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CR4 - Thread: Power Generator Sizing

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hien.nguyenquocPower-User

Join Date: Sep 2009Location: BH, VietnamPosts: 438Good Answers: 1

P ower Genera to r S iz ing08/05/2010 12:20 PM

Hi everybody, I am trying to size one diesel power generator to feed the 01 cooling water pumps. Its capacity is 160kW by Soft starter.

Another 03 ones are also water pumps with 30kW each and star/delta starters.

Question is that what I should keep my eyes on during the calculation. Since I have many references that give me different methods. One saidwe just add up the total load then give some percent to compensate for the efficiency of the generator, one said I need to consider the

starting current of these pumps, one said I need to consider the starting capacity of the pumps and if the generator can delivery enough power

for starting. It might be OK. I am considering about the starting current of motors that use soft starter. It starting current may increase up to

3 or 4 percent of the nominal current. So, we will need to consider that or we just can ignore it because the starting time is quite short?

I might have something in the mind already but I want to post it here to get your advices from experienced people.

Tks,

Hien

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#2 "Re: Power Generator Sizing" by msamad on 08/05/2010 6:42 PM (score 3)


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UnredundantGuru Re: Power Genera to r Siz ing

08/05/2010 12:36 PM

Some generators voltage regulators are negatively affected by soft starters and VFDs. I would suggest contacting the generator manufacturer

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Join Date: Feb 2008Location: The "Sunny sideof the street".Posts: 1309Good Answers: 42

concerning these issues.

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msamadGuru

Join Date: Feb 2009Location: Houston, USAPosts: 956Good Answers: 236

Re: Power Genera to r Siz ing08/05/2010 6:42 PM

I would like to ask you to see this thread first: http://cr4.globalspec.com/thread/45119

Now, come to your point:

You have total load 160 KW + 3 x 30 KW = 250 KW

I am assuming all are continuous. So, in terms of capacity, you at least need a generator of the capacity 250 KW.

Now consider these two possible worse starting scenarios:

(1) All of the 30 KW motor running and you will be starting the 160 KW motor (soft starter)

(2) 2 of the 30 KW motor and the 160 KW motor running and you will be starting the third 30 KW motors (start delta starter)

In the scenario (1), you need 2 to 3 (average 2.5) times capacity of generator to start it. So, the generator capacity required is 30 x 3 + 2.5 x160 = 490 KW

In scenario (2), you need almost 4 times (worst case like DOL starting), capacity to start it. So, the generator capacity required is 160 + 2 x 30

+ 4 x 30 = 340 KW.

The scenario (1) is the worst case starting scenario. So, based on this scenario, the capacity required is 490 KW. You can select 500 KW

generator.

However, if the system allows you to start 160 KW motor every time keeping all other 3 motor stopped. Then you can select 160 x 2.5 = 400

KW generator.

You also need to consider the future load and add it on top of this required capacity.

- MS

__________________"All my technical advices in this forum must be consulted with and approved by a local registered professional engineer before implementation" -

Mohammed Samad (Linkedin Profile: http://www.linkedin.com/in/msamad)

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Re: Power Genera to r Siz ing08/06/2010 9:27 AM

MS, tks for youe reply,

Hien

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Ms, it is me again for the question.

As I referred to ABB soft starter handbook, it is implied that normally the sort starter's stating current is around 3 t o 4 t i m e s FLC. Since

the motors are rather big so could you explain it again?

Nevertheless, I referred to some other manual provided by CAT - generator supplier. They recommended one calculation method based on

NEMA std and i t i s so comp l i ca ted . To do do this, I must gather many information such as SkVA/hp, RLC, Motor code... But these things

are just applied within the US and we are not. In fact, do we need to follow this procedure? I took my looks over some suppliers like CAT,

Siemens... they also recommended the same procedure. To be concluded, I found that whatever the method they are using, the original is

to based on the starting capacity of the motor.

I have been struggling on how to grasp one good and simple method of generator sizing. I dont want to design by myself and I want to

follow some standardized procedures but it seems impossible.

Hien

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msamadGuru



The calculation that I posted in comment # 2 is a type of short-cut method. Here is more detail calculation for the generator capacity

(though the final result is same):

Loads:

Water Pump 1 motor: 30 KW, 0.8 p.f, 380V, 57A



Cooling Water Pump motor: 160 KW, 0.8 p.f, 380V, 304A, (assumed starting p.f = 0.25, DOL stating current is 6 times of FLA)

(The p.f and voltage are assumed 0.8 and 380V respectively)

Befo re the s ta r t i ng o f the Coo l i ng Wate r pum p:

Total running load (rKW): 90 KW

Total running KVA (rKVA): 90/0.8 = 112 KVA

D ur ing s ta r t i ng o f the C oo l i ng Wate r pump:

DOL Starting Cooling Water KVA (sKVA) = (1.732 x 380 x 6 x 304 x 1.2)/1000 = 1440 KVA

Soft Starter Starting Cooling Water KVA (sKVA) = 1440 x 0.6 = 864 KVA

Starting Cooling Water KW = 1440 x 0.25 = 360 KW

Soft Starter Starting Cooling Water KW = 864 x 0.25 = 216 KW

(The multiplier 1.2 is for 20% contingency and 0.6 for soft starter starting, it is assumed that the soft starter reduces 40% of starting

current, starting current 4 times of FLA)

Total KVA during starting (sKVA) = 864 + 112 = 976 KVA

Total KW during starting (sKW) = 216 + 90 = 3 0 6 K W

(The starting time of the motor is considered 15 seconds)

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Af te r the s ta r t i ng o f the C oo l ing Wate r pum p:

Total running load (rKW): 90 + 160 = 2 5 0 K W

Total running KVA (rKVA): 250/0.8 = 312 KVA

Now, you have to select a generator whose continuous capacity is 312 KVA and momentary capacity (for 15 seconds) is at least 976 KVA.

