Short time Paralleling in M-T-M configuration

[Charz]

When would be short time paralleling (momentary paralleling) required in M-T-M configuration?
Do I need to have synchro check relays in both the Mains and the bus-tie for this?
What are the things I need to consider while doing this?

 

[xptpcrewx]

When would be short time paralleling (momentary paralleling) required in M-T-M configuration?
Do I need to have synchro check relays in both the Mains and the bus-tie for this?
What are the things I need to consider while doing this?

Momentary paralleling is required anytime you want to avoid momentary service interruptions when transitioning between split bus and emergency configurations.

Sync check relays are a must. Bus differential protection, multi source ground fault, ZSI and RELT functions are all ideal.

You should consider the equipment/device duty and withstand ratings of the gear since the fault current is basically now doubled. Also, selective coordination between main, tie and feeder breakers can provide some degree of reliability with closed transition systems.


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[ron]

Closed transition (short time paralleling / momentary paralleling / make before break) is never required, you can always do open transition (break before make).

 

[xptpcrewx]

Closed transition (short time paralleling / momentary paralleling / make before break) is never required, you can always do open transition (break before make).

You are tight about there not being an installation requirement to have a closed transition system, but there are reliability and serviceability requirements which are usually driven by a customer, design/specification, industry, or continuous industrial process. (Think hospital, data center, emergency systems, etc)


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[paulengr]

You would think sync check wouldn’t be required but once in a while somebody makes a mistake and swaps two phases or puts a delta transformer in for wye. Otherwise it is never needed on a basic MTM fed from a common source. Each of the other protective functions mentioned may or may not make sense. I’m a minimalist...the more protective functions you have the more nuisance problems you will have.

 

[xptpcrewx]

You would think sync check wouldn’t be required but once in a while somebody makes a mistake and swaps two phases or puts a delta transformer in for wye. Otherwise it is never needed on a basic MTM fed from a common source. Each of the other protective functions mentioned may or may not make sense. I’m a minimalist...the more protective functions you have the more nuisance problems you will have.

Feeding a MTM from a common source is what doesn’t make a whole lot of sense...
At the end of the day, there is a common source somewhere, but there needs to be adequate separation of primary feeders.

Can MTM using common sources be done while still offering some advantage? Sure. But this isn’t the proper application of a MTM substation arrangement.

Keep in mind, sync check isn’t only for preventing parallel operation with swapped phases or the rare case where a delta transformer gets installed as you’ve mentioned.

For power transformers with OLTC’s, the voltage between each source can be at different magnitudes due to loading, settings differences or equipment failure. Persisting single phasing conditions may also exist with open fuses. Also consider it may be the utility that has swapped phases in service fed MTM applications. You wouldn’t want to parallel under any of these conditions. I’ve seen them all.

All the functions listed make sense. RELT or bus differential is required per the NEC. Not having MSGF is actually well known to cause both nuisance tripping and desensitized operation on 3PH, 4W systems. ZSI may be the only practical way selectively coordinate with downstream and upstream equipment.

Of course if you do not know how any of these things work and don’t properly set up or commission the system to begin with, you can expect nuisance tripping regardless (minimalist or not).


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[ron]

You are right about there not being an installation requirement to have a closed transition system, but there are reliability and serviceability requirements which are usually driven by a customer, design/specification, industry, or continuous industrial process. (Think hospital, data center, emergency systems, etc)

I design a lot of critical facilities, and you would be surprised how many decide to go with open transition when they realize the additional cost of the higher short circuit ratings that come along with ~2x fault current.

 

[jim dungar]

I design a lot of critical facilities, and you would be surprised how many decide to go with open transition when they realize the additional cost of the higher short circuit ratings that come along with ~2x fault current.

Just a side note, that has nothing to do with the paralleling question. I am seeing a bit of a trend to using a MTTM arrangement. This allows a tie device to be totally deenergized for maintenance, because of the two ties being in series. In a standard MTM arrangement the Tie device always has one side energized which often forces a complete outage for routine maintenance.

 

[xptpcrewx]

I design a lot of critical facilities, and you would be surprised how many decide to go with open transition when they realize the additional cost of the higher short circuit ratings that come along with ~2x fault current.

So the ATS’s when switching back over to normal from being on emergency are open transition???


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[xptpcrewx]

Just a side note, that has nothing to do with the paralleling question. I am seeing a bit of a trend to using a MTTM arrangement. This allows a tie device to be totally deenergized for maintenance, because of the two ties being in series. In a standard MTM arrangement the Tie device always has one side energized which often forces a complete outage for routine maintenance.

But isn’t one side of each tie still energized with the MTTM arrangement?


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[zbang]

Only when both mains are on- open one main and open both ties, and the tie closer to the open main is now serviceable.

 

[xptpcrewx]

Only when both mains are on- open one main and open both ties, and the tie closer to the open main is now serviceable.

But you have to drop half the bus load. Doesn’t seem practical or worth it just to service the opposite end of the tie. It’s really no different than a MTM setup.


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[zbang]

See post #8. And if you're going to service either main, you have to drop the loads on that bus section; no way around it that I can see. With an MTM setup, how do you service the tie without dropping both buses?

