Resources/education for sizing generator with diverse load

[Paulsey88]

Hi All,

I am in the process of sizing a standby generator(s) for a large commercial building, approx 1 mil. square feet. Mixture of offices, warehousing, labs (think data center), & some small cafeteria areas. I've made good progress on areas I am familiar with such as offices, lighting, etc. However I am still feeling overwhelmed, especially with accounting for the HVAC equipment. There is a menagerie of roof top units, ground mounted chillers, air handlers, exhaust fans, terminal units, etc. Also have sizable UPS & battery charger loads. Chillers are the largest motor load I've identified. There is metering data available but only for the last few months. I am looking for resources to better understand & educate myself, hoping someone like waross will come around have suggestions. Current resources:
IEEE Orange Book - Emergency & Standby Power Systems for Industrial & Commercial Applications
IEEE Gold Book - Design of Reliable Industrial & Commercial Power Systems
CAT Specsizer Tool & documentation
Electrical Design Guide for Commercial Buildings by William Clark

Anyone have other books, guides, classes, etc they consider key to understanding sizing generators for a application of this size? Any advice is appreciated.
RP

 

[waross]

Cat spec sizer is probably the guide to use.
Some caveats and pitfalls to avoid.
Ratings;
Generator limits:
Maximum KVA Limit: Rated current limited by I[sup]2[/sup]R heating of the winding, and the saturation voltage of the winding..
Useful KVA Limit = rated winding current times the voltage in use.
A set may be rated 100 Amps per winding at 120/208 Volts wye or 277/480 Volts wye.
At 100 Volts per winding the KVA will be 83.1 KVA at 277/480 Volts wye,
but only 72 KVA at 120/208 Volts.
Be sure that your set is KVA rated at the voltage that will be used.
One of my customers got burned when a set was ordered at a given KVA at 120/208 Volts and the set delivered was rated at the given KVA at 480 Volts.
When we needed a 75 KVA set at 120/208 volts and got a set that was rated at 240 Volts, we had a hard time making up the deficit.
208V/240V = 87%, in this case 65 KVA
The job was a movie theater and the actual load was over 65 KVA.

How is the set rated? A standby set is normally rated at 100%
A prime power set is de-rated 10%. (Actually 100%/110% = 9.1%)
A 110 KVA standby set will be de-rated to 100 KVA for prime power duty. A 10% overload is allowed for one hour out of twelve.
A 100 KVA prime rated set and a 110 KVA rated standby set may be the same machine.
However the prime set may have accessories added to the same basic engine to facilitate the much greater duty.
These may or may not include:
A larger oil sump. (Fairly common.)
An oil cooler or a larger oil cooler.
Larger fuel filters.

Using CATSpecsizer.
Specsizer will give you a set size based on the maximum allowable voltage drop during motor starting.
The default allowable voltage drop is fairly conservative. You may change the allowable voltage drop setting in the sizing software.
It is my understanding that a naturally aspirated engine tends to take block loading a little better than a turbo aspirated set.
But don't take my word for it. Run a large motor starting on a lightly loaded NA set and run the same conditions on a TA set.
If your findings are not in agreement with mine, please tell me. Thanks.

Battery chargers. Personal opinion.
NO
Monthly exercise should be adequate for the engine mounted alternators to keep the battery charged.
An inspection program that includes a battery load test should identify failing batteries in time to avoid failed starts.
Hint: Disconnect the fuel solenoid wire and let the set cycle through the automatic start cycle two or three times. Then reconnect the solenoid and start the set. Run long enough to recharge the batteries.
With no maintenance and no battery charger the batteries will eventually fail and the set will not start when needed.
With no maintenance and with a battery charger the batteries will eventually fail and the set will not start when needed, AND THE BATTERIES WILL EXPLODE!
I used to connect the factory supplied trickle charger when I installed a new set.
After cleaning up the acid all over the generator room after a battery had exploded, I never again hooked up the trickle charger and disconnected any that I encountered.
Fortunately there was never any one in the room when one of the batteries exploded.
If you must connect a trickle charger it may be well to cover your assets by getting the owner to sign off on a release in which he agrees that he understands that lack of proper care may result in a battery explosion, with possible property damage and personal injury.

Some sets come with a circulating block heater. The advantage is twofold.
You are assured of the set starting in cooler weather.
At rest the coolant is above ambient temperature and above the oil temperature.
When the set starts the oil cooler initially warms the oil.
I think that warmer oil is a good thing.
You may add a circulating heater if you wish.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Paulsey88]

Bill,

Thank you for sharing. Understood on the ratings & generator limitations. The insight on the battery chargers & block heaters was not something I had considered. I appreciate the first hand experience and examples.

CAT Specsizer. I picked up on the sizing being dictated by the maximum allowed voltage sag. They only have a few "walk through' videos but they are good. I am checking on various equipment to see if I can allow greater voltage dip during startup.

Any insight on how to handle large HVAC loads? I am working with the facilities engineer to see if we can delay startup and add HVAC loads in discrete steps using building automation system. Chiller is a larger unit (~350 ton).

Any must read white papers or literature on the subject? I know nothing beats in the field experience, but anything I can learn in my 'off' time is great.

Best,
RP