DG Sizing Calculation 4

[kousikpatra]

I have following loads to be connected with a 415V, 50hZ, 3 Ph. Diesel Engine driven Generator:
1) 2 nos. 110kW Induction Motor with DOL starter,
2) 1 no. 22 kW Induction Motor with DOL starter,
3) 1 no. 18.5 kW Induction Motor with DOL starter,
4) 1 no. 15 kW Induction Motor with DOL starter,
5) 1 no. 15 kVA UPS,
6) 10kW Battery Charger for DC Load and
7) 50kW Lighting Load.

The criteria shall be that the largest load (110kW motor) shall be able to start while the rest of the loads are ON. please note that the permissible voltage dip is 15%.

Can anyone help me finalise the kVA rating of the DG Set?? May please assume standard values for calculation.

 

[itsmoked]

You will get a lot of opinions on this question.

A conservative generator would be:

330kW
110kW
22kW
18kw
15kW
15kVA
10kW
50kW
------
570kW

Call it a 600kW.
Put a timer between the two 110kW motors so that they can never start simultaneously.

Your voltage dip is a very 'hard to pin down' number because it is mostly controlled by the generator's voltage regulator and your wiring.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

I used the same criteria as itsmoked before I scrolled down and saw his response.
That would be 713 KVA. I suggest either a 700 kVA. or a 750 KVA set.
Consider the Permanent Magnet Generator option. This is a PMG that supplies a special Automatic Voltage Regulator. They do a better job of keeping the voltage up under block loading than a standard AVR. I spec. PMGs when I have to start a large residential air conditioner with minnimum disturbance to residential lighting.
BTW I have used this sizing criteria for over 15 installations with no problems.
respectfully

 

[itsmoked]

How'd you get 713? I agree most generators are rated in kVA.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

I fudged a little bit. All but the smallest load are rated in kw. so I assumed the UPS load in kw alsow.Generators in this size range are usually rated at 80% Power Factor. I divided our kw figure by 0.8 to get KVA rating. The answer was 712.5 KVA. If I had taken the time to do the calculations properly the answer would have been 708.75 KVA. Still a choice between 700 KVA and 750 KVA
The 750 would be a good choice because we have not considered the efficiency of the motors. However it is a rare occasion when all the motors are running at maximum rating simultaneously.
The final answer on size is the old standby, "It depends".
An industrial installation with the computers supplied by the UPS could probably get by with the 700KVA set. A residential installation for the local, demanding, millionare or a medical facility may be better with the 750KVA set to keep light dimming to an absolute minimum.
Respectfully

 

[sitl]

I would be very careful about these calculations. You have two motors 110kW, which are significant load since the diesel generator does not produce stiff power. In contrast to a utility grid, a genset powered system is small—measured in hundreds of kilowatts rather than thousands of megawatts—so, mechanical and electrical inertia are much lower, too. Each load now represents a significant percentage of genset capacity, which means that less inertia is available to nullify load induced electrical distortions. Given the characteristically high impedance of a genset power source, one important criterion when sizing a genset is providing enough capacity to minimize voltage and frequency dips when large loads come on line.
The rating of diesel generator will change a lot depending on the load steps that you may introduce in order to lower the kVA of genset. Anyway, I would never calculate genset without help from its manufacturer. They should have software for those calculations, and Cummins even gave me one version of it (unfortunately it is several years old).

Also, bear in mind that 15% voltage dip will result in 32% lower starting momentum of the motor, therefore reducing its capabilities of starting the load.

 

[kousikpatra]

Thnaks everybody for your valuable comments. It seems a 750kVA DG may be just sufficient. However, I guess, I also need to take concurrence from the engine/alternator manufacturers before finalising the rating.

 

[rbulsara]

Also review faq237-766

 

[waross]

