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Is it true that transformer's oil shouldn't be circulated (for cooling) cause it will change its properties and may cause a failure in isolation, if so does water cooled transformers has a special oil or two separate oils ( one in the core for isolation and one in the shell for cooling) ? ( the oil is cooled by heat exchanger by water )

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    \$\begingroup\$ Surely the purpose of the oil is to remove heat from the transformer core to the cooling fins by convection, ie circulation. It would be difficult to prevent circulation of some sort. Where have you read that this is a bad idea? Do you have a link? \$\endgroup\$ – Steve G Oct 25 '17 at 8:03
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    \$\begingroup\$ Power transformers generally circulate the oil by simple convection, with external fans blowing over the housing or radiator coils. The reason to not use any external circulation pumps is to avoid contamination of the oil from mechanical wear products. \$\endgroup\$ – R Drast Oct 25 '17 at 8:42
  • \$\begingroup\$ @steve a senior colleague told me so, I told him that i have seen a transformer with an oil pump and oil/water heat exchanger but he insisted that it doesn't exist and oil must be static \$\endgroup\$ – Chebhou Oct 25 '17 at 8:56
  • \$\begingroup\$ This is the transformer that I have seen specialtrasfo.com/rectifier%20transformers.html \$\endgroup\$ – Chebhou Oct 25 '17 at 8:59
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    \$\begingroup\$ In your linked datasheet the answer is given: "In larger units cooling is mostly obtained by forced circulation of either oil and water (OFWF) or oil and air (OFAF) in a cooler connected by oil pipes to the transformer tank." \$\endgroup\$ – Transistor Oct 25 '17 at 18:46
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Using a pump may cause cavitation in the oil, i.e. bubbles. Those could indeed significantly change the heat transfer properties of the oil. Contamination, metal filings, from the pump mechanism can also be problematic.

However, to say the oil does not circulate is an error.

In an Oil-Immersed Self cooled Transformer the physical construction of the plumbing causes the oil to circulate by convection.

enter image description here

Because hot oil is lighter than cold oil, it tends to rise inside the core. At the same time the oil in the tubes is cooled by the surrounding air and becomes heavier and falls, being replaced with the hot oil from the core. As such, when the transformer is working the oil continuously cycles through the transformer.

However, there is a limit to the effectiveness of this method and larger transformers do use a pumped oil system with a secondary water cooled radiator. Presumably, the cavitation effects are factored into these designs and sufficient filtering is used to contain contaminants. The other nice thing about this type is the radiator can be remote from the transformer.

enter image description here

Sources.

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  • \$\begingroup\$ Good answer. To add to this, the cooling mode of a transformer built to IEC / Australian standards is identified on the nameplate. "ONAN" is "oil natural, air natural" i.e. passively cooled. "ONAF" is "oil natural, air forced" i.e. cooling fans. "ODAF" is "oil driven, air forced" i.e. oil is pumped through the radiators, AND there are cooling fans on those radiators. \$\endgroup\$ – Li-aung Yip Nov 30 '17 at 14:08
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    \$\begingroup\$ Note - transformers with a driven-oil cooling system (i.e. oil pumps) don't necessarily have a water cooling system. Most power transformers I've dealt with have radiators for air-cooling. \$\endgroup\$ – Li-aung Yip Nov 30 '17 at 14:11

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