A Review of Emerging Dielectric Fluids for Sustainable and Resilient Power Transformers ^†

Abstract

This paper reviews emerging dielectric fluids for power transformers, including natural and synthetic esters, silicone oils, gas-to-liquid oils, and nanofluids, driven by environmental regulations, fire safety concerns, and the need for extended asset life. The review synthesizes technical data from standards and field experience, including a case study of an Eskom transformer energized in 2016 with natural ester fluid. Analysis confirms these fluids offer significant benefits in fire safety, biodegradability, and dielectric performance, with the case study demonstrating natural esters’ effectiveness in preserving solid insulation. However, trade-offs involving cost, material compatibility, and operational protocols require careful management.

1. Introduction

Power transformers are fundamental to the reliable operation of electrical transmission and distribution networks. The insulating fluid within a transformer serves three critical functions: electrical insulation, heat dissipation, and arc suppression. For over a century, mineral oil refined from crude petroleum has been the ubiquitous choice due to its proven performance, availability, and cost-effectiveness [1,2]. However, the evolving landscape of the power industry is creating new demands that challenge the supremacy of mineral oil. Key drivers include:

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Environmental Regulations: Stricter rules regarding fluid spills and end-of-life disposal are prioritizing biodegradable and non-toxic alternatives.

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Fire Safety: The moderate flash point (130–150 °C) of mineral oil is a significant risk in dense urban areas and indoor installations.

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Asset Life Extension: Utilities are seeking technologies that can extend the operational life of aging transformer fleets.

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Grid Resilience: Increasing load demands and the integration of renewables require fluids with enhanced thermal and dielectric performance.

In response, a new class of emerging dielectric fluids is being developed. This paper reviews these advanced fluids - natural esters, synthetic esters, silicone oils, GTL oils, and nanofluids—evaluating their potential to address these modern grid challenges and their role as an enabling technology for a more sustainable, reliable and resilient power system. This review is further enriched with a long-term case study on a natural ester-filled transformer at Eskom that has been in service since 2016, providing a window into the performance of this technology from commissioning through nearly a decade of operation.

2. Emerging Dielectric Fluids: Types and Characteristics

A. Natural Esters

Derived from vegetable oils (e.g., soybean, rapeseed, canola), natural esters are triglycerides. Their key advantages include a high flash point (~300 °C) and excellent biodegradability (OECD 301 compliant) [3,4,5]. They are particularly suited for environmentally sensitive areas. Field studies have demonstrated an additional, significant benefit: their hygroscopic nature allows them to absorb moisture from solid paper insulation, thereby slowing cellulose aging and extending transformer life [6,7]. A primary limitation is their higher viscosity and lower oxidative stability compared to mineral oil, which can necessitate design modifications or additive treatments.

B. Synthetic Esters

Chemically engineered to offer superior performance, synthetic esters (including polyol esters) provide high flash points, exceptional thermal and oxidative stability, and low pour points, making them suitable for diverse climatic conditions [8]. Their biodegradability profile is similar to natural esters. The main barrier to widespread adoption is their high manufacturing cost, typically 2.5–4 times that of mineral oil [9].

C. Silicone Oils

Silicone oils, based on polydimethylsiloxane (PDMS), offer superior fire safety (flash point >300 °C) and remarkable thermal stability across a wide temperature range (−60 °C to 200 °C) [10]. They are non-toxic and have a long service life. Their primary application is in high-fire-risk locations such as urban substations and underground systems. The high procurement cost and potential compatibility issues with certain sealants are their main drawbacks.

D. Gas-to-Liquid (GTL) Oils

GTL oils are synthetic hydrocarbons produced from natural gas via the Fischer–Tropsch process. They offer high purity, improved oxidation stability, and higher flash points than mineral oils, representing an upgrade path with a better safety and environmental profile without a complete shift to ester chemistry [11]. Their biodegradability, however, is moderate compared to esters.

E. Nanofluids

Nanofluids represent a cutting-edge advancement where nanoparticles (e.g., TiO₂, Fe₂O₃, h-BN) are dispersed in a base oil (mineral, ester, or silicone). The infusion of nanoparticles can significantly enhance the dielectric strength and thermal conductivity of the fluid [12,13]. This technology promises improved overload capacity and compact transformer design. However, challenges related to the long-term stability of nanoparticle dispersion, high costs, and a lack of standardization currently limit their application to pilot studies and niche projects.

A comparative summary of key properties is provided in

Table 1. Comparison of key properties of dielectric fluids.

