal sampling

Fair: More frequent sampling

Poor: Check source of water

Subjectmatter, 43 Name, Division 2012-02-26

variation in saturation water content with oil

temperature and acidity

0

50

100

150

200

250

0 10 20 30 40 50 60

oil temperature during operation (ºC)

saturatio n w ater con ten t of o il (mg /kg)

saturation water content in unused oil (log Ws =

7.0895 - 1567/T)

typical saturation water content in oxidised oil with

acidity of 0.3 mg KOH/g

44 Subjectmatter, Name, Division 2012-02-26

Acidity (mgKOH/goil)

F, G Not a routine test.

0,15 to >0,30

0,30

C <0,15

0,10 to >0,20

0,20

B, E <0,10

0,10 to >0,15

0,15

O, A, D <0,10

Good: Continue normal sampling

Fair: More frequent sampling. Check the presence of

sediment and sludge. An inhibited oil that reached fair values

has possibly lost its oxidation protection.

Poor: Starting from a value of 0,15, Reclaim the oil or change

the oil

Subjectmatter, 45 Name, Division 2012-02-26

Dielectric dissipation factor at 40 Hz to 60 Hz at 90 ºC

F, G Not a routine test.

E <0,10 0,10 to 0,30 >0,30

D <0,01 0,01 to 0,03 >0,03

B, C <0,10 0,10 to 0,50 >0,50

O, A <0,10 0,10 to 0,20 >0,20

Good: Continue normal sampling.

Fair: More frequent sampling. Check other parameters.

Poor: Reclaim oil, change the oil

46 Subjectmatter, Name, Division 2012-02-26

Resistivity (GΩm)

E >3 0,4 to 3 <0,4

D >50 10 to 50 <10

B, C >3 0,2 to 3 <0,2

O, A >10 3 to 10 <3

At 90 ºC

E >60 7 to 60 <7

D >800 250 to 800 <250

B, C >60 4 to 60 <4

O, A >200 20 to 200 <20

At 20 ºC

Good: Continue normal sampling

Fair: More frequent sampling. Check other parameters.

Poor: Reclaim the oil or change the oil

Subjectmatter, 47 Name, Division 2012-02-26

Example of variation of resistivity with oil temperature

48 Subjectmatter, Name, Division 2012-02-26

Inhibitor content

<40 % of

original value

40 % to 60 %

of original

value

>60 % of

original value

Good Fair Poor

Good: No action require if not indicated by other properties

Fair: (where acidity <0,08 mgKOH/g and IFT >28 mN/m).

Consider re-inhibition to original base line level based on

local experience

Poor: Continue to use and monitor as uninhibited oil, reclaim

or change the oil

Subjectmatter, 49 Name, Division 2012-02-26

Passivator content (mg/kg)

<50 and

decreasing at

>10mg/kg/year

50-70 mg/kg or

<70 mg/kg,

with a

significant rate

of decrease of

>10mg/kg/year

>70 and stable,

(rate of

decrease

<10mg/kg/year

)

O, A, B, C,

D, E, F

Good: less frequent monitoring acceptable

Fair: maintain regular monitoring

Poor: remove the source of corrosivity by changing the oil or

by removing corrosive compounds by mean of suitable oil

treatments or passivat the oil

50 Subjectmatter, Name, Division 2012-02-26

Sediment and sludge (%)

Where sediment is detected

recondition the oil

Where precipitable sludge is

detected reclaim oil

Alternatively if more

economical, or other tests

dictate, replace the oil

No sediment or precipitable

sludge. Results below 0,02 %

by mass may be neglected.

All

Subjectmatter, 51 Name, Division 2012-02-26

Interfacial tension (mN/m)

Good: Continue normal

sampling.

Fair: More frequent sampling.

Poor: Check the presence of

sediment and sludge.

<22

<20

22 to 28

20 to 25

>28

>25

O, A,

B, C,

D

Inhibit

ed

Uninhi

bited

Refer to Manufacturer's best

practice.

F, G Not applicable.

E Not a routine test.

52 Subjectmatter, Name, Division 2012-02-26

Corrosive sulphur

For corrosive oil:

- conduct risk assessment

- reduce the corrosivity of the

oil by adding a copper

passivator (e.g. triazole

derivates)

or

- remove the source of

corrosivity by changing the oil

or by removing corrosive

compounds by mean of suitable

oil treatments.

Corrosiv

e

Non

corrosive

O, A, B,

C, D, E,

F

Good Fair Poor

Category Recommended Action Limits Recommended Action

Subjectmatter, 53 Name, Division 2012-02-26

Particles (counting and sizing)

If the breakdown voltage and

water content are near or

outside the limit for the

appropriate equipment

category and the particle

number is higher than the

limits for any of the size

ranges, filter the oil.

O, A, Informative test

B, C,

D, E,

F

Not require for category G

54 Subjectmatter, Name, Division 2012-02-26

Flash point

Equipment may require

inspection. Investigate.

