• Dr Joerg Friedel

• Product Application Specialist

• Shell Technology Centre

• Hamburg, Germany

An inhibited transformer oil based on

Shell gas-to-liquids (GTL) technology

The evolution of hydrocarbon

transformer oils

2013 Transformer oils based on GTL

technology

1970 Hydrorefining

1940–1950 Use of synthetic inhibitors

c. 1920 Sulphuric acid refining

1960 Use of polychlorinated biphenyls for

transformer oils

1980 Use of ester fluids

Shell GTL transformer oil

• An inhibited transformer oil based on Shell GTL technology

• Exceeds the requirements of IEC 60296 §7.1 (high grade)

The Shell GTL process

Methane + Oxygen Carbon monoxide Fischer–Tropsch distillates Water Hydrogen

Catalyst

Condensate

LPG

Ethane

120,000 bbl/d

GTL naphtha

GTL kerosene

GTL NP

GTL base oils

GTL gas oil

140,000 bbl/d

Conversion of natural gas to clean,

high-quality liquid products using

proven technology

Gas

processing

Synthesis gas

manufacturing

Fischer–

Tropsch

synthesis

Products

CH4

O2

Syngas

CO + 2H2 CH2

Raw

natural

gas

Current uses of GTL products

Diesel fuel

Low emissions, biodegradable

Kerosene/naphtha

Aviation, heating

Passenger car engine oils

Shell PurePlus technology

Turbine oils

Thermally stable, excellent

surface properties

Transformer oils

GTL products:

Light GTL

fractions

GTL products:

GTL base oils

Typical data showing the properties that

differentiate Shell transformer oil

Standard IEC 60296 Table 2

+ Section 7.1

Density at 20°C, kg/m³ ISO 3675 max. 895 805

Kinetic viscosity at 40°C, mm²/s ISO 3104 max. 12 9,6

Kinetic viscosity at –30°C, mm²/s ISO 3104 max. 1,800 382

Flashpoint PM, °C ISO 2719 min. 135 191

Pour point, °C ISO 3016 max. –40 –42

Total sulphur content, mg/kg ASTM D 5185 max. 500 <1

DBPC content, % 0.4 0.2

Volatility 107°C, 22 h, wt% ASTM D 2007 0.75

Dielectric dissipation factor (DDF) at 90°C IEC 60247 max. 0.005 <0.001

Oxidation stability (500 h/120°C) IEC 61125C

Total acidity, mg KOH/g max. 0.3 0.02

Sludge, wt% max. 0.05 <0.01

DDF at 90°C max. 0.05 0.001

Transformer oil requirements

Ageing resistance must be maintained over the oil’s lifetime. Therefore, oxidation stability is key

(GTL based oil has five times higher oxidation stability)*

Dissipate heat

High heat conductivity

(9% higher)*

Low viscosity in the cold

(200 mm²/s lower at –30°C)*

Electrical insulation

High impulse breakdown voltage

(approximately 50 kV higher)*

Low conductivity

Protection

Prevention of copper corrosion

(<1 ppm sulphur)

Maintaining low acidity to protect

paper insulation

Diagnostic information

Detect impurities via measurement

of interfacial tension

Detect heat or electrical issues

using dissolved gas analysis (DGA)

