Transformer Ratings

[mull982]

I have a transformer which is a 230kV-4.16kV Delta/Wye transformer. The transformer ratings on the nameplate are as follows:

Type: ONAN/ONAF/ONAF

MVA: @55deg C Rise - 15/20/25 MVA
@65deg C Rise - 16.8/22.4/28 MVA

I'm assuming that this means that at the first rating of 55deg rise the 15/20/25 MVA ratings corrospond to the three types of cooling listed in "Type"
The second set of ratings of 16.8/22.4/28 corrospond to the ratings operating at a 65deg C rise with the three cooling methods listed in "Type". Do I understand this correctly?

With the 55deg rating does this mean that these ratings are good for anything 55deg rise and below such as lower temperature values? Does 55deg rise refer to 55deg above ambient of 40deg? If this is the case then these values are also the ambient values at 40deg?

For the second rating, I'm assuming these are the values for temperaturs up to a 65deg rise? Is there any downfalls to operating continuously at this rating, such as transformer life or other possible damage?

 

[jim dungar]

Yes, you are basically correct.

Transformer life is primarily affected by its temperature and time profile. The longer transformer insulation is hot, the shorter its life. The higher you load a transformer the less reserve capacity you have.

 

[kingpb]

You can request a curve from the transformer manufacturer that will tell you what the ratings are for different ambinet temperatures below 40 deg C. You will see that colder ambients will allow for higher loading. This isn't considered very ofthen becasue the transformers are ususally purchased with ratings to meet the output at the standard ratings. But in very hot climates, this could become a factor. As Jim said, you can ooperate hotter, it just shortens the life.

 

[bob]

MVA: @55deg C Rise - 15/20/25 MVA
@65deg C Rise - 16.8/22.4/28 MVA
This rating is used by most utilities when operating their transformers. They will operate using the 65C OA/FA/FA rating or in your case the 28 MVA. This is done year round without any reduction in life of the transformer. Of course during the summer months is when the load is the highest. The rest of the time the transformer operates underloaded. There are some utilities that operate the transformers beyond the 65C OA/FA/FA rating. Their thinking is that even though the life is shortened, something less that 30 yrs, it is cheaper than installing new transformers to share the load.
I do not think you would damage the transformer as long as you are below the
28 MVA rating. How much load will you have on the transformer?

 

[mull982]

MVA: @55deg C Rise - 15/20/25 MVA
@65deg C Rise - 16.8/22.4/28 MVA
This rating is used by most utilities when operating their transformers. They will operate using the 65C OA/FA/FA rating or in your case the 28 MVA. This is done year round without any reduction in life of the transformer. Of course during the summer months is when the load is the highest. The rest of the time the transformer operates underloaded. There are some utilities that operate the transformers beyond the 65C OA/FA/FA rating. Their thinking is that even though the life is shortened, something less that 30 yrs, it is cheaper than installing new transformers to share the load.
I do not think you would damage the transformer as long as you are below the
28 MVA rating. How much load will you have on the transformer?

I am in the process of determining the load that we are thinking about adding, I just wanted to make sure I understood these terms first. Basically our plant runs about 18MVA and we are thinking about adding another 2MVA to bring us around 20MVA. From what I've seen above this would not be any problem, for we can go up to 28MVA continuously without any long term damage.

My question is why give all these ratings? Why not just say the transformer is rated for 28MVA and turn the various stages of fans on as the loading increases. Alot of people here at the plant are under the impression that this transformer can only go up to 15MVA but in reality it looks like it can go up to 28MVA with no problems.

I'm assuming the ambient is the temperature surrounding the transformer? Or is this the temperaure inside of the transformer surrounding the windings?

 

[jim dungar]

From what I've seen above this would not be any problem, for we can go up to 28MVA continuously without any long term damage.

What if the cooling fins of the transformer are coated with dust which will act like an insulator? What if your cooling fans fail to operate?

When you purchase motors do you always run them at their maximum service factor?

Also, remember the conductor losses of the transformer go up by the square of the loading, 28MVA would be equal to a 166.67% loading so you should expect to see your conductor losses be 278% of normal full load losses.

