Increasing MVA Without Fans 7

[Mbrooke]

Is there such a thing as a transformer's MVA being increased past the ONAN rating without fans- such as making the radiators much larger?

 

[Mbrooke]

If we had asked for a self cooled 240 MVA transformer

DUDE! Thats neat. Wish I knew this earlier in life.

So being 15% smaller, the unit gets hotter but no loss of life?

Forgive me for the cyclic nature- but this goes against everything I thought I knew.

 

[prc]

1) The overload capability of IEC transformers is listed in loading guide IEC 60076-7 ed2.0-2018 for cyclic over loading, short time overloading etc. Loss of life will not be "slight reduction" but massive depending on the magnitude of overloading. Please refer to the standards for the detailed explanation. If the transformer is wet, it can be a sure ticket for failure

2) In case regular overloading is desired by user, usually it will be expressed by user and OEM design accordingly. For example, track side supply transformers are loaded 50 % and up to 100 % for short periods. So spec call for 50 % overloading for 15 minutes and 100 % for 5 minutes with slight increase in hot spot temperatures. OEM design for this overload, test such overloads at factory. Solar transformers are also specified with slight overloading part of the time in a day.

3) No,240 MVA ONAN transformer will not have been 15 % less costly or of less weighing than 280 MVA. It reflects the cooler capacity only.

 

[Mbrooke]

I mean 15% smaller.

 

[Golfex]

Good discussion here about transformer sizing.
I have another question regarding the fans and its corrosion when installed in marine environment (offshore platforms).

Is there any solution of fans with low maintenance and a long lifetime?

 

[prc]

Yes.Reputed transformer fan makers have fans suitable for such duty. With stainless steel or polymer fan blades and extra surface treatment to stand the corrosion.

 

[bacon4life]

Mbrooke- Unfortunately, there is no such thing as "no loss of life." The transformer is always aging. At high temperature, the transformer will eventually fail from thermal decomposition. At lower temperatures, the thermal degradation continues at a very low rate. The transformer will instead fail from conditions such as rust, corrosion, obsolesce of accessories, through faults, vibration, leaks, and moisture. For my particular example per IEEE C57.91, operating at 58C rise instead of 80C rise theoretically increases the transformer life by a factor of 10x to more 200 years.

PRC-If I were to start requiring a winding rise of 58C instead 80C when purchasing ONAN transformers, would the only additional cost be for a little larger cooler? I had assumed the OEM would make some pretty fundamental design changes to achieve such a low winding rise. Maybe something in our specifications have been unusual, and therefore drives a large difference in winding rise for ONAN versus ONAF2? I hope we can figure out how to reconcile my limited observations as customer with your extensive experience as an OEM.

 

[prc]

Why to reduce hot spot rise to 58C instead of 80C? Designers always try to reach the same hot spot rise for ONAN and ONAF cooling modes. Some times they fail to achieve that optimum target. True if you want a 60 MVA ONAN and 100 MVA ONAN, it is not cooler capacity alone, some other parameters are also adjusted, eg Current density, additional oil directing washers etc.

 

[argotier]

Very interesting thread. I can't really add nothing new to what prc already said, but I'll try to recap the basics to shed some light on the problem (hopefully).

Regarding the OP original question:

Is there such a thing as a transformer's MVA being increased past the ONAN rating without fans- such as making the radiators much larger?

It is only possible, without increasing the loss of life rate, if the transformer doesn't reach the windings temperature rises (medium and hot-spot) and the design limit is the top-oil temperature rise. This can be checked from heat run tests or manufacturer calculations.

Why?
Every transformer has its particulars and designing a balanced heat dissipation system that allows for simultaneous optimum temperature rises (for top-oil, medium winding and winding hot-spot) is very difficult. Usually only one of the three mentioned reaches its limit and therefore defines the design.