Fluid Type	Flash Point (°C)	Biodegradability (OECD 301)	Relative Cost (Mineral Oil = 1)
Mineral Oil	130–150	~25% (Poor)	1.0
Natural Ester	~300	>95% (Excellent)	2.5–3.5
Synthetic Ester	>250	>90% (Excellent)	3.0–4.0
Silicone Oil	>300	Low to Moderate	3.0–4.0
GTL Oil	160–180	Moderate (~30–50%)	1.5–2.0
Nanofluid	Depends on base fluid 1	Depends on base fluid 1	4.0+ (High)

¹ Properties of nanofluids vary based on the base fluid (mineral oil, ester, or silicone) and nanoparticle type/concentration; flash point and biodegradability are largely determined by the base fluid component.

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3. Maintenance and Field Experience

The transition to eco-friendly fluids, particularly natural esters, is supported by growing field evidence. Maintenance protocols for esters are similar to those for mineral oils but require fluid-specific interpretation of results [14].

3.1. Interpretation and Monitoring

To assess the condition of the ester oil-filled transformers, three key parameters must be interpreted differently than mineral oil filled units:

• Dissolved Gas Analysis (DGA): Natural esters exhibit “stray gassing” of methane and ethane at low temperatures (80–120 °C), which is normal and should not be misinterpreted as a fault [7].
• Water Content: Esters can hold significantly more water in a solution without a proportional loss of dielectric strength, shifting the monitoring focus to relative saturation rather than absolute ppm values [6].
• Acidity: A rising acid number in esters is primarily due to the formation of non-aggressive fatty acids, unlike the sludge-forming acids in oxidized mineral oil.

3.2. Case Study: Performance of an Eskom Natural Ester Transformer from Commissioning

The long-term performance of a natural ester fluid is evaluated using operational data from an Eskom power transformer (88/6.6 kV). This 88/6.6 kV, 20 MVA power transformer located in an industrial area in Mafikeng, South Africa’s North-West Province serving an industrial load. This unit is a particularly valuable case study as it was energized in 2016 with the natural ester fluid, providing data from the very start of its service life. The monitoring data from 2019 to 2025, summarized in

Table 2. Summary of key insulation parameters for Eskom natural ester transformer (2019–2025).

Parameter	Range (Min–Max)	Mean	IEC In-Service Limit
Dielectric Strength (kV)	62–82	70	≥30 2
Moisture (ppm)	6–30	17	≤200 2
Acidity (mg KOH/g)	0.05–0.08	0.06	≤0.10 3
Degree of Polymerization (DP)	>910 4	–	≥500 5
Hydrogen (H2, ppm)	1–19	6	–
Methane (CH4, ppm)	140–845	402	–
Carbon Monoxide (CO, ppm)	176–636	316	–
Carbon Dioxide (CO2, ppm)	7616–44,512	22,694	–

² Per IEC 62770 (unused natural ester). ³ Per IEC 62975 (in-service caution threshold). ⁴ Single measurement recorded in April 2025. ⁵ Typical minimum recommended DP for reliable transformer operation.

, captures the transformer’s condition through its early and mid-life operation.

3.2.1. Dielectric Strength Analysis

The data confirms excellent and consistent dielectric integrity over this period, with the Dielectric Strength maintained between 62 kV and 82 kV above the IEC 62770-2024 minimum requirement of 30 kV for unused natural esters.

3.2.2. Moisture and Relative Saturation

The fluid’s inherent ability to manage moisture is evident, with varying Moisture Readings (9–30 ppm) which is below the IEC limit of 200 ppm for new ester oil, causing no detrimental effect on dielectric strength, underscoring the importance of monitoring relative saturation.

3.2.3. Acidity and Chemical Stability

The acid number fluctuates between 0.05 and 0.08 mg KOH/g, which is below the IEC 62975 in-service caution threshold of 0.10 mg KOH/g. These results represent normal, mild aging of the ester itself; the fatty acids formed are non-corrosive and do not promote sludge.

3.2.4. Solid Insulation

The condition of the solid insulation remains exceptional. The Degree of Polymerization (DP) value of >910 measured in 2025, after nearly nine years of service, indicates minimal paper degradation and is comparable to new paper insulation (DP~1000–1200). This finding provides powerful, real-world evidence for the life-extending property of natural esters when used from commissioning.

3.2.5. Dissolved Gas Analysis (DGA) Trends

Key DGA diagnostic findings against IEEE C57.155-2014, visible in

Table 3. Key DGA diagnostic findings against IEEE C57.155-2014 framework.

