From an efficiency perspective: the difference between Oil-immersed transformers and Dry-type transformers

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).