Comparative Analysis of Synthetic and Natural Ester Fluids

The main difference between synthetic and natural ester fluids is their origin and molecular structure. Synthetic esters are chemically engineered for thermal stability and oxidation resistance, natural esters are derived from renewable vegetable oils for environmental friendliness and biodegradability. This affects their performance, cost, and application in different transformers.

INVEXOIL company, a leader in oil technologies, makes insulating fluids more sustainable through its “Transformer Oil Regeneration System” and “Transformer oil regeneration services based on IEC 60296 & IES 60422 standards“, for optimal performance and environmental compliance. This article will go through the details of synthetic and natural esters and their application in different transformer systems.

The 17 Main Differences Between Synthetic and Natural Ester Fluids:

1. Molecular Structure and Composition
2. Chemistry (chemical behavior)
3. Water Interaction
4. Volume Resistivity
5. Acid Limits
6. Thermal and Fire Properties
7. Oxidation Stability
8. Low-Temperature Behavior (pour point)
9. Biodegradability and Environmental Impact
10. Equipment Lifespan and Reliability
11. Loading Flexibility and Network Optimization
12. Aging Behavior and Degradation
13. Breakdown Voltage and Mixing Behavior
14. Diagnostic and Fault
15. Transformer Application
16. Operating Conditions
17. Retrofilling

1. Synthetic and Natural Ester Fluids: Molecular Structure and Composition Differences

Natural esters are derived from vegetable oils and are mainly triglycerides with 3 fatty acid groups attached to glycerol. Their fatty acid profile is determined by the source oil, such as soybean or sunflower, resulting in variations in their physical and chemical properties. For example, natural esters have a molecular weight of 850-950 g/mol and an unsaturation level of 120-140 mg I2/g. These properties affect their degradation and moisture absorption.

Synthetic esters are chemically engineered, using pentaerythritol and saturated fatty acids. The 4-group molecular structure gives them more stability. Synthetic esters have lower iodine values (less than 5 mg I2/g) 1 since they are saturated and have a molecular weight of 950-1000 g/mol. This engineered composition gives them better thermal and oxidation stability than natural esters.

The physical and chemical differences between these two fluids also affect solubility properties such as 2-FAL solubility. Natural esters have higher solubility due to their higher polarity and unsaturation while synthetic esters have lower solubility due to their saturation and higher oxidation stability.

2. Chemistry Differences of Synthetic and Natural Ester Fluids

Natural esters have unsaturated fatty acids that are prone to oxidation and need antioxidants to prevent degradation. These unsaturated bonds introduce a level of reactivity that is not present in synthetic esters which are composed of saturated fatty acids only. Synthetic esters are free of natural impurities and have a chemically inert profile under normal operating conditions making them more stable in the long run.

The synthetic esterification process produces uniform molecular chains with consistent chemical behavior, while natural esters may have slight variations based on the feedstock used. This variation can affect their aging and compatibility with specific transformer designs.

Related Article: Understanding Transformer Oil Types for Optimal Performance and Sustainability

3. Water Interaction Differences of Synthetic and Natural Ester Fluids

Water interaction is a factor in transformer performance:

• Natural Esters: Have high moisture absorption capacity which helps in managing water contamination in the transformer system by absorbing water from the solid insulation. This property can extend the life of cellulose insulation.
• Synthetic Esters: Have lower moisture absorption rates but higher water saturation limits than natural esters. This reduces the risk of free water formation at higher temperatures and improves dielectric performance under extreme conditions.

Moisture content in natural esters can go up to 1000 ppm without affecting dielectric strength while synthetic esters can maintain high performance at below 400 ppm.

4. Volume Resistivity Differences of Synthetic and Natural Ester Fluids

Volume resistivity measures a fluid’s ability to resist electrical conductivity:

• Natural Esters: Have lower initial volume resistivity due to residual ionic impurities from their natural sources. However, this can be improved through purification and additive incorporation.
• Synthetic Esters: Have higher volume resistivity due to their chemically pure composition and are more suitable for high voltage applications where minimal electrical losses are critical.

At 20°C synthetic esters have volume resistivity above 10^12 Ω·m while natural esters are between 10^10 to 10^11 Ω·m.

5. Acid Limits of Synthetic and Natural Ester Fluids

Acid limits are key to transformer health:

• Natural Esters: Higher initial acidity due to free fatty acids and need to be monitored closely to prevent rapid degradation.
• Synthetic Esters: Negligible acidity (<0.01 mg KOH/g) and stable over long-term use, less maintenance required.

The industry standard acid limit for safe operation is 0.2 mg KOH/g, natural esters may reach this limit faster than synthetic esters under the same conditions.

