tie-switch

[augie47]

We have an industrial facility with two service points. There is a "tie switch" between services "A" and "B". The system is "kirk-key" controlled to prevent the tie from being closed with both services energized. Knowing the ingenuity of maintenace personnel, the "what if" question arose.
Both of the plant service transformers are fed by the same primary
line and both are phased alike, however, the impedeance is different.
In the highly unlikely event that this tie switch was ever closed with both services energized, what would be the result ?

 

[hillbilly]

I'm not a expert on transformers, so this is just a opinion.
If I read the question right, nothing would happen, there's no potential between the corresponding phase conductors of each transformer.
Either that or something really big happens.
steve

 

[zog]

The key interlock system is installed because your main breakers could not interupt a fault with both transformers in paralell. If you have two 3000kVA transformer they each can put out about 50,000A of fault current, so with both mains and the tie closed you have about 100,000A of available fault current. Your main breakers probally have interuption ratings of 50,000 to 65,000A. The results could be catastrophic.

Your guys need to be trained on the operation and hazards of your power system to be permitted to even enter that substation per OSHA 1910.269 (b).

 

[kingpb]

With the impedances of the transformers different, you would have circulating currents. The current is added to the load current, so if either transformer were loaded very heavily, you could get into a situation where you are overloading the transformer. The higher current also contributes to a higher I^2R loss.

Also, the transformers would not share the load equally.

Typically, if you are going to operate transformers in parallel, they will have LTC's (Load Tap Changers) but they would have also been specified, designed, and built to try and have nearly matching impedances, as close as realistically possible.

If you are talking about short time operation in this manner, due only to accidental overiding of the interlock, your probably safe.

If this is going to be an ongoing problem, then you probably want to add electrical interlocks, that would cause an automatic trip of the other main if the tie is closed on a live line/live bus situation, or have main contacts wired into the closing coil circuit of the tie in a manner that would block close when you have voltage on both sides of the tie.

There is certainly the fault levels that need to be addressed as ZOG pointed out.

 

[infinity]

I thought that the purpose of a Kirk Key system was to prevent any possible connection between the two power sources?

 

[krameshkannan]

There is no wrong in parallaling the 2 transformers since the normal load & fault current will be shared by both the transformers according to their impedances.
So it is absolutely safe to parallel the trasformer.
As for as my knowledge is concerned by paralling the transformer the fault current does not increase

 

[kingpb]

I thought that the purpose of a Kirk Key system was to prevent any possible connection between the two power sources?

That's what my folks thought the lock on the liquor cabinet was supposed to do also, but did it keep me out?

 

[krameshkannan]

Yes there is a danger in the way of 2 power sources having different voltage levels.Then we can't parallel the trasformers.So it depends on the nature of the power source.
If infinity can provide the details about the power sources it will be easy to analyse the problem

 

[ron]

The resultant fault current of the paralleled transformers will be lower (resistance in parallel), so the fault current on the secondary of the transformers will be higher.

 

[kingpb]

Yes there is a danger in the way of 2 power sources having different voltage levels.Then we can't parallel the trasformers.So it depends on the nature of the power source.
If infinity can provide the details about the power sources it will be easy to analyse the problem

Wait a second, first you said there was no problem, and now you say there is, so which is it.

I can prove by calculation that the fault currents could be considerably higher by coming off the same system and simply closing the tie. Just as ZOG pointed out.

Please explain yourself.

 

[augie47]

switch

switch

thanks for the answers....(especially kingpb @ liquor cab--my point exactly).
It is highly unlikely the switch will ever be closed...We were thinking if that ever did happen the results would not be catastophic. Just confirming our thoughts. Thanks.

 

[ramdiesel3500]

For those who don't understand the "kirk-key" switch arrangement, I'll try to explain. Several years ago, it was common for engineers to design a building with two electrical service feeds. Each feed was set up with enough capacity to supply the entire building. This was for the purpose of increasing reliability of service. If one of the utility transformers failed, the other would then be able to carry the entire load.
Now each service had a main switch with a key lock (called a Kirk Key) than could only be closed so long as the key was present in the lock. A third switch was also present in the switch gear called a tie-switch. This switch, when closed, would connect the main bussways of the two services. This switch also had a kirk-key lock.
The critical part of this arrangement is the logic that only two of the three switches can EVER be closed at the same time. All three locks are keyed alike. So long as only two keys exist, the system is safe. What generally happens though is that when these systems are manufactured, the manufacturer of the locks install a key in every lock so that three keys are supplied. One of them is supposed to be destroyed when the system is put into service. What actually happens in most cases is the key ends up in a managers desk with all his other spare keys "just in case we lose one". Yeah right! How on earth could you lose a kirk key? Anyway, I know of at least one death that has resulted here in my home town as a result of the "third" kirk key being pulled from a desk drawer and used to close the third switch!

 

[jtester]

quoted from a Westinghouse Distribution Transformer Manual

" If greater capacity is desired, two trannsformers of the same or different kva ratings may be connected in parallel if the following conditions are met:
1. Voltage ratings are identical.
2. Tap settings are identical.
3. Percent impedance of one is between 92.5% and 107.5% of the other. 4. Frequency ratings are identical.
5. The angular displacement is identical."

I also agree that the fault currents will rise.

Jim T

 

[zog]

"What generally happens though is that when these systems are manufactured, the manufacturer of the locks install a key in every lock so that three keys are supplied. One of them is supposed to be destroyed when the system is put into service. What actually happens in most cases is the key ends up in a managers desk with all his other spare keys "just in case we lose one". Yeah right! How on earth could you lose a kirk key? "


Yep, I have seen this many, many times. The equipment installer usually leaves all the keys in the locks. When doing aceptance testing (Which is what I do), we always remove the proper keys so the interlocks will do what they were intended to do.

Kingpb, I missed the thing about impedances not being matched, so yep, there may be circulating currents.

FYI, "Kirk" key is a brand name, like Klenex or Jello that gets used alot.