Oil-Immersed Transformer
A transformer using mineral oil as insulation and cooling medium, most common for outdoor power and distribution.
An oil-immersed transformer uses mineral oil for both insulation and cooling—the most common type worldwide.
Oil serves dual functions: electrical insulation (dielectric strength 30-40 kV/2.5mm) and heat transfer. Circulates by natural convection (ONAN) or forced (ONAF/OFAF) per cooling class.
Advantages: higher ratings (1000+ MVA), better overload, longer life, lower cost/kVA. Disadvantages: fire risk, environmental concerns.
QDTB manufactures from 30 kVA to 500 MVA with premium oil, advanced insulation, complete preservation and protection.
Related Terms
Distribution Transformer
A transformer stepping down voltage from the distribution network (11-33 kV) to utilization voltage (400/230V) for end consumers.
Power Transformer
A large transformer (>10 MVA) used in transmission networks for stepping voltage between system levels.
Cooling Class (ONAN/ONAF/ODAF)
Standardized codes describing transformer cooling methods: O=Oil, N=Natural, A=Air, F=Forced, D=Directed flow.
Buchholz Relay
A gas-actuated protective relay between transformer tank and conservator detecting internal faults by sensing gas accumulation.
More Terms Starting with “O”
On-Load Tap Changer (OLTC)
A device changing transformer tap positions under load without interrupting supply for automatic voltage regulation.
Overexcitation
A condition where core flux exceeds design level due to excessive voltage or reduced frequency.
Oil Preservation System
Equipment preventing moisture and oxygen from entering transformer oil, including conservators and sealed tanks.
ONAN Cooling
The simplest transformer cooling method using natural oil circulation and natural air convection, requiring no pumps or fans.
More in “Transformer Types”
Autotransformer
A transformer with a single winding acting as both primary and secondary, providing voltage conversion through electromagnetic induction and direct electrical connection.
Amorphous Alloy Transformer
A distribution transformer using amorphous metal core material achieving 60-70% lower no-load losses than conventional silicon steel cores.
Box-Type Substation (Prefabricated)
A factory-assembled compact substation integrating transformer, HV/LV switchgear, and protection in a weatherproof enclosure.
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