Why?
Simply put (too simplified I'm afraid) for a general ONAN/ONAF case, the two cooling modes work very differently.
In ONAN the heat transfer coefficient for the radiators is very poor and the oil flows within the transformer at a low rate. This means that the transformer works with relatively high oil temperatures and tends to be the design limit the top-oil rise (usually, but not always).
In ONAF mode, the radiator-air heat transfer improves greatly and the oil flow within the transformer also increases, lowering the working oil temperatures (compared to ONAN) and the windings (the main heat source) can be refrigerated more efficiently. BUT with the MVA increase, that the ONAF imposes, the windings temperatures start to increase greatly (always relatively speaking to ONAN) and tends to define the design in this mode.

The bottleneck of it all are the windings. For an already made unit, the windings will have a defined number and size of "cooling ducts" to refrigerate them from within. So, even if you can lower the oil temperatures with more radiators or attaching fans, the windings will get hotter and the "internal oil flow" wont be enough to keep them from exceed the limit temperature rises.

If someone is still awake after reading all this... hope it helped.

 

[Mbrooke]

Thats the answer I was seeking, thank you!!!! Love learning about this stuff.

 

[JASGripen]

Can the specs of the oil be changed to increase the MVA of the ONAN?

 

[prc]

1) There is no change in cooling duct or changes in internal flow with ONAN and ONAF cooling modes.

2)To understand the features better, let us take a specific case. Consider an ONAN transformer that manufacturer uprate to ONAF with 30 % more kVA.

3) Permissible Temperature rises are same for both modes ie as per IEC Top oil rise 60K, Average winding rise = 65 K and hot spot winding rise 78 K

4) Now let us see changes inside transformer with 130 % loading.Copper Losses increase by square 1.3 ie 1.69 times. So total loss ( core +copper losses) increase compared to ONAN rating will be 1.6 times

5) Top oil rises to 87 K for 1.6 times of earlier loss. Hot spot winding gradient ( temperature difference between winding hot spot and oil)goes up by 1.3 raised to 1.6 (=1.5 times)due to 30 % more current ie 1.5x 18= 27K ie hotspot rise goes up to 87+27=114 K from 78 K ( 60+18)earlier. So designer has to bring down the top oil temp by 36 K ( ie 51 K for oil ) to reach permissible limit of 78 K for winding hot spot This is done by partially adding more number of radiators and putting fans to blast air over the surface of radiators ( air flow increases heat dissipation by about 30-80 % ) Better cooling is not due to higher rate of oil flow through windings but more from heat convection from radiators that increases temperature drop from top to bottom of radiator.

6) By using esters you can allow more oil rise. But still winding hot spot will be a limit unless you change winding insulation to Nomex from kraft paper. In reality designers take a lower current density so that even at ONAN the winding temperature gradient over oil will be modest, much less than 18K (78-60)

 

[argotier]

You have two steps for the overall cooling:

Windings (Main heat source) --(1)--> Oil --(2)--> Air (through radiators).

Both will transfer heat mainly by convection as Q = h*A*ΔT (being Q: heat transferred; h: heat transfer coefficient; A: surface area in contact; ΔT: difference between high and low temperatures of the bodies in contact).

With ONAN and ONAF modes both h and ΔT will be modified, in both steps of the cooling, so it depends not only of the temperature of the oil but also its flow rate (as it changes h).

Can the specs of the oil be changed to increase the MVA of the ONAN?

6) By using esters you can allow more oil rise. But still winding hot spot will be a limit unless you change winding insulation to Nomex from kraft paper. In reality designers take a lower current density so that even at ONAN the winding temperature gradient over oil will be modest, much less than 18K (78-60)

Now that you mentioned, there is a way to increase MVA of a transformer using esters! (though the standards recommends for this case an agreement between manufacturer and user).

There are studies that suggests that the combination of esters and thermally upgraded paper results in an further upgrade of the papers thermal class, property that allows for transformers to work with higher temperature rises (both oil and winding). This can be used for: design smaller units at same MVA, increase an existing transformer power or to increase its life (but without increasing the temperature rises).

I dont know if this property is very used in practice.

Some info about it:

Link