Why Are Oil-immersed Transformers Still Dominant in Power Grids?

Despite the emergence of alternative technologies, oil-immersed transformers continue to be the dominant choice for high-voltage power transmission and distribution networks globally. Their enduring prevalence stems from a confluence of well-established technical advantages, operational reliability, and economic factors deeply embedded in power system engineering.

1. Unmatched Insulation and Cooling Performance:
The fundamental design of oil-immersed transformers leverages dielectric mineral oil (or increasingly, less-flammable ester-based fluids) as both an electrical insulator and a coolant. This fluid performs several critical functions:

Superior Dielectric Strength: Oil provides significantly higher dielectric strength than air, allowing for more compact designs and higher voltage ratings within a manageable physical footprint. This is paramount for high-voltage transmission equipment.

Efficient Heat Dissipation: Transformers generate heat due to core losses (hysteresis and eddy currents) and load losses (I2R losses in windings). Circulating oil efficiently absorbs this heat from the windings and core. Natural convection or forced circulation (fans, pumps) then transfers the heat to the external radiators or heat exchangers, effectively managing operating temperatures. This efficient cooling directly enhances lifespan and maintains performance under load.

Protection and Preservation: The oil bath protects cellulose-based paper insulation (used on windings and between layers) from oxidation and moisture ingress, significantly slowing its degradation. Oil also helps extinguish minor internal arcing events.

2. Proven Robustness and Long Service Life:
Decades of rigorous engineering, standardized manufacturing (governed by standards like IEEE C57.12.00, IEC 60076), and extensive field experience have honed oil-immersed transformers into exceptionally robust and reliable assets.

Durability: They are designed to withstand demanding environmental conditions, electrical stresses (including short circuits), and thermal cycling over decades. Typical design lives range from 25 to 40 years or more, with many units exceeding expectations with proper maintenance.

Well-Understood Maintenance: The condition of both the oil and the paper insulation can be effectively monitored through well-established diagnostic techniques like Dissolved Gas Analysis (DGA), furan analysis, moisture-in-oil measurement, and regular electrical testing. This allows for predictive maintenance and informed decisions on life extension or replacement.

3. Economic Advantages in Scale and Manufacturing:
The maturity of oil-immersed transformer technology translates into significant economic benefits, particularly for high-power applications:

Cost-Effectiveness for High Ratings: For large power transformers (LPTs) and medium-sized distribution transformers, the cost per kVA is generally lower than dry-type alternatives, especially at higher voltage and power levels. The materials (steel, copper, oil, cellulose) and manufacturing processes are optimized for high-volume production.

Established Supply Chain: A vast global supply chain exists for the materials, components, and specialized manufacturing equipment required for oil-immersed transformers. This ensures availability and competitive pricing.

Repair and Refurbishment: The industry possesses deep expertise and established facilities for repairing and refurbishing oil-immersed units, often extending their useful life cost-effectively compared to full replacement.

4. Handling High Power Densities:
In applications demanding very high power density – especially crucial in transmission substations where space can be constrained – oil-immersed designs excel. The superior cooling efficiency of liquid allows more power to be handled within a smaller physical volume than air-cooled (dry-type) alternatives rated for the same voltage and power.

Addressing Challenges and the Future Landscape:
It is acknowledged that oil-immersed transformers present challenges, primarily concerning environmental protection and fire safety:

Environmental Concerns: Leaks or spills of mineral oil can pose environmental risks. This has driven the adoption of more biodegradable ester-based fluids, improved tank sealing technologies, and stringent regulations for containment (dikes, catchment basins).

Fire Risk: Mineral oil is flammable. Mitigation strategies include using less-flammable fluids (silicone, esters), installing fire suppression systems, implementing strict safety protocols, and physical segregation within substations.

While dry-type transformers and alternative technologies like SF6 gas-insulated units have found important niches, particularly indoors, in urban areas, or for specific lower-power applications, they have not displaced oil-immersed transformers in core grid applications. Dry-types typically face limitations in voltage rating (especially above 35kV) and power rating compared to oil-immersed units, and can be physically larger and less efficient for the same ratings.The dominance of oil-immersed transformers in power grids is not a matter of stagnation, but a testament to their proven ability to meet the demanding core requirements of electrical networks: high efficiency, exceptional reliability, long service life, manageable maintenance, and cost-effectiveness, particularly for high-voltage and high-power applications. Decades of continuous refinement have optimized their design, manufacturing, and operation. While environmental and safety considerations drive ongoing innovation in fluid technology and containment, and alternative technologies continue to evolve for specific use cases, the unique combination of insulation, cooling, robustness, and economics ensures that oil-immersed transformers remain the indispensable workhorses of the global power infrastructure.