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If a 7MW genset is needed, which is better, to choose three 3.5MW generators or four 2.5MW ones? If take into account gas or diesel generators, is the answer similar?

A lot depends on your load profile and your ability to bring the spare generator online to meet your load requirments. In both cases if one of the operational generators fails you are still unable to meet your total anticipated load and the total system may crash. If the 2.5KW and 3.5KW generators have the same same failure rate then the opportunity for that crash is approximately the same.

The 3 x 2.5KW system would have some spare capacity when all there are running compared to the 2 x 3.5KW system.

If you run all the generators continuously the 3 x 3.5KW system would have more spare capacity (3.5KW) total spare) than the 4 x 2.5KW system (3.0KW total spare).

Not sure which system would prove to be more reliable and also which would be more efficient. You would need to do some detailed analysis on the two different options.

I can see arguments for both arrangements. I am assuming that the generators are installed on a balanced system so their frequencies and phases are properly matched. Also you may want to look at how to handle the arrangement were you can not supply your full required load. That may help you determine what is better.

Finally there is the matter cost for each arrangement. What is the difference in aquisition and operations costs for each of the two arrangments.

Not an easy problem sence your the ideal situation may depend on a variety of variables.

Niel Leon
engineering.com