Yes; possible.

Other than re-rating the temperature rise, fans are probably the easiest and cheapest.
You get a lot of redundancy as well.

Bill
--------------------
"Why not the best?"
Jimmy Carter

@PRC: Tell me what you know! :)

In hydro plants, a power transformer often is cooled with a water heat exchanger.

I've heard of that- its a neat idea.

And it works very well indeed.

Added thought via edit: absent an existing cooling tower or cooling water system already present for other purposes, however, you'd just be incurring needless extra expense and complication...

As per IEEE, transformers are designed for ONAN rating and then upgraded for ONAF by putting up fans.In IEC world, transformer is designed for ONAF rating and de-rated with out fan cooling. Definitely fan cooling is economical than adding more or bigger radiators.But better contact OEM before such over loading. It is not only cooler,but many other parts of transformers are to be checked whether designed for such overloads.

Any idea by how much larger radiators help?

Larger radiator won't help much. Design of natural cooling between winding and oil as well as between oil and ambient have to match each other.

I agree with electricuwe; larger rads will only be of limited help since they are subject to the law of diminishing returns, since a lower rad bottom temp will only increase natural oil flow very slightly.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

Cheapest method would be to use thermal couples and IEEE equations to determine how much capacity the transformer really has. There are utilities that do dynamic loading calculations. There are co-ops that run their transformers harder on cold days without any calcs. The nature of the load can let you ride it harder as well. A transformer has a lot of oil that has a lot of thermal intertia. If you are just trying to run out a peak, you might not have to do anything more than just be aware that you can't handle a certain loading for longer than X hours. If you contact the manufacture, they probably rated the transformer really conservative with like 110 F ambient temp. Some utilities undersize their transformers for their peak and live with loss of life due to overloading it because according to their money calculations, that is the cheapest method and a lot of times transformers are changed out to growing load and not because they reached their end of life.

Mbrooke,

You want to avoid fans, are you also keen to avoid forced oil circulation using pumps?

From noise perspective I'm ok with forced oil pumps. However the industry is moving away from them due to oil contamination. If someone can offer pumps that won't do so I'll take them.

If noise is your issue with fans, have you looked into low noise fans? We purchased an ONAF transformer with a sound level of only 50 dB for a substation very closed to a residential area. By comparison, similarly sized transformers seem to run 57 dB to 65 dB on the ONAN rating. If you need just a small increase rather than the typical the typical 33/66% increase, mounting just a few very low speed fans may be feasible.

Any good low noise fans out there which you might recommend?

1) Fan cooling also increases cooling only by reducing the bottom oil temperature going back in to tank. Fan cooling is recommended from cost angle. There are users who ask for 100 MVA units only with ONAN cooling.
2)Yes.ONAF is to be preferred over ODAF (oil pump &fans).My factory regularly supply 500MVA 400 kV 3 phase autos &1500 MVA 765 kV Auto Banks with only ONAN/ONAF cooling.
3) Special low noise transformer cooling fans are made by almost all reputed makers. If you mention the area where you are looking, I can suggest makes. But better leave these to transformer makers who can do professional job
4) DM 61850,sorry to say, your idea is not safe. Any wet, old transformer, if overloaded can result in failure by bubble formation from winding.

Those 100MVA ONAN only units- how are they designed differently if it all? Are they basically a 100 ONAN / 140MVA ONAF unit without fans? Or an 65MVA core designed around a system that can cool without fans?

As I mentioned earlier, in IEC world we design units for max rating. In fact, there is no difference in active part between a 100 MVA ONAN unit or 100 MVA ONAF unit. With ONAF unit, we will provide less number of radiators and put fan to increase heat transfer from radiators to air. This will bring down bottom oil temperature further, increasing thermal head for cooling inside oil.

Does the 100MVA ONAN have a smaller core than the 100MVA ONAF is my question? And if so by how much typically?

No change as per IEC standards. As per, IEEE, slight increase ( may or may not) in core dia with ONAN rating.

So as I see it, its possible to squeeze 60MVA out of a 40MVA core just by increasing the radiator size?

PRC,

It is done and done regularly with some industrial customers and utilities. The manufacture's rating is conservative and it is not unusual to rate transformers dynamically based on the weather conditions. People do studies to try to determine how much slack they have based on present oil temperature, ambient temperature, and things like acceptable loss of life

.

Here is a study by USBR.

https://www.usbr.gov/power/data/fist/fist1_5/vol1-...

I found this:




It seems like North American standards require a 133% safety factor?


I have to find the other report, but I know for a fact some utilities will overload 50 and 60MVA transformers for the loss of units on the basis the overload will last a few hours and anticipating a 1% life reduction. I believe the number is 125% but have to find out.


One question I have for PRC: if I order a 100MVA ONAN only unit- will fans being added later make a difference and by how much? Basically what I'm asking is if an ONAN only unit is simply a OA/FA/FA2 trafo just without FA/FA2 listed and perhaps slightly larger radiators for a maybe 2% increase?

We purchased the low noise fans directly from the transformer manufacturer, so I do not have any specific recommendations.

No, it is not as simple as adding fans. As an example, we have one transformer that has a 76C winding rise on the 400 MVA ONAF2 rating but only a 58C winding rise on the 240 MVA ONAN rating. Manufacturers model all three stages of cooling to find out which case has the hottest temperatures.

