The Unbalanced Delta

Status
Not open for further replies.
R

rattus

Senior Member
We have discussed this before, but I think it is worth repeating for those who may have missed it:

Consider a delta secondary with a single load across one of the windings. How is the current shared among the three secondary windings? This is a real, not ideal, transformer.

Rattus
 
Re: The Unbalanced Delta

This is a good practical question, and is a basis for NEC[02] Subsection 240.4(F).

Hint: An "ideal" transformer will closely appoximate a "real" transformer in one setup and not quite so well in others, but is still will help one understand why the other cases are problems.

Edit Add: Actually, an ideal transformer is a close enough approximation in any case.

[ March 25, 2005, 10:53 AM: Message edited by: rbalex ]
 
Re: The Unbalanced Delta

Bob, my learned friend, this is a case where the ideal and real do perform differently.
 
Re: The Unbalanced Delta

Rattus, we have done this many times. If we serve a business with a small open delta and then they grow, they are installing more load and we go to a larger open delta bank. The next time, we have to close the delta and we have a situation where the 3? loads are split 1/3 on each transformer. However, the 1? load is split with 1/3 on each of the kickers and the lighter gets 2/3 of the load. Yes, I know that it adds up to 4/3.

By the way, we call the larger transformer the lighter since it serves the lighting load and the smaller transformer the kicker since it "kicks up" the high ?. When the bank is changed to a full delta configuration, we still refer to the transformers in the same way. :D
 
Re: The Unbalanced Delta

Did you check it empirically; and if you did, did you carefully check your polarities?

Edit Add: I didn't know Charlie was going to sneak in before me.

[ March 25, 2005, 12:49 PM: Message edited by: rbalex ]
 
Re: The Unbalanced Delta

Charlie, another learned friend, this is a textbook example with three identical transformers and a single load. I don't think the load is shared quite like you say. The addition of other loads can change the results however.
 
Re: The Unbalanced Delta

I'm assuming this is how you intend for it to be wired:

1phase%20load%20delta.gif


I've used "conventional" figures - and connections.
 
Re: The Unbalanced Delta

Nice diagram Bob; it will do nicely although the primary could have been a wye. Yes, the dots are conventional.
 
Re: The Unbalanced Delta

rattus

I apologize for not getting back sooner.

I'd like to clarify something before we go on. Are you under the impression that the primary connection has no effect on the distribution of the delta secondary currents?

That is, do you believe the distribution currents in the delta secondary of a "real" three-winding transformer loaded only on a single-phase is the same whether the primary is wye or delta, assuming the primary voltages are maintained reasonably symmetrical?
 
Re: The Unbalanced Delta

Originally posted by rattus:
We have discussed this before, but I think it is worth repeating for those who may have missed it:

Consider a delta secondary with a single load across one of the windings. How is the current shared among the three secondary windings? This is a real, not ideal, transformer.

Rattus
Click to expand...
We basically have two voltage sources in parallel.
One is the winding that matches up directly with the load.
The other is the combination of the two other windings.

By superposition, each "source" would carry half the current (load).

That's for ideal, in the real world the two sources would have different internal impedances, but without looking closely, I would have to agree with Bob that the "ideal" would be close enough of an approximation for most sparkies.

Now calculate the kVA's...
 
Re: The Unbalanced Delta

Engy, I contend that one source has twice the resistance and leakage inductance as the other, therefore the load is shared 2:1 with a real transformer. With an ideal transformer, there is no way to predict how the load will be shared, but it matters not since each transformer has an infinite KVA rating.

Bob, I understand that there might be some harmonic currents in the neutral, but I can see no difference in the way the loads are shared. Show me, my gg grandparents were from MO. Yes, I have been shown up and down before--lots of times.
 
Re: The Unbalanced Delta

Rattus,

If you can, I'd suggest you try it yourself empirically.

Even the "handedness" of the delta makes a difference when the primary is wired "wye," and a "delta-delta" (as drawn above)will operate differently yet. Actually "wye-wye" has some interesting quirks too, "real" or "ideal," if you "connect the dots" on one side and connect the other polarity on the other.

The only restrictions I'd request is that the primary source be "stiff" enough to maintain relatively symmetrical voltages on the primary windings and the single secondary winding not be overloaded. Measure the secondary currents in each winding - not the line - and carefully note the phase differences in the two "unloaded" windings compared to the "loaded" one and compare them all to a primary reference current. OK you can measure all the secondary line currents too - and compare them with the same primary reference.

If you can't - CROSSMAN - we have another set of experiments for you. :D
 
Re: The Unbalanced Delta

Bob,

I don't have access to a lab, but I might be able to set up a PSPICE simulation. But of course, you have to pay attention to the polarity dots.

A few years ago a story ran in the local paper about this old farmer with a 40 year old combine. All the chain and belt guards had long since disappeared, and when the reported asked the farmer if that was not dangerous, the replied, "Well, you have to know not to put your hands in them moving parts!"

There will be phase shifts in two of the windings and none in the third (resistive load that is).

Must get busy on the PSPICE to remember what I have forgotten.
 
Re: The Unbalanced Delta

I'm not familiar with the PSPICE simulator (program?)

But if you tell me the results, I'll tell you if you they're valid. Make sure you can can conform it to the restrictions I mentioned and it is extremely important that all three winding sets be "modeled" properly. Can it model magnetizing currents?

The secondary delta winding currents will react significantly different depending on the primary configuration, "wye" or delta.

The "handedness" of the delta will significantly affect the current distribution in the primary windings if they are "wye" connected. Try it both ways; while maintaining the primary "wye" configuration, and keeping the load arbitrarily on the same winding "a", check the results when the "dot" on the loaded winding is connected to the "tail" of "b" and then check what happens when it wired in the opposite hand; i.e., to the "tail" of "c." It will be critical to make sure the primary phasing and magnitude is maintained for you to get valid results.

If I get the time today, I'll try to post the schematics to use. We are just beginning our commissioning program on my project and time is not always readily available. I was there from six in the morning till six in the evening yesterday. Its already 4:30am now and I'm on my way to the jobsite in a few minutes.
 
Re: The Unbalanced Delta

Bob, SPICE was developed at Berkeley and was widely used in IC design. In the old days, a mainframe computer was required. Newer versions have been developed (PSPICE) which run nicely on todays PCs.

The transformer model uses all linear elements, so it will not simulate the harmonics from the non-linear core.

I have the student version (free) which is limited in the number of circuit elements and nodes, so I may not be able to do all three phases, so that would not be very good.

Crossman needs something to do though.
 
Re: The Unbalanced Delta

Rattus,

These are the configurations you should try:

1phase%20load%20delta%20Secondary.gif


The first two are fairly common configurations. The third is "valid" but fairly uncommon in US domestic systems, although it's a relatively common IEC/CENELEC setup.
 
Re: The Unbalanced Delta

Just to be a bit anal? :( My delta symbol is showing up as a D :(

[ March 30, 2005, 04:09 PM: Message edited by: engy ]
 
Re: The Unbalanced Delta

For what its worth, by convention, the "delta" - "wye" symbols represent static "phasors" rather than vectors similar to what I did on the "confusing" diagram. Note "a" lined up with "A," "b" with "B" and "c" with "C."

threephase%20holt.gif


However, I left them simply as "designators" for this exercise on purpose; eventually, I'm sure where going to want to talk about primary-secondary, phase-shift and I didn't want to give anything away prematurely.
 
Status
Not open for further replies.
Top