Got a question about main-tie-main systems. We have one of these in our plant where the HV bushings on one of the transformers went bad. Therefore we had to open that particular main, and close the tie. We have new bushings installed and we're ready to put the transformer back into service. The question I have is, can we re-energize the main while keeping the tie closed? Or are we back-feeding the other main? Do we have to open the tie-breaker, and then re-energize that main?
Main-Tie-Main Question
- Thread starter adamscb
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Saturn_Europa
Senior Member
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- Fishing Industry
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- Electrician Limited License NC
There are three breakers. Two mains and one tie. Only two can be closed at one time.
Open tie breaker then close main.
This MCC or switch gear doesn't have Kirk keys?
See picture for our system. And I'm not sure if it has Kirk keys. I think we will have to just open/close breakers.
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This is a good question. I would say that the best way would be as described by Saturn. It may be possible to parallel the 2 transformers but they would need to be on the same utility feed, the same kva, use the same voltage taps and have same impedance. Also the transformers phasing would have to be the same, if not, bad things would happen. I would not try it unless you know that it was set up to switch that way. I have only seen it done by first opening the tiebreaker then closing the other main. I think it usually has a kirk key setup to prevent out of sequence operation.
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big john
Senior Member
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- Portland, ME
There are very likely to be interlocks that prevent all three breakers from being closed simultaneously.
If the interlocks are not present then it definitely may be designed to close both mains prior to opening the tie, I have seen that done. But like Coop said, you need to be very sure of how it is installed because you are paralleling the two transformers.
If the interlocks are not present then it definitely may be designed to close both mains prior to opening the tie, I have seen that done. But like Coop said, you need to be very sure of how it is installed because you are paralleling the two transformers.
Tony S
Senior Member
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- Resting under the Major Oak UK
I’ve done this a few times with no problems but only when the HV supply to the transformers has been on the same bus. Bare in mind, during the time the transformers are paralleled the available fault current is liable to exceed the rating of the LV panel. It’s a calculated risk and only you can make the decision.
I’ve got the same procedure to do over the christmas break to take transformers out of service for maintenance. The three breakers are key interlocked but as an authorised person I have master keys.
I’ve got the same procedure to do over the christmas break to take transformers out of service for maintenance. The three breakers are key interlocked but as an authorised person I have master keys.
I assume that you lost power on that line for some time when the bushing failed. If so the time that you would be down on that side of the switchgear in order o make a transfer would be much less. Taking that in consideration is it imperative that you take a risk of having a catastrophic in paralleling both sources in oder to make the transfer?
No, but they should be close. They will share the load in proportion to their impedances.
I would be concerned about the "should be" statement. Is it worth the risk?
When power had to have been lost because of the original failure the brief time it takes for a loss of power to open the tie breaker and close the second source should not be an issue compared to risking a catastrophic failure to be a hero.
That would be like splitting hairs there, knowing no two transformers are exactly the same! "should be close" can be taken taken as "very little difference", IMO.
See example 3, page 5 of http://static.schneider-electric.us...Transformers/General Documents/7400DB0701.pdf.
Actually look at all of the different schemes for paralleling.
Actually look at all of the different schemes for paralleling.
Huh? :?
That is a relative statement. Even a slight phase shift could end up in a massive failure but it appears as though you would be willing to take the risk of a posibleloss of life and thousands of dollars of losses in addition to a massive down time.
Makes sense to me all for disconnecting powr for r a very brief moment in order to open the tie breaker and close the second one breaker. And, just what would you define as to what very little difference would be allowable?
Where are you getting a phase shift from?
OK. What assurance is there that it will be within that parameter? Is it worth the risk rather than a very brief power outage in order to open the tie and close the main?
You walk up and look at the transformer nameplates.
Should there be a failure.becuse of a slight out of phase failure and a posible loss of life then yu you are innocent as yo simply walked up and liked at the nameplates. It wasn't you fault. And to think that all yo u would have had to do is to briefly open the tie breaker first and then close the main . Power would have Ben last for an extended length of time due to the original failure and much less than a minute to make here transfer. Why would there be K-K intlks supplied to prevent paralleling sources?
Your posts are painful to try to decipher. Now you are creating K-K interlocks out of thin air as well as an impossible phase shift.
Interesting. There are those who don't understand the reason and design of double ended switchgear. It is careless to asume that both sources are feed such that they can be paralleled. The purpose of the design is based upon 2 separate sources such dependent upon the reliability of each source not that hey can be paralleled. You are assuming that both sources originate from a single source where the primaries of each transformer are being supplied in an identical way. If power is lost from that source what would be the advantage of double ended switchgear if that source of power of lost? Yes, if both ends of the switchgear are feed from the same source in a basically identical manor then you may be correct to 'assume' that you would be able to parallel both ends of the gear without incident.
I asked a question if there is an issue with a very brief loss of power in the time that it takes to open the tie breaker and then close the second end of the switch gear and not risk the possibility of a failure if the two sources are parallel? It does't surprise me that I got no answer.
I have sold double ended switchgear, both LV and MV, for nearly 20 years for a major manufacturer and all were specified and built with K-K interlocks to prevent the paralleling of the two separated sources. I wonder why? For what purpose? Are you aware as to what the purpose of K-K inrlocks are? It intregued me that you would be willing to assume both the risk and liability to attempt to parallel the two separate sources and condone others to do it.
It is important to consider things beyon the end of ones nose and the distribution system and how and why it has been designed. It just isn't limited to the (2) transformers which supply power to the switchgear. It starts upstream from there to their souce of power. Do you know for what that is? Has it been designed such that both sources can be paralleled? Are you assuming that they are?
If you are assuming the results may be more than 'painfull.'
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