To improve cooling for the power transformer, its capacity will be increased. Is it true?

[WSO]

I have discussed this issue with an engineer about this topic. I think that to improve the efficiency of cooling system for the power transformer should be expand the thermal limit and allow for overloaded the transformer. So, the engineer ask me for the references. How to explain him?

 

[collies99]

Transformers are already factory tested and its design are not not conducive for firld modification.

Transgormer need to operate within the design limits or basically what is stamped on the nameplate. Operations outside these guideline will either compromise transformer life and insulation system integrity.

many books on power transformer, please peruse st least one.

 

[ScottyUK]

That's not strictly true - plenty of transformers have been uprated by adding larger cooler banks. Gaining 10% - 20% of rating is fairly typical. I'm not exactly sure what the original question was though.

 

[WSO]

Some customers ask engineer to reduce cost by using smaller capacity with improving cooling system instead.

I think it is incorrect ideas.

 

[waross]

Many transformers have a natural air rating and a forced air rating. A very few also have a forced oil rating but let's ignore these few for now. Some transformers are purchased without fans and run at their ONAN (Oil-natural, air-natural) rating with the intent to upgrade in the future to the ONAF rating (Oil-natural, air-forced).
Some transformers are sized and purchased based on their ONAN rating and never upgraded.
Some transformers may be purchased based on their ONAF rating with a full compliment of cooling fans.
Some transformers may be purchased based on the ONAN rating but upgraded to ONAF in the future by the addition of fans to accommodate increased loads.
An ONAF transformer will generally have greater losses than an ONAN transformer with the same load. You can get the specs of the transformers under discussion and with the local energy tariffs calculate an increased dollar value for operation of an ONAF transformer compared to an ONAN transformer.
Many transformers have a dual, ONAN/ONAF rating. If a transformer does not have an ONAF rating then it may be a code violation in many jurisdictions to overload it regardless of increased cooling.

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

 

[cranky108]

Sort of. If you increase the cooling of the oil it will increase the capacity some. The issue is the oils ability to carry heat away from the transformer core. And at some spot near the core the oil dosen't circulate very well, so you have a hot spot that can fail if it's cooling is not increased. So by adding additional cooling on the out side you increase the capacity some.

Try this, add a soaker hose on the outside of the transformer.

 

[ScottyUK]

If you can face the additional complexity - and can engage the OEM to assist - then provision of forced circulation will generally allow greater increase in rating than adding larger radiators and/or adding fans because the limiting factor in removing heat quickly become the convection-driven oil loop. I personally wouldn't attempt to add forced oil circulation without the involvement of the OEM.

 

[VTer]

It seems that we all agree that additional cooling does help in increasing capacity. The very difficult thing to do is to determine how much additional capacity forced air cooling will provide if it is not specified on the nameplate as Future ONAF rating and if the OEM is no consulted. In either case I would not try continuously loading the transformer over its ONAN rating without consulting with OEM.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[marks1080]

Speaking very generally....

When we talk about overloading a transformer what we are really talking about is how hot the unit gets. Heat and heat dissapation is the single most important factor when talking about keeping a transformer healthy. Suppliers will give you nameplate ratings but they will not help you determine (at least in my experience) how much the unti can be overloaded and for how long.

Practically, if there were a way to super-cool a transformer you could go way past its MVA rating. The heat is what damages insulation.

Most utilities have different ratings based on time. For example, if a transformer is rated for 100MVA, the utility may allow the unit to run at 200MVA for a very short duration (say 10 minutes). A unit may have a continuous rating, a 10 day rating, a 2 day rating, a 10 hour rating, 2 hours rating, 15 minute rating, 2 minute rating, etc... These ratings may be above any beyond the manfacturers recomendations.

It's tricky business what you are talking about doing. To do it properly, you really want to use some sophesticated software to calculate all the variables. If you screw up it might mean blowing up a transformer.

If this is something you think you might have to eventually look into get professional help. If you are unsure about what you are doing it would probably be a mistake to attempt to change the units rating.