*Shell GTL transformer oil compared with inhibited naphthenic oil

Base fluids – Changes and challenges

Inhibited insulating oils

High-performance

base fluid, consistent

in supply, composition

and performance

Very low sulphur

content

Higher stress on insulating oil and paper

Demanding design of

transformers

Compactness, load,

voltage, efficiency

Increased severity in

operation and higher

reliability of

transformers

Ambient and high oil

temperatures,

service life,

protection

Resistance to degradation – Inhibited

GTL versus conventional uninhibited oil

• IEC 61125C – The induction period is reached when the volatile

acidity significantly exceeds 0.1 mg KOH/g

• Inhibited oils show predictable and best resistance to degradation

• Monitoring antioxidant concentration gives an indication of the oil

condition before significant quantities of acids develop and

potentially attack the insulating paper

Inhibited GTL

Uninhibited conventional oil

Shell GTL transformer oil’s cooling

properties

• Customer transformer trials* showed comparable or better cooling

properties for GTL-based oils compared with conventional oils

• GTL-based oils have a higher heat transfer coefficient than

naphthenic-based oils for laminar and turbulent flow, although the

differences are small

• Modelling showed the advantages of GTL-based oil, especially

under high-stress conditions

• The parameters influencing cooling properties are

– Specific heat capacity

– Thermal conductivity

– Viscosity

– Density

*Customer trials run by equipment manufacturers and utilities

Superior oxidation stability

GTL inhibited

Naphthenic, inhibited

Naphthenic, uninhibited

Oxidation stability test IEC 61125C

Induction period IEC 60296 § 7.1 (0.1 mg KOH/g volatile acid)

Typical RPVOT results (min.) ASTM D 2112:

Naphthenic inhibited transformer oil 405 min

GTL inhibited transformer oil 750 min

500 1,000 0

Resistance to degradation

• GTL inhibited oil versus conventional inhibited oil

• UIEC 61125 C extended oxidation stability test

– Test run for standard 500 h. When the inhibitor content fell to

approx 50% of initial value, the antioxidant was topped up to

initial level (refer to IEC 60422); later regular inhibitor topping up.

Test run for approx 2,180 h (more than four times the usual

duration)

• Extended resistance to degradation in normal service and when re-

inhibited.

Resistance to degradation

GTL inhibited oil

Acidity 0.18 mg KOH/g

Sludge <0.01 wt%

Oil loss 0 wt%

Naphthenic inhibited oil

Acidity 0.96 mg KOH/g

Sludge <0.01 wt%

Oil loss 24 wt%

Results from Shell data

• GTL inhibited oil versus conventional inhibited oil

Electrical properties – Lightning

impulse breakdown

• Lightning impulse breakdown voltage testing

• Needle-plane and needle-sphere electrode configurations (gap

typically 25 mm, using positive and negative impulses 4 kJ; 50 µs)

• Testing at the University of Manchester, UK

• GTL inhibited oil and naphthenic inhibited oil (water content <10 ppm)

Electrical properties – Lightning

impulse breakdown

Needle – plane

12.5 L oil

Tungsten needle tip radius 50 ±5 μm

Brass plane electrode 200 mm diameter

Needle – sphere (IEC 60897 method A) 300 ml oil

Steel needle tip radius 7–2 μm ellipse

Brass sphere electrode 12.5 mm diameter

Shell GTL transformer oil offers high-

voltage impulse stability

• GTL transformer oil offers higher resistance than naphthenic oils

against high-voltage impulses caused by lightning or switching

because of its low aromatic content

0

50

100

150

200

250

Needle -sphere +ve

Needle -sphere -ve

Needle -plane +ve

Needle -plane -ve

Naphthenic GTL10 mm gap

Source: University of Manchester

Average breakdown voltage in kilovolts (gap 25 mm unless specified; 4 kJ; 50 µs)

Foaming and air release improved

Classic naphthenic oil Classic naphthenic oil New GTL-based oil New GTL-based oil

10 seconds shaking to incorporate air

Comparison flash point COC and

evaporation loss

• Significantly higher flash point and reduced volatility provides

additional safety

0 50 100 150 200

Diala S2 ZX-U

Paraffinic Oil

Diala S4 ZX-I

°C

Evaporation loss ASTM D 972

22 h at 107°C

ASTM D 5800

1 h at 250°C

Naphthenic oil 26% 100%

GTL 0.75% 40%

GTL

Paraffinic

Naphthenic

Water saturation versus temperature

• No significant difference in water solubility compared with

conventional transformer oils

(Measurement Schering Institute, Vaisala Humicap MM 70)

Comparison with mineral-based

transformer oils

GTL-based transformer oil

Dissolved gas analysis

(DGA)

