Size for Diesel engine generator - Urgent need

[testingpump]

Dear all ,

Can you please help me in selecting the appropriate size for a diesel engine generator ,The generator will be used in a booster pumping station as a main source of power , The continuous generator will be used mainly to run a vertical turbine pump with a motor of nominal output power 150 HP , 3000 RPM , 1.15 SF , 400 V , 50 Hz, and 201 a nominal current. Assume all other load for the remaining secondary applications like lighting about 5kw. , I appreciate all your efforts if you can help me in this issue showing all your calculation for this selection of the correct size.

The confirmation of the correct size of required generator is important as the original specified generator for my project was 220 KW (for standby application not continuous), BUT due to the change of the motor speed from 1800 rpm to 3000 rpm that caused an increase of the starting current, inaddition to the change of application from standby to continuous. So I need to confirm what is the required correct size of continuous generator and please attach your calculaion.

Thanks for your cooperation

 

[testingpump]

3 phase motor

 

[waross]

Will the generator start the pump? My rule of thumb for an absolute minimum size is twice the motor rating. That would be 230 KW with an allowance for the service factor. This is one generator, one motor. Expect large frequency and voltage drops when starting.
The load on the generator will pull the frequency down and as the frequency drops, the Under Frequency Roll Off feature of the Automatic Voltage Regulator will reduce the voltage.
If it works, it will probably be ok. Don't be surprised if your mercury vapor lamps extinguish when the motor is starting.
If the generator is in place and it starts the pump, I would continue using it.
If a new generator is to be purchased, I would use;
150HPx.75=112.5KW
112.5KWx3=337.5KW
337.5KWx1.15SF=388KW.
I would use a 400 KW machine for good starting and minimum effect on other circuits.
Expect other good recomendations.
respectfully

 

[LionelHutz]

What method is being used to start the motor?

I would expect the specified generator to be OK using a soft-starter or a VFD. DOL starting probably requires a bigger starter.

A soft-starter will reduce the starting current to about 2.5 times the motor FLA and should take < 10 seconds to start. A typical genset can provide about 1.5x rated current for 10 seconds. So, if you figure the ratios out you need a genset about 2 times the motor kW rating or 1.5 times the motor HP rating.

For DOL, you can assume 6 times motor FLA so doing the same as above it shows you would need a genset rated about 3.7 times the motor HP to start with minimal voltage drop.

Of course, if you want hard numbers for the above then you need to investigate the starting of that actual motor and pump to determine it's current profile using the soft-starter or DOL.

A VFD will draw the motor FLA + harmonic currents equal to about 30% of the motor FLA. The genset manufacturer should be able to easily help with that decision.

 

[peebee]

Generators will usually have a "starting kVA" rating in addition to their normal kW/kVA ratings.

Your motor starting kVA will be about 6 or 7 times normal running load kVA. You could get a more precise number by using locked rotor amps, RLA... starting kVA = RLA x V x sqrt(3). Some starting methods (reduced-voltage, VFD, solid state, etc) will reduce the starting kVA, possibly even to a value less than normal running current.

The generator starting kVA must be higher than the motor starting kVA. The normal generator kVA and kW rating must be higher than the motor kVA and kW normal running electrical load.

 

[waross]

My comments were based on Direct online starting.
Much of the motor starting current is reactive and the generator will withstand a current overload for the duration of the starting period.
If your main load is one large motor and any other loads can accept under voltage and under frequency, there are a few tricks that will help an undersized generator start a motor DOL.
One technique is to dial in as much droop as you can to the governor. Then set the governor to autogenous operation. The droop will help the starting and then the autogenous feature will take over and correct the frequency.
Without an autogenous capable governor, you can set the frequency a few Hz fast and use lots of droop.
respectfully

 

[peebee]

Sorry, typo in my last post, that should have been LRA, not RLA. LRA will usually be about 6 or 7 times RLA.

