Double Ended Substation / Kirk Key Needed?

[quogueelectric]

Hear-hear! Welcome to the "Global" economy with electrical design being done in India and Korea.

The companys I have worked for who think they are saving money by hiring the youngest fastest yet most inexperienced workers are usually burned by the double edged sword . My favorite saying is there is never enough time to do it right yet there is always enough time to do it twice.

 

[weressl]

The companys I have worked for who think they are saving money by hiring the youngest fastest yet most inexperienced workers are usually burned by the double edged sword .

Will it catch up with companies like GE?

 

[tony_m]

What about the generators?

What about the generators?

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?

Not to beat a dead horse, but another reason you can't remove your kirk keys is your generators. Without the interlocks in place you could potentially have generator power meet utility power. That could make for some fun times! :grin:

 

[dlbarker2]

Double Ended Substation Kirk Key Issue

Double Ended Substation Kirk Key Issue

Many good responses here, but no one mentions that on a 4000 amp main totaling up 4000 amp worth of breaker ratings (not load, but ratings) doesn't mean the 4000 amp bus is loaded to 4000 amps. As a matter of fact, it better not be loaded to 100% of the all breaker ratings. Just because it's a double ended substation, doesn't mean each breaker should be or needs to be rated to carry ALL of the code calculated loads of both sides when you close the tie breaker, but that is frequently done if the criticality of the systems warrants it. The actual running load on 4000 amps worth of breaker ratings is probably somewhat less than 2000 amps, but you can check this with the panel meter and see what you've got. The Code Calculated Load for the 4000 amps worth of breakers might be as high as 2800 - 3000 amps (75% of the breaker rating is pretty high), but I'd be surprised if it were much more than that. Closing the tie on a total load of 3500 to 3800 amps would not present any problem for the main breaker carrying that total load, but if it were much more than that, you'd want to consider load shedding prior to closing the tie breaker. Also, the main question, if both of your mains are fed from the same transformer there is no problem closing the tie before opening a main as long as you're sure they're phased properly, which is sounds like you've properly checked. The available fault current shouldn't increase, because you're still dealing with the same transformer let-through current, and the feeders are still sharing the loads pretty much the same as they were before. Just make sure there isn't a generator associated with feeding either or both sides of the substation, because that would definitely be out of phase with the utility source, and would be disasterous if you closed a tie on that source.

 

[mjoneill]

Hello. Considering this system from the Utility's view point, most Kirk key interlock systems are required so that the customer's facility does not inadvertently create a parallel connection between two sources from the Utility system. The protection provided by the Kirk key interlock usually prohibits the customer from creating the parallel by forcing supply sources to be opened before a parallel is made or is set up in such a way as to require interface between the customer and the supply Utility to allow for a temporary parallel of the Utility?s system through the customers substation bus. Since all supply is now from one source, the Kirk key interlock would not be needed for this purpose.

The normal load capability and the fault duty capability for these circuit breakers appear to be in question here. A 4000 amp circuit breaker is rated for approximately 40000 amps of load carrying capability. The circuit breakers fault duty capability is usually much higher at 20,000 amps or more. Last, for a substation supplying a load, it is not unusual from the utility side to have a 2000 amp CB supplying two or more 1200 amp CBs on one bus. Not sure what level of diversity is allowed for your particular installation however.

Just some thoughts, hope this helps.

 

[zog]

Also, the main question, if both of your mains are fed from the same transformer there is no problem closing the tie before opening a main as long as you're sure they're phased properly, which is sounds like you've properly checked. The available fault current shouldn't increase, because you're still dealing with the same transformer let-through current, and the feeders are still sharing the loads pretty much the same as they were before. .

Thats what we thought at first too until the 1 line was posted, take another look.

 

[zog]

A 4000 amp circuit breaker is rated for approximately 40000 amps of load carrying capability.

Huh?

The circuit breakers fault duty capability is usually much higher at 20,000 amps or more.

Huh? again

Not sure what you are trying to say here, maybe I am misunderstanding your post, maybe I havent had enough coffee, or maybe you are misunderstanding breaker ratings.

 

[io748]

Just to clear up the confusion on this one: The low voltage double ended substation has two transformers (one on each end). The primary on each transformer is connected to the same source.

Thats what we thought at first too until the 1 line was posted, take another look.

