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Dry-type Transformer and Oil-immersed Transformer

The biggest difference between oil-immersed transformer and dry-type transformer is whether there is “oil”. Because the oil is liquid and has fluidity, the oil immersed transformer must have a shell. Inside the shell is transformer oil. The coil of the transformer is soaked in the oil. The coil of the transformer can not be seen from the outside. Another feature is that there is an oil conservator on the oil immersed transformer and transformer oil is stored inside, but now the new oil-immersed transformer is also produced without an oil conservator. The dry-type transformer has no oil, so the shell is not needed, and the coil of the transformer can be seen directly.

In order to facilitate heat dissipation of oil-immersed transformer, that is, to facilitate the flow and heat dissipation of internal insulating oil, a radiator is designed outside, just like a heat sink. The dry-type transformer does not have this radiator, and the heat dissipation depends on the fan under the transformer coil, which is a bit like the indoor unit of household air conditioner.

Oil-immersed transformers are generally installed indoors or outdoors due to the need of fire prevention. The dry-type transformer is generally installed in the low-voltage distribution room, side by side with the low-voltage distribution cabinet.

Power transformer


Different packaging forms, dry-type transformer can directly see the iron core and coil, while oil-immersed transformer can only see the shell of the transformer.

Different lead forms

Most dry-type transformers use silicone rubber bushing, while most oil-immersed transformers use porcelain bushing.

Different capacity and voltage

Dry-type transformers are generally suitable for power distribution. Most of them have a capacity of less than 1600KVA and a voltage of less than 10kV. Some of them have a voltage level of 35kV. The oil-immersed transformer can achieve all capacity from small to large, and the voltage level can also achieve all voltages. The UHV 1000kV test line under construction in China must use oil immersed transformer.

Insulation and heat dissipation are different

Dry-type transformers are generally resin insulated and cooled by natural air, and large capacity transformers are cooled by fans. The oil-immersed transformer is insulated by insulating oil, and the heat generated by the coil is transferred to the radiator (fin) of the transformer by the circulation of insulating oil in the transformer.

Applicable place

Dry-type transformers are mostly used in places that need “fire prevention and explosion-proof”, which are easy to be used in large buildings and high-rise buildings. The oil-immersed transformer may have oil spray or leakage after “accident”, resulting in fire. Most of them are used outdoors, and there are places where “accident oil pool” is excavated.

Different cost

For transformers with the same capacity, the purchase price of dry-type transformer is much higher than that of oil immersed transformer.

The model of dry-type transformer generally starts with SC (epoxy resin casting encapsulated type), SCR (non epoxy resin casting solid insulation encapsulated type) and SG (open type).

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