transformers - a definitive reference?

Status
Not open for further replies.

megloff11x

Senior Member
Can anyone recommend a definitive and straightforward reference for the various ways we employ transformers and why?

The missing phase that acts like it's still there in a delta as long as you keep load under 57%.

corner grounding

center grounding a delta and the high leg - why did we do this?

blocking harmonics wye-delta

connecting delta loads to wyes.

virtual grounding.

1001 installation schemes - which are legal and good and which aren't and why?

and the illustration & explanation of the center tapped 240 to the home.

etc.

Engineering texts tend to overcomplicate matters to show off math you'll never use again in real life. Many of the references for installations stick to fuse and conductor sizing for the most common cases. The historical reasons behind some of the more esoteric configurations seem to be lost or getting lost. And making decisions when you have a square peg and a round hole combination are tough.

Is there a book like the excellent one on GFCI/AFCI but on transformers?

Matt
 

coulter

Senior Member
Matt -

megloff11x said:
Can anyone recommend a definitive and straightforward reference for the various ways we employ transformers and why?...
You are really asking for a lot. If you find one I would be interested.:smile: . Second answer: Try Chapman, Electrical Machinery Fundamentals. But it does have a lot of math.

megloff11x said:
...Engineering texts tend to overcomplicate matters to show off math you'll never use again in real life. ...
They do that because the mathematical models are what we work with. The physicists do the research, and come up with the formulas that describe how the phenomena behaves. Without the math, it's hard to understand the model.

One often hears, "Well, in theory that is the way it works, but that isn't necessarily the way it works in the field."

My general (unspoken) response is, "It's not a theory, its a model. And the model is based on what happens in the field. If what you are seeing doesn't fit the model, you don't understand the model (and the limitations)"

The model is important

carl
 

drbond24

Senior Member
I'm with you Matt; I'd love to have such a book.

Second answer: Try Chapman, Electrical Machinery Fundamentals. But it does have a lot of math.

This is the book I have on my desk, but it only has a chapter on transformers. It covers a few topics well, but does not go into great detail. I have found it lacking quite frequently.
 

coulter

Senior Member
My take on some of your questions:
megloff11x said:
...The missing phase that acts like it's still there in a delta as long as you keep load under 57%....
That statement baffled me completely. There is no model I know of that supports that. Say we have a 1000kva 13.8D/480Y xfmr fed with a fused disconnect (commom installation) and we loose one of the 13.8kv fuses. You don't have 57% available - you got nothing available. Oh there is voltage on the 480, but the phases are horribly skewed and un-useable to drive a motor. If it's important to not do this to the system (ie you don't want burned up motors), one installs a phase loss relay and shuts the xfm down.

megloff11x said:
...corner grounding...
Never understood that one. Forty years ago (and more) corner grounded 480D was popular over 480Y. I have no clue why. I can't think of any reason to install one today - except if that's the xfmr you have and you can't wait or can't afford another one.

megloff11x said:
...center grounding a delta and the high leg - why did we do this?...
Again popular 40 years ago. I only have one clue. Small industrial installations need 3ph for the motors, and the motors were not much bigger than 30hp and there was only a small single phase hotel load (small office).

Why not 480? Probably money. You need an extra xfm for the 120/240V load. Maybe 480V distribryion was more money then.

Why not 208Y? It wasn't very popular 40 years ago - I don't know why not. Maybe has to do with the available motors. I never saw any 200V motors - lots of 440/220V still in use. I did some research on one installation (1969). As I recall, per the mfg, a 30hp, 460/230 motor on 208V was only good to 25hp.

I can't think of a reason to put one in today.

megloff11x said:
...blocking harmonics wye-delta...
I have to let one of the whizzes answer this one. My limited understanding is: Third harmonics circulate in the delta winding and are gone - not reflected back to the utility (or supply) In a Wye they are additive in the neutral and are reflected back to the supply. (I have to stop, I've told you more that I know:-? )

megloff11x said:
...connecting delta loads to wyes. ...
I don't know where this came from. A 3ph load that doesn't have a neutral, and by inference doesn't need a neutral reference, couldn't care less if the supply is Y or D. It can't tell. Unless you get a ground fault. Then you could tell the difference between grounded Y, impedance grounded Y, ungrounded D, corner grounded D. Just in case it wasn't apparent, I speaking of 480V, 208Y is always grounded.

