Criteria for oil selection suitable for

use as insulating liquid in UHV oil

filled equipment.

Dr B Pahlavanpour, Nynas AB, South Africa

5/19/2014

What are the requirements on a Transformer Oil?

Provide cooling

Act as electrical insulator

Meet expected in-service behaviour

Last the lifetime of the transformer

Fulfill standards and specifications

3 5/19/2014

Provide cooling

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Low viscosity better oil impregnation

Oil with low viscosity and low viscosity index impregnate insulating

paper faster than oil with high viscosity index.

Naphthenic oil impregnate insulating paper faster than paraffinic.

Saves time for manufacturer

Avoid presence of air bubble between paper layers

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Calculation of the natural oil

circulation speed on basis of

the Laws of Bernoulli, Newton,

Reynolds and Prandtl

w = Flow Speed

f = Calculation Factor

∆T = Temperature difference v = Kinematic Viscosity at

Operation Temperature

Virtual Oil Wheel with natural circulation

v

Tfw

Oil viscosity is the major parameter other than design that can influence the cooling!

T2

T1

w P

5/19/2014

0

250

500

750

0 20 40 60

a

Viscosity [n]

a = f n [ W / m2K ]

70 15 9 3

414

497

792

202

HEAT TRANSFER COEFFICIENT

5/19/2014

Viscosity index

Low viscosity index oils are

thinner at high temperatures

Operating temperature

normally between 40-80°C

Low viscosity index oils thus

give better cooling at higher

temperatures

Naphthenic oils have lower VI

compared to paraffinic oils

= Paraffinic oil

= Naphthenic oil

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Changes of viscosity with temperature

Temperature Oil A Viscosity Index

25

Oil B Viscosity Index

110

40 C 7.60 7.60

70 C 3.40 4.10

100 C 2.00 2.50

5/19/2014

Transformer Operating Temperature for two oils with the same viscosity at 40 C

70

40

T [°C]

Dissipation Heat [W/h]

5400 5600

Oil Viscosity

[mm2/s]

2.8

2

T=2.7°C

Operating temperature of

transformer cooling

system is dependent on

transformer oil viscosity

Index:

1 mm²/s higher operating

viscosity is leading to

approximately 3°C higher

operating temperature

5/19/2014

Hanging Paper Wetting

Level of oil wetting checked over

72 h

Two oils compared

Paraffinic

Visc. 40 ˚C 10.6 cSt

Visc. 20 ˚C 22.5 cSt

Naphthenic

Visc. 40 ˚C 9.1

Visc. 20 ˚C 19.5

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Results over time

Naphthenic

Paraffinic

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Wax crystallisation at low temperatures prevents oil circulation and cooling

which can cause damage to the transformer

Low Temperature Properties

Paraffinic oil Naphthenic oils

– 40

0

20

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Oil with high pour point

Have high viscosity index

Bad performance at low temperature

Poor cooling at high temperature

Once wax is oxidised, produces sticky sludge which damage insulation

paper

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Act as electrical insulator

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AC Breakdown Voltage

Breakdown voltage is a measurement of the oil

ability to withstand electrical stress

A high value indicates that the oil is a

good insulator

Water and particle content influence

test results

Indicates the cleanliness of the oil not the quality

Poorly refined oil can have a high breakdown

voltage

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Impulse Breakdown

The test is designed to simulate a lightning pulse striking a

transformer

It measures the break down behavior with DC impulses

This property is affected by the degree of oil refining

Could be seen as an inherent quality property

It is not affected by contaminants in oil

This property is usually not included in the specifications;

exception ASTM D3487

Test methods: eg ASTM D3300

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Impulse Breakdown vs. Polyaromatics

