question on transformer configuration advantages

[stcalle]

Ok here is the scenario:

I have a 900ft between an EDR (electrical distribution room) and a Communications Room. I will be runnning a feed from the DB(distribution board) in the EDR to a DP(distribution panel) in the Comm. Room. I use 208/120VAC in both rooms. For voltage drop purposes, I will be stepping the voltage up before the 900ft conduit run to 480V then stepping it back down to 208/120VAC. Can someone tell me the pros and cons to the following setups:

1) At the EDR end, using a delta primary - delta secondary step up transformer (112.5kVA), then only running 3 cables (500kcmil) in the conduit. Then using a delta primary - wye secondary step down transformer (112.5kVA) at the Comm. Room end.

2) At the EDR end, using a delta primary - wye secondary step up transformer (112.5kVA), then only running 3 cables (500kcmil) in the conduit but grounding the neutral. Then using a delta primary - wye secondary step down transformer (1125.5kVA) at the Comm. Room end.


I would like to add that there will be no loads tapping the 900ft run where the voltage is 480/277VAC. The 480VAC is only for tranmission purposes so allow less conductors to be ran using 208/120VAC.

thanks

If there is any unclarities, please let me know

[ March 21, 2005, 12:46 PM: Message edited by: stcalle ]

 

[charlie]

Re: question on transformer configuration advantages

I will bow to the knowledge of the engineers here but I see no reason to ground the secondary side of the 480 volt, step up transformer if it is a wye configuration. I would take only 3 conductors and do it ungrounded in either case.

 

[steve66]

Re: question on transformer configuration advantages

You may find you are limited by off the shelf transformer configurations. If you use a delta-wye step up, and a delta-delta step down, they will need different winding ratio's to get back to your original voltage.

I know you want a delta-wye for the stepdown to cancel out harmonics. And why not consider stepping up to 600v or even higher and reducing the wire size some more.

Steve

 

[charlie]

Re: question on transformer configuration advantages

The problem with voltages higher than 600 is the higher voltage insulation (probably shielded) and special terminations (elbow terminators or stress cones). At 600 volts, you will be running less than 200 amperes and I am thinking the maybe one circuit of 500 kcmil Al. will hold the load and voltage drop (I didn't do the voltage drop calculations).

 

[stcalle]

Re: question on transformer configuration advantages

Ok, good info guys, I spoke with one of my colleagues and the specific question that I forgot to ask directly was as follows:

Should the transformer at EDR side,the step up transformer, be a delta primary - wye secondary or be a delta primary - delta secondary? In the above scenario this is the difference between the two. I wanted to know which transformer configuration is ideal and what may be the pros and cons of using either configuration.

Just as charlie has stated, he sees no reason to ground the neutral of the wye connection on the secondary side of the step up transformer so then would using a delta primary - delta secondary transformer be ideal, since you then don't need that fourth wire?

I hope this is a little more clear

thanks

[ March 22, 2005, 08:09 AM: Message edited by: stcalle ]

 

[charlie]

Re: question on transformer configuration advantages

You don't need the forth wire in either case. I would get whichever is available and most economical and I would go for 600 volts.

 

[stcalle]

Re: question on transformer configuration advantages

Originally posted by charlie:
You don't need the forth wire in either case. I would get whichever is available and most economical and I would go for 600 volts.

So you're saying that the delta primary - delta secondary is preferred? The thing is if I used a delta primary - wye secondary configuration, I will have 4 wires (3 phases and a neurtral), but since the transformer at the Comm. Room end is going to be a delta primary - wye secondary step down transformer, I do not need to run the extra cable.

Going back to my question, I wanted to know if someone knew the electrical theory background on why a delta primary - delta secondary transformer would be better then using a delta primary - wye secondary in this scenario.

Thanks