Diesel generator sizing

It depends on the load.

May be you mean how the generator must be sized in conjuction with the diesel/gas engine. Normally about 10% oversize of the generator is used.
Beware: calculate generators in apparant power (VA) and engines in real power (W). The real power supplied by the generator depends on the load. Calculate with a power factor of 0.8

s7xp.com said:

Beware: calculate generators in apparant power (VA) and engines in real power (W). The real power supplied by the generator depends on the load. Calculate with a power factor of 0.8

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I've seen generators stall when trying to start large motors. If your load is a lot of small motors coming on one at a time 80% PF is probably fine. If your load is a few large motors or a lot of motors coming back on at once, it's probably not enough. This is an area where I would consult the factory technical support staff. Make sure you know how loads will be coming on, and what each load is.

It is also depending if the generator is in island mode or parallel to the grid. Otherwise, like Tom mentioned, check the load and how it the load start up (like e-motors, chillers). Do not use very large steps at once in island mode.

It is possible to order a generator that will take a 100% load dump - they just put a bigger engine on the alternator. I saw one just recently where the client specified a 150% load dump for 7 seconds - the generator crowd just supplied a bigger generator that would do the job.


You have to size a generator to the load/expected future load. Load dumps are also a huge consideration or the engine will slow down to a point where it cannot recover and it waill justv stop. I have seen this quite a few times.

Danesh.
I suspect that BobB is deliberately trying to keep this simple but if your load contains a large proportion of motors or non linear devices like drives/inverters then selecting a suitable generator set involves much more than just picking the right size of engine and alternator. Load acceptance capabilities depend on many factors including the generator excitation system, reactances (especially the sub transient reactance) and whether the engine is normally aspirated or turbo/supercharged. My advice would be to speak to a number of generating set manufacturers and provide them with a detailed load profile. The manufacturers will assume that your load is linear unless you tell them otherwise.
Andybr

Try down loading the sizing software from this link.

http://www.kohlerpower.com/industrial/solutions/software.htm?sectionNumber=13261&nodeNumber=1&contentNumber=104

It is pretty simple really at the end of the day Andy. Work out your maximum load now and in the future and when the generator is pretty well loaded, decide what the largest load you have can be load dumped on the generator and the type of load. That may be a large motor, or a UPS, or whatever. At the end of the day the calculation HAS to take into account the largest load that can be dumped on the generator at any time. I know of many generators where the first step can be a maximum of 40% and the second step 60%. I know of others where the first step can be 100% - oversize the generator. The generator guys need to know maximum load, maximum possible load dump at any time, maximum possible first step load dump etc etc. At the end of the day they just oversize the alternator/engine to suit the application. Just costs more bucks.

Bob.
Having spent quite a number of years carrying out this type of calculation for a large generating set manufacturer I agree that your philosophy is OK as long as the load is resistive. The problem with starting motors, energising transformers and supplying very non linear loads is that the reactive currents can be very high and this impacts on the alternator excitation rather than the prime mover. If the excitation system cannot cope then the voltage collapses and the set will normally trip on undervoltage or even overspeed if the sudden loss of load is severe enough. Alternators for this type of application are normally excited by a permanent magnet generator (PMG). I am sure that you will have seen references to this before. My advice to Danesh is still to speak to the manufacturers.
Andybr

Andy, I fully agree with you here. I do control system design, build panels, program PLCs etc for diesel generators (standby and power station) for a living. I thought I made it clear that it does not matter what the load is, the generator guys have to be fully informed so that they can size a generator to suit. That is why I mentioned large motor - UPS etc. Apparently I did not make it as clear as it should be.

I have not seen other than a PMG for some years now. Have worked with lightly turbo charged, heavily turbo charged engines for some 15 years. MTU are generally very heavily turbo charged - engines are physically smaller - do not start and load if cold - bang in the night otherwise. CATs are generally more accomodating but not always. Cummins are turning out some nice stuff these days than can often take 100% load steps. Horses for courses.

Sorry if I caused confusion but I thought my words clearly indicated that all load factors must be given to the generator guys.

Bob.I have been out of the generation field for a few years now but I remember MTU engines as being wonderful bits of engineering. I believe the 12 and 16 cylinder engines were originally used in submarines and they ran as smooth as silk and very quietly. I also worked with Cats and many of my ex colleagues work for Cummins now so I would have to be nice to them. Towards the end of my Generation time I worked almost totally with Gas Turbines on Co-Gen projects at HV and I have particularly fond memories of those days. Anyway, I have hijacked Danesh's thread enough for now.
Andybr

Very Thanks for yor answers, I want to design emergency powwer for 4*200KW + 4*132 KW electro pump and about 10KW lighting.
What is your answer?

Regards,
Danesh

You haven't got hold of this yet have you ?

You haven't given any of the required info - change over time , likely duty cycle etc .

Go to the guy who will supply the genset and give him your requirements

Thanks,

I need this genset in order to providing emergency power for a water pump station, and change over time is not matter, and duty is about 3-10 hours

Is there any Genset guy?



Danesh.

10BaseT. said:

You haven't got hold of this yet have you ?

You haven't given any of the required info - change over time , likely duty cycle etc .

Go to the guy who will supply the genset and give him your requirements

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