Tie Breaker

[Humble2000]

A 600v bus is fed from two transformer connected each side of the bus through breaker. A tie breaker is between the east and west side of the bus.
Customer is reaching the maximum capacity of one of the transformer. I asked why we can't close the tie breaker and transformers can share the load. They looked at me like it was a very dumb question!!!
It raises many issues, higher fault level. That was only one concern. I appreciate any feedback on the subject.

 

[davidbeach]

If the gear was not specified with closed tie operation in mind it will not have the interrupting ratings to deal with closed tie fault currents. Closed tie operation subjects all loads to any issues that would be seen by either bus (downstream faults, etc.) rather than only half the loads. Running closed tie will also allow current to flow through from one primary to the other primary; depending on the nature of the two sources this many be of no consequence or it could result in significant through current. Lots of other issues; if everything was addressed in the original design for closed tie, no problem but if not it could be a major undertaking.

 

[alehman]

Under some conditions it may be possible to operate the transformers in parallel. There are several issues with this arrangement, so it may be much simpler if you could divide the load between the transformers so the tie may remain normally open.

To operate with the tie closed (transformers in parallel), you will need specialized overcurrent protection. In north America, this is commonly referred to as a "spot network". This commonly requires directional current sensing. This would be a job for an experienced engineer.

 

[davidbeach]

It could be done without the need for network protectors, that would be an additional mess, but there would certainly need to be directional protection on both mains.

If the original design was done with closed tie operation in mind, it is highly likely that present operation would be closed tie. Since the present operation isn't closed tie it is highly unlikely that you can make closed tie operation work.

 

[dwftx34]

Another thought many systems with tie-breakers and two utility transformer have two different grid feeds. If this is the case then the closure of the tie would cause a fault across the tie. Most systems with ties on the secondary side of the utility transformers is to provide a redundant source in the case of a grid or transformer failure.

 

[dinkelja]

Does the double ended substation have some variety of interlock? One purpose being to prevent backfeeding of a faulted primary feeder. All of ours are spec'd with simple Kirk-Keys. I would investigate trying to balance the loads better before moving to a different topology. Do you have any data on power flow?

 

[rbulsara]

While technically it is possible to parallel the transformers, by closing the tie there are other aspects to be considered.

1. If the dual ended substation (two-transformer) design was originally implemented for redundancy, that is to maintain power upon failure of one of the transformers or the service, operating in them parallel defeats that requirement. So paralleling the two units may not be a correct strategic answer.

2. Are the transformers fed from two utility services? Most utility companies would not permit parallel operation of customer transformers on two separate feeders, regardless of technical feasibility.

I think most other points are made earlier.

 

[apowerengr]

Humble - can you swap a couple feeder loads and create a better load-share between the transfomrers thus allowing the tie breaker to remain open?

 

[Humble2000]

Both transformer have same grid feed. Transformers are exactly the same.
Yes, there is a key interlock between the tie and each breaker on the secondary of each transformer, meaning one should be open to close the tie.
Switching loads is not easy. There are cables that need to re-rout and other complications.
dinkelja"
One purpose being to prevent back feeding of a faulted primary feeder.
You mean if a fault happens at the primary of one transformer the other transformer will contribute to that?
or you mean it prevents back feeding of a faulted secondary feeder?

 

[Borti]

The first thing you should check is the short circuit rating of the low voltage switchgear. It is very likely that it is not high enough to allow for the paralleling of the two transformers.

 

[peebee]

I agree with Borti and others above. Parallelling the two transformers will roughly double the available fault current, which may well exceed the rating of the equipment.

Another note, which I did not see mentioned above -- presumably, the whole reason that a double-ended substation was originally provided was to ensure redundancy in your transformation. If your load has grown to the point where it exceeds the rating of a single transformer's maximum rating, then you have lost that redundancy. Which is just fine, so long as you realize that this has happened. In the event of a failure, you will be down until the transformer is replaced; if you have redundancy, all you need to do is switch to the other transformer.

Here is another consideration, which might well be a solution for you: fan rating. Do either of your existing transformers have fan cooling? If not, it could probably be added. That will usually buy you about 12% additional capacity. If you don't have fans, this may be an easy out for you.

 

[peebee]

And regarding your Kirk keys -- they are a safety feature, there to prevent paralleling your two transformers. Think twice (and check with an expert, in person, not just via engtips.com) before disabling them.