transformer fluid options and condition monitoring -...
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Copyright 2016 M&I Materials Ltd. Products of M&I Materials Ltd.
Transformer fluid options and condition monitoring
STLE Toronto Section September 2016
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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
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Transformer fluid classes & Options
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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
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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
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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
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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
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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.
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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.
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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
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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
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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
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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
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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
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Applications of Ester FluidsElectrical Utility
• Power
• Distribution
• Retrofilling / retrofitting
Wind Energy
• Offshore / Onshore
• Offshore Substation
Oil & Gas
Rail
• Traction / Trackside
• Underground
Marine
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Silicone Liquid
• Manufactured from chemicals
• Repeating part of structure
shown in box
• Viscosity depends on the chain
length
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Properties of Silicone Fluids
• Increased fire safety
• Poor environmental protection
- not biodegradable
• No corrosive sulphur
• High oxidation stability
• Suitable for breathing and
sealed transformers
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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
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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
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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
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Ignition Resistance
Mineral Oil 3 mins
Synthetic ester 3 mins
Mineral Oil 4 mins
Synthetic ester 70 mins
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Environmental Protection
• Legislation
• Demonstrates social responsibility
• Reduced or simplified containment
• Reduced clear up costs for spills
• Potentially lower insurance premiums
• Preserving the planet
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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)
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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
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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
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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
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ASTM D2112 Oxidation Stability Testing
• Pressurised Vessel Oxidation
Test
• Oil + Copper Catalyst +
Oxygen @ 90psi
• Temperature = 140˚C
• Time for Pressure = 65psi
• DOBLE Engineering USA
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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)
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Retrofilling with Alternative Fluids
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Retrofilling with Alternative Fluids • Retrofilling is the replacement of the fluid in the transformer with an
alternative fluid. Can be done on site.
• Increased fire safety
- 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
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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
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Interaction of Transformer Fluids with Solid Insulation
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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 -
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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
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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)
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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
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Dissolved Gas Analysis with Alternative Fluids
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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)
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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)
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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
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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
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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
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Duval Triangles
• Mineral oil and synthetic ester fault boundaries have been modified
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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
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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
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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
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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