Double Ended Substation / Kirk Key Needed?

[io748]

My facility has 9 double ended substations (M-T-M) that have kirk key interlocks. When our facility was constructed it was fed from two substation transformers, whose primaries were connected to the same feed from the utility. Some time later one of the transformers was removed, and both main feeds into the plant were connected to the same source.

Question 1: It has been suggested to me that the kirk key system could be bypassed safely, since both sides of each substation are fed from the same source. This is desirable as I could test the main breakers and switches with no outages. Does anyone see any issues with bypassing the kirk key system, given this situation?

Question 2: Each main breaker is 4000A, and the tie breaker is 4000A. The feeder breakers on one side total 4000A. So if you have the tie breaker closed and one main open, 8000A of breakers will be fed from one 4000A main. Why would the founding fathers of this plant have not double sized the main breakers?

As an ex-controls guy who is just starting a new role as a facilities electrical engineer, any input is appreciated.

 

[wireguru]

doesnt seem kosher. without the keying, you could close both mains and the tie and have 8000a available to either side. another thing would be to make sure the phasing didnt get messed up when the xfmr was removed and the feed connected to the other xfmr, that would make quite the boom if both mains were closed.

 

[zog]

My facility has 9 double ended substations (M-T-M) that have kirk key interlocks. When our facility was constructed it was fed from two substation transformers, whose primaries were connected to the same feed from the utility. Some time later one of the transformers was removed, and both main feeds into the plant were connected to the same source.

Question 1: It has been suggested to me that the kirk key system could be bypassed safely, since both sides of each substation are fed from the same source. This is desirable as I could test the main breakers and switches with no outages. Does anyone see any issues with bypassing the kirk key system, given this situation?

Question 2: Each main breaker is 4000A, and the tie breaker is 4000A. The feeder breakers on one side total 4000A. So if you have the tie breaker closed and one main open, 8000A of breakers will be fed from one 4000A main. Why would the founding fathers of this plant have not double sized the main breakers?

As an ex-controls guy who is just starting a new role as a facilities electrical engineer, any input is appreciated.

The size of the mains are designed for te toal load on both sides in a typical M-T-M configuration. The total frame sizes added up of the feeders are not that important and what you have is typical. What is more important is your total load and the settings on all the feeders being coordinated with the main and tie settings.

Now back to your bypassing the key interlocks question. As I understand both of your mains are now being fed from the same transformer, is that correct? A sketch of your primary distribution system would help here or if you like I can come over there and take a look at what you have.

 

[jim dungar]

Question 2: Each main breaker is 4000A, and the tie breaker is 4000A. The feeder breakers on one side total 4000A. So if you have the tie breaker closed and one main open, 8000A of breakers will be fed from one 4000A main. Why would the founding fathers of this plant have not double sized the main breakers?

The intent was to provide redundancy not additional capacity.

 

[io748]

Yes, both mains are fed from the same transformer. To verify that phasing is correct I checked voltage between A-A, B-B, and C-C on both sides of one of the low voltage boards, and had negligable difference (2-3 volts). On top of that the tie breakers have been used before in the past with no issues (motors running backwards, etc.). And on top of all that, we have a tie breaker between our two mains that is used regularly when we run our gensets (we run about 10MW of gensets in parallel for peak shaving from time to time). My feeling is that the kirk keys were originally installed and needed because we had two utility substation transformers. Since we only have one now, and since phasing is correct, would it not be safe to bypass the kirk keys and close the tie with both sides energized?

The size of the mains are designed for te toal load on both sides in a typical M-T-M configuration. The total frame sizes added up of the feeders are not that important and what you have is typical. What is more important is your total load and the settings on all the feeders being coordinated with the main and tie settings.

Now back to your bypassing the key interlocks question. As I understand both of your mains are now being fed from the same transformer, is that correct? A sketch of your primary distribution system would help here or if you like I can come over there and take a look at what you have.

 

[zog]

Yes, both mains are fed from the same transformer. To verify that phasing is correct I checked voltage between A-A, B-B, and C-C on both sides of one of the low voltage boards, and had negligable difference (2-3 volts). On top of that the tie breakers have been used before in the past with no issues (motors running backwards, etc.). And on top of all that, we have a tie breaker between our two mains that is used regularly when we run our gensets (we run about 10MW of gensets in parallel for peak shaving from time to time). My feeling is that the kirk keys were originally installed and needed because we had two utility substation transformers. Since we only have one now, and since phasing is correct, would it not be safe to bypass the kirk keys and close the tie with both sides energized?

From what you are describing I would tend to agree with you, they were needed in the old configuration and are not necessary anymore. Then again, I could be missing some info here so dont just go with the "Zog said it is OK" theory.

 

[iwire]

Is the busing rated for 8000 or 4000 amps?

If not 8000 amp busing what NEC section allows supplying it with that level of over current protection even for short periods of time?