 

[bob]

My question is why give all these ratings? Why not just say the transformer is rated for 28MVA and turn the various stages of fans on as the loading increases. Alot of people here at the plant are under the impression that this transformer can only go up to 15MVA but in reality it looks like it can go up to 28MVA with no problems.

I'm assuming the ambient is the temperature surrounding the transformer? Or is this the temperature inside of the transformer surrounding the windings?

There are some things that are based on the basic 55C ratings. Example fault current is calculated using the %Z and the 15 MVA rating. Concerning the fans, they are usually all turned on in advance of the months that your peak load occurs and not in stages. The temp is the surrounding ambient conditions. For your information the 65C temp is the winding temperature and not the oil. You should have a temp monitor for this. I assume you have oil gauges for monitoring also.

 

[kingpb]

Also make sure this is not being used as a double ended feed, e.g. Main-Tie-Main. Adding the extra load may mean you are not going to be able to use it the way it is initially intended and some load shedding would be in order.

The reason I say this, is that it would be out of the ordinary for an engineer to design this much spare capacity into a transformer, unless knowledge of future growth was known at the time. Even then, 10MVA of "future" growth is quite of bit.

 

[mull982]

What if the cooling fins of the transformer are coated with dust which will act like an insulator? What if your cooling fans fail to operate?

When you purchase motors do you always run them at their maximum service factor?

Also, remember the conductor losses of the transformer go up by the square of the loading, 28MVA would be equal to a 166.67% loading so you should expect to see your conductor losses be 278% of normal full load losses.

So if the cooling fins fail to operate are we then damaging the transformer if we run it at 65deg rise without any additional cooling?

 

[jim dungar]

So if the cooling fins fail to operate are we then damaging the transformer if we run it at 65deg rise without any additional cooling?

The 28MVA rating assumes you have ideal heat transfer by convection. Anything that reduces your heat transfer will reduce your possible loading.

Any time I suggest loading a transformer above its base (self cooled) rating, with no foreseeable reduction in loading, I usually tell my customers to put a replacement transformer on order.

 

[mull982]

The 28MVA rating assumes you have ideal heat transfer by convection. Anything that reduces your heat transfer will reduce your possible loading.

When you say "reduce possible loading", that means that the transformer will still physically put out the required 28MVA or whatever is demanded, however it will be damaged while doing so.

Any time I suggest loading a transformer above its base (self cooled) rating, with no foreseeable reduction in loading, I usually tell my customers to put a replacement transformer on order.

What value or what percentage is recommended for loading on a transformer for a continuous rating?

 

[jim dungar]

When you say "reduce possible loading", that means that the transformer will still physically put out the required 28MVA or whatever is demanded, however it will be damaged while doing so.

I would not say it will be damaged, I would say its life will be shortened.

What value or what percentage is recommended for loading on a transformer for a continuous rating?

In general my initial install design target is 80% of the self cooled rating. This allows room for future growth as well as provide room for short time (could be months) continuous demands.

You need to consider all of the possible cost associated with continuous loading a transformer above 100%. As I mentioned before the operating costs can sky rocket (you can often cost justify for a second transformer rather). If all of the capability of the transformer is used up in normal operation you have no ability to add future or emergency loads; what is the lost opportunity cost of not being able to quickly expand the facility? Don't forget replacements, it seems that the larger the transformer rating the longer the replacement lead time.

 

[kingpb]

A transformer will not see any degradation in life by operating it at it's higher (max) rating and at maximum temperature rise. They are designed for these ratings. e.g. you can operate at 28MVA @ 65 deg C rise over 40 deg C ambient 24/7 without any damage. To say that you shouldn't go above the self-cooled rating is way, way being over conservative, and leaving capacity on the table. Which costs a lot of money.

Typically, on transformer with this type of rating, going from 55 to 65 deg C rise gives you approx. an additional 13% increase in power capability. It used to be, that the extra 13% was the spare capacity. And the design was for nearly the max. all fans on, or oil pumps on, etc at the 55 deg C rating.

