Transformer cooling

[Designmen90]

Hello everyone,
when we talk about power transformers and their cooling we have fans and oil pumps. As I can realize, the transformer is at first cooled by fans and if the fans are not enough, the pumps starts. Am I right? Now, is there two pumps for both direction? One for drawing the warm oil from transformer and second for injecting the cooler oil? Well, if pumps works, there is oil flow indicator, too. Is this indicator activated when the process of cooling is done or in some other situation?

 

[David Prosser]

There are lots of different ways to cool transformers. Most transformers on my networks are oil cooled in air with a few oil and water. I've not come across any that have needed fans or pumps but they are available.

Based on the coolant used the cooling methods can be classified into:

1. Air cooling
2. Oil and Air cooling
3. Oil and Water cooling

1. Air cooling (Dry type transformers)

• Air Natural(AN)
• Air Blast (AB)

2. Oil cooling (Oil immersed transformers)

• Oil Natural Air Natural (ONAN)
• Oil Natural Air Forced (ONAF)
• Oil Forced Air Natural (OFAN)
• Oil Forced Air Forced (OFAF)

3. Oil and Water cooling (For capacity more than 30MVA)

• Oil Natural Water Forced (ONWF)
• Oil Forced Water Forced (OFWF)

 

[Moley]

Most of my work is on transformers at power stations. You have various types of cooling, mainly rads with no fans or pumps (ONAN), rads with fans and no pumps (ONAF) and rads with fans and with pumps (OFAF). On a normal generator Tx you would have OFAF, Oil Forced (pumped) and Air Forced (fans). These are controled by WTIs (Winding Temperature Indicators) and OTIs (Oil Temperature Indicators), both of which have probes in 'pockets' on the Tx which then operate a dial which in turn operate either micro switches or mercury switches which control the pumps/fans. These WTI/OTI units can have 6 settings, fans on & off, pumps on and off, alarm and trip. On larger Txs you will have two banks of radiators, one on each of the longer sides of the Tx. Each bank of rads will have fans and two pumps. Normall only one pump is used. the second being a spare incase the first fails. The fans are normally wired to all run together (although with a timed start so that they don't all try to start at once). The oil flow indicators are just that, indicators to show a pump is running. They serve no other purpose apart from flagging a fault if a pump is supposed to be running.
So, the probes in the tank detect the Tx temperature which operate the WTI/OTI units. If the temperature is low then no pumps or fans need to run. Once the temp rises above the temperature set it'll turn on fans then pumps. If it gets too high it'll send an alarm to the control room. If it gets too high it can trip the unit out.

 

[Designmen90]

Normall only one pump is used. the second being a spare incase the first fails.

Thanks for nice answer.
If one pump is used, where the warm oil goes and how? I suppose that the pump "pumps" the cooler oil into transformer, but what is with warm oil which are already in transformer?

 

[Julie.]

Thanks for nice answer.
If one pump is used, where the warm oil goes and how? I suppose that the pump "pumps" the cooler oil into transformer, but what is with warm oil which are already in transformer?

It's like your car radiator, depending on the size of the transformer, the oil is pumped from the transformer enclosure to a large radiator - where it is cooled naturally, or via forced air fans, then returns to the transformer enclosure once cooled as a closed loop.

Lots of different systems, but basically if it's an oil insulated transformer, the hot oil circulates naturally or forced, and is cooled either naturally or forced

 

[Moley]

It just circulates. It goes from the transformer into the radiators then goes back into the transformer tank. It's the same oil just going around and around.

 

[Designmen90]

It just circulates. It goes from the transformer into the radiators then goes back into the transformer tank. It's the same oil just going around and around.

So this one pump is there just to make the this (around and around) proces faster? But if this proces is fast, will this oil have enough time to cool?

 

[David Prosser]

So this one pump is there just to make the this (around and around) process faster? But if this process is fast, will this oil have enough time to cool?

If it's been designed properly then yes.

 

[Simon47]

If the transformer is oil cooled the the first heat transfer us from the winding to the oil. Warm oil is less dense than cool oil, so natural convection can circulate the oil - if possible/practical the designer will aim for natural convection as that avoids all the reliability/maintenance issues that go with moving parts.
On very small transformers, some fins on the outside of the case are enought to cool it. Warm oil will rise off the windings, spread out, and as it cools by contact with the case, it will flow down and back into the bottom of the windings.
Next step up would have loops of pipe the the oil flows down through - increased surface area.
Go bigger still and youvstart to see radiators - with things arranged so the oil will still circulate by natural convection. Again, if a passive setup is practical then no pumps or fans to minimise maintenance.
The downside to a passive setup is size - larger everything basically. If space (or other factors) doesn't permit, then pumps and/fans will be used. Cooling is still the same - heat in the windings transfers to oil, then heat in the oul transfers to air or water, it's just that fluid movement is mechanically boosted.
As for how a pump operates. If you pump oil into one end of a pipe - then it has to come out the other end. So uf the pump is (say) pushing warm oil down a pipe and into a radiator - the that oil will come out the other end of the radiator and down the other pipe back to the transformer tank. So inly one pump needed - but there may well be two (side by side) for redundancy.

 

[pc1966]

The passive approach is known as thermosiphon and you see it in home hot water systems (before combi boilers typically the pump was only for central heating radiators, and thermosiphon for the heat exchange to the hot water tank), or in some very old cars engine cooling radiator that did not use a water pump.

 

[Julie.]

The passive approach is known as thermosiphon and you see it in home hot water systems (before combi boilers typically the pump was only for central heating radiators, and thermosiphon for the heat exchange to the hot water tank), or in some very old cars engine cooling radiator that did not use a water pump.

It used to fascinate me as a child, the fireplace/range in the main living room, had the hotplate to one side, oven to the other, and a small tank right at the back, and loads of levers; shift them and the heat was directed behind each of them to a greater or lesser extent.

The hotplate and oven made sense, but I could never work out how the small tank actually heated the main hot water cylinder upstairs!

Obviously as I got older, it changed from childhood fascination to complete hatred!

Cleaning all the brass, and blacking the range seemingly every weekend!!!!

 

[R-fur]

It used to fascinate me as a child, the fireplace/range in the main living room, had the hotplate to one side, oven to the other, and a small tank right at the back, and loads of levers; shift them and the heat was directed behind each of them to a greater or lesser extent.

I remember my gran having one of those when I was a kid, she used to bake in it. My last house had one in the kitchen, I never tried baking in it but the swing out trivets were great for keeping coffee warm and the oven was good for drying boots.

Oil cooling (Oil immersed transformers)

• Oil Natural Air Natural (ONAN)
• Oil Natural Air Forced (ONAF)
• Oil Forced Air Natural (OFAN)
• Oil Forced Air Forced (OFAF)

I agree with David, with the addition that you sometimes see L rather than O as in LNAN the L stands for liquid which is usually silicon or mydel.

As others have said the oil circulates naturally by thermosiphon, the oil heats up in the transformer and rises, then in the radiator it cools and falls. Often if there is a pump fitted it pumps the oil in the opposite direction to the natural flow. Apparently this is more efficient but I have never understood how.