DGA interpretations can use same tools as for

traditional hydrocarbon oils (e.g., with a Duval

diagram)

Failure detection In the case of a transformer failure (e.g., through

partial discharge), hydrogen will be generated

and a Buchholz relay can release an alarm

Material compatibility Compatibility given for materials that are

compatible with mineral oils – same substance

class as mineral oils (hydrocarbons)

Water solubility Comparable with naphthenic transformer oils

Compatibility with

naphthenic oils

No issues observed in many tests with used and

unused oils

DGA evaluation – Duval diagrams

Source: Schering Institute

PD: Partial discharge

D1: Discharge low energy

D2: Discharge high energy

T1: thermal failure <300°C

T2 Thermal failure 300–700°C

Partial discharge Partial discharge

low energy

Hot spot

300–700°C

Hot spot >700°C

Absolute gas concentration is lower for the GTL oil compared with the naphthenic oil

Shell GTL oil

Shell naphthenic oil

Ease of use: Compatible and miscible

• No miscibility, compatibility or solvency issues found

• GTL-based transformer oils can be used alongside traditional oils

• Top-up performance even better than for naphthenic oil

Oil + Cu Oil only Oil + Cu + air

Topped up with GTL oil

Topped up with naphthenic oil

Oven test, 35 days at 100°C

Comparison with inhibited naphthenic

oil B

Shell GTL oil, % 5 50 85 100

Naphthenic oil B, % 100 95 50 15

Density, kg/m³ ISO 3675 869.2 866 837.4 816.4 807

Flash point PM, °C DIN EN 22719 138 143 151 167 188

Kinetic viscosity at 40°C,

mm²/s ISO 3104 8.95 8.958 9.183 9.483 9.560

Breakdown voltage, kV IEC 60156 72 75 75 78 80

DDF 90°C IEC 60247 0.005 0.0004 0.0004 0.0004 0.0002

Oxidation stability IEC 61125C

500 h

Acidity, mg KOH/g 0.14 0.03 0.02 0.02 0.02

Sludge, wt% <0.01 <0.01 0.02 0.01 <0.01

DDF 90°C 0.023 0.020 0.005 0.001 0.001

Oxidation stability IEC 61125C (500 h, 120°C)

Comparison with uninhibited

naphthenic oil A

Shell GTL oil, % 5 50 85 100

Naphthenic oil A, % 100* 95 50 15

Density, kg/m³ ISO 3675 873 868.2 838.5 816.7 807

Flash point PM, °C DIN EN 22719 135 143 159 171 188

Kinetic viscosity at 40°C,

mm²/s ISO 3104 10 9.9 9.72 9.64 9.56

Breakdown voltage, kV IEC 60156 72 77 82 80 80

DDF 90°C IEC 60247 0.0005 0.0004 0.0007 0.0007 0.0002

Oxidation stability IEC 61125C

500 h

Acidity, mg KOH/g 1.12 0.52 0.02 0.02

Sludge, wt% 0.35 0.04 <0.01 <0.01

DDF 90°C 0.052 0.016 0.009 0.001

Oxidation stability IEC 61125C (500 h, 120°C)

Shell GTL transformer oil

• Performance in the field

Positive experiences with Shell GTL

transformer oil

For applications in

Distribution

transformers

Power transformers

Reactors

Instrument transformers

Traction transformers

Good performance – no issues

observed; functions as expected

for a high-grade oil

No design or maintenance

changes required (possible

optimisation and maintenance

reduction being explored!)

