From a process perspective, the core difference between Oil-immersed transformers and Dry-type transformers lies in the construction method of the insulation system and the winding encapsulation process. Simply put: oil-immersed transformers achieve insulation and heat dissipation by being "immersed in oil," while dry-type transformers achieve insulation by being "encapsulated with solid materials." 1.Insulation system construction process (the most crucial difference)Oil-immersed transformersThe windings are "barely wound": copper wire is wrapped with cable paper or corrugated paper, and is not fully encapsulated after winding. Insulation is filled with oil: all gaps between the windings and the core, between the windings, and between the windings and the tank are filled with insulating oil. Key process: Vacuum oil injection—After the transformer body is dried, filtered and degassed insulating oil is injected under vacuum to ensure that the oil completely wets all insulation gaps and removes air bubbles. Dry-type transformers (taking the mainstream epoxy resin casting type as an example) The windings are "solidified": after the copper wire is wound, it is placed in a mold, and epoxy resin is poured under vacuum. After curing, it forms a hard insulating entity. Insulation relies on epoxy resin: the resin itself is both an insulating material and a structural support. Key process: Vacuum casting—the epoxy resin is mixed with curing agent and filler under vacuum and then injected into the mold to eliminate air bubbles and ensure that the insulation layer is dense and defect-free. 2.Winding Manufacturing Process： Oil-Immersed Transformer No overall encapsulation after winding Interlayer insulation is done with cable paper, and the ends are insulated with insulating cardboard or laminated wood. The wound coil is directly fitted onto the iron core, and then the entire coil is immersed in oil. Dry-Type Transformer Vacuum casting or vacuum pressure impregnation (VPI) is mandatory after winding. Low-voltage coils are mostly wound with copper or aluminum foil to reduce eddy current losses. High-voltage coils use enameled round or flat wire, and are then cast with epoxy resin. After casting, curing treatment (temperature curing) is required to allow the resin to reach its final strength.   3.Assembly and Sealing Process： Oil-Immersed Transformer After the transformer body (core + windings) is assembled, it must be installed in a sealed oil tank. The oil tank welding requirements are strict, and leak testing (air pressure or water pressure test) is required. Installation accessories: oil conservator, radiator, gas relay, dehumidifier, etc. Finally, vacuum oil filling is performed. Dry-Type Transformer After the transformer body is assembled, an oil tank is not required. The protective outer shell (metal plate or mesh cover) is installed directly, mainly for protection rather than sealing. Temperature control probe and cooling fan (optional) are installed. No oil conservator, no radiator, no gas relay   4.Testing Procedures： Oil-Immersed Transformers In addition to routine electrical tests, oil hydration tests are required (breakdown voltage, water content, acid value, chromatographic analysis, etc.). A sealing test is required (to check for tank leakage). Partial discharge measurements are usually performed after oil filling. Dry-Type Transformers No oil hydration test required. Focus on partial discharge levels (typically ≤10pC, advanced European level ≤5pC). Insulation resistance and absorption ratio tests are required to check for moisture.

In Africa, the demand for oil-immersed and dry-type water treatment systems differs significantly—oil-immersed systems overwhelmingly dominate in terms of quantity, while dry-type systems represent a niche growth driver characterized by "small scale and high growth rate." They are not competing in the same arena, but rather serving entirely different scenarios. In terms of unit quantity, the African transformer market is characterized by "Oil-immersed transformers being the majority".   l Why is there a much greater demand for oil-immersed containers in Africa? Africa's natural environment and socioeconomic conditions are almost tailor-made for Oil-immersed transformers: 1. Adaptability to extreme high-temperature environments: Temperatures in Africa (especially North Africa, the Sahel, and the Gulf region) often exceed 50°C. Oil-immersed transformers use insulating oil as their cooling medium, and their thermal management system remains stable at high temperatures; Dry-type transformers rely on air convection for heat dissipation, which significantly reduces their efficiency in high-temperature environments. This is the core physical reason why Oil-immersed transformers dominate in North Africa, the Sahel, and the Gulf region. 2. Outdoor Scenarios Dominate Africa's electricity demand primarily comes from: Rural electrification (outdoor pole-mounted/ground-mounted transformers)、Mining microgrids (open-pit mines)、 Distributed photovoltaic power stations Urban power grid expansion (outdoor box-type transformers). Over 90% of these scenarios are outdoors, and Oil-immersed transformers outperform Dry-type transformers in terms of cost-effectiveness, strong overload capacity, and weather resistance. 3. Cost Sensitivity Standard oil-immersed distribution transformers cost approximately $15-30/kVA, while Dry-type transformers cost $30-50/kVA, more than double the price. In most African countries and projects with tight budgets, Oil-immersed transformers are preferred over Dry-type transformers whenever possible. 4. Technological Maturity and Ease of Maintenance In Africa, there is a shortage of local skilled workers. Oil-immersed systems are mature equipment with globally applicable inspection and maintenance systems. Dry systems, on the other hand, require more technical expertise from maintenance personnel should problems occur.   However, with the increase in urbanization rates in Africa, the accelerated construction of data centers in South Africa, Nigeria, and Kenya, and the development of commercial real estate, the proportion of dry-type roofing will continue to climb from the current penetration rate of ~17%, and is expected to reach 30%+ in the commercial building sector around 2030. Even so, in terms of unit numbers, the overwhelming advantage of oil-immersed roofing in Africa is unlikely to be shaken in the next 10-15 years.