Hello sitl
I was under the impression that the criteria used by itsmoked and myself took your points into consideration.
As for checking with the vendor, the last set that I installed was a disaster. The vendor told the customer that a set was 50% larger than it actually was. (Wrong capacity conversion from three phase to single phase).
I had sized the set and provided quotations from reputable dealers. The salesman was easily able to beat the quotations I had provided by supplying a much smaller set. After a lot of BS from the salesman (On international phone calls) he referred me to the service department to be instructed on how to connect the set for the proper output. The end of that was an e-mail from the service department to myself stating that the capacity of the set was exactly as I had said and not what the salesman had said.
The end of that was a substantial rebate to the customer to correct the error.
The set had been shipped to Central America and returning it was not a viable option.
The owner now has to curtail his power usage when on generator power.
I can think of 10 sets that were sized and installed after the owners consulted with the vendor and all 10 were undersized. I installed 7 of them. A few years later, I replaced 7 of them and 1 was replaced by someone else. The other two owners curtail usage and must inhibit the generators from starting automatically. I sized the 7 sets that I installed and the other owner copied my sizing. There have been no more problems.
I still consult with the factory and I don't hesitate to call the British Isles, the U.S., or Europe from Central America if I need help, but I do my own sizing, thanks. My sizing is based on the load. A salesmans sizing is too often based on getting the sale.
yours

 

[BJC]

Caterpillar, Cummings and Kohler all have generator sizing programs. They are free and they work. I like the Cummmings program. It gives me the same answer I get if I mess around with it for hours. It allows you to play "What if". You can move start times for some motors, change the allowable voltage drop etc.
Cat and Kohler give nearly the same results. They all answer with the catalog number of that particular manufactures gen set. Most have an outline drawing of the genset so you can see if it fits in the space the architech gave you.

 

[waross]

Thanks BJC
Just for kicks, BJC can you run the Cummins program on the information provided by the Original Poster and confirm the sizing that Itsmoked and myself have done? Thanks.
I have been doing this since before the internet was readily available. I aquired a John Deere CD years ago with motor sizing information that confirmed the methods I was already using.
I guess I should upgrade.
Incidently the 10 set disaster was with a generator line marketed by one of the three you mentioned.
respectfully

 

[BJC]

It tells me 880KW.
I put the lighting on the first step, all the motors on the second. the additional 110 kw motor on the next and the UPS on the last. IME the 15% voltage drop is the killer.
If I put every motor on its own stop I get 700KW.
If I put everything on the first step I get "No genset is avalable that....."
I don't have the Cat program loaded at the moment. The Kholer program tends to size generators slightly smaller that the other two. Kholer tends to use faster prime movers.
A vendors program you might expect to sell you more generator than you need but I have not found that to be so.
If you size the generators manually or any other way and later put a recording ammeter on them the seldom reach the loads you calculated.

 

[itsmoked]

Thanks BJC; Just confirming 880kW and not 880kVA?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[BJC]

Thats 880 KW
The program says I need a Cummings model 880DFHD. It'sa 12 cylinder, 1860 cubic inches.
Thats at sea level

 

[waross]

Hi guys
Thanks for the feedback BJC.
I see our mistake. Itsmoked and I live in NEMA land and when we see kw we assume input. In IEC land kw of a motor is output. If we take our 570 kw and add 20.% for efficiency and power factor we get 684 kw. That tracks well with your figure of 700 kw. That would be within 2.3%
684/0.8=855 KVA.
I would say a minimum set size of 850 KVA. All loads started seperately. It looks like we had a valid sizing criterea but used the wrong numbers.
A Permanent Magnet Generator excitation syatem will go a long way to holding up the voltage.
Without seeing the actual project it is impossible to say if the rating should be conservative or if a marginally sized set will suffice.
BJC; The different set sizes your program recommended for different starting schedules, and your comments about loading after the set is installed illustrate this.
I'm embarrased about the input-output mistake. I recently saw a couple of quotes about forgiveness on the forum and they are weighing heavy on my mind.
Respectfully

 

[BJC]

waross
If I change all the starters to solid state the program comes up with a Model 550DFGB 550 KW.
I am lazy and have used these programs enough now that I trust them somewhat.
I sometimes use both or all three of them.
They also make it easy to play "what if". In this case it would be to evaluate the cost of a larger genset against using solid state starters.

 

[sitl]

to waross:
to be honest, when I saw your calc., I was little doubtful about the size of your genset, therefore I had to post. Since you clarified NEMA and IEC difference in input data, you got genset which is closer to the power that I would expect for this type of load.
I tend to use at least two programs when I calculate gensets, bearing in mind that those programs in their calculations also comprise influence of UPSs, harmonics etc, therefore being more thorough than common calculations.

 

[waross]

Hi sitl:
Actually, when I do a size a generator, and I do quite a few, I use the current information from the equipment nameplates, or a figure that I know to be valid. Hence no NEMA vs IEC confusion.
Thanks for the heads up. I will have no excuse if I make this mistake ever again.
respectfully