Substation Tie breakers

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Lady Engineer

Senior Member
Location
New Jersey
Hi everyone,

I have a problem that I haven't encountered before. I've been doing MV substations for a little bit now, but one thing I've never seen is an undersized tie breaker.

I'm doing a Waste Water Treatment Plan upgrade. I may need to add another unit substation. There is one MV outdoor substation that feeds 4 unit substations, which are all located indoors. Every tie bus breaker (480V) in these substations is sized for half of the bus rating. I've always used fully rated tie breakers, but can't seem to find a reason to provide a tie half the size. The bus is rated for 2500A at 480V, the secondary breakers are rated at 2500A, but the tie bus breaker is rated 1200A.

What do you think? :?
 

templdl

Senior Member
Location
Wisconsin
Hi everyone,

I have a problem that I haven't encountered before. I've been doing MV substations for a little bit now, but one thing I've never seen is an undersized tie breaker.

I'm doing a Waste Water Treatment Plan upgrade. I may need to add another unit substation. There is one MV outdoor substation that feeds 4 unit substations, which are all located indoors. Every tie bus breaker (480V) in these substations is sized for half of the bus rating. I've always used fully rated tie breakers, but can't seem to find a reason to provide a tie half the size. The bus is rated for 2500A at 480V, the secondary breakers are rated at 2500A, but the tie bus breaker is rated 1200A.

What do you think? :?
I've quoted quite a few myself and have never quoted anything as such as I can recall. Is the main breaker feeding each 2500A main but section 2500A? If so then each section is 2500a so if you open one of the main breakers because if a loss of power that means that the other section's main 2500A breaker remains closed. Then, when you close the tie breaker that means that the 2500a main must not only supply the loads for its 2500A bus but also is being asked to supply the loads of the other 2500A bus. As such it may make sense to protected the 2500a main breaker by assuring that it wouldn't be tripped by supplying the additional loads if the other bus.
 

Lady Engineer

Senior Member
Location
New Jersey
I've quoted quite a few myself and have never quoted anything as such as I can recall. Is the main breaker feeding each 2500A main but section 2500A? If so then each section is 2500a so if you open one of the main breakers because if a loss of power that means that the other section's main 2500A breaker remains closed. Then, when you close the tie breaker that means that the 2500a main must not only supply the loads for its 2500A bus but also is being asked to supply the loads of the other 2500A bus. As such it may make sense to protected the 2500a main breaker by assuring that it wouldn't be tripped by supplying the additional loads if the other bus.


Here's the deal...

There are two secondary breakers each 2500A. The primaries (2) are each 600A each. The tie is the only breaker undersized. It looks like the tie is N.O., but not sure.
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
It depends primarily on how the tie-breaker is used. If it?s just a ?dumb-switch? and doesn?t provide actual fault overcurrent protection, it may be fine; it only has to be able to carry and interrupt the load current of the more highly loaded bus. In any case, it is subject to Section 110.9, particularly the second paragraph.
 

ron

Senior Member
It depends primarily on how the tie-breaker is used. If it’s just a “dumb-switch” and doesn’t provide actual fault overcurrent protection, it may be fine; it only has to be able to carry and interrupt the load current of the more highly loaded bus. In any case, it is subject to Section 110.9, particularly the second paragraph.
Agreed, it only has to carry the load of one side to the "other" source. No need to be fully rated for ampacity. Short circuit ratings need to be full. I often make it fully rated so by default I get a spare main when withdrawn.
 

zog

Senior Member
Location
Charlotte, NC
Agreed, it only has to carry the load of one side to the "other" source. No need to be fully rated for ampacity. Short circuit ratings need to be full. I often make it fully rated so by default I get a spare main when withdrawn.

Yep, took the words out of my mouth. The tie only has to carry load for one side (Mains have to suppy both is sub is in single ended mode)so it can be smaller. You can save a little money that way. But when a main goes down and you need a spare you will wish you had the tie the same size.
 

templdl

Senior Member
Location
Wisconsin
Here's the deal...

There are two secondary breakers each 2500A. The primaries (2) are each 600A each. The tie is the only breaker undersized. It looks like the tie is N.O., but not sure.

Do you have a one-line of one for one of the lineups that you can scan and post? Usually there is a "main" breaker that feeds each of the bus, each bus providing power to a number of feeder breakers then a tie breaker between the 2 bus which will tie tihe two bus together when closed. What are the sizes of the mains? You have inferered that the mai (primary) breakers are 600a each and the tie breaker 1200a.
Your question would be a piece of cake to answer if we could see a one line.