 

[ScottyUK]

Soft starters produce huge harmonics during the starting phase. Generator AVRs do not like harmonics, especially when they are a significant proportion of the load: if you choose to use a soft starter be sure to state the nature of the load to the supplier and contractually tie them to providing a generator which is unconditonally stable with the load you are presenting. They'll probably object to this clause, so argue until an acceptable compromise is achieved.


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Sometimes I only open my mouth to swap feet...

 

[dpc]

I don't think you're going to get anyone here to do this calculation for you. At least I hope not. We can provide some suggestions and guidance, but no one on this forum is in a position to determine the proper size with any degree of certainty.

My recommendation is to contact two or three engine-generators suppliers, get their forms for providing load data, fill these out and then have them recommend a generator size for you. They will be happy to help you, and I suspect that the answers will be very similar.

 

[testingpump]

Thanks alot for all your help ,

Regarding the motor it will be with soft start which will reduce the starting current , Today I contacted two generators suppliers , Caterpillar and Perkins , I got two different answers as both companies have two different softwares for calculation with different parameteres and safety factors , one of them gave me 230 Kw and the other 270 kw . But I'm confused why this difference if we are talking about load calculation that should depends on constant load of the motor ?

 

[ScottyUK]

Other factors such as governor and AVR response times play a part, plus the design of the alternator itself. Plus the manufacturers have the benefit of many years experience of doing this sort of thing: they have a lot of case histories of what works and probably more than a few of what doesn't.


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Sometimes I only open my mouth to swap feet...

 

[wbd]

I would err on the conservative size and go with the larger genset of 270kW. Even if it costs more but it works, it would be less costly in the long run of a smaller one that didn't perform well. Plus there may be room for added growth. Just my 2 cents worth

 

[jraef]

Also be careful as to the manufacturer of the soft starter. Some do not work well on smaller sized generators because they are very sensitive to frequency fluctuations. Ask for a technician who can tell you if the soft starter uses "phase lock loop" voltage sensing. If it does, you will need to size the genset for Across-the-Line (DOL) starting anyway to avoid frequency drift.

 

[Marke]

Hello testingpump

Sizing a generator for a single motor application is not too difficult provided that you understand the generator and the motor and starter and how they operate.
I have some information on my site at