From the perspective of raw materials, the fundamental difference between Dry-type transformers and Oil-immersed transformers lies in the different insulating media and winding encapsulation materials, which determines the composition of their entire material system.   Core insulation medium: oil or air + solid insulation Oil-immersed transformer Main insulation material: Mineral insulating oil (refined from petroleum fractions, such as naphthenic oil and paraffinic oil The insulating oil performs two main functions: electrical insulation and heat dissipation. The oil fills all gaps between windings, between windings and the core, and between windings and the tank. Dry-type transformers Main insulation material: Solid insulation system, no liquid insulation used Three common technical approaches: Epoxy resin casting type (mainstream): Windings are vacuum-cast and wrapped with epoxy resin Nomex paper insulation type (H-class Dry-type transformers): DuPont aromatic polyamide paper + air Glass fiber impregnation type: Glass fiber reinforced resin Heat dissipation relies entirely on air convection (AN) or forced fan blowing (AF)   Core Material: Basically the same Both cores use cold-rolled grain-oriented silicon steel sheets (CRGO), processed by stacking or winding. There is no fundamental difference in this raw material. Some high-end Dry-type transformers use amorphous alloy cores to reduce no-load losses, but this is not a dividing line between product categories.   Differences in materials between fuel tank and structural components Oil-immersed transformer Oil tank: Welded steel plate, inner surface coated with insulating varnish Accessories: Oil conservator, radiator/cooling pipes, gas relay, explosion-proof pipe, desiccant (silicone filter), oil filter valve These components are themselves important raw material components. Dry-type transformer Enclosure: Protective enclosure made of aluminum alloy or steel plate (IP20/IP23), no oil tank No oil conservator, no radiator, no gas relay Only a temperature control system (PT100 sensor + temperature controller) and cooling fan (optional) Overall material usage is significantly less than that of oil-fired transformers   Oil-fired transformers rely on "oil" for insulation and heat dissipation, so the windings can be bare and the oil tank must be thick and heavy; dry-fired transformers rely on "epoxy resin sealing" for insulation and "air" for heat dissipation, so the overall materials are lighter and cleaner, but the cost of epoxy materials themselves is not low.