6. Synthetic and Natural Ester Fluids: Thermal and Fire Properties Differences

Thermal properties of insulating fluids are important for transformer performance. Natural esters have high fire points, above 350°C. Synthetic esters have slightly lower fire points, between 305°C to 315°C. These values are due to their molecular structure: natural esters triglyceride composition gives higher heat endurance, and synthetic esters engineered composition prioritizes oxidative stability.

The heat capacity (Cp) and thermal conductivity (k) are also different. Natural esters have Cp of 2.1 J/g·K and k of 0.17 W/m·K at 25°C. Synthetic esters have slightly higher values, Cp of 2.3 J/g·K and k of 0.18 W/m·K. These values contribute to better cooling performance of synthetic esters under extreme conditions.

Table: Flash point for different types of esters

Liquid	Flash point ISO 2592	Fire point ISO 2592
Mineral Oil	160°C	170°C
Synthetic Ester	275°C	316°C
Natural Ester	316°C	360°C

Regarding viscosity, natural esters have a kinematic viscosity of 33 mm²/s at 40°C, synthetic esters have a slightly lower viscosity of 28 mm²/s. At low temperatures (-10°C), viscosity increases dramatically, and synthetic esters (~730 cSt) have better flow properties than natural esters (~330 cSt).

7. Oxidation Stability Differences between Synthetic and Natural Ester Fluids

Oxidation stability is key to the long-term life of insulating fluids:

• Natural Esters: Need antioxidants to resist oxidation degradation. Without them, natural esters will form sludge and acids that can impair transformer performance.
• Synthetic Esters: Inherently resistant to oxidation due to their saturated molecular structure, less need for additives, and longer fluid life.

The “real issue” with oxidation stability is with natural esters’ exposure to environmental factors like oxygen and high temperature. This requires close monitoring and additive management, synthetic esters offer a more passive and reliable solution.

8. Low-Temperature Behavior (Pour Point) of Synthetic and Natural Ester Fluids

Pour point determines low-temperature usability:

• Natural Esters: Crystallize below -25°C, not suitable for colder climates.
• Synthetic Esters: Remain fluid down to -40°C, ideal for transformers in sub-zero environments.

Table: Pour point for different types of esters

Ester Type	Synthetic ester (MIDEL 7131)	Canola natural ester (MIDEL eN 1204)	Soybean natural ester (MIDEL eN 1215)
Pour Point	-56 °C	-31 °C	-18 °C

Below -25°C, natural esters crystallize, not usable in colder climates. Synthetic esters remain fluid due to their engineered molecular structure, and better performance in sub-zero environments. Synthetic esters also have higher viscosity at -10°C (~730 cSt vs ~330 cSt for natural esters), which may impact flow properties under this condition.

9. Synthetic and Natural Ester Fluids: Biodegradability and Environmental Impact Differences

Natural esters are fully biodegradable with over 97% biodegradation in OECD 301B tests. Being bio-based they have a much lower carbon footprint than mineral oil or synthetic esters.

Synthetic esters are also biodegradable over 90% but not biobased. They are more energy-intensive to produce so have a carbon footprint similar to mineral oil. Despite this, they are considered environmentally safe due to low ecotoxicity.

Natural esters have better water partition coefficients (log Kow ~2-3) so they have lower environmental accumulation. Synthetic esters have log Kow ~4 so moderate bioaccumulation potential. This slight disadvantage is offset by lower release rates in transformer operations due to less maintenance and better oxidation stability.

10. Equipment Lifespan and Reliability of Synthetic and Natural Ester Fluids

Equipment lifespan is dependent on insulation health:

• Natural Esters: Extend cellulose insulation lifespan through better water management but need to be monitored for acidity and oxidation.
• Synthetic Esters: Consistent performance and less maintenance for high-stress applications.

Natural esters may extend the lifespan of transformers in normal conditions, synthetic esters are better for demanding applications.

11. Synthetic and Natural Ester Fluids: Loading Flexibility and Network Optimization

• Natural Esters: Good loading flexibility in normal conditions but underperform in peak loads due to thermal limitations.
• Synthetic Esters: Better performance in variable loads and high temperatures for network optimization in extreme scenarios.

12. Synthetic and Natural Ester Fluids: Aging Behavior and Degradation

12.1 Acidity Increase

The acidity of insulating fluids increases over time due to degradation, mainly hydrolysis. During thermal aging natural and synthetic esters behave very differently:

• Natural Esters: Acidity increases sharply in the first 1960 hours of aging and reaches 0.8 mg KOH/g. This is due to the rapid hydrolysis of triglycerides and the release of fatty acids.
• Synthetic Esters: Acidity is stable in the early stages of aging and then increases significantly after 2800 hours and exceeds 1.0 mg KOH/g. This is due to the onset of hydrolytic degradation of their more stable ester bonds.

