Diesel Generator Sizing Calculation

[SK07]

I am in the process of sizing Diesel Generator Set. Expected rating is close to 4000 kVA.

It is to be used for Continuous Duty (24 hours a day and through out year) at a remote location since Main power from utility Company is not available.

I would appreciate if any one has a sample detailed sizing calculation and could post it here or send on my email ID.

Thanks

 

[ScottyUK]

Get hold of Cat's Specsizer software. It's free, generally conservative in its estimates, but does a reasonable job and is straightforward to use.

 

[waross]

I concur with Scotty.
If your load will vary, you may consider a load controller running multiple sets on an as needed basis. If there will be extended running at below about 25% of rated load, the fuel savings gained by running a smaller set may cover the extra cost of hardware.
Common terms are Prime Power and Standby Power.
A Standby rated set will not allow overloads.
A prime rated set will allow a 10% overload for one hour out of 24.
A 4000 KVA Prime rated set may be the same base model as a 4400 KVA Standby rated set.
The basic Standby rated set may have extra options such as an oil cooler and a larger engine sump.
You may have warranty issues if you use a Standby rated set in a prime power application.
I have seen Prime Power sets with the engine over sized an additional 25%.
The reason was to lengthen the time between engine rebuilds.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[SK07]

Thanks Scotty.

I did use Caterpillar sizing software and have the results in front of me.
I have also done manual calculation but my calculated rating is much less (almost 35%) than CAT results.
The idea of writing on this Forum was to get sample of some manual calculation to cross check and finalise the DG Rating.

My Loads are as,
Step 1: Transformer Load (Lighting and other small motor loads) - 500 KW
Step 2: Motor Load - 550 KW, DOL, Starting Current = 6 times Full Load Current
Step 3: Motor Load - 550 KW, DOL, Starting Current = 6 times Full Load Current
Spare - 25% (Approx. 400 KW)

Running P.F: 0.8
Starting P.F: 0.3
Voltage: 3300 Volts,
Frequency : 50 Hz

 

[ScottyUK]

I'm not surprised that Cat have given a generator size larger than you calculated. I usually get similar results. Their solutions are always conservative and will give good results, but don't forget that they're in the business of selling generators. ;-) I'd be very comfortable knocking at least 10% off their size estimate, which often results in an alternator a full frame size smaller.

I hope Mike will comment on Specsizer, he knows their products very well.

 

[waross]

Quick and dirty;
Total 2000 kW
Add 1000 kW for starting one motor = 3000 kW
@0.8 PF = 3750 KVA
Plus 10% for prime rating. = 4125 KVA
I have sized quite a few smaller standby gen sets with this method. All have been running for years with no capacity problems. (Except where the owner has increased the load.)
When Spec Sizer became available I ran some sample calculations for existing sets. Spec sizer said that none of them would run.
Some comments for a set that size.
1> I wouldn't size the set without knowing the load profile.
2> A loaded 4 kW set may use over 300 US Gallons per hour of fuel. That may be over $20,000 per day.
3> If this is for an installation where the heavy load exists for only 8 or 9 hours a day, I would be spending a few hours going over fuel consumption charts. It may be economical to use two sets, a large one for loaded hours and a smaller set for light load hours.
The load profile may matter.
Another option may be multiple sets based on N+2 For continuous duty you may consider taking sets out of service for overhauling.
Our rule of thumb on sets from 350 kW to 600 kW was 15,000 hours to top end overhaul and a complete overhaul at 30,000 hours. One set running continuously will use up 15,000 hours in under two years. Another reason to use smaller sets if possible.
Mikes comments are welcome.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

I will agree with Bill and Scotty that the CAT program is pretty conservative, comes from getting sued more than once or twice. You will find the Cummins sizing program (along with Kohler and Generac) usually comes pretty close to the CAT sizing recommendations for the same reasons. Starting a 500kW motor across the line can vary demand quite a bit depending on what that motor is driving. A sizing program that starts you up high and lets an experienced sales engineer "save you money" is not a bad marketing tool properly used. In reality I don't use anyone's programs much, like Bill and others here on this site I have been doing this for awhile, but I wouldn't give you a firm recommendation with what little info you have provided.

This is really one of those issues with a whole lot of "it depends". When you get up in size to where you are at you are considering a pretty large unit, so some things to consider,

How long on site? A year, two, five or more?
How "remote" is site? is there a concern about being able to transport your generator on and off site?
What kind of fuel is available and it's cost?
Are there any environmental regulations at site?
Who is going to take care of it?
As pointed out above, load profile and duty factor have a big impact on actual selection. You say "motor load", what kind of load is it driving?

Have you considered a rental option? Folks like CAT, Cummins and Aggreko(and others depending on where you are in the world) are pretty good at providing this kind of site power for many applications all over the world. They provide the units, temp cabling, transformers, fuel storage and other equipment, and in most cases also provide the maintenance. In the size range your talking you have a choice at being at the top end of the high speed market or the lower end of a medium speed engine/generator set going with a single unit.

From years of being in the rental business, off the cuff I'd say a you may want to consider a multiple unit setup of modular power units, like three units at 1200-1500 ekw and possibly a housekeeping set of about 300-500 ekW depending on what has to run all the time. Just a quick guess on what you provided, but in reality you need to provide a lot more info to get a good recommendation. There are a number of good controllers available at a reasonable price that continue to make a multiple unit solution more attractive.

Hope that helps, Mike L.