I've never seen a Y primary except on one utility 34.5 distribution. I put up a thread on this subject. Never really got any good discussion, and no references.


megloff11x said:
...virtual grounding. ...
I'm assuming you mean grounding transformers, zig-zag and such. I understand the theory, not the model:rolleyes: Hopefully somebody will pitch in and enlighten us.

megloff11x said:
...and the illustration & explanation of the center tapped 240 to the home...
Jim D has pretty good explanations in the lengthy thread. His explanations were a lot better than mine. I was too focused on making sure any model fit the normal 3ph stuff I work on - not necessary, didn't help.

megloff11x said:
... And making decisions when you have a square peg and a round hole combination are tough. ...
Yes, they are tough. Those are the ones where I get to learn something new.

carl
 

megloff11x

Senior Member
I've got Chapman and several other related books. I'm looking for "one stop shopping" as far as summary, history, etc. goes. Right now the whole story is spread out over several sources, and "tribal knowledge."

I borrowed an old copy of Ugly's, which has quite a bit on the hook em' up side.

The GFCI/AFCI book that Mike sells has an excellent discussion of history and how it works for those types of circuit protection devices. It also has a nice summary of power around the world.

I also have the "Smart Motion Cheat Sheet" which is now available as a PDF file. It has all the unit conversions and equations for setting up your gearing, belts, rack & pinion, leadscrews, etc.

I'm looking for something like the two items above, only for transformers.

Maybe I'll have to condense it myself.

My curiousity on the open delta (I tend to butcher my terminology heinously) is, how did first they figure out this worked? Did someone calculate and demonstrate? Did a transformer somewhere blow a phase open and it ran just fine that way until someone noticed, and then went back and did a bunch of math to figure out why and slapped his forehead?

The more esoteric configurations came from some bright or not so bright idea.

If I condense something I'll post it here for review. You can never catch your own boo-boos.

Matt
 

coulter

Senior Member
Matt -

Stuck my foot in it again. You didn't need any of what I wrote.

However, I just ordered three books from ABB, transformers, transformer testing, and transmission and distribution. I send a review when they show up. This may be of interest.

carl
 

russ

Senior Member
Location
Burbank IL
Open delta is popular in my area. (just two transformers)
I assume is was originally installed by the utility company because it was cheaper than a wye, which required three transformers.

I think all they had to do was add a second smaller transformer to the existing single phase that was already in place.
 

drbond24

Senior Member
Pierre C Belarge said:
Try

Transformer Principles and Applications

By ATP/NJATC

After some searching on Amazon.com, I found a used copy of Transformers, Principles and Applications by the American Technical Society. It also says Dunlap, Siefert, Austin on the front. Is this the same one you're talking about?

Anyway, the one I'm talking about was printed in 1972 so it is pretty old (to me anyway; that's before I was born!) but it does a better job than the Chapman book previously mentioned in this thread.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
what ??

what ??

drbond24 said:
I'm talking about was printed in 1972 so it is pretty old (to me anyway; that's before I was born!) but it does a better job than the Chapman book previously mentioned in this thread.

Easy Now whipper-snapper:D
Them words might get you in trouble on this forum

augie<<<<< old fart
 

masterelect1

Senior Member
Location
Baltimore
corner grounded transformer

corner grounded transformer

The EC has installed the wrong transformer for the application; a 480/240 delta/delta ungrounded,as in 3ph/3 wire, which in my opinion is a dangerous set-up. I believe the "out" for the EC is to corner ground the xfmr. secondary to give the OCPD a ground fault reference.

As I am totally unfamiliar with corner grounding,( I have never been exposed to it) I ask the following (embarrassing) question:

Can the ground be an EGC or does it have to be a seperate GEC and what size must either wire be?

My feeling is that I can use the EGC, bond to any phase and use a #6 wire for the grounding.:confused:

Thanks in advance,
John
 

coulter

Senior Member
Note to moderator: Does this need to be moved to a new thread?

masterelect1 said:
The EC has installed the wrong transformer for the application; a 480/240 delta/delta ungrounded,as in 3ph/3 wire, which in my opinion is a dangerous set-up. I believe the "out" for the EC is to corner ground the xfmr. secondary to give the OCPD a ground fault reference.

As I am totally unfamiliar with corner grounding,( I have never been exposed to it) ...

John -

Some questions:

1. How do you know this is the wrong transformer for the application? What do the prints/equipment specs call for?

2. Why is an ungrounded 240D inherently dangereous?

From what you have posted, I can't tell there is anything dangereous, not to code, or unreliable. Maybe some ground detectors need to be installed.

