transformer fluid options and condition monitoring -...

Copyright 2016 M&I Materials Ltd. Products of M&I Materials Ltd.

Transformer fluid options and condition monitoring

STLE Toronto Section September 2016

Copyright 2016 M&I Materials Ltd. www.midel.com

Topics Covered

• Transformer Fluid classes & Options

• Comparison of Fluid Properties

• Fire Safety

• Environmental Protection

• Fluid Moisture Tolerance

• Oxidation Stability

• Retrofilling With Alternative Fluids

• Interaction of Transformer Fluids with Solid Insulation

• Dissolved Gas Analysis

• Fluid Standards


Transformer fluid classes & Options


Fluid Filled Transformer

• Electrical insulation in a transformer consists of fluid and solid materials

• As a state of art one can have different options of fluid and solid materials in a transformer to make the transformer fit for purpose

• Picture shows a general construction of a transformer


Fluid –Function as a coolant and electrical insulation

Solid – Function as an electrical insulation with or without a liquid

• As a current state of the art liquid & solid insulation combination provides the best technically adept and and economically viable solution in transformers for most applications

• Common fluids in transformers – Mineral oils, Esters, Silicone liquid

• Common solid insulation materials – Cellulosic Paper, Aramid, other synthetic materials

Fundamental purpose of the insulating materials


Fluid Classes IEC 61039

• IEC 61039 has replaced previous standard IEC 61100

• Classifies fluids by fire point and net calorific value

• Mineral oil is class O

• Synthetic esters class K

• Natural esters class K

• Silicone oil class K

Fire Point Net Calorific Value

O ≤ 300°C 1 ≥ 42 MJ/kg

K > 300°C 2 < 42 MJ/kg

LNo Measurable

Fire Point3 < 32 MJ/kg


Mineral Oil Chemical Structures


Mineral Oil from Crude Oil

Mineral oil is a distillation cut from crude oil which is further hydrotreated to reduce aromatics and to remove impurities such as sulphur.

Typical

Transformer Oil

280 - 370°°°°C


Paraffinic and Naphthenic Mineral Oils

Paraffinic Oil • Contains substantial quantities of naturally occurring n-paraffins

• Has a relatively high pour point and may require the inclusion of

additives to reduce the pour point

• Properties include higher resistance to oxidation, more viscous, and

less volatile

• Paraffinic oils oxidise slower than naphthenic oils

• The oxidation product (or sludge) is insoluble & will precipitate at the

bottom of the tank and may obstruct the transformer cooling system.


Paraffinic and Naphthenic Mineral Oils

Naphthenic Oil • Contains lower level of naturally occurring n-paraffins

• Has a low pour point and requires no additives to reduce the pour point

• Provides better viscosity characteristics

• Has more polar characteristics than paraffinic oil

• Naphthenic oil is more easily oxidised than paraffinic oil

• The oxidation product (sludge) in the naphthenic oil is more soluble than in paraffinic oil, hence is less likely to be precipitated in bottom of the transformer, and thus is less likely to obstruct convection circulation of the oil.


Additives

• Added to delay the oxidation of oil.

• Uninhibited oils: Must be free of additives to improve oxidation stability.

• In the USA, IEEE accept that < 0.08% inhibitor can be classified as

uninhibited

• Inhibited oils: are insulating oils which have antioxidants


Synthetic Ester

• The building blocks for

synthetic esters are alcohols

and acids

• Can have 2 to 4 ester groups,

depending on raw materials

• Ester linkage in boxes

• All R chains in synthetic

esters are saturated for

oxidation stability


Key Properties of Synthetic Ester

• High fire point

• Low pour point

• Biodegradability

• High oxidation stability

• Superior moisture tolerance

• Paper lifetime enhancement

• Suitable for breathing and sealed transformers

• Suitable for cold climates


Natural Ester

• Made from vegetable sources

• Triglyceride with three ester groups

• R chains can be saturated or

unsaturated

• Viscosity depends on saturation of

carbon chains

• Oxygen stability depends on saturation

of carbon chains


Properties of Natural Ester Fluids

• Increased fire safety

• Greater environmental protection

• Superior moisture tolerance

• No corrosive sulphur

• Lower oxidation stability

• More suited to temperate climates & indoor locations

• Suitable for sealed systems (+ bagged conservators)

