1) Tie two 4000A 480V switchboards with a busduct. 2) Tap a 2000A feeder to (e) SWBD

[fandi]

Hello All,
I wonder what the potential issues would be if:
1) Tie two 4000A 480V switchboards (let's call MSB-1 and MSB-2, separately fed from (2) 3-833kva utility transformers) with a 4000A busduct:
These two are 5ft facing each other. The peak combined loads in 1 year of both MSBs are less than 2000A. To maximize the new Cogen, I'd like to connect the Cogen to both MSBs. Meaning I plan to remove one upstream 3-833kva transformer and tie the two MSBs via an approx 10ft 4000A busduct.
For ground fault protection, I'll remove the neutral disconnect link at the switchboard (let's call MSB-2) that its upstream transformer is being removed. This to avoid the two MSBs to have two different neutrals connected to ground.

2) The Cogen switchboard has its own main 2000A motor operated breaker. Can I tap that 2000A Cogen switchboard directly to the one of the two existing MSBs mentioned above? The Cogen is about 100ft away from the MSBs.

Thanks a lot.

 

[Tony S]

The transformer total O/P current is 3005A per bank of three. What would be the total loading for all three boards?

One thing I don?t like is having transformers stood idle for any length of time. I was always a bit twitchy re-energising them.

If you intend to set this up so you can do a live switch over of the feeders the maximum prospective fault current could become a problem.
Plus there is the danger of paralleling the utility feeds. We could do this but needed the DNO?s (POCO) permission as they had to juggle about with their local 33KV bus.

I?ve done live feeder swaps, I can?t say I was happy about it as I knew I was exceeding the boards MPFC. The chances of a heavy fault developing in the few seconds it takes to operate the breakers is remote but it?s there.


I?ve just had another thought. Have fun with the copper work if the boards are facing each other. Somehow you?re going to have to reverse the bus duct connections at one end.

 

[fandi]

Thanks for your info. There's no live switching nor parallel feeds as the work will be done during a utility power shutdown. The spare transformer will be removed by the utility company. We only install the bus duct between the MSBs.

 

[kingpb]

Hello All,
I wonder what the potential issues would be if:
1) Tie two 4000A 480V switchboards (let's call MSB-1 and MSB-2, separately fed from (2) 3-833kva utility transformers) with a 4000A busduct:
These two are 5ft facing each other. The peak combined loads in 1 year of both MSBs are less than 2000A. To maximize the new Cogen, I'd like to connect the Cogen to both MSBs. Meaning I plan to remove one upstream 3-833kva transformer and tie the two MSBs via an approx 10ft 4000A busduct.
For ground fault protection, I'll remove the neutral disconnect link at the switchboard (let's call MSB-2) that its upstream transformer is being removed. This to avoid the two MSBs to have two different neutrals connected to ground.

2) The Cogen switchboard has its own main 2000A motor operated breaker. Can I tap that 2000A Cogen switchboard directly to the one of the two existing MSBs mentioned above? The Cogen is about 100ft away from the MSBs.

Thanks a lot.

In general, don't see any fatal flaws in the concept. You will need to verify the fault levels since you will have utility power and cogen power on the MSB(s), and of course protection scheme will need to be coordinated.

If this project is in the LADWP territory (Going by your location), I assume they are on board and you are aware of their cogen interconnection requirements.

 

[fandi]

In general, don't see any fatal flaws in the concept. You will need to verify the fault levels since you will have utility power and cogen power on the MSB(s), and of course protection scheme will need to be coordinated.

If this project is in the LADWP territory (Going by your location), I assume they are on board and you are aware of their cogen interconnection requirements.

Thank you very much for your input. SCE (the utility company) and the City Plan Checker are reviewing the drawings and I'm still waiting for their comments.
I'll make sure the MSBs will be sufficient AIC ratings for the utility and cogen combined fault current.
The thing is it's normal to connect a smaller downstream panel to a switchboard but I've never seen any application that connecting two 4000A MSBs (one is GE, the other one is Square D) given the ground fault protection will be taken care of.

 

[ron]

2) The Cogen switchboard has its own main 2000A motor operated breaker. Can I tap that 2000A Cogen switchboard directly to the one of the two existing MSBs mentioned above? The Cogen is about 100ft away from the MSBs.

When the Cogen is down for service, the 2000A connection to the MSB's will not be protected. You might be able to rationalize using a tap rule in 240.21, for connecting 2000A worth of interconnection to a 4000A main

 

[fandi]

Thanks, Ron.

 

[JoeStillman]

Removing the bonding jumper from one switchboard may not be enough to have proper ground fault protection. Will there be "objectionable neutral currents" from the cogen?

 

[fandi]

Removing the bonding jumper from one switchboard may not be enough to have proper ground fault protection. Will there be "objectionable neutral currents" from the cogen?

I have to confirm with the Cogen manufacturer but let's assume there is then what can I do to prevent that?
Thanks.

 

[Tony S]

This is what I referring to with the copper work required for two facing boards using a bus-duct. I wouldn?t like to tackle it and ensure the MPFC withstand is maintained. I have done it using 500mm? singles in trefoil for 2x2MVA boards.

 

[JoeStillman]

I have to confirm with the Cogen manufacturer but let's assume there is then what can I do to prevent that?
Thanks.

As a starting point, I would want to look at how the neutral busses and bonding jumpers are configured in the original main-tie-main arrangement. There is usually some scheme to prevent a ground fault on one source from tripping the main on the other.

Check out this white paper.