I know some other posters have different experience than I do, But in 40+ years of industrial experience I have never seen a corner grounded 240D. I can't think of a single reason why anyone would want one.

carl
 

masterelect1

Senior Member
Location
Baltimore
coulter said:
Note to moderator: Does this need to be moved to a new thread?



John -

Some questions:

1. How do you know this is the wrong transformer for the application? What do the prints/equipment specs call for?

ANS---Specs call for "Y" 240 center ground, no neutral (grounded) conductor. straight 240 volt application.

2. Why is an ungrounded 240D inherently dangereous?

ANS---A 240 or 480 3wire 3 phase ungrounded system presents a shock potential as a patial ground on each of any 2 phases could have a potential voltage difference that could lead to a shock if the 2 partially grounded phases were touched at the same time. Also, presents an arcing potential at the point of the partial ground.

From what you have posted, I can't tell there is anything dangereous, not to code, or unreliable. Maybe some ground detectors need to be installed.

ANS.---It violates the code @ 250.21 if it does not meet the exceptions, which it does not, unless ground detectors are installed as you cite. It is however reliable if you want your facility to run with an unintentional ground, such as in a manufacturing enviroment.

I know some other posters have different experience than I do, But in 40+ years of industrial experience I have never seen a corner grounded 240D. I can't think of a single reason why anyone would want one.

-----I have not seen one either, hence the advice seeking. My understanding is so that the system will have a ground reference to trip the OCPD.

-----By the way, I also come from 30+ yrs industrial, so I am not familiar at all with the corner ground philosophy.

John
 
Last edited:

coulter

Senior Member
I had two pages written and decided I would again cheat george out of his duty :D

Most of what I am hearing is you are not particularly familiar with ungrounded systems and think they are inherently dangerous. This is no sin ? unless you get stuck.

I don?t know of any peer reviewed information that shows ungrounded systems to be any more dangerous than grounded systems. Bonded is important and necessary ? not grounded. There are plenty of threads here the support this.

Forcing the change to a corner grounded system (or a grounded Y either) without understanding the effects on the equipment does not seem like a good idea.

Don?t confuse restriking, arcing, ground fault issues that are inherent with 480D systems with any similar issue with 240D systems. It isn?t there and does not happen.

The issues I see are one regulatory, and one engineering spec. Both have engineering solutions:

1. Specified installation is ungrounded and requires ground fault detectors to meet NEC 250.21. Okay EC submits request to engineering. Engineering responds with change order to install three transformered 240V pilots. Inexpensive and simple.

2. 139/240Y transformer specified, 240D transformer installed. Contractor requests permission to substitute a 240D xfm because a 139/240Y is not readily available. Engineering has two options, ?Approved? or ?Disapproved, get specified transformer?.

I?ll bet they approve it. It is likely the equipment can not tell the difference between ungrounded 139/240Y and ungrounded 240D

Personal opinion is ungrounded 240D is safer than corner grounded 240D. If the system is good, capacitive coupling is pretty high impedance and likely balanced showing 139V to ground. If the system has a fault, the ground detectors show up immediately - even a relatively high impedance fault will show up imediately. But there is no extra danger. In fact it is now the same as if it were a corner grounded system. Not good to tie into a hot phase, but better than ----

Corner grounded 240V. There is no coupling or high impedance. You know the phase voltage to ground is 240V.

masterelect1 said:
... "I have never seen a corner grounded 240D. I can't think of a single reason why anyone would want one." (quote from carl)

-----I have not seen one either, hence the advice seeking. My understanding is so that the system will have a ground reference to trip the OCPD...
I don't agree with this. Bonded absolutely required. Grounded - no benificial effect known to me. Grounding doesn't trip breakers - Bonding does.

edited to fix formatting and grammar

carl
 
Last edited:

BigPower

New member
Location
Seattle, WA
Hope this helps

Hope this helps

I work for a utility and teach night school for the apprentices. I use Transformers by Kenneth Edwards and Transformers and Motors by George Schultz. Both are good references for transformers and are not overly complicated.

Our utility currently installs open delta banks in areas where growth is expected. When the demand increases, adding the third bank increases the 3 phase bank capacity almost 75%. We also use the center-tapped delta with a high leg in some locations because of the variety of voltages avaliable. We also install a center-tapped 480 'A' bank for the same reasons.

As far as the virtual grounding, we have a 13kV delta primary in our underground network. We use a zig-zag transformer (also called an interconnected wye) to provide a ground reference for protective relays.
 
Status
Not open for further replies.
Top