• High pour point

• Cost effective solution for managing risk


Applications of Ester FluidsElectrical Utility

• Power

• Distribution

• Retrofilling / retrofitting

Wind Energy

• Offshore / Onshore

• Offshore Substation

Oil & Gas

Rail

• Traction / Trackside

• Underground

Marine


Silicone Liquid

• Manufactured from chemicals

• Repeating part of structure

shown in box

• Viscosity depends on the chain

length


Properties of Silicone Fluids


• Poor environmental protection

- not biodegradable

• No corrosive sulphur

• High oxidation stability

• Suitable for breathing and

sealed transformers


Overview of Transformer Fluid Applications

• Key: A = Largely used, B= Less commonly used, X = Currently not used

Mineral Oil Silicone Liquid Synthetic Ester Natural Ester

Power

TransformersA X B B

Traction

TransformersA A A X

Distribution

TransformersA A A A

Instrument

TransformersA B B B


Comparison of Fluid Properties


Transformer Fluid Properties

Property Mineral Oil Silicone Liquid Synthetic Ester Natural Ester

Flash Point ˚C 160 >300 >250 >300

Fire Point ˚C 170 >350 >300 >350

Fire Classification O K K K

Oxidation Stability Medium High High Low

Moisture Tolerance Low Low V.High High

Biodegradability Low Low Readily Biodegradable

Readily Biodegradable

Corrosive Sulphur Possible None None None

Pour Point ˚C - 50 <- 50 - 50 to - 60 -15 to -31

Breakdown Strength IEC

60156 2.5mm (kV)

>70 >50 >75 >75


Fire Safety


Fire Safety Advantages of Using Alternative Fluids• High degree of fire safety

• Fluids extremely difficult to ignite

• Very low risk of pool fires (none reported)

• Very low risk of smoke hazard (esters)

• Lower installation costs

• Potentially lower fire protection costs

• Possible insurance benefits


Ignition Resistance

Mineral Oil 3 mins

Synthetic ester 3 mins

Mineral Oil 4 mins

Synthetic ester 70 mins


Environmental Protection


Environmental Protection

• Legislation

• Demonstrates social responsibility

• Reduced or simplified containment

• Reduced clear up costs for spills

• Potentially lower insurance premiums

• Preserving the planet


Environmental Impact of Fluids

• Impact can be assessed by laboratory testing

• OECD classifies Readily Biodegradability (at least 60% degradation in

28 days and this must happen within 10 days after a sample exceeds

10% biodegradation.

• Very stringent test standard

• It is assumed that compounds will rapidly biodegrade in aquatic environments under aerobic conditions (in the presence of micro organisms)


Comparison of 28-Day Biodegradation


Advantages of Readily Biodegradable Fluids

• Safer for the environment

• Does not persist (decomposed by living organisms)

• Possibility of reduced or simplified containment

• FM Global® - larger volume before containment required for biodegradable fluids

• Use demonstrates commitment to social responsibility


Fluid Moisture Tolerance


Fluid Moisture Tolerance

• Moisture can ingress through headspace in breathing transformers

• Through leaks & during maintenance

• Oxidation aging of paper releases water

• Wet transformers

• Have a shorter life –rate of paper aging increases with increasing moisture content

• Prone to dielectric faults

• Require maintenance


Moisture Content Limits (IEC)


Oxidation stability


Oxidation stability

• Oxidation

• Fluid degraded by reaction with Oxygen from the air

• Leads to increase in acid value

• Can increase fluid viscosity

• Produces sludge in mineral oil

• Can lead to polymerisation or gelling in natural esters

• Short chain acids (2-5C) due to oxidation is soluble in water and affects transformer life


ASTM D2112 Oxidation Stability Testing

• Pressurised Vessel Oxidation

Test

• Oil + Copper Catalyst +

Oxygen @ 90psi

• Temperature = 140˚C

• Time for Pressure = 65psi

• DOBLE Engineering USA


ASTM D2112 Oxidation Stability – Test Results

Typical

Natural esters

< 40

Synthetic

esters

421

High Oleic

Natural esters

197

Silicone fluids

>450

Mineral oils

300

0

50

100

150

200

250

300

350

400

450

500

Fluid Types

RB

OT

Te

st

(Min

s)


Retrofilling with Alternative Fluids


Retrofilling with Alternative Fluids • Retrofilling is the replacement of the fluid in the transformer with an

alternative fluid. Can be done on site.


- Significantly improves fluid fire point

• Greater environmental protection

- Replaces a poor environmental fluid with a much better one

• Asset Life

- Retrofilling also enhances moisture tolerance & extends paper life

- Reduces maintenance

- In particular synthetic ester is very oxidation stable and is suitable for breathing transformers


Pre-Retrofill Checks

• Used to establish if the transformer is suitable for retrofill

• Visual check of the transformer

• Check of previous oil testing results

• Furan analysis can give guide to paper condition


Interaction of Transformer Fluids with Solid Insulation


Miscibility of Transformer Fluids

• Miscibility gives an indication of the compatibility between fluids

• Non miscible fluids will separate into two distinct layers

• It is important to note that although mineral oil and silicone liquid are miscible a mixture may cause foaming under vacuum

Synthetic Ester Mineral Oil Natural Ester Silicone Liquid

Synthetic Ester -Fully

MiscibleFully Miscible Not Miscible

Mineral Oil Fully Miscible - Fully Miscible Miscible

Natural Ester Fully MiscibleFully

Miscible- Not Miscible

Silicone Liquid Not Miscible Miscible Not Miscible -


Solid Insulations

• Cellulose• Most common solid insulation

• Wide range of applications

• Used at low to moderate temperatures

• Aramid paper • More robust than cellulose

• Used with high temperature fluids (K class)