Parameter	Eskom Results	IEEE C57.155 Significance	Remark
Acetylene (C2H2)	0 ppm (all samples)	Indicator of arcing/high-energy discharge	No arcing faults in 9 years
Hydrogen (H2)	1–19 ppm	Below typical thresholds	No corona/partial discharge
Methane (CH4)	140–845 ppm	Stray gassing signature	Not a fault condition
Ethane/Ethylene	Low/consistent	Thermal fault gas ratios	No increasing thermal fault
CO/CO2 Ratio	Declining trend	Paper aging indicator	Solid insulation preserved
Duval Triangle	T2 Zone	Thermal fault 300–700 °C	Consistent with stray gassing

, show normal stray gassing patterns for an ester fluid, with no presence of acetylene (C₂H₂), indicating no arcing faults. The stable and low levels of Carbon Monoxide (CO) and Carbon Dioxide (CO₂) suggest very slow thermal aging of the cellulose. Furthermore, the fluid itself shows excellent aging characteristics, with low Acidity (≤0.08 mg KOH/g) and stable Interfacial Tension. This case study demonstrates that natural esters can effectively preserve both the solid and liquid insulation systems, ensuring long-term reliability and validating their use in new transformers for sustainable asset management.

3.2.6. Summary of Findings

The Eskom transformer demonstrates that natural esters deliver on their promise of fire safety, environmental benefit, long-term cost savings due to reduced maintenance intervention and unplanned transformer trips as well as life extension.

4. Discussion: Trade-Offs and Application-Based Selection

The selection of an optimal insulating fluid is a multi-criteria decision. There is no universal “best” fluid; the choice must align with specific application priorities:

• Urban/Indoor Settings: Fire safety is critical. Silicone oils and esters are the preferred choices.
• Environmentally Sensitive Areas: Natural and synthetic esters are ideal due to their rapid biodegradability and low toxicity.
• High-Voltage/High-Load Applications: Synthetic esters offer proven stability, while nanofluids represent a high-potential future technology.
• Cost-Constrained Upgrades: GTL oils and hybrid fluids (mineral/ester blends) offer a middle ground with enhanced safety or performance at a moderate cost premium.
• New transformers for maximum longevity, as demonstrated in the Eskom case study, specifying natural esters in new transformers can be a strategic investment for maximizing asset life and reducing long-term maintenance.

The transition to advanced fluids requires careful assessment of materials. Silicone oils, while thermally stable, exhibit compatibility issues with certain nitrile rubber gaskets and sealants, often necessitating a switch to fluorocarbon elastomers (FKM) or PTFE. Natural esters, due to their higher solvency power, can soften or swell less-resistant enamel coatings on windings. Manufacturers must verify paint, gasket, and adhesive compatibility prior to retrofilling or new filling.

The higher viscosity of natural esters at low temperatures requires a review of pump start-up settings and cooling system design to ensure adequate flow. Furthermore, maintenance personnel must be trained to interpret DGA results using specific ratio methods for esters (e.g., IEC 62975), rather than classical mineral oil methods (e.g., Rogers Ratios), to avoid false positive fault diagnoses.

The primary trade-offs involve balancing superior fire safety and environmental performance against higher initial costs and, in some cases, more complex maintenance and compatibility requirements.

5. Conclusions and Recommendations

This review underscores a paradigm shift in transformer insulating fluids, driven by the imperatives of safety, sustainability, and long-term asset value. Emerging fluids like esters and silicone oils are no longer niche alternatives but are proven technologies capable of mitigating key modern grid risks. The case study of the Eskom transformer, operational since 2016 with a natural ester, provides compelling evidence that these fluids can preserve critical paper insulation in like-new condition for extended periods, fundamentally altering the life-cycle cost calculus.

To facilitate their adoption, the following steps are recommended:

• Conduct Phased Pilot Programs: Utilities should initiate targeted pilot projects, including specifying advanced fluids in new equipment, to generate localized data on the performance and total cost of ownership.
• Establish Fluid-Selection Guidelines: Develop a structured decision-making framework to select fluids based on application-specific priorities (e.g., fire safety in urban areas, environmental protection in environmentally sensitive spaces, retrofitting, or new builds for life extension).
• Prioritize Retrofitting and New Build Studies: Further investigate the technical and economic criteria for retrofilling existing transformers with natural esters. Simultaneously, promote the specification of natural esters in new transformers as a default strategy for fleet-life extension and sustainability.

By integrating these emerging dielectric fluid technologies, utilities can build a more resilient, safe, and sustainable power grid for the future.