• Dr Joerg Friedel • Product Application Specialist • Shell Technology Centre • Hamburg, Germany
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)
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