Recently I was studying that for 3 Transformer each of 100 MVA, 500 kV/132 kV transformer, three different cooling methods are being used:

  1. For first one we use Force Oil cooled
  2. Second uses Self-oil cooled
  3. 3rd one uses Forced air cooled The content said that forced oil cooled transformer is heavier than force air cooled and self-oil cooled is the lightest one. Why is it so?
  • \$\begingroup\$ Compare oil weight to air weight and report back your findings. \$\endgroup\$ – winny Jun 9 '17 at 18:36
  • \$\begingroup\$ It because of the exponential ratio of volume to surface. It's also why your ears are small whilst an elephant's are large. \$\endgroup\$ – Paul Uszak Jun 10 '17 at 12:44

This big transformers tend to be oil immersed. This oil is mainly used as an insulator, but also helps to cool the transformer. The oil insulates, supresses corona effects and is also the coolant.

In self-oil cooled there's ussually a thermosiphon effect. The hot oil goes up and is circulated through the radiator with no additional parts due to a siphon effect.

Air force-cooling would add a fan to the radiator previously mentioned, but the oil stil circulates through siphoning. So it will be heavier than previously because we have extra parts.

Force oil cooled means that we do not expect the siphon effect to circulate the oil but we add an oil-pump. This oil is then ussually cooled through an oil/water heat exchanger. This means it's heavier still than before with the pump, heat exchanger, etc.

The heavier, the more cooling power you have in this case (and more expensive as well)

Hope it helps.

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    \$\begingroup\$ That's being too simplistic and indicates the transformer designer did not do a good job. If you use forced air cooling the radiator does not need as much surface area as with convection cooled - this could reduce weight. The transformer itself could also be designed to be smaller since it has better cooling and so also be lower in weight. A similar argument applies to forced oil circulation. Early car engines used natural circulation but required huge radiators and large amounts of coolant - modern ones can achieve the same cooling with much less weight overall. \$\endgroup\$ – Kevin White Jun 9 '17 at 20:00
  • \$\begingroup\$ Transformers are not cars. And these radiators are ussually more like fins. Transformer size of this amount of power are not mandated by cooling requirements. The size is determined by other factors. The size of the cooling tank is what could vary. \$\endgroup\$ – Andrés Jun 9 '17 at 20:07
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    \$\begingroup\$ Why did the designer decide they needed forced cooling then if natural convection would do? If natural convection wasn't adequate then the fins were not large enough! \$\endgroup\$ – Kevin White Jun 9 '17 at 20:09
  • \$\begingroup\$ @KevinWhite: Because adding forced cooling is a cheap way to get 20-30% extra capacity out of a transformer without increasing the size of the windings or core. Why wouldn't the designer do it? \$\endgroup\$ – Li-aung Yip Jun 10 '17 at 11:54
  • \$\begingroup\$ @Andrés has the correct answer here. I've added more details in my answer below, including references to standards, and some typical ratings/sizes. \$\endgroup\$ – Li-aung Yip Jun 10 '17 at 12:09

Oil-filled transformers can be cooled in three ways:

  • ONAN: Oil Natural, Air Natural. The transformer is filled with oil, which circulates naturally by the "thermosiphon" effect. Air circulates naturally, by convection, over the tank and cooling fins.
  • ONAF: Oil Natural, Air Forced. The transformer oil circulates naturally, and a fan is provided to blow air over the radiator fins. The fan is controlled by a thermostat, and the fan turns on when the oil/winding temperature rises above some threshold, probably around 70C. This typically extends the transformer's thermal capacity about 20%.
  • ODAF: Oil Driven, Air Forced. The transformer oil is circulated by a pump - again, controlled by a thermostat. A radiator fan is also provided, again, controlled by a thermostat.

(The codes ONAN, ONAF, ODAF have standard meanings given in IEC / AS 60076.2.) enter image description here

Small transformers, such as those found on pole-tops and in kiosk substations, tend to be ONAN. The transformer has no active cooling equipment, which means there's no fans or pumps to maintain out in the field.

Medium transformers in the 1 MVA - 20 MVA range tend to have cooling fans. A typical rating is 10/12.5 MVA, ONAN/ONAF - you get an extra 2.5 MVA capacity if the fans are turned on. The extra maintenance is no problem, because these are located at substations, which are convenient to work at.

Larger transformers in the 25 MVA+ range tend to have both fans and oil pumps. A typical rating is 36/48/60 MVA ONAN/ONAF/ODAF - you get an extra 12 MVA capacity if the fans are turned on (ONAF), and an additional 12 MVA if both the fans and oil pumps are on (ODAF).

The reason for the difference in weight is at least partly due to the extra weight of the fans and pumps themselves.

I'd also hazard a guess that the iron cores have to be bigger to handle the increased magnetic flux required to transfer the additional MVA's of power. The size of the iron core is dictated by the required power capacity, not cooling considerations.

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