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Generator sizing advice
4

Generator sizing advice

Generator sizing advice

(OP)
I have a doctor relative who runs a clinic + operation theater from his house. He needs to buy a DG for the attached list of loads (Excel file).

I was thinking around 3 phase, 40 KVA. Any tips / suggestions ?

Muthu
www.edison.co.in

RE: Generator sizing advice

(OP)
The DG will be a residential zone enclosure. Should the DG then be derated ?

Muthu
www.edison.co.in

RE: Generator sizing advice

Hi edison123.
You will be cutting it a little close. Not much diversity may be allowed. That's not to say that there won't be a diversity factor, but I don't see a lot that I would take a chance on derating.
Have you considered the possibility a a load of light fuel and the resulting loss of power? As an example, if you get a load of winter diesel you will lose about 10% of your power. Have you considered aging of the engine? Worn rings and loss of compression, injectors worn and not at 100% efficiency?
A 40 KW machine rated for prime power would give you a +10% cushion.
I have seen generators rated at +25% to allow for ageing. That would put you at 50 KW.
I would try to sell a 50 KW, standby rated set.

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

RE: Generator sizing advice

(OP)
Thanks Bill. I was assuming a diversity factor of 0.8 for a total load about 40 KW and hence my 40 KVA choice. This will be a standby generator running for about 4 hours a day. The price difference between 40 & 50 KVA is not much - about $ 400. The supplier claims there is no derating because of the residential enclosure. How should I probe him further on this ?

Muthu
www.edison.co.in

RE: Generator sizing advice

Does this doctor's house really have a 3 phase service?

RE: Generator sizing advice

(OP)
Yes, in India, a load over 5 KW is required to have 3 phase only supply by utility terms.

Muthu
www.edison.co.in

RE: Generator sizing advice

I also agree with Bill's advice. Also have some room for growth. Once one has a generator, they always come up with more load. Also account for some imbalance, more important on smaller systems, which will result in more load on one of the phases and the Gen size needs to be 3 times the highest phase.

 

Rafiq Bulsara
http://www.srengineersct.com

RE: Generator sizing advice

2
Muthu,

I think Bill and Rafiq are correct, a little bigger may be better, as Rafiq said, imbalance will likely be a big issue since it's unlikely anyone has been concerned about load balancing.

On the enclosure, the quieter the enclosure the more retrictive to air flow, usually gen manufacturers don't derate power based on enclosure type, but they do reduce the ambient capability, sometimes in very small print.  

As a quick example, a generator as a free air unit, no enclosure, is rated for 105 degrees F, add a standard enclosure, ambient capability drops to 95 degrees F, change the enclosure to critical grade and ambient capability drops to 85 degrees F.

Since you'll likely have higher loads on hotter days, this can become quite an issue, just ran into this recently at a high end residence and the installing electrical contractor had no idea what ambient capability was, so when the end user demanded the quietest unit available for his home in back country SoCal, and the first time it got hot the unit overheated in about 2 hours.  The original speced unit was rated at 105 degrees F, but when the enclosure got changed, it dropped to 85 degrees, electrical contractor is eating the cost of changing the unit because of his ignorance.  Of course now he knows, but a pretty expensive lesson.

Hope that helps,

Mike L.

RE: Generator sizing advice

(OP)
Thanks Rafiq. Good point about load distribution.

Thanks Mike. So I got to grill the supplier about the ambient rating ? In my place, the summers go as high as 45 deg C and that's when the power is most used.

Ok. Assuming a full load of 40 KW with a reasonably balanced loads and with an enclosure, would you say a 50 KVA set should do it ?

Muthu
www.edison.co.in

RE: Generator sizing advice

I wasn't aware of the ambient issues Catserveng. This may save my assets one day.
Thanks.
Have another  star.
Yours
Bill

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

RE: Generator sizing advice

I've never seen electric loads decrease.  Ever.

If he has any "dollars" at all left in his budget, I'd recommend a combination of three "margins" - not all three added together, but combined certainly.

Electric growth: 15% - 25%.

Power derate and load uprate (heat causes higher loads, less production as mentioned.  Not as great a factor if no AC, but in a doctor's office, some cooling is needed.)  At least 15% margin.

Starting surge: Depending on how many electric motors are mixed in, possibly 15 - 20% above max power.  If not, the starting surge of his "last" motor (and with Murphy's Law always applicable) that last motor will come on in middle of the hottest part of the hottest day when every other load (lights and instruments) are already running ...


)

RE: Generator sizing advice

A note on the A/Cs. many A/Cs now have a random time delay before starting after a power failure. If the A/Cs don't have this feature consider adding it with timing relays so that the A/C starting is staggered on the generator.
Given the smallish sizes of the A/C units I don't see a starting problem as long as they don't all try to start as soon as the generator comes on line.