12.2 (2-FAL) Solubility

2-furaldehyde (2-FAL) is a key indicator of cellulose degradation and its solubility varies:

• Natural esters have higher 2-FAL due to their higher polarity and compatibility with degradation by-products. After 2800 hours of aging 2-FAL in natural esters is 1.5-2 times higher than in synthetic esters.
• Synthetic esters have lower 2-FAL solubility, so DGA (dissolved gas analysis) is less effective for diagnostics.

12.3 Dissolved Gas Analysis (DGA)

Gas generation in insulating fluids during aging is critical for fault diagnosis. DGA results show:

• Natural Esters: TCG is 20 times higher than synthetic esters after 1960 hours of aging. Ethane (C2H6) is the main gas, followed by methane (CH4) with values above 1200 ppm.
• Synthetic Esters: Gas is much lower; ethane and methane are below 100 ppm.

CO2 and CO levels are slightly higher in natural esters due to more cellulose degradation. After 2800 hours CO2+CO is 1400 ppm in natural esters vs 900 ppm in synthetic esters.

12.4 Oxidation Stability

Synthetic esters have better oxidation stability than natural esters due to their engineered molecular structure. Natural esters rely on antioxidants to slow down oxidation, synthetic esters resist oxidation inherently, remain stable in viscosity, and reduce acid formation over time.

13. Breakdown Voltage and Mixing Behavior Differences of Synthetic and Natural Ester Fluids

Breakdown voltage measurements show different behavior when mixed with mineral oil:

• Natural Esters: Mixing natural esters with mineral oil (new or aged) reduces their breakdown voltage. 90% natural ester and 10% aged mineral oil samples had some of the lowest values (~60 kV)
• Synthetic Esters: Synthetic esters have higher breakdown voltage than natural esters when mixed. Even after blending synthetic esters meet the IEC 61099 standard of 45 kV.

13.1 Variability

Synthetic esters have more variability in breakdown voltage when mixed with mineral oil as seen in the standard deviation of the test results. A sample with 60% synthetic ester and 40% mineral oil had a standard deviation of 23% of the dielectric performance.

14. Diagnostic and Fault Analysis of Synthetic and Natural Ester Fluids

14.1 Duval Triangle Interpretation

Duval Triangle applied to DGA results shows thermal fault (T1, <300°C) as the main issue for both esters. This method works for both fluids, but natural esters have more gas generation so more robust fault detection than synthetic esters.

14.2 Partial Discharge Resistance

Natural esters have higher partial discharge inception voltage than synthetic esters so better resistance to electrical stress. This is good for transformers operating in high-stress environments.

15. Transformer Application Comparison of Synthetic and Natural Ester Fluids

15.1 Breathing vs. Non-Breathing Transformers

• Natural Esters: Good for sealed transformers where moisture ingress is controlled.
• Synthetic Esters: Good for both breathing and non-breathing transformers due to low moisture uptake.

15.2 Distribution vs. Power Transformers

• Natural Esters: Used in distribution transformers for cost and environmental benefits.
• Synthetic Esters: Used in power transformers where extreme conditions and reliability are critical.

16. Operating Conditions

Natural esters are good for moderate climates, synthetic esters are essential for harsh or variable climates.

17. Retrofilling

Both esters can be retrofilled but synthetic esters are more compatible with existing mineral oil systems so easier transition.

When Natural Ester Is Better Than Synthetic Ester?

Natural esters are better than synthetic esters in applications that prioritize environmental sustainability, cost, and moderate operating conditions. Good for eco-friendly utilities and distribution systems in a stable climate.

Table: Summarizes the key properties of natural and synthetic esters

Property	Natural Ester	Synthetic Ester
Base Fluid Type	Vegetable oil	Pentaerythritol ester
Fire Point	>350°C	~305-315°C
Biodegradability/Toxicity	Best in class	Readily biodegradable
Biobased/Carbon Footprint	Yes/very low	No/similar to mineral oil
Cellulose Paper Lifespan Increase	Up to 12x	Marginal
Thermal Class Increase	15-20°C	None
Dielectric Properties	Equivalent to mineral oil	Equivalent to mineral oil
Partial Discharge Inception Voltage	Higher than mineral oil	Lower than mineral oil
Crystallization at -25°C	Yes	No
Viscosity at -10°C	~330 cSt	~730 cSt
Fluid Oxidation Stability	Adequate	Good
DGA Diagnostic Capability	Robust	Limited

Conclusion

In synthetic and natural ester fluids comparison, each has its own advantages for specific applications. Natural esters are good for eco-friendliness, biodegradability and partial discharge resistance, suitable for applications that require sustainability. Synthetic esters are good for oxidation stability, low temperature performance and extended equipment life, suitable for harsh operating conditions. The choice between these fluids depends on the operating priorities, transformer design and environmental considerations. The developments by INVEXOIL in oil regeneration and compliance to international standards makes it more important to choose the right transformer fluid for performance and sustainability.