 

[WastefulMiser]

Hear-hear! Welcome to the "Global" economy with electrical design being done in India and Korea.

I worked closely with counterparts in India who made 1/10. Not GE ... but a competitor. The arrangement wasn't fair to either party -- but management got to boast about saving X-dollars.

 

[MJJBEE]

Another Solution

Another Solution

Just to clear up the confusion on this one: The low voltage double ended substation has two transformers (one on each end). The primary on each transformer is connected to the same source.

OK Let's try to solve the original problem really quickly. I am making a few assumptions about loads etc on this 480V switchgear. But if there are sufficient motor loads and a short voltage sag is acceptable. You could try a fast transfer relay at each Load Center. This would prevent the violation of the NEC because you open the feeding source first then close in the backup source (IE Tie Breaker) all within a few cycles. You would still have to remove the Kirk key however the fast transfer relay would not allow you to transfer between sources while leaving the bus energized. This is a fairly common item and there are several manufacturers.

 

[gpedens]

double ended trouble

double ended trouble

Paralleling the two 13.2kv/480 transformers would be the concern I have when you close T on your sketch. If i remember correctly it was mentioned there was a 2 or 3 volt difference between phases of the 2 xfmrs on the 480 side. If the bus impedance is is 0.001 this could give a circulating current of 2000 to 3000 amps plus the load current. Paralleling xfmrs is difficult. They need to have the same impedance and tap voltage. I would not want to operate with the tie closed unless properly evaluated to insure currents remain within limits. Example lets say both plant loads are 2000 amps and there is the 2 to 3 volt difference and bus impedance of 0.001 and xfmr impedances are close the same. With the 2 to 3 volt difference when the tie is closed the other 2 breakers will have the 2000 amp load plus the 2000 to 3000 amp circulating current between the 2 xfmrs. the breakers M, M, and T will each be carrying 4000 to 5000 amps as well as each xfmr or you could end up with one transformer carrying most of the load rather than 1/2. This example is a bit extreme but I have seen overheated buses due to transformer mismatches. If properly designed the tie can be closed with no problem. This is just basically what we call a breaker and a half scheme for a switchyard or substation.

 

[weressl]

If i remember correctly it was mentioned there was a 2 or 3 volt difference between phases of the 2 xfmrs on the 480 side. If the bus impedance is is 0.001 this could give a circulating current of 2000 to 3000 amps plus the load current.

Sounds like new math to me.

Bus impedance has hardly anything to do with the value of the circulating current, bus impedance difference does. ANSI does allow a +/- variation within the stated impedance, so even transformers having the same nominal impedance will have circulating currents, when paralleled.

 

[gpedens]

double ended trouble

double ended trouble

Sorry about that. It was late I was tired. I should have said circuit impedance. Which would include the internal impedance of the two transformers plus bus between them and the difference in the tap settings or turns ratios of the xfmrs. Impedance mismatches tend to make one xfmr hog the load and voltage mismatches tend to drive the circulating current. Think of it as paralleling batteries. For example a AA cell and D cell alkaline or a D cell alkaline and a D cell nicad. We typically dont operate plant loads with both feeds crosstied. If one is lost it will auto transfer. if we want to manually transfer, we with close the tie to do a live bus transfer, then open the other. In the above posts the kirk key could probably be defeated and the tie closed. It was designed in it can be designed out. Kirk keys can be used to insure a proper operating sequence for personnel safety reasons or just to prevent equipment damage. Its just a matter of looking at the pros and cons and doing the fault calculations. A fault on one load side might take out both sides with the tie closed. Transformers can be paralleled, Kirk keys may or may not be needed. I can tell you from personal experience getting a duplicate Kirk key requires a signed waiver.

 

[jrannis]

Zog, your assumption is correct that all 3 breakers will be closed for a short period of time. Just long enough to go from the tie after it is closed to one of the mains and open it. (iwire I agree with your point that if it is not safe/not compliant with code, it does not matter how short the time is)

Also it can be assumed that the system is correctly designed for one side to carry the load of both sides indefinitely. This has been done before for extended periods of time in the past (so I am told) when the switches on one side have needed maintenance.

I am attaching a quick sketch of the system, please pardon its crudeness. Let me know if I have left out any needed information.

It seems like you would have a written procedure for operating the gear and using the tie breaker. Does one exist for your plant?