It's all about what you service, when, and what you can afford to shut down to do it. If a complete shutdown is preferred, or at least tolerated, that's one thing (or if you plan on never servicing the gear), but if you need to keep some things running, the design becomes different. Heck, you might have some loads on an transfer switch between bus sections just for this purpose.

 

[xptpcrewx]

See post #8. And if you're going to service either main, you have to drop the loads on that bus section; no way around it that I can see. With an MTM setup, how do you service the tie without dropping both buses?

It's all about what you service, when, and what you can afford to shut down to do it. If a complete shutdown is preferred, or at least tolerated, that's one thing (or if you plan on never servicing the gear), but if you need to keep some things running, the design becomes different. Heck, you might have some loads on an transfer switch between bus sections just for this purpose.

This is exactly my point. You aren’t really getting anything with an extra tie breaker. Basically a similar issue as servicing any main or feeder breaker.

If you are ok with dropping half the load, then MTM works fine. You can drop the load and rack out the tie on a live bus (one side only). Utilize RELT and/or a remote racking mechanism. With shutters in place, service the cubicle. There really isn’t anything you need to do to the bus that would not require a complete outage anyway.


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[jim dungar]

This is exactly my point. You aren’t really getting anything with an extra tie.
...
Utilize RELT and/or a remote racking mechanism. With shutters in place, service the cubicle. There really isn’t anything you need to do to the bus that would not require a complete outage anyway.

My customers will not allow you to work in a cubicle whose only protection from voltage is a set of shutters which can bypassed and manually opened.

It is not uncommon to use both sides of a MTM lineup to feed the same equipment (for redundancy) rather than supply additional load. Breaker racking and conductor terminations are items with much high probability of failure than transformers and switchgear bussing. I know of several people that think of MTM as a way to have two full rated services, but they forget that total capacity is really limited by the rating of the Tie.

 

[xptpcrewx]

My customers will not allow you to work in a cubicle whose only protection from voltage is a set of shutters which can bypassed and manually opened.

It is not uncommon to use both sides of a MTM lineup to feed the same equipment (for redundancy) rather than supply additional load. Breaker racking and conductor terminations are items with much high probability of failure than transformers and switchgear bussing. I know of several people that think of MTM as a way to have two full rated services, but they forget that total capacity is really limited by the rating of the Tie.

Reasonable point however shutters can be safely locked out. And yes, having redundant feeds to equipment is the ideal setup. In my opinion a property designed MTM arrangement will size the tie for the largest total connected load on either bus and further coordinate tie breaker settings with each main. Tie breakers need not have the same continuous current rating as the mains but it’s smart to size everything the same and have them be interchangeable as needed for parts, maintenance, simplicity and future expansion...


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[ron]

So the ATS’s when switching back over to normal from being on emergency are open transition???

Yes sir, open transition both directions. That is the only way to avoid having to account for the extra fault current in the downstream short circuit equipment rating. Don't get me wrong, I do have clients that have applications that use closed transition, but many are open.

 

[zbang]

My customers will not allow you to work in a cubicle whose only protection from voltage is a set of shutters which can bypassed and manually opened.

And that's kind of it, if the customer wants a higher level of interlocks/lock-outs and is willing to pay for those, why argue? Kind of like scissor lifts, OSHA says you don't need fall protection to work in one, but the site/customer might.

 

[Russs57]

I think I am failing to understand something here. It could be terminology but I assume we are talking main-tie-main normal power switch gear. If that is the case, my job responsibilities require I correct my misunderstanding so please bear with any dumb questions. Assume I have rack out mains and tie breaker (I understand that likely changes everything that has been said in this post).

1) Why can't I rack out the tie breaker/switch and service it at anytime when both main feeders are available?

2) If both service feeders are available and I can tolerate a "blink", why can't I open a main, close the tie, rack out the open main, and service it at anytime?

3) When both service feeders are available, and both main breakers are closed, wouldn't the open tie breaker/switch have voltage present on both sides of it?

FWIW I am at a hospital. We pay the extra money for a non-interruptible POCO source. I expect working POCO ATS's with at least two preferred and one emergency feeder in their vault. All my important loads will have ATS's with generator power and all my really critical loads will have UPS's between those ATS's and loads. Personally I would want to know when something is wrong in the vault/sub station even if it means a partial loss of normal power and the inconvenience of opening a main and closing a tie. If it was automatic I'd be afraid something might go unnoticed. Staffing and skill level isn't what it used to be

 

[ron]

1) Why can't I rack out the tie breaker/switch and service it at anytime when both main feeders are available?

2) If both service feeders are available and I can tolerate a "blink", why can't I open a main, close the tie, rack out the open main, and service it at anytime?

3) When both service feeders are available, and both main breakers are closed, wouldn't the open tie breaker/switch have voltage present on both sides of it?

It would be easier to do this with a picture, but if you rack out the tie breaker and service it when both main feeders are available, the line and load sides of the tie breaker are not de-energized and can't be serviced. The only way to de-energize the line and load sides of the tie, is to de-energized one of the sources and open the tie for an extended period of time to service that end of the tie, and that would only happen if it were magically not in the same cubicle with the "other" side of the tie that is still energized, which would make the overall task of servicing the tie energized, and not doable in most company's work practice's.

That's why the addition of a second tie, makes it possible to perform maintenance to one side of the tie at a time without being in the energized cubicle.