Adding short circuit on closed transfer?

[AusLee]

Hi,

I have a 1000 kVA generator and a 1000 kVA transformer.

They both feed an open transition ATS in the main distribution board.

Since it is open transition, only 1 source is active at a time. The short circuit rating of the MDB is 30kA, sufficient for either source.

Now the client wants to substitute the open transition of the ATS to a closed transition ATS. The time both sources are online together during the closed transition is aroun 100 milliseconds. Will this require subsituting the whole MDB with another one with double the kA rating?

 

[RonShap]

You need to calculate the contribution from both sources for a closed transition application.

 

[AusLee]

So i need to add the two short circuit values, in which case the existing MDB will be unsuable as it is and has to be upgraded or substituted?

My feeling is: since they will be closed together for 100 milliseconds, what are the chances for a short circuit on the downstream network to happen at that particular moment in time?

Suppose we are working on generator because of a power failure, and the power has "returned", then downstream is fine because we are already using and supplying it, and the upstream is fine because the relays that sense the mains voltage return will not trigger the flip unless this side of the network has been diagnosed ok ... ?

 

[AusLee]

I am not sure how to interprete this last sentence in paragraph 4.6.1 of IEEE 666

"However, if the alternative source being paralleled is an onsite source, such as an engine-driven generator, short-circuit design calculations are usually based on this paralleled condition, if it exists for extended periods."

Please advise.

 

[dpc]

By the letter of the NEC, both sources must be counted for short circuit ratings. However, you can apply for a variance from the local authority based on the very brief time that the two sources are paralleled.



"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[Skokian]

There may be ground fault trip considerations.

Also, does the generator have synchronism capabilities to prevent out of phase connection to the grid thru the 1000 KVA transformer. Is this a medical facility?

The ATS mfr. can lend guidance. I'm familiar most with Zenith/GE, Russelectric.

http://www.geindustrial.com/cwc/Dispatcher?REQUEST=PRODUCTS&famid=3

http://russelectric.com/

Regards . . .

Jim S. Nasby

 

[davidbeach]

You need to do a study with both sources in parallel. The resultant fault current, particularly the SLG fault current can easily be higher than the sum of the two sources each considered separately. A strange, but true, result.

 

[ScottyUK]

Skokian,

It's an open transfer scheme. No synchronising problems.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[mcgyvr]

Scotty,
He wants to switch to closed transition ATS...

 

[ScottyUK]

So it is... I'll get my coat...


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[GenSwgrEng]

You do not have to take both sources into account in your short circuit analysis if parallel operation will last less than 100ms. If parallel operation is going to last more than 100ms, you not only need to take into account the combined available fault current of both sources, but you need a slew of additional protective relaying on both the generator and transformer. Assuming the transormer is connected to the utiilty, they will likely want 32, 40, 69, 47, 27 and 81 relays. They will also want proof that your system will isolate from, and cannot be reclosed to a dead utility. (They'll probably want that regardless of if it's 100ms or extended parallel operation.)

However Skokian is very correct that you need to examine your ground fault protection. Specifically, you need to determine if you are switching the neutral, and if your generator is neutral-bonded. Ideally you will have a solid neutral and leave your generator neutral unbonded. (It is bonded at the transformer through the solid neutral of the ATS.) If you have a switched neutral on the ATS, you need to have a neutral bond on the generator, but you may need to modify the operation of the ground fault system while in parallel operation. There are a variety of ways of accomplishing this depending on what type of ground fault detection you are using. It can get ugly.

Engineers are always honest in matters of technology and human relationships. That's why it's a good idea to keep engineers away from customers, romantic interests, and other people who can't handle the truth.

 

[AusLee]

Hi GenSwgrEng,

Thank you for your reply. Can you please indicate which standard mentions the 100ms limit?

Thank you.