http://www.LMPhotonics.com/genset.htm

Firstly, the genset is made up of four important components, the Engine, the Alternator, The Governor and the AVR.
The Engine produces KW and has a short term overload margin that can vary from as low as 120% up to 300%. The governor regulates the speed of the engine to allow for variations in load and minimise variations in output frequency. The engine must produce enough KW to cover the shaft load on the motor, the losses in the motor, the losses in the starter and the losses in the cables.
The Alternator produces KVA and it must produce sufficient current to enable the motor to start. The minimum start current required is dependant on the minimum start torque required by the pump at all speeds to full speed, the aility of the motor to convert amps into torque at all speeds to full speed and the ability of the starter to control the current applied to the motor. The AVR controls the excitation applied to the motor to regulate the voltage and cope with changes in load without variations in voltage.
Alternators have anshort term overload capacity, typically between 120% and 300%.
Some alternators are "self excited" That is, the excitation is taken from the output voltage generated. The initial flux is residual flux in the rotor which generates a small voltage which is fed back to increase the excitation and produce more flux and more output voltage etc. The problem with the self excited system, is that when the alternator is overloaded, the voltage drops due to the impedance of the alternator. The falling voltage reduces the excitation and the voltage falls even further. - The output voltage very quickly collapses for a small overload. Better systems use a permanent magnet generator to provide the excitaton and this results in a far superior short term overload performance.
AVRs also come in a number of flavours, at one end or the scale, there is the single phase half wave peak rectifier type which effectively samples once per cycle, and at the other end of the scale, there is the three phase full wave averaging detection system that in effect is continuosly sampling. The single phase peak reading system is very sensitive to transient loading and harmonics and does not tend to perform well on motor starters. The three phase full wave averaging AVRs usually give the best performance for motor starting applications.
So, to determine the size, calculate the start curent required and he start time. Then find the overload capacity of the alternator and size accordingly. In the case of a submersible pump, you will probably need somewhere in the order of 250% - 300% current to start. If you use an Alternator with a three phase averaging AVR and a PMG exciter, you could have an averload capacity as high as 300% in which case, the Alternator rating could be equal to the KVA rating of the motor.
The engine will need to provide sufficient KW to start the motor. Determine the peak shaft load during start, and on a pump, this will be at full speed and will be 100% load. Then determine the losses in the motor during start. If the motor has a full load efficiency of say 90%, then at full load, there will be a copper loss in the order of 5% - 7% of the motor rating. During start, the copper loss will be much higher. If the loss was say 7% at full load, the during start with a start current of 300%, the copper losses in the motor will be 7 x 3 x 3 = 63%. Determine the losses in the alternator the same way, probably similar in magnitude to the motor losses. The cable losses must also be considered. If the cable has been sized for a 5% voltage drop, then there is 5% loss at full load. During start at 300% current, the cable losses will be 5 x 3 x 3 = 45%, so we have a total KW requirement of 100% (shaft power) + 63% (motor copper loss) plus 63% (alternator copper loss) plus 45% (cable losses) plus say 20% (iron loss in motor and alternator etc) giving a total load on the motor of 281% of the motor rating. If the engine is capable of a short term overload capacity of 300%, then you could rate the engine at close to the input power rating of the motor plus starter plus cable loss (= motor rating / efficiency plus say 10% for motor loss plus 10% for alternator loss plus 5% for cable) If the engine has a 120% overload capacity, then you would need to rate the engine at around 2.4 times the motor power rating.
So, there is no easy rule of thumb, you need to do some engineering. Often, I find that people go for the cheapest option on a price per KW basis, and end up with a large machine to start the pump, but a machine with a higher cost per KW will offer a much better overload capacity and require a much smaller machine to do the job with a lower installed cost.
It is best to try to operate the engine at around 70 - 80% load if possible. Oversizing the engine (selecting a machine with a low overload capacity) will result in more frequent maintainance and higher running costs.

Best regards,


Mark Empson

http://www.lmphotonics.com

 

[petronila]

Hello Testingpump,

your application is a motor, and like Peebee sayds, for motor starting (loaded or unloaded ) you will need several times the FLA, so is important the motor´s KVA this is called the starting KVA or locked rotor current value.You can find this KVA in name Plate like CODE

SKVA= Code Letter x HP . Due to at starting the motors have a Low PF from 0.3 to 0.4 you could calculate the KW like KW= SKVA x PF.

Go to the name plate find the code (or ask to the motor´s manufacturer) and calculate he motor KW then add the Ligth´s KW and you will havethe Rated KW.

Regards

Petronila

 

[powerjunx]

testingpump,

There's a lot of factors and preferences why both manufacturer has different generator sizing results, most factors were already mention above.

For me since you had connected loads other than motor such as lighting, better select the higher rating as Wbd suggested. The higher rating has something to look at the "load growth factor" in your system not just simply looking at the motor load as constant.

 

[waross]

To make your day even more interesting, Caterpillar often supplies Olympian sets in this size range. Olympian sets are manufactured by F G Wilson which is now owned by Cat. They use Perkins engines in the F G Wilson sets.
Also, the same set will have different ratings depending on whether it's rated Prime, Standby, or Continous.
It is possible that you were actually quoted on identical sets with different ratings. Not likely, but possible.
You are probably starting to realise that sizing a gen set is often a series of compromises and educated guesses.
The continous rating is the most demanding.
Some changes between a standby set and a continous set may be:
Continuous, Lower rating on the same alternator, 25% oversized motor, larger oil sump, and an oil cooler.
These options are not always fitted and some may be fitted on a standby set.
The lower rating and the oversized motor are common.
respectfully

 

[rbulsara]

Refer to faq237-766