Currently, IEEE C57 standards do not really recognize the 55 deg C rise rating anymore. Manufacturers will still accept a spec with it stated that way, but it really doesn't mean anything. They will give you a transformer with 65 deg C capability, and only using a 55 deg C rise just means it runs cooler.

 

[mull982]

I would not say it will be damaged, I would say its life will be shortened.



In general my initial install design target is 80% of the self cooled rating. QUOTE]

In a case like mine when the transformer only gives a 55deg/65deg rating do you take the 55deg rating to be the ambient rating? If not, how is the ambient rating determined based off of these given values or what value would you used 80% of?

 

[bob]

In a case like mine when the transformer only gives a 55deg/65deg rating do you take the 55deg rating to be the ambient rating? If not, how is the ambient rating determined based off of these given values or what value would you used 80% of?

I believe the design is based on 40C ambient. Check with the mfg to be sure.

 

[kingpb]

Yes, the 55 deg C means that at 95 deg C (55 + 40) the transformer is capable of 15MVA loading without any additional cooling other than the ambient air around the transformer. To go above 15MVA (without degradation) you need to turn on the first stage of cooling. The cooling also works the other way as well, say you have 15 MVA on the transformer and the prolonged ambient temp is say 50 deg C, then the cooling should also come on. This is taken care of in the control cabinet of the transformer, using temp inputs.

If you allow it to go to 105 deg C (65 +40) then you can get approx. an additional 13%.

 

[jghrist]

I agree with kingpb. The 55?C rise rating is a throwback to the days over 30 years ago when transformer insulation was not as good. It would only withstand 100?C (30?C ambient + 55?C average winding rise + 15?C hot-spot rise over average winding). Newer insulation will withstand 110?C.

When loaded to 28 MVA continuously with a 30?C ambient and both stages of fans running, the average winding temperature will reach 65?C and the hot-spot temperature will reach 110?C. The transformer will operating at this temperature without additional (from design) loss of life.

What the temperature is with lower loads depends on how the temperature switches are set that turn on the fans. If these are set to switch on at the lower temperature rise, then the 16.8/22.5 MVA ONAN/ONAF ratings are meaningless because the temperatures will not reach 65?C before the fans switch on to cool the transformer.

 

[mull982]

I have just looked at a power study from a couple of years ago and found that the Max MVA plant load at that time was aprox 17MVA. (I'm sure its grown since then)

With this 17MVA I'm assuming it was at a 55deg rise? Is there any way of telling?

So lets say its currently loaded up to 17MVA. This is above the ambient rating and probably into the single fan cooling stage. How much more load can safely be put on this transformer. From what I've heard, it sounds like 11 more MVA to bring us up to the max rating of 28MVA at 65deg?

 

[mull982]

I guess I should also mention that I have another similar transformer that has only one rating which is 15/20/25 MVA @65deg rise.

This only has a 65 deg rise rating. I'm assuming that this transformer actually has less capacity because it has less of a rating a 65deg. If that is its rating at 65deg what would its ambient rating be? 15MVA?

When reviewing a power study I see a transformer has two rating options. Nominal KVA which is listed at 15000 and Full load KVA which is listed at 25000. Are the 65deg ratings always used for these nominal and full load KVA's?

 

[jghrist]

I have just looked at a power study from a couple of years ago and found that the Max MVA plant load at that time was aprox 17MVA. (I'm sure its grown since then)

With this 17MVA I'm assuming it was at a 55deg rise? Is there any way of telling?

So lets say its currently loaded up to 17MVA. This is above the ambient rating and probably into the single fan cooling stage. How much more load can safely be put on this transformer. From what I've heard, it sounds like 11 more MVA to bring us up to the max rating of 28MVA at 65deg?

What do you mean by "assuming it was at a 55deg rise"? With 17 MVA loading, the only thing you can say is that the average winding temperature will be less than 65?C (assuming a 30?C ambient). If the load were 28 MVA constantly, then the average winding temperature would eventually get to 65?C with all fans running.

If you knew the loading for the previous day, all of the thermal constants of the transformer, and when the fans were running, then you could calculate the transformer winding temperature, but why? If your load is less than 28 MVA, you are not overloading the transformer.