Optimizing Transformer Capacity for Increased Loads

[bonobig]

Hello, I have a 1000 kva 13.8/400v transformer. I want to add additional loads to its 60% present load. The nameplate says max. ambient temp. is 55 deg. Cel. and max temp. rise is 50 deg. Cel. Can I load it to 90% continuous load after the additional capacities, and, is temp. rise equivalent to max. operating temp.?

Thanks,
bonobig

 

[bonobig]

I need help for the matter.

 

[Babadag]

Never the less 13.8kV it is North American practice since secondary is 400 V
in my opinion it is about a dry-type transformer designed according IEC standards. See: IEC 60076-12 /2008 Power Transformer Part 12: Loading guide for the dry-type power transformers chapter. 5.9 Loading equations
If the manufacturer standard is USA standard you may consult
IEEE Std C57.96/1999 [there is a 2013 revised edition]
IEEE Guide for Loading Dry-Type Distribution and Power Transformers
6.2 Equations for calculation of temperature, load, and loss of life
In order to calculate the temperature rise we need more information like actual hot-spot temperature, the ventilation system AN =natural AF=forced the actual ambient temperature and other.
Let’s hot-spot temperature for rated 1000 kVA will be 105 oC for 90%[900 kVA]
the hot-spot will be only 97oC if the ambient is the same for AN rated ventilation system.

 

[Asymptotic]

Hello, I have a 1000 kva 13.8/400v transformer. I want to add additional loads to its 60% present load. The nameplate says max. ambient temp. is 55 deg. Cel. and max temp. rise is 50 deg. Cel. Can I load it to 90% continuous load after the additional capacities, and, is temp. rise equivalent to max. operating temp.?

Thanks,
bonobig

What is the elevation above sea level? At high elevations derating is necessary.
How long has it been in service?
Is this an oil-filled transformer? If yes, is oil analysis performed regularly?
Is it equipped with a top oil temperature gauge?
If so, does the gauge have a peak temperature pointer? What is the maximum temperature? Reset the pointer, and check again after several days of operation. Record the maximum ambient air temperature over this period as well as transformer oil temperature..

It is best to perform a study to determine what the load actually is over the course of several days, but at first glance it ought to be possible to operate at 90%. The two main limiting factors are allowable insulation system temperature, and current ratings (not only of the windings, but also of the connection bushings and tap changer).

When operating close to rating it is a good idea to install temperature-controlled cooling fans on the heat exchanger fins. The rule of thumb is life is reduced by half for each 10°C rise in transformer and oil operating temperature.

Do a search on the term "oil filled transformer loading", and read up. These https://www.usbr.gov/power/data/fist/fist1_5/vol1-5.pdf from the US Dept. of the Interior are a good place to start. Also, if applicable, consult with the organization that does your transformer testing and oil analysis. They will be able to advise you based upon their experience and knowledge of your transformer's ratings and condition I suggest bringing in a professional engineer to investigate this question, and advise you further.

 

[Babadag]

Thank you Asymptotic. It is possible it is an oil-filled-or other liquid- transformer, of course. However, modern installation does not employ yet[usually] liquid filled transformers up to 2500 kVA.

 

[Asymptotic]

Thank you Asymptotic. It is possible it is an oil-filled-or other liquid- transformer, of course. However, modern installation does not employ yet[usually] liquid filled transformers up to 2500 kVA.

Record what is on the transformer nameplate. It will indicate dry or oil-filled, ventilation type, %Iz, winding BIL, and other essential data.

 

[jim mcnamara]

@jim hardy - do you see how to help, so far this may not be what the OP needs? ...or so it was reported to me.

 

[magoo]

To one of the initial questions of the OP:
Maximum temperature rise is an increase in temperature above the ambient. In your case, it would be a 50 degree rise (C) over a 55 C ambient or 105 C at full output or 1000 kVA. If you restrict the loading to 90% (or 900 kVA), the temperature rise will be less than 50 C.

 

[Tom.G]

If you restrict the loading to 90% (or 900 kVA), the temperature rise will be less than 50 C.