 

[cranky108]

Have you consulted the IEEE transformer short term overload tables? Basically if you are only overloading it for less than 8 hours, you do have some flexability without adding any additional cooling.

 

[ptcabb]

Most utilities have different ratings based on time. For example, if a transformer is rated for 100MVA, the utility may allow the unit to run at 200MVA for a very short duration (say 10 minutes). A unit may have a continuous rating, a 10 day rating, a 2 day rating, a 10 hour rating, 2 hours rating, 15 minute rating, 2 minute rating, etc... These ratings may be above any beyond the manfacturers recomendations.

True from a thermal point of view, it's about thermal mass, environmental conditions, the pre fault / overload temperature and duration of the overload. Even so, treating the transformer as an uniform object that heats and cools down as a block is at best incorrect (and incidentally a reason I2t thermal overload protection functions are largely pointless on larger (>5MVA) units..) Like mentioned before, it's about the hot spots and even if the insulation at the hot spots is able to handle the raised temperature, the insulation experiences forced aging and you'll reduce the life expectancy of your transformer.

More interestingly (to me at least), even if you'd over rate your transformer for short or medium term applications, how would you handle the protection system around it? With smaller transformers that are fused off, the fuse ratings would become an issue. A little larger units without differential protection are usually part of selectivity schemes, so even if you'd increase protection margins by using for example different setting groups, it could have a big influence on up- and downstream protection schemes. And once you get to the units with differential protection, the additional overload will increase transformer losses quite substantially, leading to an increased differential current. Your bias/restrain curve could compensate for that, at a cost of desensitizing the protection function. And the chance for failure is just in such a situation where insulation breakdown might occur. No way you'd be able to detect inter turn winding faults before they escalate into phase to phase/ground faults.

 

[veritas]

Two further points:

1. The OEM's ratings are usually based on certain standard conditions as per some applicable standard such as IEC 76-6, e.g. ambient daytime temperature of 40 degrees C. If your temperature is substantially below this then there is scope of some degree of OL for certain periods of time. After all night times are better than day times and winter more suited than summer to allow OL's.

2. Couple of years ago I came across a formula that qualified the loss in trfr life span due to loading above the OEM's decalred rating. Forgot the formula but I remember that if increased to 10% above rating more than 50% of the time, the lifespan was only reduced by a few days (not more than a month) over a 20 year period

But these are generalisations and I agree with many of the comments above that hotspots play and important role, the involvement of the OEM is recommended, knowing to which standard the trfr has been manufactured, how much OL is desired and when - time of day, nature of OL, i.e. cyclic, constant, intermittant, etc.

 

[collies99]

I think it is 10 degrees C above hot spot temperature for North American built transformer, based on IEEE rating class and built to NEMA standard ?????

 

[bacon4life]

WSO
C57.91 IEEE Guide for Loading Mineral-Oil-Immersed Transformer provides guidance and formulas for calculating the expected lifespan for transformers at various loadings. The lifespan of paper is cut in half for every 8 degree C increase in temperature, hence the desire to limit high temperatures to only short periods. Correspondingly, the lifespan of the paper doubles for every 8 C decrease in temperature. Unfortunately transformers operated much colder than nameplate won't last forever, other components fail instead.

There is nothing inherently wrong with using a smaller transformer with significant cooling as long as the owner evaluates all the factors including the shorter lifetime, the cost of I^2*r losses, and the risks of a cooling system failure.

ptcabb
How can I^2*R losses impact a current differential relay? Usually transformer differential have margins between 20 and 60% to account for CT ratio error, LTC error and CT saturation. Even looking at real power instead of current, transformers are usually more than 99% efficient.

 

[mikekilroy]

Does no one trust the mfgr's installed thermostats? Of course if they do not exist....

Have you consulted the IEEE transformer short term overload tables? Basically if you are only overloading it for less than 8 hours, you do have some flexability without adding any additional cooling. BE CAREFUL with such generalizations..... as others have correctly stated, thermal mass is different for different size xfmrs: From my xfmr design experience, I would swag an 8 hour thermal time constant to be about a 400kva size: smaller = shorter, larger = longer.