Low degree

of refining

1

0.1

0.01

120 140 160 180 200 220 240 260 280 300

Polyaromatics HPLC wt % Polyaromatics

wt%

Aromatic

content wt%

Impulse

breakdown kV

0.1 5 >300

0.2 7 282

0.7 10 220

0.30 10 196

0.75 10 148

Impulse breakdown kV

negative polarity High degree

of refining

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Dielectric dissipation factor (DDF)

and power factor

It is a measure of the dielectric losses in oil, in

other words the amount of energy dissipated as

heat

This property is dependant on

Temperature

Frequency of the AC-field

Impurities (such as oxidation products)

Particles

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How to ensure high electrical insulation

Production technology

Dedicated handling

system

20 5/19/2014

Meet expected in-service

behaviour

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Important factors for In Service Behavior

oil

Facilitate

transformer

diagnostics

Low ageing rate

Interact positively

with paper

Leave copper

unaffected

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Oxidation behaviour –

Inhibited vs Uninhibited oils

Oxidation behavior of different types of oil

5/19/2014

Transformer Oil Ageing vs Temperature

70°C 60°C

<50 °C

Time

Neutralization Number

(mgKOH/g)

The rate of oxidation increases with temperature – hence the importance of cooling

as this will extend the life of the transformer oil, and the transformer itself

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Oxidation Stability - test methods

Uninhibited

Standard Grade Oils

Inhibited

Super and High Grade Oils

ASTM D2440 Type II oils

IEC 61125C fully inhibited oils

IEC 61125C trace inhibited oils

IEC 61125C uninhibited oils

164 hours 164 hours

500 hours

332 hours

25 5/19/2014

Last the lifetime of the

transformer

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What is the lifetime of a transformer

dependent on?

Design and materials

Manufacturing and installation

Operating conditions

Ambient temperature

Follow up and maintenance

5/19/2014

Interaction of Paper and Oil

Transformer oil is a

replaceable product

Reclamation or replacement

But the Kraft paper in a

transformer is not replaceable

Life of

Transformer Life of the

Paper =

28 5/19/2014

Fulfill standards and

specifications

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Specifications for unused oils

IEC 60296 and ASTMD3487

are the most widely used

International standards for

insulating oil today

5/19/2014

Specifications for unused oils

International standards

IEC 60296/03

ASTM D3487

National specifications

CAN/CSA-C50-08 (Canada)

AS 1767.1-1999 (Australia)

JIS C 2320-1993 (Japan)

Equipment manufacturers ABB, Siemens, IranTransfo, Areva

Utilities and other bodies PGCIL, NGC, ESKOM, TEIAS,

Doble

5/19/2014

Super grade insulating oil

It is over and above all requirements of IEC60296 and ASTM D3487

Low viscosity, good cooling property

Impregnates insulating paper faster

Low viscosity index, better cooling for highly loaded transformers

High resistance to oxidation, transformer can run at high temperature.

Low pour point, better oil flow

High solvency power, avoids precipitation of oxidation products.

Long service life, protecting insulating paper

Long service history (India, China, Russia)

2014-05-19

Power transformer 100 MVA, Value $1,500,000 - 35,000 lit oil

Unit A (cheap oil) Oil Cost $63,000 (4.2%)

Unit B (good oil) Oil cost $71,000 (4.7%)

After 10 years Oil change $71,000 Handling $35,000 Loss of transmission $70,000

40 years service

Sum: $239,000 Sum: $71,000

Mean probability of damage of insulation through 40 years 0.5% /Year $300,000 0.1% /Year $60,000

Total: $539,000 (36%) $131,000 (6%)

Transformer oil life cycle assessment

5/19/2014

Summary of specified Transformer oil

requirements

Cooling Electrical insulation Life time

Viscosity Breakdown voltage Oxidation stability

Pour point Dielectric dissipation factor (DDF) Inhibitor content

Viscosity index Impulse breakdown Solubility

Resistivity

Water content

Material compatibility Health, safety and environment Others

Sulphur content Flash point Density

Acidity IP 346

Interfacial tension,

IFT

Corrosive Sulphur Furanic compounds

Aromatic content Gassing tendency

34 5/19/2014

Taking oil further