Assuming adequate cooling. Thar going to be many Watts of thermal energy to get rid of.

 

[magoo]

If the nameplate says that the transformer has a 50 C rise over a 55 C ambient, then the transformer and its cooling will be able to handle that at 100% load. If you don't have adequate cooling, then it won't. If the transformer meets the applicable standards, then the cooling system part of the transformer should be designed for the temperature rise and maximum ambient temperature on its nameplate.

For 1000 kVA, from my experience I would expect the transformer would be self cooled, i.e., it would have radiators of sufficient size to cool the transformer without the need for fans or pumps.

 

[jim hardy]

@Babadag and @Asymptotic have given good advice.

If i understand the questions

Can I load it to 90% continuous load after the additional capacities,

"AFTER" additional capacities?? I guess by that you mean "INCLUDING" them?
Loading a transformer that close to its ratings means there's not much margin left for days when voltage runs high or you have a lot of harmonics in the load, or it's just a hot summer day and the building it's in is even hotter. Were it in my factory i'd not push it that hard.
I like to run small ones like yours about 2/3 rated .

Realizing that a 1000 kva transformer isn't an insignificant piece of gear, ,,,,
If you plan to run it that near capacity i would take a real close look at the ventilation for the room and install temperature monitoring with alarms on the exhaust air. Might even add cooling fans.

Make sure the location it's in is well ventilated.
We had a 50,000 KVA transformer that ran hot in our 6 month long summers.. Fixed it by replacing the diamond deck plate right above it with grating.. Hot air wants to rise so we gave it a place to go .

is temp. rise equivalent to max. operating temp.?

No. That is how much the temperature of the windings will rise above inlet air assuming it's free to circulate through the transformer.
-------------------------------
@Babadag
to clarify just a little for OP:

In order to calculate the temperature rise we need more information like actual hot-spot temperature, the ventilation system AN =natural AF=forced the actual ambient temperature and other.
Let’s hot-spot temperature for rated 1000 kVA will be 105 oC for 90%[900 kVA]
the hot-spot will be only 97oC if the ambient is the same (AS?_jh) for AN rated ventilation system.

In other words,

forced air cooling will lower temperature from 105 to 97 C , prolonging life of the insulation.

Did i understand your paragraph correctly ?
--------------------------------------------------------------------------@bonobig: Old US saying "When all else fails, read the directions."
Please find the transformer's technical manual and post a link to it or cut and paste the relevant pages.
Pictures of the installation would help.

You are asking us for permission to load up a transformer and we have no basis to grant it.
What does the transformer's installation & operation manual say ? What kind of area is it in ?
------------

old jim

 

[jim hardy]

@Asymptotic posted a great link in # 4, here's a snip..

At 90% load keep the room below 99 degrees F. Good luck with that in a steam plant in the tropics.
Your transformer instruction manual should have a similar guideline .

 

[Asymptotic]

Make sure the location it's in is well ventilated.
We had a 50,000 KVA transformer that ran hot in our 6 month long summers.. Fixed it by replacing the diamond deck plate right above it with grating.. Hot air wants to rise so we gave it a place to go .

A great example where seemingly little things having enormous consequences.

Jim makes an excellent point about harmonics, and their contribution to temperature rise.

Other resources for the OP:
A Comparative Study of IEC 76 and ANSI C76 on Transformers

National Grid - Transformer Replacement Program Implementation Manual. This is specifically for dry type, low voltage transformers ranging from 15 KVA to 1000 KVA, but worth a read, and is the source of this graph.

ANSI C57.96 specifies a 20 year expected life at 100% load under a batch of other assumptions including an elevation of less than 1000 meters (3300 feet), freely flowing air convection, and an average ambient of 30°C which does not exceed 40°C.

In short, a cooler transformer lives longer.

It is also the reason for asking how long this transformer has already been in service. If your transformer has 20 years on it already, it is inadvisable to add more load, but rather to evaluate it's replacement with a higher capacity unit.