Generator N+1 Question...

[RobbieR]

I have a design question that I've never seen before.

We have an existing office building that we?re modifying for a client.

Currently there are two generators serving different portions of this building: a 250kW and a 500kW. We need to tap into the 500kW (utilizing a spare breaker in the genset) and run a separate feed to a new ATS to feed the portion of the building that the 250kW generator is feeding for an N+1 type configuration. The 250kW Generator is currently serving a 400A 480V Distribution board through an existing ATS and the 500kW Generator is currently serving a 800A 480V Distribution board through another existing ATS.

1) Under normal conditions, if utility is lost, the 250kW generator would run for its dedicated space through the 400A Distrubution board and the 500kW generator would run for its dedicated space through the 800A Distribution board.
2) However, if the 250kW generator does not start, or is down for maintenance, I need the 500kW generator to pick up the load that the 250kW would serve.

How would this be controlled?

I'm assuming the downstream breaker for the 500kW generator would be Normally Open on the 400A Distribution board. But where would the signal come from to close this breaker if the 250kW generator failed to start? It sounds simple, but what am I missing?

Thanks!

 

[MIEngineer]

Install a second ATS for your two generator feeds, the load side would supply the emergency-side of your existing ATS. Not very elegant but I've seen it done and it works.

Install a PLC-controlled switchgear to utilize the three feeds independently.

 

[kingpb]

Another issue you did not mention and need to consider, is that when the 250KW is down or does not start, the 500kW would need to start and pick up the 250kW load.

Also, if the 250kW is down, and the 500kW is operating, how are you going to load shed from the 500kW machine in order to be able to pickup the 250kW loads?

If the breakers you are working with are not electrically operated type, then load shedding is going to be an issue, unless you can isolate a whole panel or multiple panels.

Some or all of this may be done with transfer switches, but those take up space, are costly, and may add some transfer time to your operation that you cannot afford.

Adding another 250kW unit is probably the simplest, but now you may have space constraints.

Putting in new switchgear with electrical operated breakers, or use an MCC to give you switching flexibility combined with a PLC based digital control system is probably the best.

 

[mcelweec@gmail.com]

Tie- bus switchgear is probably what you need.

 

[hillbilly1]

Install a second ATS for your two generator feeds, the load side would supply the emergency-side of your existing ATS. Not very elegant but I've seen it done and it works.

Install a PLC-controlled switchgear to utilize the three feeds independently.

I agree, this would be the simplest way to do it, or if the load is locally monitored 24 hours, and does not require immediate redundancy, a manual transfer switch may also work.