Like all thermal limited devices, there are 2 ways to exceed this limit: FAST and SLOW.

Fast is like 10-20x rating so it heats so fast a hot spot causes failure before a thermostat or thermistor can react due the thermal mass and location.

Slow is like 1.2 or 2x rating so it heats slow enough for the mass to heat evenly, allowing a well placed thermal device to catch it.

So, I assume this thread was about how much one can push a thermal device (xfmr) over long term, so slow; if it designed for 100%, then it should not exceed thermal switch at this level. If one needs to push it, why not trust the thermal device? Add your fans, cooling, etc., and load it 1.2-2-3x rating, and if thermal device says too hot, you found your answer? My gut feel on xfmrs like we build in 10-500kva range is if you blow a lot of cool air over a xfmr it may increase capacity 20-30%

 

[DanEE]

When a transformer is spec'd by the manufacturer, in addition to cooling / winding IR & core hysteresis/eddy heating considerations, how much margin is designed into the magnetic core.. i.e. on overload at what point does the core begin to saturate?

 

[ScottyUK]

Core flux density is primarily a function of system voltage, not load current, so increasing the current won't significantly alter the state of the core magnetics. Modern core design is usually optimised by computer modelling so there often isn't a lot of margin in the core design.

 

[veritas]

Of course the maximum OL is limited to what the protection has been set to. My philosophy is usually that the protection must allow maximum loading of the trfr + some margin. Maximum loading is taken as highest declared rating. Typically, I would set the OC relay to 150% of rating. Trfr is allowed to be overloaded to 130% of maximum rating for short periods. With the older EM relays, reset ratios of 0.9 are common so a setting of 144% of rating was required, thus the 150%. Reset ratio of modern relays are much closer to 1 but I guess the 150% settings has stuck with me.

This is my standard approach in the abscence of specific directives from others.

This is of course based on the premise that trfr loading is an operator and not protection issue. Protection should only come into play for overcurrent and not overload conditions. Overloading should be taken care of by trfr guards such as winding and oil temperature.

Sorry for digressing from the main focus of this thread but I thought this an interesting slant to it all.

 

[prc]

Unfortunately the question itself is not clear."To improve the efficiency of cooling system for the power transformer should be expand the thermal limit and allow for overloaded the transformer. So, the engineer ask me for the references. How to explain?" What is the intention? To overload transformer or improve the cooling efficiency?

As far as over loading is concerned ( say cyclic overloading,emergency over loading or overloading to take advantage of lower ambient etc) standards are very clear and give adequate guide lines. The following are the standards on Overloading of Transformers to refer to:
Oil Immersed Transformers- IEC 60076-7 ed1.0(2005-12)/ IEEE C57.91-2011 Dry type Transformers- IEC 60076-12ed1.0(2008-11)/C57.96-1999.

Old Arhenius law of ageing factor of 2 for every 8C change in hot spot temperature has been changed. Today IEC follow 6C and IEEE 7C for an ageing factor of 2.The latest position on various aspects of overloading are covered in above standards, with a detailed bibliography.The standards also give extent of permissible overloadings for different types of transformers with loss of life.

It should be remembered that standards give overloading guidelines based on a dry paper insulation. But old transformers will contain a lot of water in paper and this will bring down permissible hot spot temperature drastically.With higher moisture content, ageing will be much faster and chances of dielectric failure with bubble formation is more.So it is prudent to avoid overloading of old transformers with out a thorough dryout.

Regarding overloading the magnetic circuit- that may happen with change in V/f of system or inappropriate tap changing and not with overloading.The general understanding today is that with a flux density of 1.72T at rated voltage and frequency, transformers are suitable for 110 % continuous overfluxing,125 % for one minute and 140 % for 5 seconds.

 

[ScottyUK]

prc,

You might add IEC 60354:1991 Loading guide for oil-immersed power transformers to your list of references.