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.