 

[io748]

Understood, thanks for the input.

From what you are describing I would tend to agree with you, they were needed in the old configuration and are not necessary anymore. Then again, I could be missing some info here so dont just go with the "Zog said it is OK" theory.

 

[jghrist]

It may be a little difficult to discern the original intent of the Founding Fathers :roll: without the equivalent of the Federalist Papers, but my guess is that the Kirk Key interlocks were needed because the fault level with both transformers paralleled was too high. With only one transformer in service, the fault level will not exceed the ratings and the interlocks are not necessary.

If my understanding is correct (a one-line would be helpful), both mains are now served by the secondary of the remaining transformer. If this is the case, I can't think of a situation where defeating the interlocks would be useful, even if it were possible.

 

[io748]

Thanks to everyone who pointed out that I would be doubling the fault current that could be put on the switchgear buss for a short time, the current capacity of the buss is something that I hadn't considered.

jghrist, the reason this was brought up was so there would be no outages. When the key system is used, one of the main breakers has to be opened before the tie can be closed, which shuts off half the board for a few minutes.

 

[WastefulMiser]

What manufacture makes 8000A switchgear?

 

[Pierre C Belarge]

Another thought.
When it was changed over from two transformers to one, was the one transformer changed to a large transformer?

I do not want to rain on the 'party' here, but... there is just too little info supplied to us here to really help give a solid and safe answer.

Example:
When the prior engineers made the change, why did they not remove the Kirk Keys? That would give me reason to think.


Zog has offered to travel over and take a gander...this decision is not a small one, and I for one would jump at the opportunity.

 

[zog]

Is the busing rated for 8000 or 4000 amps?

If not 8000 amp busing what NEC section allows supplying it with that level of over current protection even for short periods of time?

Nothing would ever see 8000A. Not sure where you get that from. Maybe the OP's statement of the total frame size ratings of the breakers?Me thinks he was a little confused on that part.

The Mains are 4000AF so nothing will ever see more than that, no matter what the configuration. Relax codemaster

 

[zog]

What manufacture makes 8000A switchgear?

No one, that I have ever seen anyways, and I see a lot of switchgear. That was a misconception.

 

[iwire]

Nothing would ever see 8000A. Not sure where you get that from. Maybe the OP's statement of the total frame size ratings of the breakers?Me thinks he was a little confused on that part.

The Mains are 4000AF so nothing will ever see more than that, no matter what the configuration. Relax codemaster

Two 4000 amp mains supplying 4000 amp busing is a problem, the load those busses supply is irrelevant.

Two 4000 amp mains in parallel is a problem.

 

[Pierre C Belarge]

Two 4000 amp mains supplying 4000 amp busing is a problem, the load those busses supply is irrelevant.

Two 4000 amp mains in parallel is a problem.

Bob
Unless the transformer is only rated for 4000 amps.

Would this not also be considered overcurrent devices in parallel as per 240.8?

 

[iwire]

Bob
Unless the transformer is only rated for 4000 amps.

If I have a 75 KVA 480 to 208Y/120 transformer it's secondary rating is 208 amps, can I supply a 225 amp MLO panel with it?

Would this not also be considered over current devices in parallel as per 240.8?

IMO yes, both sources now come from the same transformer, with both ends and the tie closed the OCPDs are in parallel.

 

[Pierre C Belarge]

If I have a 75 KVA 480 to 208Y/120 transformer it's secondary rating is 208 amps, can I supply a 225 amp MLO panel with it?

My thoughts were that the transformer may not be able to supply the 8000A.



IMO yes, both sources now come from the same transformer, with both ends and the tie closed the OCPDs are in parallel.

............

 

[io748]

Agreed...Zog, I would definitely take you up on your offer, do you ever find yourself in the Rocky Mount area?

Pierre, what other info would help? I'll sketch out a one-line tomorrow to attach.

Thanks to everyone for the input.

Another thought.
When it was changed over from two transformers to one, was the one transformer changed to a large transformer?

I do not want to rain on the 'party' here, but... there is just too little info supplied to us here to really help give a solid and safe answer.

Example:
When the prior engineers made the change, why did they not remove the Kirk Keys? That would give me reason to think.


Zog has offered to travel over and take a gander...this decision is not a small one, and I for one would jump at the opportunity.

 

[Pierre C Belarge]

io748

Sometimes it is just plain ole difficult to put into words what you actually have in your facility. It sounds like a large place and there may be some complexity that you see everyday, yet we cannot see it here. It may be a small detail, but could make a ton of difference.
Hence, I mentioned about why the prior group did not remove the Kirk Key(s).

I am not saying you are incompetent, just that this may be way too complex for us to provide you with the proper answer.
From Zog's posts, it seems to me he would be just the ticket you are looking for. There is nothing like "in person".

Good luck with this, and please keep us informed as to the results.