I have attached 2 pictures of 2500KVA double ended unit substations each which happen to have (2) 3200A each w/3200AT mains and a 3200A w/3200AT tie breakers.
I do have a one line for it but it's a 'D' size drawing and too large to scan.
 

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Lady Engineer

Senior Member
Location
New Jersey
Do you have a one-line of one for one of the lineups that you can scan and post? Usually there is a "main" breaker that feeds each of the bus, each bus providing power to a number of feeder breakers then a tie breaker between the 2 bus which will tie tihe two bus together when closed. What are the sizes of the mains? You have inferered that the mai (primary) breakers are 600a each and the tie breaker 1200a.
Your question would be a piece of cake to answer if we could see a one line.

I have attached 2 pictures of 2500KVA double ended unit substations each which happen to have (2) 3200A each w/3200AT mains and a 3200A w/3200AT tie breakers.
I do have a one line for it but it's a 'D' size drawing and too large to scan.

The secondaries are the mains, and they are 2500A each. The Primary breakers on the 1500KVA transformers are 600A each. The tie, as I stated is 1200A.

I do substations all the time, I just have never used an undersized tie breaker. Not really too complicated, my friend, I just wanted to know why would anyone use a undersized tie breaker.

I just installed a Unit sub with (2) 3000A mains, and a 3000A tie two months ago. I don't undersize my tie, usually.
 

templdl

Senior Member
Location
Wisconsin
The secondaries are the mains, and they are 2500A each. The Primary breakers on the 1500KVA transformers are 600A each. The tie, as I stated is 1200A.

I do substations all the time, I just have never used an undersized tie breaker. Not really too complicated, my friend, I just wanted to know why would anyone use a undersized tie breaker.

I just installed a Unit sub with (2) 3000A mains, and a 3000A tie two months ago. I don't undersize my tie, usually.
Congratulations on the 3000a unit sub as those jobs don't come along too often. Great job!!
I agree with what one of the other guys said regarding interchangeability though. If it is critical swgr then you want to keep your options open if one of your sec. Mains go south.
Another thought, if the tie is undersized and you want to provide full capacity to the other bus you're SOL. Other than saving money of a smaller tie breaker they could have shot themselves in the foot as far as versatility is concerned. If your customer knows what the limitations are then there should be no issue other be a bit unusual.
I guess you guys have been getting a bit of cool weather out there lately.
 

Lady Engineer

Senior Member
Location
New Jersey
Congratulations on the 3000a unit sub as those jobs don't come along too often. Great job!!
I agree with what one of the other guys said regarding interchangeability though. If it is critical swgr then you want to keep your options open if one of your sec. Mains go south.
Another thought, if the tie is undersized and you want to provide full capacity to the other bus you're SOL. Other than saving money of a smaller tie breaker they could have shot themselves in the foot as far as versatility is concerned. If your customer knows what the limitations are then there should be no issue other be a bit unusual.
I guess you guys have been getting a bit of cool weather out there lately.

I'm glad you said that, because I just didn't understand it. You guys have cleared that one up. Here's a question: Is it worth the extra money? Getting the full use of the bus seems more logical, you think?

Side Note: Yeah, I don't know what happened to it. Yikes, please bring back the warm weather. :)
 
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Lady Engineer

Senior Member
Location
New Jersey
I do not have a clue about how to answer your answer your question, but I wanted to say " Hi. How ya doing?" I have not seen you post in awhile.

Hey Jumper! How are you? I've been pretty busy here, but I search for the answers on the website. Have you been busy, too?

You know what...sometimes engineers think they know everything until they find out they don't know everything. lol In my case, I have a lot on my shoulders with the utility, but I try to keep the attitude that there's always something you can learn.
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
... Here's a question: Is it worth the extra money? Getting the full use of the bus seems more logical, you think?
It depends on the purpose of the tie. Having done several FMEA (Failure Modes and Effects Analysis) as recommended by NFPA 70B, Section 26.5 over the years, including the largest refinery in the world, and battling CMP1 over Section 110.9 implications for short-circuit duties, I’ve come to the conclusion that eliminating the tie altogether is probably the most cost effective, reliable and safest solution.

Most people don't recognize that, for continuous process facilities, it's more important to provide spare load capacity than spare source capacity.