Maximum decrease 10

%

All

Categories

Subjectmatter, 55 Name, Division 2012-02-26

Summary of typical actions

Perform risk assessment and

correct as necessary

- Passivation

- Oil change

- Reclaiming

Corrosive - Oil corrosiveness

Sulphur

PCB PCB contamination detectable - Refer to local regulations

Restore original additive

concentration according to

recommendations

-Reclaiming

-- or change the oil

- High colour value

- Low IFT

- High acidity value

- High dissipation factor value

- Presence of sediments or sludge, or both

- Low inhibitor content, for inhibited oil

- Low passivator content for passivator

containing oil

Chemical

- High water content - Reconditioning

- Low breakdown voltage value

- High particles content

- Turbid (not clear)

Physical

Key factors Symptoms Corrective actions

56 Subjectmatter, Name, Division 2012-02-26

Reconditioning

Reconditioning is a process that eliminates or reduces physical

contamination by means of physical processes (filtration, drying,

degassing etc.)

Reconditioning reduces the particle and water content of the oil

The process may also remove some dissolved gases and other

components such as Furanic compounds

it is recommended that the oil should not be vacuum treated at

temperatures over 85 °C.

If it is desirable to reduce particles or free water, cold treatment at

atmospheric pressure may be appropriate.

Subjectmatter, 57 Name, Division 2012-02-26

Conditions for processing inhibited and/or passivator containing

mineral insulating oils

85 280

80 200

70 80

60 30

50 15

40 8

Temperature (°C) Minimum Pressure (Pa)

58 Subjectmatter, Name, Division 2012-02-26

Vacuum – Treatment of Insulating Oil

Pressure [Pa]

Working Temperature [°C]

100000

10000

1000

100

10

1

0 40 50 65 100 150 200 250 300 350

Water Insulating Oil Working Point

Subjectmatter, 59 Name, Division 2012-02-26

Removing water from liquid and solid insulation

On-line methods

Advantages:

• Possible

for

transform

ers

that

cannot

withstand

vacuum

• Transformer

remains in

service

Disadvantages:

Off-line methods

Advantages:

Efficiency

• Speed

Disadvantages:

• Transformer has

to be taken

out of

service for the

duration

of the process

• Cost

60 Subjectmatter, Name, Division 2012-02-26

Reconditioning equipment

Filter

Filtering equipment usually forces oil under pressure through

absorbing material such as paper or other filter media.

Centrifuge

use for rough bulk cleaning where large amounts of contaminated

oil are to be handled.

Vacuum dehydrator

It is an efficient means of reducing the gas and water content of a

mineral insulating oil to very low values

Subjectmatter, 61 Name, Division 2012-02-26

Oil reconditioning application to electrical

equipment

Direct reconditioning

The oil is passed through a purifier and then stored in suitable clean

containers

Reconditioning by circulation

The oil is circulated through the purifier, being taken from the

bottom of the tank of the electrical equipment and re-delivered to

the top

It is useful for removing suspended contaminants

62 Subjectmatter, Name, Division 2012-02-26

Oil Reclamation

This is a process that eliminates or reduces soluble and insoluble polar

contaminants from the oil by chemical and physical processing.

The resulting product should be evaluated on critical parameters

Reclamation of oils of moderate to high acidity will usually result in oils

with a lower oxidation resistance than the original new oil

IEC developed new standards for reclaimed oil IEC62701

Subjectmatter, 63 Name, Division 2012-02-26

Replacement of oil in electrical equipment

Lower than 72.5 KV

A small extra quantity of oil is needed to rinse the interior of the tank

and the immersed parts

It is essential that the tank and the surfaces of conductors and

insulators be cleaned effectively and subsequently be kept free from

fibres

Possibly up to 10% of the original oil might remain adsorbed in the

solid insulation and its contaminants may take some time to migrate

into the new oil

64 Subjectmatter, Name, Division 2012-02-26

Replacement of oil in transformers rated 72,5 kV

and above

Use manufacturers recommendation

Removed used oil from main tank as much as possible

Rinse the winding with clean hot unused oil (hot flashing)

Apply vacuum with care (manufacturers recommendation)

Subjectmatter, 65 Name, Division 2012-02-26

Passivation

Metal passivator is added as a stock solution, dissolved in insulating

oil

The stock solution can be added via an oil processing unit or other

suitable equipment.

This treatment is recommended for unused and in-service insulating oil

66 Subjectmatter, Name, Division 2012-02-26

Renewal of additives

Oil reclamation remove oxidation inhibitors

Additives shall be replaced in the reclaimed oil after the reclaiming

process and before the equipment is re-energized

The most widely used additives are 2,6-di-tert-butyl-paracresol (DBPC)

and 2,6-di-tert-butyl-phenol (DBP).

Metal passivators will also be reduced or removed due to their polar

nature.

Subjectmatter, 67 Name, Division 2012-02-26

Decontamination of oils containing PCBs

Contaminated oil may change and replaced by unused oil

There are several processes and techniques available for either onsite

and off-site decontamination of PCB contaminated oils

All PCB decontamination methods, either off-site or on-site, have

to be applied by skilled companies complying fully with local

regulations.

68 Subjectmatter, Name, Division 2012-02-26

Dehalogenation process

Several process are available

Dehalogenation processes using sodium and lithium derivatives

Dehalogenation processes using polyethylene glycol and potassium

hydroxide (KPEG)

Dehalogenation in continuous mode by closed circuit process

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