Buck/Boost Transformers

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wilbur101

wilbur101

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Ok, I've got a problem. Several actually, but I am only looking here for help with the technical one. Anyway, I am working on an expansion project for an industrial manufacturing client in Virginia. The client has an existing distribution and wants me to use a 480V, 3phase 1200A feeder from an existing substation for a feed to a new building. The building is approximately 800 feet away. I do not like doing this, but this is what the client is mandating. There will be a voltage drop of about 5-6%. The client is also locked into using Square D equipment. The client wants to use a Buck/Boost transformer to resolve the voltage drop problem. The client's design has the 1200A feeder going to a 1200A I-Line panel and then an 800A breaker in the I-Line panel feeds out to a 500kVA transformer. There is a second 800A breaker in the I-Line panel that feeds another 500kVA transformer. Has anyone ever done anything like this? Does Square D make a Buck/Boost transformer large enough (I can't find one and the local rep is just shy of useless on this topic)? Square D literature seems to indicate that 2 transformers will be required for each buck/boost - this seems like it will require a huge amount of space. I always thought buck/boost transfromers were used for smaller loads or dedicated loads and not building feeds. Any help or advice is greatly appreciated.
 
Re: Buck/Boost Transformers

I do not know how large a buck boost is available.

I do know that voltage drop is directly related to amperage. Unless the load is absolutely stable there is no way know how much the voltage drop will be at any given time.

You say there will be a voltage drop of about 5-6%. Is that during the production process and all loads are on or is that when the building is not in use.

In my opinion this plan will not work.
 
Re: Buck/Boost Transformers

Using a load of 800 amps, 664 kva, 4 ckts of 500 kcm has a VD of 2.75% What size load are you using? If this is a industrial customer and is going to be billed a demand basis on kva from the utiltiy, plant capacitors will reduce the VD and improve the power factor. 800 ft is a long way to go.
 
Re: Buck/Boost Transformers

Using a load of 800 amps, 664 kva, 4 ckts of 500 kcm has a VD of 2.75%
Click to expand...
I come up with 7- 500 kcmils per phase just to get 800 amps @ 480 volt 800' and that is with 5% VD. :confused:
 
Re: Buck/Boost Transformers

if you install capacitors thatn your demand for reactive power will compensate at the plant. Actuall I would like to know the power quality which will determine your quality of power and heating losses
 
Re: Buck/Boost Transformers

If I read your post correctly I would consider changing that taps on the transformer. It probably comes standard with (2)2-1/2%AN and (4)2-1/2%BN taps. You can compensate for the 5% normal voltage a 5% below normal tap, which means that the voltage is 5% low, which will bring the voltage back up to a point that you are looking for. Voltages will vary with the load, it's a given. As the load decreases the voltage will rise.
 
Re: Buck/Boost Transformers

Wirenut
"Bob, Forgive me for asking a dumb question, but how will plant capacitors reduce voltage drop in this application? " Any question is a good one because it makes one think.
I am assuming that since the customer is an industrial manufacturing client that the load would consist of a large motors. If the plant power factor is say 0.80 then the addition of the caps would raise the PF to say 0.99 and reduce the reactive current flow and reduce the VD.

Iwire
I ran the caculations again using a PF of 0.95 and PVC conduit an came up with 2.2%.
Why don't you show the calcs and lets see what you did.
Bob

[ November 10, 2004, 08:34 PM: Message edited by: bob ]
 
Re: Buck/Boost Transformers

Originally posted by bob:
Iwire
I ran the calculation's again using a PF of 0.95 and PVC conduit an came up with 2.2%.
Why don't you show the calcs and lets see what you did.
Bob
Click to expand...
I used a voltage drop calculator, it has always worked well. :eek: :p

I am much closer to your numbers now. :cool:

Actually the software came up with 1.6% VD - 800 amps, 800', 480 volt, 4 - 500s

Would this be the result of not considering power factor?
 
Re: Buck/Boost Transformers

Iwire:

It is hard to say what power factor your software is using. It might be 80%, or it might be some other number.

I like to look at my Square D slide card (They call it a "Motor Data Calculator", but it has a lot more on it. Free if you know a distributor who has one to give away). It has voltage drop for each wire size at both 95% and 80% power factor and both 1 phase and 3 phase.

Something about voltage drop this calculator shows has always puzzled me. I think I will post that as a new topic.

Steve
 
Re: Buck/Boost Transformers

A buck boost transformer, in general, will be a recipe for disaster. This is a first year power engineering problem. Follow this reasoning, I think some other contributors have touched on it also.

Under no load there will be little or no voltage drop. Under full load, you will see your largest drop. A buck boost transformer will be in the circuit and will raise the voltage by the boost amount at all times. If the utility provides you with 480 +\-5% and you add a 5% boost, under no load you could have as much as 528 volts, 480 + utility 5% + boost of 5%. Clearly this is too high, based on the standard of 480 +\-5%. If voltage problems relating to load change were as simple as a buck boost transformer, utilities would not deal with voltage regulators, line drop compensation, etc. There are solutions, but buck boost is one I would not recommend.
 
Re: Buck/Boost Transformers

Jtester I was about to post the same thing. Voltage drop cannot be over come by boosting the voltage or resetting the taps on the transformer as this invites over-voltage when loads are turned off. The only true way to over come a voltage drop problem is to use the correct wire size for the distance of the run and current. or like the utility's do . transform the voltage up to a voltage where it wont be a problem and then transform it back down.
But the cost of a 576kva transformer ouch. :eek:
I think up sizing the wire will be cheaper.

[ November 11, 2004, 07:46 PM: Message edited by: hurk27 ]
 
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