Proven to be able to detect

failures in transformers by

initiating Buchholz relay alarm

Since 2013, Shell GTL transformer oil has been used to fill

thousands of transformers across three continents

Properties of Shell GTL transformer oil

in use in a transformer

• GTL oil still bright with no signs of sludge after 18 months

Date CO2 CO H2 CH4 C2H2 C2H4 C2H6 TAN BDV DDF IFT

10 Jul 12 1,178 221 164 3.5 0.01 2.3 1.4 0.06 61 0.207 22.2

23 Oct 12 1,595 126 6.2 5.3 0.01 3.9 3.5 0.06 73 0.264 22.6

30 Jan 13 1,342 100 202 2.4 0.01 2.4 1.9 0.06 60 0.299

15 May 13 1,153 139 174 2.8 0.01 2.1 1.7 0.06 83 0.121 23.6

04 Jul 13 79 2 3.1 0.6 0.01 0.1 0.1 0.01 88 0.061 37.4

28 Aug 13 182 17 12 1.2 0.01 0.4 0.3 0.01 83 0.021 36.3

11 Dec 13 547 43 89 1.9 0.01 0.6 0.5 0.01 89 0.034 36.4

26 Jan 14 79 2 0.5 1 0.01 0.2 0.1 0.01 78 0.034 32.2

24 Mar 14 169 35 6.1 1.5 0.01 0.3 0.3 0.01 84 0.022 32.2

02 Jul 14 625 75 9.8 2.7 0.01 0.3 0.2 0.01 72 0.022 35.2

25 Nov 14 473 76 1.9 1.8 0.43 0.4 0.4 0.01 66 0.034 33.7

Power plant station unit transformer (10 years old)

Hawker Siddeley (1986), 132 kV, 90 MVA, free breathing. Changed to Shell GTL oil June/July 2013.

Vacuum applied January 2014. No issues

Source: EDF UK

Shell GTL transformer oil performance

after one year in service

• New transformers filled with Shell GTL transformer oil; first oil

analysis after one year in service

Type N2 O2 CO2 CO H2 CH4 C2H2 C2H4 C2H6 C3H6 C3H8 TAN BDV DDF BHT IFT

380-kV grid transformer

(350 MVA) 53,423 24,974 226 114 2 1 0 0 0 10 0

380-kV grid transformer

(350 MVA) 6,849 4,262 191 51 0 1 0 0 0 0 0 0 75 0.001 0.21 48

380-kV grid transformer

(350 MVA) 3,006 1,639 74 18 0 0 0 0 0 0 0

30-kV reactor

(50 MVA) 62,753 31,889 429 74 3 0 0 0 0 0 0

30-kV reactor

(50 MVA) 38,678 15,444 178 36 0 1 1 1 1 0 0 0 78 0.0009 0.22 47

30-kV reactor

(50 MVA) 63,935 31,640 436 88 3 7 0 12 0 3 0

30-kV reactor

(50 MVA) 57,350 28,661 131 41 4 7 0 8 0 0 0 0 86 0.004 44

30-kV reactor

(50 MVA) 68,358 34,739 378 85 4 7 0 0 0 0 0

30-kV reactor

(50 MVA) 54,609 24,606 888 69 7 6 0 2 18 0 0 0 72 0.0002 0.2 46

30-kV reactor

(50 MVA) 5,788 3,422 95 42 1 0 0 1 0 0 0 0 86 0.004 0.23 42

Source: Amprion/Siemens

Shell recommendation for use of Shell

GTL transformer oil

• In the case of a change from a conventional transformer oil to Shell

GTL oil

Vacuum treatment and filtration No change to Shell naphthenic oil

Oil reclamation No change to Shell naphthenic oil

Top up with Shell GTL or naphthenic oil Up to 10% no objection*

Fill old transformers 15% remaining oil acceptable*

Oil monitoring As described in IEC 60422

Interpretation of DGA As described in IEC 60599

Concentration measurement of

antioxidant As described in IEC 60666

Summary – benefits of Shell GTL

transformer oil

1Compared with conventional inhibited transformer oils 2In ambient conditions

High flash point:

Additional safety

Virtually no sulphur:

Risk of corrosion

caused by oil sulphur

eliminated

Oxidation stability –

less acid formation:1

Longer transformer

lifetime expected (with

lifetime filling)2

Higher impulse

breakdown voltage:

Lower probability of arc

formation, reduced oil

ageing1

Transformer lifetime

Oxidation stability –

less sludge formation:1

No blockage of cooling

drains, maintains low

temperature