generator (diesel + alternator) sizing

[gepedo]

This may be an extremely stupid question, but I'm kind of out of my element on this one.....as I'm more familar with communication systems.

The building I'm currently working out of has a backup generator (in case of power failure). It is currently max'd out, with respect to the building load...and I've been assigned to address this problem. The two options that I have are:

1) Reduce the load on the generator.
or
2) Upgrade the genset to satisfy the current load.

Option 2) seems rather expensive and option 1) alone seems to be a tremendous amount of work (the building is very old). Upon further inspection, I happened to notice that the diesel unit was rated for,

470 H.P @ 1500 RPM
535 H.P @ 1800 RPM

while the alternator was rated at,

350 KW, 438 KVA, 1800 RPM, 1214 AMP

I realize that 350 KW is equiv. to 470 H.P....which is the rating of the diesel at 1500 RPM.

Since the rating of the diesel at 1800 RPM is 535 H.P and the rating of the alternator at 1800 RPM is 470 H.P....there is a mismatch between the two.

Is this a normal practice for the generator to be sized slightly smaller than the diesel ?

If not, would it be worth looking at upgrading the alternator ?

I realize that this alone wouldn't satisfy the load requirements, but coupled with a small load reduction, just might do the trick.

Any input would be much appreciated.

 

[fsmyth]

I would concentrate more on the generator specs.
IMHO, the prime mover should always have a bit of
headroom, and there is a reasonable amount here.
Note that if the system is "old", as you say, then
it is quite possible that the motor is not making
rated horsepower, and/or there are other cumulative
inefficiencies in the generator and transport.
There must surely be a way to unburden the
supported load (i.e., drop one or more HVAC's)
for the duration, without a great deal of
trouble. No?
<als>

 

[rbulsara]

Yes it is normal to have engine sized bigger than the alternator. Efficiency of conversion is one reason. But real practical reason is the engines are manufactured only in certain cylinder sizes, 4, 6, 8, 12 and 16 etc. For example an 8-cylinder engine can be used for alternator sized from 500kW to 1000kW. A 6-cylinder engine may be too small for 500kW (this is just an example not the fact).

So it may be possible to upgrade the alternator in some cases. But it depends on many other factors as to type of load served, largest motor load on the unit, etc. At end of the day engine only provide the real kW part of the alternator not the reactive part (KVAR). Refer to faq237-766.

Also look out for warranty and liability issue. Always review this with the generator manufacture of course.

 

[itsmoked]

gepedo; The engine MUST be rated larger than the generator or you will have stability problems. If you look at say a 10kw generator you will see things like 20+ hp required to run it.

This is not wasted hp.

I don't know, (others can confirm), maybe you could run a parallel generator. Both come on then auto parallel, or you cut up the facility into two sections one for each.

 

[ccjersey]

How much motor load is there on the set?

Usually the snag with a maxed out generator is motor starting current, and VS Drives or soft starters (cheaper if you do not gain anything from the variable speed), or possibly power factor correction to reduce the VAR from the generator could help there.

I am assuming the generator's KVA rating is higher by about 25% than KW rating. I believe that is all the power factor correction could possibly gain you, and practically you won't get all of that 25%. If the engine is not up to the extra, still nothing gained.

Easiest of all is to prioritize the loads in the building as was suggested above.

Jim

 

[ScottyUK]

One possible solution is to implement a selective load shedding controller. Typically these have three or four levels of load-shedding: the critical load which is never shed, then two or three others which are dropped in reverse order of importance. To give you some idea, we implemented such a scheme in our control room:

Level 0: UPS loads + battery charging, UPS cooling, switchgear tripping supplies, fire detection & suppression.

Level 1: High priority process monitoring, high priority comms equipment, IT servers (on their own UPS)

Level 2: Central HVAC plant, non-maintained lighting.

Level 3: Office power, kitchen facilities.

If you are supporting awkward loads like large UPS input rectifiers then you would be well advised to speak to the generator manufacturer. Some generator control systems - usually not the very dumb or very sophisticated ones - react badly to large non-linear loads like rectifiers. Common practice is to over-size the alternator and specify a three-phase averaging AVR. The genset manufacturer should be able to advise you. Make sure you identify the nature of your load, or better still allow the supplier to study the load and propose a solution, and detail this in the contract because you will need that if the genset reacts badly to the load and it goes to litigation.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[itsmoked]

ScottyUK; Sort of off topic, but then I think the topic has been answered. You mention in your response that in your scheme the UPS is non-sheddable. With large UPS's I can image them adding greatly to the generator load to recharge during the crisis. Do they ever delay recharging while on generator?

 

[ScottyUK]

Yes, there is an option to tell the UPS to suspend or reduce battery charging while on generator supply. We have a short duration battery - only about 10 min per module - so we oversized our generator to get some charge back into the battery and didn't use the option. Less risk if the generator falls down! We're quite lucky that there's a lot of generator-friendly load in the building from the HVAC plant - quite a few small 3-ph induction motors and approaching 100kW of electric heater banks. When we lose that lot the quality of the load presented to the generator deteriorates - lots of 5th and 7th harmonics from the UPS modules, plus some 3rd from the single phase rectifer loads in all the DCS and computing equipment - hence my earlier comments about making sure the supplier knows what load the genset has to support. A light harmonic-laden load is worse than a heavy linear load as far as the AVR is concerned.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[itsmoked]

Thanks ScottyUK.

 

[rbulsara]

gepedo:

I know this forum focuses on technical side, but the business end is equally important. Part of the goal of expericenced engineers (specially consulting) should also be to impart practical nuiances of the trade so that new generation of engineers are enriched.

My two cents from that point of view is that when advising the Owner of solutions, start with the best soulution, however costly it may seem and let them tell you they can not afford it. Do not start with 'value engineered' soultions off the bat that. Once owner asks to look for other alternatives, they will realize that they gave up something in return for cheaper solution. If you start with cheaper one, the Owner may assume that you have advised the best option which may turn out not to be.

So in my book, only option would be to upgrade the generator and there may be multiple ways to do that. But never messing with existing stuff to look like a hero for a only to fall on our face later.

 

[itsmoked]

Good point rbulsara.

1) I would say a clear audit of the loads would be the first item.

This may show that there are a large number of pointless loads running in the "carry the whole place" existing solution. Just saying a bigger generator, while the simplest, may actually be a lousy solution in itself.

Rather like a doctor giving treatment before examination.

2) Once #1 is done the customer and only the customer can say we want it all or what part of the load do they want covered.

3) Once this is stated then an estimate of how much it might cost to modify the wiring could be arrived at.

4) Then a cost for scrapping a good generator and replacing it with a larger one can be estimated.

5) Then the correct "business" solution may present itself.


Course this is all a mute point.

The OP's question had lots of info(nice) but boiled down to prime mover hp verse generator rating. Not how to change the site. So I don't quite know what specifically you are speaking to rbulsara... Or maybe you are just speaking generally?

 

[rbulsara]

itsmoke:

It was a general note, but directed to OP. I did try to answer the technical question and so did others.

What prompted my last response is my belief that this is a prime example of type of projects that eventually becomes a "mess" in the Owner's eyes. Its too easy for us 'Engineeers' to get carried away in effort to come up with a 'technical' solution which does not make a good business sense.

This point is far too important to ignore, as success of the individual and our profession depends on it. I feel responsible to provide a 'right' advise as well when someone has asked for it.