750 kva transformer

[jschultz]

Can anybody tell me why most engineers do not want to use a 750 kva dry-type transformer? None of the ones I talked to could give me a reason not too, but all of them had never done it.

I have a building that has 17 residential units on each of 6 floors. The developers do not want to give us the space for transformers on the residential floors, so I have to use 208v bus duct. 3 floors worth of load goes over 500KVA. So I was trying to figure out what I should do with the distribution. I do not have enough room in the meterclosets for another busduct. I already had planned on two bus ducts each served by a 500kva transformer. But the loads have increased since the initial design concept. One guy in the office said, "why don't you use a 750KVA transformer for each bus duct?". I said, We never do any over 500KVA. I called my boss and he said "never seen it done". I called a couple of other engineers and they all said, never seen it done. But no one could give me a good reason not to.

Can one of you tell me why I should not, or why I can?
The 750's are big and heavy but will fit in the room I have currently. (3) 500's will not fit.

Some people suggested substations. That seemed to get bigger and would not fit in the room.

 

[charlie b]

Re: 750 kva transformer

I have no fundamental objection to using a 750. In fact, while working for a previous employer, I performed an engineering review of an apartment building that was having problems with lights flickering and other issues. We recommended replacement of the existing 500 KVA transformer with a 750.

By the way, when you say that ?3 floors worth of load goes over 500KVA,? I presume you are talking about the calculated service load. If that is the case, the utility will probably give you a 500, or even a 300, to serve the building.

 

[jschultz]

Re: 750 kva transformer

i have 6 floors worth of residential load. 2 busducts. Each busduct serves 3 floors. The calculated demand load is just over 500kva for 3 floors.

the utility is going to give us a 500 or 750. They did not know for sure last I talked to them.

[ November 12, 2004, 02:42 PM: Message edited by: jschultz ]

 

[charlie]

Re: 750 kva transformer

A 500 kVA and smaller transformer will have an impedance of around 2% but a 750 kVA and larger transformer will have an impedance of 5.75% or more. That drastic increase in impedance will effect the voltage drop to the point that you will have problems with keeping the voltage high enough. If you change the taps, the voltage will be much too high at night. A 750 kVA will give you a lot of trouble in a residential setting.

 

[coulter]

Re: 750 kva transformer

Hi guys and girls, I'm brand new at this forum, but let me take a lick.

J -
Nothing wrong at using a 750kVA if that is what you need.

Since I don't have to do much commercial/residential (thankyou God I'll guess that you are 208V, 3ph on the secondary and primary is either 4160V or 480V.

Given that, my biggest concern is the available secondary short circuit current. If you really, truly had to use a 500kVA, 208V, 2%Z xfmr, you have about 69kA SSC to deal with (assumes infinite primary buss). I have only seen one in the last 30 years, and hope it is the last. I ended up using current limiting fuses to get the SSC down to where I could use a 65kA secondary main. 5%Z is a lot easier to handle. Most general purpose dry types are 4%Z to 6%Z. I checked Hammond, SQD and Teal. 2% would be special order for all.

So, looking at a 750kVA, 208V, 5.5%Z, SSC is 41.6kA. Much easier to deal with. 42kA equipment will do fine.

Now for the voltage drop: Again I don't do much commercial, so I don't know what the normal power factor would be. If you are 1.0pf, the The Vd will be about 1.5% and if you are .9pf Vd should be about 3.4% (from IEEE Gray book) Interestingly, the percentage doesn't change much with the xfmr rating. Of course the load current does, so if your load is say 1500A (110A more than fla for a 500kVA, 208V) then with a 750kVA installed, the Vd is reduced by 1500/2100 or 3.4X1500/2100 for 2.4% at .9pf.

Again, I am not familar with the normal load swings for commercial/residential, so what ever the normal swing is, set the taps at mid range for 208V. So if you are set for 208V at 50% fla and the load swings from 1/3 to 2/3 fla on the 750kVA, that would put you at less than 1% high to 1% low. And if the load does go to near zero, you are still less than 2% high. That would be great in anybodys book.

Hope this helped, but again, I am new at this, so please give me a critique - such as "correct but useless", "goofed up", or "oh - okay"

carl

 

[kiloamp7]

Re: 750 kva transformer

Nothing wrong with a 750KVA if that is what you need. But what is the primary voltage?

A plain jane dry-type with a medium voltage primary will have appx. 5.75% Z. That is the standard & it works for the whole country, generally. If the primary is 480V., then I believe the Z would be somewhat lower.

If you are talking Power Company-type and/or liquid-filled, well that may be a different story.

 

[jschultz]

Re: 750 kva transformer

480v primary. Transformer owned by building. Utility is supplying at 480v.

 

[jschultz]

Re: 750 kva transformer

Cutler Hammer DT-3 150 degree
500 - 4.9%
750 - 6.3%

so they are not too far apart, assumming i looked them up correctly.

My boss still does not like the 750 idea, still does not have a good reason, other than never done it in the past so we aren't doing it now. We ended up having the Mech designer sharpen his pencil and take some fluff out of the condensing units and now I am just below 500 on each bus duct.
The utility company is serving the entire building with one 750KVA transformer. The NEC and Utility Demand factors are completely different. Especially for residential situations.