Selectivity on a 100kW Diesel generator

[mshields]

I am specifying a 100kW, 208Y/120V diesel standby generator to serve a classroom building at a local University. I am working with the Eaton breaker combinations to achieve0.01 selectivity on the Emergency Branch. Here I should tell you that I plan to have a 300A optional standby ATS and a 150A, Article 700, Emergency branch.

The key to finding breakers that will work in series, from the 450A main to the 150A Emergency branch and lastly to the 20A-1P breakers mostly for emergency lighting, is low fault current. If the fault current is low enough, one can find the breakers using the manufacturer's selectivity combination tables (or via plugging into SKM - it matters not). But here's where I'm running into a problem. Caterpillar is telling me that the reactances on these relatively small generators is no higher than 8%. I need higher than that to make this work.

Does that 8% number sound right to you? I have always thought generators were more typically in the 16% range.

Any suggestions would be appreciated.

thanks,

Mike

 

[ron]

It depends on where they get the alternator. I was just working with an 8% Kato last month.

 

[mshields]

thanks Ron

thanks Ron

at least lets me know, my guy is probably not mistaken. It just seemed low to me.

 

[mbrooke]

It wouldn't surprise me. Especially if the engine to alternator ratio is small. I know of for example Kohler using the same engines on 20 to 50kw genets. All that they change is the alternator. By reactance you mean available short circuit current?

 

[mshields]

short circuit

short circuit

I'm asking about reactance because short circuit is a function of it.

But here's another question: We take the impedance of a generator as being it's reactance, I assume because this vector quantity has a comparatively negligible Resistance component. Is that accurate.

This generator is 250feet away from the breakers it needs to coordinate with. Obviously this adds a Resistance to the equation. Should I be calculating this differently, i.e. taking that into account?

Thanks,

Mike

 

[ron]

You want the x"d (subtransient reactance (direct)) value. That is the ability to produce short circuit current in the fast transient time period the equipment is rated for. The impedance of the feeder from the generator to the load needs to be added.

http://cumminspower.eu/www/literature/technicalpapers/PT-6008-GeneratorReactances-en.pdf

 

[mbrooke]

I'm asking about reactance because short circuit is a function of it.

But here's another question: We take the impedance of a generator as being it's reactance, I assume because this vector quantity has a comparatively negligible Resistance component. Is that accurate.

This generator is 250feet away from the breakers it needs to coordinate with. Obviously this adds a Resistance to the equation. Should I be calculating this differently, i.e. taking that into account?

Thanks,

Mike

If you are doing selective coordination you always factor anything that might limit or raise your bolted fault current. A 250 foot distance is something to consider at this voltage/current. FWIW, lower fault currents are easier to coordinate with. If you knew your true bolted fault current you actually know exactly where to disregard time current plots (ie if you know the fault current doesn't go over 8000amps you can disregard selective coordination attempts above that say what might happen at 12,000).