• Used in demanding applications e.g. Traction


Cellulose Solid Insulation

O

O H

OHOH

O H

O

O H

OHO

O H

O

O HO H

OHO

O H

n-2

Chemical Structure of Cellulose

O

O H

OHOH

O H

O

O H

OHO

O H

O

O HO H

OHO

O H

n-2

Chemical Structure of Cellulose

• Cellulose pressboard and paper is the most commonly used solid insulation in fluid filled transformers

• Cellulose is a natural polymer made up of glucose rings linked together in chains

• After drying new cellulose has 1000 to 1200 glucose rings in each chain (DP value)


Cellulose Degradation in Transformers

• Paper strength is often indicated by Degree of Polymerisation or DP value

• Water attacks the links connecting glucose units in cellulose reducing the polymer chain length

• As paper ages the DP value reduces, through depolymerisation

• Depolymerisation of paper weakens the winding mechanically

• Once the winding is weakened short circuit stress can cause mechanical failure

• Reducing the rate of depolymerisation extends the life of the transformer

• Increasing moisture & temperature accelerates ageing of paper

• Short circuit faults will cause failure


Dissolved Gas Analysis with Alternative Fluids


Fault Gas Formation in Mineral Oil

Bond Bond EnergykJ/mol

C - H 413

C - C 348

C - O 358

C = O 799

C = C 614

Si - O 368

Si - C 301

Hydrogen

• H·+ H· = H - H

Methane

• CH3·+ H· = CH4

Ethane

• CH3·+ CH3· = C2H6

Also

C2H4 (Ethylene)C2H2 (Acetylene)


Synthetic Ester Fault Gas Formation• Hydrogen

•H·+ H· = H – H

• Methane

•CH3·+ H· = CH4

• Ethane

• CH3·+ CH3· = C2H6

•

•C2H4 (Ethylene)

• C2H2 (Acetylene)

Also

• CO (Carbon monoxide)

• CO2 (Carbon dioxide)


Key Indicator Gasses

• High Energy (Temp >700˚C)

• Acetylene C2H2

• Medium Energy (Temp 300 – 700˚C)

• Ethylene C2H4, Ethane C2H6

• Lower Energy (Temp <300˚C)

• Hydrogen H2, Methane CH4

• Cellulose Degradation

• Carbon Monoxide CO, Carbon Dioxide CO2

• Esters also give off CO and CO2 without paper


Fault Diagnosis Methods

• Ratio methods

• Rogers Ratio

• Doernenburg Ratio

• Optimised for mineral oil – seem to be falling out of fashion

• Duval Triangle – commonly used

• Uses C2H2 + C2H4 + CH4 in triangular plot

• Michel Duval has published triangles for

• Mineral oil, silicone fluid, synthetic esters and natural esters


How to use the Duval Triangle • PD = Partial Discharge

• T1 = Low thermal fault (<300°C)

• T2 = Medium thermal fault (300-700°C)

• T3 = High thermal fault (>700°C)

• D1 = Low energy electrical discharge

• D2 = High energy electrical discharge

• DT = Intermediate thermal fault or electrical discharge

•TRENDING IS IMPORTANT

EXAMPLE

Gas ppm Ratio %

C2H4 81 81/150 54

CH4 27 27/150 18

C2H2 42 42/150 28

Total 150Acetylene

EthyleneMethane


Duval Triangles

• Mineral oil and synthetic ester fault boundaries have been modified


The Duval Triangle for Silicone and SE


Importance of Good Sampling

• Ideal sampling method is syringe with tap

• Gasses can be lost in transit

• Especially CO and H2

• Gasses can be introduced from atmosphere

• O2, N2, CO2

• Galvanic reactions can cause high H2

in samples

• Galvanised fittings must be avoided on sample port


CO2 / CO Ratio

• With MINERAL OIL

• Can be used to indicate if cellulose is involved in fault.

• CO2/CO ratio 3-11 is normal, average around 7

• <3 indicates a local high temperature fault

• A large quantity of CO produced in degradation of paper

• >11 indicates general overheating

• With ESTERS

• More care is needed as esters can produce CO and CO2 without

paper

• Cigre WG 47 (M.Duval) looking at this more closely


Conclusions

• DGA can be performed using non mineral oils (Non-MO)

• The range of gases from Non-MO are the same as for MO

• DGA key gasses are similar for esters and mineral oil

• Duval Triangle – fault boundaries changed for each fluid type Synthetic esters, Natural esters.

• CO/CO2 ratios need extra care


Fluid Standards


Fluid Standards

• Mineral Oil• IEC 60296 New Mineral Oil• IEC 60422 In Use Mineral Oil

• Synthetic Ester• IEC 61099 New Ester Fluids• IEC 61203 In Use Ester Fluids

• Natural Ester• IEC 62770 New Ester Fluids• American Standard ASTM D6871-03

• Silicone Liquid• IEC 60836 New Silicone Liquid• IEC 60944 In Use Silicone Liquid


Q & A and DiscussionsManjunath Ramakrishna

M&I Materials

Hibernia Way, Trafford Park

Manchester M32 0ZD

United Kingdom

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