1. No-load loss Po – basically at the same level Po mainly depends on the core material and the design magnetic flux density. For the same efficiency class, Oil-immersed transformers and Dry-type transformers use the same level of cold-rolled grain-oriented silicon steel sheets (CRGO) or even amorphous alloys, so Po can be very close. 2. Load Loss Pk – Oil-immersed transformers have an advantage at "actual operating temperature" This is the key point of divergence: Oil-immersed: Winding heat is quickly carried away by the oil, resulting in uniform oil temperature and more controlled hot spot temperatures. Resistance temperature rise is relatively controllable ⇒ R(T) in Pk = I²R(T) won't spike too high. Dry-immersed (epoxy resin cast SCB): Although F-class (155℃)/H-class (180℃) insulation materials allow for higher temperatures, for lifespan management, the control system usually keeps the winding temperature within a more conservative range (e.g., ≤120~130℃ long-term). Once it approaches the threshold, it will limit the load or force the fan to stop running at full speed. Result: With the same conductor quantity and design margin, the Pk of an oil-immersed transformer is more likely to stabilize at a lower effective value during long-term high-load operation – this is not due to "different copper wires," but rather a more powerful heat sink. 3. Heat dissipation determines a hidden efficiency factor: "Available Overload Efficiency". This means that if your load characteristics are consistently high (>75%), the overall operating efficiency curve of a hydraulic transformer will look better and be more durable than that of a dry-type transformer. Conversely, if the load rate fluctuates at a low to medium level year-round, the efficiency difference between the two will be minimized.   In short, efficiency isn't a difference in label between oil-immersed and Dry-type transformers, but rather a difference in the matching of operating conditions. Oil-immersed transformers excel in "lower overall losses and larger thermal margins under high load rates and long operating times"; Dry-type transformers excel in "maintenance-free operation, cleanliness, safety, and comparable efficiency under reasonable load rates (avoiding pushing the limits). What truly drags down efficiency is never the type itself, but rather—two mismatches: overloading (powering up a small load while the transformer is underutilized) or underloading (overheating and limiting the load capacity).

Advantages and disadvantages of Oil-Immersed Transformers：*Advantages: Strong heat dissipation and overload capacity: The insulating oil has extremely high heat dissipation efficiency, can withstand short-term overloads, and is suitable for applications with large load fluctuations. Large capacity and high voltage level: Mature technology allows for the manufacture of transformers with extremely large capacity and extremely high voltage levels, making it the core of power transmission and transformation systems. Good insulation performance: The oil dielectric has high insulation strength, effectively protecting the internal windings. Lower initial cost: For the same capacity, its manufacturing and purchase costs are generally lower than those of dry-type transformers. Low operating noise: The oil dampens the vibration of the core and windings, resulting in relatively quiet operation.   *Disadvantages: Fire Risk: Insulating oil is flammable and may cause fires or even explosions in the event of a malfunction, requiring extremely high fire safety standards. Complex Maintenance: Requires regular monitoring of oil level, temperature, and quality (e.g., water content, acidity), and professional maintenance such as oil filtration and replenishment. Leakage Risk: Aging seals may lead to oil leaks, polluting the environment and requiring remediation. High Installation Requirements: Typically requires a separate distribution room or outdoor platform, and an emergency oil tank. Environmental Issues: Waste insulating oil is hazardous waste and requires professional recycling and disposal.   Advantages and disadvantages of Dry-type transformers： *Advantages: Safe and Fireproof: Contains no flammable liquids, especially the epoxy resin cast type which is flame-retardant and explosion-proof, allowing direct installation at load centers (e.g., indoors, within buildings). Maintenance-Free and Easy Installation: Requires no oil level or quality monitoring, essentially maintenance-free; simple structure and easy installation. Environmentally Friendly and Clean: No risk of oil leaks, no toxic gases, environmentally friendly. Good Overload Capacity (New Models): Modern design with improved heat dissipation significantly enhances overload capacity. High Adaptability: Excellent moisture resistance, suitable for harsh environments such as humid and dusty conditions (requires appropriate protection level).   *Disadvantages: Lower heat dissipation efficiency: Relying on air cooling, its heat dissipation capacity is inferior to oil cooling, and its overload capacity is still inferior to Oil-Immersed Transformers transformers of the same capacity. Limited capacity and voltage: Due to limitations in heat dissipation and insulation materials, the capacity and voltage rating of a single unit are usually lower than those of Oil-Immersed Transformers transformers. Higher initial cost: For the same capacity, the price is usually higher than that of Oil-Immersed Transformers transformers. Higher operating noise: Vibrations of the core and windings are directly transmitted through the air, resulting in relatively high noise levels. Sensitive to the operating environment: Open-ventilated (non-enclosed) types are susceptible to dust and moisture, requiring the environment to be kept clean.   In summary: The key to choosing a substation lies in balancing safety, cost, and the environment. For outdoor or stand-alone substations prioritizing large capacity, low cost, and high efficiency, choose Oil-Immersed Transformers; for indoor or densely populated locations prioritizing high safety, maintenance-free operation, and environmental friendliness, choose dry-type substations.