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

RE: Generator sizing advice

(OP)
Bill

The AC's have a start time delay of about 3 minutes whenever it is switched on.

I am going over to meet that doctor now to discuss his budget. I will call for 50 KVA.

Muthu
www.edison.co.in

RE: Generator sizing advice

(OP)
racookpe1978

That kinda factoring will give that doctor a heart attack. :)

Muthu
www.edison.co.in

RE: Generator sizing advice

Did you mean to suggest 50 KW rather than 50 KVA?
You may want to consider possible derating for the enclosure. This would be in addition to the increase to 50 KW.

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

RE: Generator sizing advice

(OP)
Bill

I got the assurance from the supplier that the DG has been derated for the eclosure and 50 KVA will meet the present requirement.

Muthu
www.edison.co.in

RE: Generator sizing advice

(OP)
Ok. Feedback. Got back from meeting the doctor. We decided on 50 KVA set in enclosure. Higher capacity ruled out due to diversity factor, wet stacking at low loads and of course, budget. The load redistribution has to be done along with some UPS for operation theater equipments.

The DE set cost comes to about $ 9,000/- excluding the civil works and the cabling.

Thanks everyone for your tips and suggestions.

Muthu
www.edison.co.in

RE: Generator sizing advice

Are we on the same page Muthu? 50 KVA does not equal 50 KW. The 50 KVA set will produce 40 KW. The prime rating will give you an additional 10% or 44KW.
Looking at your load list I wouldn't worry about wet stacking. Check the oil and look at the exhaust every 1/2 hour for the first 40 hours and about once a day for the next two or three runs. If you get wet stacking it should be evident due to oil usage. It can easily be cured on a new set if we catch it before the cylinders develop a "glaze". Wet stacking can happen but is rare with new, small diesel sets.

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

RE: Generator sizing advice

(OP)
Bill

After 50 KVA, the next available size was 63 KVA. Since the total load is only about 40 KW (not likely to increase much in near future) and since the engine supplier wanted at least 60% load at any given time, I chose 50 KVA. With a diversity factor of 0.8, the actual maximum load likely to only about 30 KW.

Muthu
www.edison.co.in

RE: Generator sizing advice

Thanks for the explanation Muthu.
I had a supplier threaten to void the warranty on a new set because of a light load. Unfortunately there was one big motor to start when the rest of the plant was already running. One of the suppliers engineers informed me that air conditioning was a good load and, with a fine disregard to the cost of fuel suggested that we add enough air conditioning to load the generator. Of course that would not have left enough reserve to start the large motor. I suggested that, given the nature of standby operation, a standby generator may have a hard life and that's life. We reached a compromise; I set up a load bank and loaded the generator for a week to wear in the piston rings and they would continue the warranty. I was up on the roof four times a day adding water to my jury rigged load bank. 280 KW will boil off a lot of water in a week.
Despite the numbers, an onsite evaluation is often the most accurate evaluation.
Yours
Bill

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

RE: Generator sizing advice

(OP)
LOL. You have contributed enough to global warming.

Muthu
www.edison.co.in

RE: Generator sizing advice

guilty as charged. cringe.

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

RE: Generator sizing advice

280 kW...global warming? cost of fuel? Try 5000 kW and 3000 kW load banks..one of them was run for 24 hrs!!

Cost were worth the mission, however I was not in favor of the 24 hr run the customer requested. But it made them happy so it is worth it (they paid for it).

Rafiq Bulsara
http://www.srengineersct.com

RE: Generator sizing advice

rbulsara -- re. " I was not in favor of the 24 hr run the customer requested" -- why (mostly just curious)?

Are you concerned that a 24 hr run would simply be wasteful, or that it would actually be harmful to the generator?

RE: Generator sizing advice

Or that it was too short?

RE: Generator sizing advice

I have put in quite a few generators and have only load banked two of them. Once to avoid wet stacking and once to cure wet stacking. Load bank runs are sometimes spec'd. Many customers and/or consultants are happy with a load test with just the building load. Given the cost of fuel and manpower, many customers will accept a 2 hour or 4 hour load test with the building load. It's a cost thing and a lack of sleep thing from the point of view of those of us who may have to spend a night sitting with a perfectly good gen-set.

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

RE: Generator sizing advice

peebee:

When I say I was not in favor, I meant that I was not insisting on it. Somewhat wasteful perhaps, and no, I was not worried about damage to the generators. Only that the requested test was over and above other acceptance tests and factory witness tests that we had performed -on load.   Considering many other factors including the application (standby), personal knowledge of the system (designed ad commissioned by us) and past experience 4-6 hrs test would also have suffice. But I was more than happy to see them run for 24 hrs.

I am all for extensive testing, check out my newsletter on commissioning. I generally get accused of detailed and extensive commissioning procedures, never for cutting corners.



 

Rafiq Bulsara
http://www.srengineersct.com

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