Also select the engine whose continuous capacity is 250 KW and momentary capacity (for 15 seconds) is at least 306 KW.

If you select a 500 KW generator (p.f 0.8), the KVA rating of the generator is 500/0.8 = 625 KVA. Every generator has some overload

capacity for short time duration. The NEMA standard generators can withstand 50% load for at least 30 sec. Considering the overload

capacity, the generator has the capability to supply 625 x 1.5 = 937 KVA for at least 30 seconds (and more than 976 KVA for 15 seconds

for sure). So, 500 KW generator should work fine for these loads.

Voltage dip is also one factor that requires selecting the generator reactances but in this type of load, it is not so important.

I also would suggest you to consult with the generator manufacturer for final selection.

N ote fo r H ien : I sent you the Caterpillar work sheet (that you emailed me yesterday) with filling out the information of you loads including

my comments. Please check your email.

- MS

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MS, I suggest to close the topic here since your answer said it all. It is very useful and I think I have got what I have been looking for.

Hope you will be always fine. Please contribute more for this forum. Thank you,

Hien

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Re: Power Genera to r Siz ing

08/27/2010 7:30 PM

I have p rov ided answ ers t o some H ien ' s ques t i ons by ema i l . Fo r fu r the r re fe rence , I pas ted th em here so tha t o the r eng ineers

can be bene f i ted i n fu tu re :

1. At this case, we can start the biggest cooling pump first, then we go for the rest pumps. The generator capacity would be much

reduced. Right? '

Yes, it will reduce to at least 50 KW to 100 KW

2. What does "voltage dip" mean? And 10, 20 or 30% are our requirement? It means if I am the customer, I will request the supplier for

this?

Voltage dip is the momentary voltage drop within the generator during first few cycles of the motor starting. It is due to the synchronous

reactance of the generator. The higher the generator capacity, the lower the reactance value, the lower is the voltage dip. See this link:

http://www.re-volt.us/pdf/CPS_info_sheet_38_Voltage_Dip.pdf. This curve is almost same as NEMA 32.18.2 (definition of voltage dip).

3. Is that Ok with 30% voltage dip? Does it affect to the controller e.g the contactor coils will be unable to hold closing? In the case thisgenerator will have to provide power for the lighting system, will 30% voltage dip be accepted?

Unless the system has very voltage sensitive loads (sensitive relay, contactor), 30% voltage dip is ok.
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4. Is voltage dip different from voltage drop and how it is different? Normally, we need the voltage drop during motor starting to be kept

within 10%. I don't understand why they have three kinds of voltage dip?

Yes. As already mentioned, the voltage dip is the momentary (few cycles) voltage drop within the generator, voltage drop is the withinthe cable and transformers.

5 . If some educational books say that we just need to gather all of starting current of the motors. Then choose the generator whose

running current is equal of higher than these total starting currents. Is that not clear and incorrect?

This is correct if all of motors are started same time. However, it is even not 100% correct.

6. I have contacted some local suppliers and they obviously don't understand about the sKVA that the generator can provide during motor

starting.

These are so technical terms; generally local suppliers don't have this level of knowledge. They generally use some short-cut calculation.

7. If all these things have been go through, do I need to double check by recalculate the "voltage drop" in the cable during motor

starting. We need it within 10%.

Yes, it is correct. You can go up to 20% for total voltage drop during starting (for NEMA motor). After starting is over, it should be not

more than 5%.

See more information in these sites:

http://ecmweb.com/training/electrical_basics/electric_sizing_gensets_large

http://www.buyerzone.com/facilities/generators/rbic-power-quality-08.html

http://www.myemergencygenerator.com/site/1324593/page/555882

http://www.gregorypoole.com/products/electricpowergenerator/Documents/LEBE5294-02%20Electric%

20Power%20Application%20Sizing.pdf

http://www.dieselduck.ca/machine/03%20electricity/alternator.htm

http://www.cashmanequipment.com/UserFiles/Uploaded/cms/motor-start-concerns.pdf

http://www.gillettegenerators.com/sizing/sizing03.html

http://forums.mikeholt.com/showthread.php?p=1161144

http://www.buyerzone.com/facilities/generators/rbic-power-quality-01.html

http://www.kwietpower.com/_docs/motor_starting/sizing_inst.pdf

http://www.kohlerpower.com/common/pdfs/83474_GensetMotorStarting_Final.pdf

- MS

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JRaefGuru

Join Date: Sep 2006Location: California, USA,where the Godless livenext door to God.Posts: 3368Good Answers: 447


Nice work.

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gkkumarAssociate

Join Date: Sep 2010Posts: 47


if we using VVVFD or VFD what will be the capacity of generator for 250 KW motor

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