As a side note, the factory testing is the only time an ONAN/ONAF/ONAF transformer operates at the ONAN design temperatures. In actual operation, the fans will turn on well before the ONAN rating is reached.

Why would an ONAF2 allow for higher winding temps than ONAN? Wouldn't it always be 76*C and fans/oil circulation/reduced loading/ect to make sure the wingdings never go above 76*C?

1) USBR document is a 30 year old document and IEEE/IEC revised Transformer Loading guides several times - latest IEC 60076-7-2018 & IEEE C57.91-2011.

2) If you order 100 MVA transformer, you will get only 100 MVA unit. Automatically you will not get overloading capacity. Of course you can over load transformers for short periods as indicated in above guides, but with loss of life.

3) It was experimentally found out that life of paper( ie tensile strength halves) is 20.5 years with a moisture level of 0.5 % for a continuous temperature of 98C or 110 C (with thermally upgraded paper) With every 6 C rise above these hot spots, life halves ie at with 108C it will be 10.25 years.

4) In service, paper absorbs moisture and with 1.5 % moisture and at 98 C also, life halves ie 10.25 years. With increased moisture, threshold temperature to start bubble release also comes down.ie overloading limit comes down.

5) Manufacturers(self being one for more than 50 years!)are not conservative or building up any extra margins during these days of cut throat competition.

6) Better to consult OEM before planning any continuous overloading to avoid premature failure of your costly transformer. It may or may not have overload capacity with increased cooler capacity.

7) OEMS design transformers for the same hot spot at various stages of cooling. But depending on various parameters, it can vary and for setting up WTI, maximum hot spot temperature is selected.

Will a 100MVA ONAN be heavier or larger than a 60/80/100MVA OA/FA/FA2 unit? And if so by how much? Thats where my curiosity lays. I've never seen an ONAN unit besides small ones 2.5MVA and under.

IN IEC world, same weight. In IEEE may be marginally heavier.

How much do the fans and supports weigh?

Bill
--------------------
"Why not the best?"
Jimmy Carter

Thanks PRC!

So errrr, can I ask a rather DIY question... Please forgive me... But does that mean I can load a 40/50/60MVA IEEE/ANSI transformer to 50 or 60MVA without fans?

Forgive me- the theory behind this is fascinating.

No you cannot. It only means, the transformer is designed to take a load of 50 & 60 MVA. It does not mean you can load without adequate loading.

That means IEC transformer burns (runs) hotter?

One transformer.

Two sets of numbers, one per IEEE and one per IEC.

The IEEE set of numbers will have both a lower MVA rating and a lower per unit impedance on the transformer base. The IEC numbers for MVA and impedance will both be higher.

Convert from one base to the other, or some third base, and you'll find the same they're the same. To get to the IEC base rating the fans have to be used. To get to the IEEE base rating you don't need to use the fans, but to get to the IEEE top rating, same as the IEC base rating, the fans are required.

Basically the IEC just ignores the lower ratings only listing the IEEE top rating? I know they do that with impedance- but with loading I'm still a bit murky.

As I understand IEEE/ANSI vs the IEC the IEEE standards force a larger core for the same base rating. Could be wrong though.

As David said; Same transformer.
The same transformer has the same core.
The IEC base rating assumes that all fans are installed and are in use.

Bill
--------------------
"Why not the best?"
Jimmy Carter

Let us quote from standard:
C57.12.10 - Recommended impedance for a 50/66.67/83.3 MVA 132 kV 550 BIL is 9.5 % with OLTC at 50 MVA base ie at 80 MVA, impedance will be 15.2 %
IEC 60076-5 - Recommended minimum impedance for a 50/80 MVA unit - 12.5 % on 80 MVA base. ie at 50 MVA 7.8 % impedance
In above case, IEEE unit will be lighter due to higher impedance ( less core, more copper); but if IEC unit were ordered with 15 % impedance at 80 MVA, active part weight will be almost same for both units.

@Waross yes, but I'm thinking a base rating based on fans. ONAN only.

@PRC- interesting perspective! Can IEC units tolerate a 133% overload for all ratings for several hours with a slight life reduction only?

The heat run tests I have seen indicate that OEMS do not design for the same hot spot temperature on each rating. For the 240/320/400 MVA example I mentioned above, the fans only added about 25/50 percent to the rating. The OEM seemed to size the cooling system for the worst case temperature rise, which was the ONAF2 rating. For the ONAN and ONAF1 ratings, the OEM just followed the IEEE requirements that the ratings be exactly 240/320/400 and the temperature rise be less than 65C/80C on each of the ratings.

Note that OEM has a strong incentive to be a little bit conservative in the thermal modeling because if the transformer exceeded the allowable rise, the transformer would have been rejected. After doing a heat run test at the factory, we asked for additional ratings to be calculated per IEEE C57.91 at various ambient temperatures ranging from 30C to 0C. Based on actual heat run data, the resulting ratings under nominal IEEE conditions of 30C were approximately 280/350/410 MVA. If we had asked for a self cooled 240 MVA transformer, it might have been more than 15% smaller.

Loss evaluation can also play into the size of core & coils versus how many radiators/fans are required. If losses are highly priced, the core/coils will be larger and less heat will need to be dissipated. Also, the weight difference between various bidders on the job often exceeds 15%.

Quote:

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.

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.

I mean 15% smaller.

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?

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.

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.

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.

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:

Quote (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?

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.

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

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

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)

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).

Quote (JASGripen)

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

Quote (prc)

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