See the attached picture. I have come across a system where you'd have bunch of transformers feeding a busduct loop. The following questions are probably silly...

1) When you wire up a system like this, are the switchboards considered to be short circuited when feeding the busduct?

2) Some of the transformers feeding the busduct have either 440V secondary or 480V secondary voltage rating. Considering the taps on these transformers are set such that the secondary nominal voltage is 470V, what is the likelyhood of circulating currents being induced when the taps are slightly off?

Except for the 69kV/4.16kV transformer, all transformers are Y (ground) - Y (ground). All breakers are closed.

Thank you for all the help.

amohammed-

Are you sure that all the breakers are intended be closed? This looks more like a system of electrically operated breakers acting as transfer switches.

To operate transformers in parallel, impedances should be matched to within certain tolerances. With just a slight phase shift, circulating currents would be evident as soon as you closed the breaker, and it wouldn't stay closed very long. I don't have a link, but I believe Square D has some literature on this topic.

All the breakers feeding the busduct are definitely closed and are not transfer switches either. The transformer impedance varies from one transformer to the other. I am not sure if there are phase synchronizers on each branch...there may be some energy auditing equipment but thats all.

I will look up SquareD for more info. Thanks for the reference.

Teach me something with your circuit. What is the purpose? & Why is this configured this way? I'll look again, but I am not knowledgeable with this type of configuraton, maybe I overlooked something.

Ok, maybe I see what's going on. Apprently, this system offers redundant systems in case of a multipul failures. I see four feeders, four tie breakers.
Is that correct?

Nope, went back and seen all breakers are closed.
Hmm, still willing to learn how this system works.
Thanks

I did not create this circuit for my curiosty. It is an actual network found in an industrial facility. The busduct system I've depicted in the piece of paper is not even half of what is actually out there! The busduct loops around the building feeding power panels, plugs & recepticles, machines etc..

I have modeled this system using PTW exactly as shown in the oneline for this thread. I am uncertain how the system is operating with all the breakers closed. Apparantly this configuration does not lead to a short circuit condition and I need to understand why . I need to know how to model this busduct correctly so that it represents the actual condition.

Hmm, I know how to model the equipment in easypower but not SKM. I am sorry to ask again, but are you sure that the breaker is energized on both ends of the same busway from different sources?

There are hundreds of systems out there. Just never seen one like this with all breakers energized and not a tie. Zog, do you have any knowledge of how this system works?

there is definately no tie breaker on this loop and yes all the breakers are closed.

Yep, my guess is this is for welding bus. Seen it in a ton of car plants. Something about the more stable voltage with paralled transformers giving you a higher quality weld. Never mind the crazy high fault currents this operation mode causes. Are you exceeding your breakers interuption ratings? Are the breakers fused? (KDON, RLX, AKRU, etc)

Every plant I have seen this in has decided to reconfigure the system after seeing the monster Ei values and the fact thier breakers can't clear a fault.

Appears this supply arrangement has gone through several "upgrades" where it has always been less expensive to add a transformer than do a total gut and redesign. Unfortunately, as Zog points out, other extremely critical items so often get overlooked when it is done by the seat of the pants in house.

The fact that the transformer %Z varies a little is not much concern. Just means one or more will reach full load before the others. If they are heavily loaded and the impedance varies to much, then you may have a problem, otherwise not. Looks like they are paralled and that is done often (with maybe 2 not 4 units).

Not an issue of short circuit of the transformers.

Just make sure that you have modelled the combination correctly in your software. Often times it is not. Your concerns about why they have not short circuited leads me to believe that you may not understand the connection.

As with the previous post, your first and foremost concern should be the bracing of the gear and interrupting rating of the protective equipment. Then move to arc flash analysis after you are sure of what you have.

Hope it helps!

Zog:The breakers (main or submain) are not fitted with any current limiting fuses however you are right on the breaker type. Almost all are GE AKRT breakers with Amptect 1A trip units. These breakers are indeed failing by a small margin. I know what my recommendations will be now!

Acobb: Appreciate your valuable input on transformer paralleling. Will look more into it. I remember learning about ferroresonance problem with these systems.

As for the short circuit question....I told you it was a silly question! After thinking through what a short circuit meant I realized why it's not one.

Thanks.

Thanks Zog. I learn something every day.

Alan, I missed the second page. Thanks also.

The AKRT is 2000A frame, an AKRU would have current limiting fuses to allow for a 200kA interuption rating. Since the breakers are not AKRU's it appears Alan is right that this system was never designed to be onfigured this way and was a series of "upgrades" over the years. I can change the breakers to AKRU, which also involves some cell modifications, so it is an expensive option. That would solve your fault current and arc flash issues. But after they see all options, I bet they just decide to "break the bus", chaeper, easier solution.

Arc Flash Calculations

Please assist me,

I am using Easy Power for our Arc Flash Studies, it has been extremely difficult to obtain info from the utility company, Easy Power has Default Data
100000MVA (150X/R) 3 Phase and 100000MVA (150X/R) for SLG Short circuit,I have run different scenarios with some data but i have not seen significate differences regarding the incident Energy at the main bus after the X-mer,(12.3kV to 0.48kV) or after the 480 V main breaker, I appreciate any input on this matter

Thanks!

1) You are using an infinite source to run your study. The incident energies you've obtained across certain buses are probably lower than what's out there. You will need to reduce your source fault contribution to something more realistic. This could mean one more scenario for you to consider.
2) I don't know what to comment on the incident energies if you don't explain what your scenarios are. If you are refering the "main bus" to the bus on the line side of the main breaker then typically you should have higher incident energies on the line side of the main breaker than on the load side.

What would a generalized more realistic example of a source fault

Thanks!

Understand your pain with the utility, but you must "endeavor to perservere". Write them a letter, if you have not done so already, and ask for the R + jX values at your service in % or PU with the MVA base and the voltage. You can also ask for the service transformer size and %Z so you can see what they ultimately give you. Just got the exact same info for a utility customer from the provider utility over the phone last week.

Seems I have led a sheltered life and many utilities are reluctant to give complete data to their non-utility customers, even though you need it as well. Haze will probably say "du-huh". I would consider adding some verbage to the letter that their refusal to release information that they have available will only serve to increase the liability for both of you.

On another note, are you sure Easy Power is 100000 MVA and not 100 MVA? Also is the X/R actually 150 and not 15? Not familiar with the software, but the values seem odd at best.

OOPS....Forgot to attempt to answer your last question. Regarding what would be a more realistic supply, this exact issue is being discussed on this forum as we speak. Search a bit.

Hope it helps!