I’ve attached a sketch with various potential distribution systems. Statistically, Figure “g” is the most reliable since it has one less component (the tie breaker) to fail. It is only slight, but it is statistically significant if the "live bus" side includes automatic starts. It’s obviously less expensive. It also is safer since the entire facility can be operated with one operating train shut down for maintanence. Most still opt for the belt and suspenders of Figure “e” but that has a modest additional safety exposure depending on the layout. If a closed transition is included, then NEC 110.9 also has a tremendous affect on the interrupting duties. View attachment Distribution.pdf
 

Lady Engineer

Senior Member
Location
New Jersey
Hey Jumper! How are you? I've been pretty busy here, but I search for the answers on the website. Have you been busy, too?

You know what...sometimes engineers think they know everything until they find out they don't know everything. lol In my case, I have a lot on my shoulders with the utility, but I try to keep the attitude that there's always something you can learn.

Cool, thanks for the PDF.
 

jumper

Senior Member
It depends on the purpose of the tie. Having done several FMEA (Failure Modes and Effects Analysis) as recommended by NFPA 70B, Section 26.5 over the years, including the largest refinery in the world, and battling CMP1 over Section 110.9 implications for short-circuit duties, I?ve come to the conclusion that eliminating the tie altogether is probably the most cost effective, reliable and safest solution.

Most people don't recognize that, for continuous process facilities, it's more important to provide spare load capacity than spare source capacity.

I?ve attached a sketch with various potential distribution systems. Statistically, Figure ?g? is the most reliable since it has one less component (the tie breaker) to fail. It is only slight, but it is statistically significant if the "live bus" side includes automatic starts. It?s obviously less expensive. It also is safer since the entire facility can be operated with one operating train shut down for maintanence. Most still opt for the belt and suspenders of Figure ?e? but that has a modest additional safety exposure depending on the layout. If a closed transition is included, then NEC 110.9 also has a tremendous affect on the interrupting duties. View attachment 6846

Cool, thanks for the PDF.

Ma'am, I am pretty sure this is what you meant to do.
 

MEP_PM

Member
rbalex,
Thanks for sharing the diagrams. I don't think I've ever seen that many different configuration shown side by side. However, I've seen about half of them installed on various projects.

Most people don't recognize that, for continuous process facilities, it's more important to provide spare load capacity than spare source capacity.
View attachment 6846

Can you further explain this statement?
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
A fully rated tie in critical processes could be necessary because you could have multiple sets of (redundant) loads that are 4 x 50%. Meaning only two need to be operating at one time, say 1A and 1B.

You would load Bus 1 with say LD1A, and LD1C and Bus 2 with LD1B and LD1D. Each main is fully capable of carrying the full load, including some spare capacity, and the plant could be fully functional running completely off of either Bus 1 or Bus 2. In this case with the tie closed, you would have to have a fully rated tie breaker since all load could be on the opposite side.

More times than not I think you would use a fully rated tie. The cost differential between say a 1200A breaker and 2000A is not going to be that much, and fully rated provides long term flexibility.
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
...Can you further explain this statement?
For properly designed continuous process operations, such as refineries, for motor driven loads, mechanical failures are far more likely than upstream electrical failures to disrupt operations.

Where redundant drives are provided, load reacquisition by reacceleration for Main-Tie-Main (M-T-M) is no better than simply automatically starting the redundant driver on the healthy partner bus. Both techniques require some degree of automatic load reacquisition. Depending on the ?stiffness? of the electrical system, some delay in load reacquisition is necessary anyway.

Actually, for any electrical load, motor driven or not, the load?s service reliability, is statistically only a bit better than the load?s independent reliability, no matter how many alternate power sources are available; i.e., if the load itself fails alternate power sources won?t help. If the load?s service is actually critical it should have a redundant online spare.

One of the implications of Section 110.9, first paragraph, is that the short-circuit interrupting rating must be based on all available sources at the line terminals for any closed transition transfers. This affects both the M-T-M and all downstream OCPDs ? even if the transition is automatically controlled, momentary and none of the M-T-M devices will "see" the combined available fault current. While it is possible that a downstream OCPD could see the combined currents, that would mean a downstream fault would have to occur during the transition, but the transition could not be the cause of ther downstream fault.

I've decided a tie has little value if the critical loads are redundant. Since operation at full capacity on one train is available, increased safety is also a benefit because one train may be completely deenergized - with no "tie" lurking in the background.
 
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