Parallel generator and Article 712.6(B)

[kenaslan]

Here is my situation. I have 2 Generators in Parallel. Lets say each generator has a 500A output breaker. Each generator will then feed an isolation switch as required by 445.18(C). From the isolation switch will run to a connection cabinet and then from the connection cabinet to a fuseable disconcert with 500A fuses. IMO there is a 1000A potential between the connection cabinet and the 500A fuseable disconnect. Therefore the wires from the connection cabinet to the 500A fuseable disconnect should be rated for 100A and the fuseable disconnect should be also rated for 1000A.

Any thoughts?
Confused and up in the Night

 

[ron]

I guess a oneline would make this easier to understand, but 445.18(C) doesn't require an extra isolating switch if you have 500A breakers already. I'm guessing those 500A breakers are electrically operated and will open and close as the generators are determined to be good sources?

I'm having trouble understanding where the paralleling bus is.

 

[kenaslan]

Here is a one line
View attachment Gen_Set.pdf

 

[kenaslan]

The isolating switch is to be able to remove and replace a generator without shutting down the other generator.

 

[ron]

In many designs, the isolation switches are physically located at the downstream parallel bus. You will need to consider the withstand rating if you keep them just switches, and you might have to add fuses or change them to breakers so the short circuit rating is high enough.

Because you have 500A fuses on the output, the generators must be N+1 so consider the load flow and what will be the design load on the paralleled bus.

You might want to operate with one of the gens with its 500A CB open so you don’t have to be concerned with extra current available on the parallel bus, but generally you would pick it to be the expected current flow.

 

[JoeStillman]

IMHO, that 500A fuse downstream of the paralleling bus is undersized. It should be 1000A.

 

[ron]

IMHO, that 500A fuse downstream of the paralleling bus is undersized. It should be 1000A.

Are you interpreting 705.12(B)(2)(3)(b) to come up with that in some way? I think the intent of 705.12(B)(2)(3)(b) is not this application.

 

[JoeStillman]

Are you interpreting 705.12(B)(2)(3)(b) to come up with that in some way? I think the intent of 705.12(B)(2)(3)(b) is not this application.

I agree with you about 705.12. I'm just saying that two 500's feeding in to another 500 means you can only use 50% of your total generator capacity. Why not make it all available?

 

[Russs57]

Is there parallel switchgear? Perhaps there is a miscommunication with the word parallel. Perhaps these generators have no provision to synchronize together?

The one line isn’t like any parallel generator installation I have seen.

 

[drktmplr12]

Is there parallel switchgear? Perhaps there is a miscommunication with the word parallel. Perhaps these generators have no provision to synchronize together?

The one line isn’t like any parallel generator installation I have seen.

agreed that parallel can mean two very different things.

scenario 1: generators are wired in parallel, but only one generator is utilized (N+1)
scenario 2: generators have the capability to synchronize. if this is the case, there would be a handful of protective relays to insure safety.

i see nothing in the op that suggests paralleling. more than likely, its #1. in this case you need a manual or automatic transfer switch as the "connection panel", or kirk keyed breakers. you absolutely need to be sure that both gens can never accidentally be connected to the same bus if there are no controls and gear for paralleling. the distribution would need to be sized according to one gen, not two.

 

[Russs57]

I find it interesting how things have changed over time. Also how one application differs from another.

For example I work in a hospital setting and my brother works on offshore boats. He is required by maritime law to run on a split bus where I’m to run on a parallel bus.

Used to be I’d have a pair of generators on a parallel bus. When one generator reached required operating parameters it’s breaker closed and all ATS’s transferred. Then other generator would synchronize in and it’s breaker would close. Not exactly N+1 but close. Gear had tie breaker and means to manually synchronize.

Newer gear has no tie breakers and no means to manually synchronize. Tier one loads hit the bus when first generator connects to bus......and so on. Don’t know what they expect me to do if other generators refuse to synchronize as tier two loads are the real life support stuff. And don’t even get me started on PLC vs mechanical relay logic. Used to be I had a print and I could “make” things work. Nowadays one lightening strike to a battery charger takes out a master PLC and I’m sunk because management won’t pay for laptop and software. Truth be told I think they are afraid I’m going to “improve” original design. And of course this stuff only happens when you have a once in a lifetime hurricane and outside help isn’t possible.

Always been some disconnect between engineers and operators. IMHO it’s now reaching a dangerous state and I’m sure bean counters are behind it.