Delta or Wye???

[sundowner]

Recently I was asked what I would expect to find on the secondary of a distribution xformer on a college campus. I replied that I would expect to see a wye connection because of the available current gain. This particular xformer had several hundred parking lot lights, few hundred dorm rooms, and a few small buildings.

My thoughts were that the lights (HPS) were the major load and power factor would need to be corrected, hence the wye secondary.


Could you guys please straighten me out on this, am I all wet or am I on the right track with thinking that a wye has current gain while the delta configuration has the voltage gain.

Thanks in advance

Steve

 

[petersonra]

Re: Delta or Wye???

I am not sure what you are referring to. A transformer changes one voltage to another. The secondary current increases over the primary current by the same ratio as the voltage decreases from primary to secondary.

e.g.-if you have a 480/3ph primary and a 240/3ph secondary, the current seen on the secondary is twice what the current is on the primary.

delta is often used where there is no grounded conductor desired, such as in many manufacturing plants. otherwise you will typically see a wye configuration since a ground fault will trip the OCPD and clear the fault.

there are other things you might see depending on what you loads look like. personally I would expect to see 208/120 for the dorm rooms. but than i don't know much about wiring up dorm rooms, but it would seem to be the most cost effective solution.

power for the the other stuff would be dictated by what equipment it is feeding, code requirements, what the utility allows, and the personal preferances of the people who designed the power system for the college. you might well see an assortment of stuff.

[ August 17, 2005, 09:52 AM: Message edited by: petersonra ]

 

[charlie b]

Re: Delta or Wye???

Where do I begin?
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• <font size="2" face="Verdana, Helvetica, sans-serif">How about here: A transformer's secondary, whether wired as a WYE or as a Delta, does not "correct" the power factor of the load. Rather, being an inductive component, it tends to contribute to the power factor problem, not to alleviate it.</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Secondly, if we disregard the power losses internal to a transformer, then the power is the same on the input and on the output. Power is voltage times current (with some other stuff thrown in, on occasion). So if a transformer steps down the voltage, then it will step up the current, so that voltage times current is the same at both sides. That is true for a WYE and for a Delta.</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Third, if you have to supply both dorm rooms and parking lot lights from the same transformer, then you will need both 120 volts and 208 volts. (Perhaps I am wrong here, but I don't think 120 volt parking lot lights are a common commodity.) You can only get both with a WYE secondary.</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Finally, if I were designing a parking lot lighting system, I would use 277 volt fixtures. That means that I would need one transformer with a 480/277 volt secondary, and a separate transformer (120/208 volt secondary) for the dorm rooms.</font>

<font size="2" face="Verdana, Helvetica, sans-serif">Now I would like to ask what the situation you are dealing with. This is apparently an actual, in-service transformer. What are you being asked to do with it? Or is this just an "academic" :roll: discussion?

 

[james wuebker]

Re: Delta or Wye???

Charlie b, I agree with you except with all the parking lights I would probolay use 480v single phase for the power .
Jim

 

[sundowner]

Re: Delta or Wye???

Thanks guys I knew I'd get some feedback. Several weeks ago I was at a job interview, one of the most difficult ones I can remember, one of the guys interviewing me asked what I would expect to find on a distribution xformers secondary with several hundred parking lot lights, I responded that it was a pretty vague question but with only lights on the sec. I would think it would be a wye connection because of the current gain available on a wye sec. That said let me say this, would'nt it be cheaper to do it that way ( to try and have a more resistive load rather than a reactive load --- power factor--- ) I remember something about this during apprentice school...but oh so many years ago.

Another thought here ...is'nt this the reason we add capacitor banks to very large MCC line ups to try and correct for the power factor, making the large (inductive) motor load, more resistive, thus the current would be in phase with the voltage.

Any thoughts on this guys

Thanks again

Steve

 

[charlie b]

Re: Delta or Wye???

Capacitors and inductors have opposite effects. If the overall load is inductive (which is what we usually see), then adding the right amount of capacitors (generally we add a "capacitor bank") will offset the inductive effect.

I won't quite say this makes the load "more resistive." That is a bit too imprecise for me. The load originally has resistance and inductance. When you add exactly the right amount of capacitance, you are left with nothing but resistance. But you haven't added any resistance, you haven't made the circuit "more resistive." I would put it this way, "you have reduced the circuit to being only resistive."