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Genset Size Estimation
2

Genset Size Estimation

Genset Size Estimation

(OP)
I have 4 nos of 300HP motors rated at 4160V. The details of each are as follows:

FLA = 39.28A
RLA = 239A
P.F. = 86.2% (100%Load), 83.9%(75%Load), 77.2(50%Load)
Eff. = 95.5% (100%Load), 95.6% (75%Load), 95%(50%Load)
RPM = 1800
S.F. = 1.15

Based from 80% loading, I interpolated the equivalent power factor to be 84.3%. Hence, each motor will be 300HP x 0.746 = 223.8kW, therefore (223.8 x 0.843)/0.955 = 188.66kW.

Motor Starting Power = sqrt(3)*239*4000 = 1655.84kVA @PF=0.15, the power will be = 248.4+j1637.1kVAR

at 80% Loading, 84.3% P.F.:

P = 188.66kW + j195.93kVAR
running kVA= 272kVA at 84.3% P.F.

For one motor starting with the rest of the motors running:

sKVA = (188.66kW + j195.93kVAR)*3 =(565.98kW + j587.79kVAR)

sKVA Total = (565.98kW+587.79kVAR)+(248kW.4+1637.1kVAR)
           = 813kW + j2224.89kVAR
           = 2368.77kVA

Neglecting losses and O/L capability, can we say that we need a genset with eKW = 813kW, kVARpeak = 2224.89kVAR

Now considering a genset capable of 130% short time O/L,

can I say that a genset with:

ekW =813kW, kVAR peak= (2224.89/1.3)=1711.45kVAR
or a 1894.73kVA genset min. rating will be ok?

Just trying to understand the concept and at least make a decent genset estimate. Thank you.

 

RE: Genset Size Estimation

This is why gen set manufacturers provide free sizing software.

RE: Genset Size Estimation

The main concern will be voltage during starting.  The voltage drop calculation is iterative since the current drawn by the starting motor is a function of the voltage which is a function of the current drawn.  

Your approach is probably conservative, but a motor starting analysis is the best approach.  You'll also need to know what voltage level is required for starting this particular motor and load.   

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

RE: Genset Size Estimation

(OP)
Actually, I am trying to estimate the generator size based on manual calculation, find a suitable genset size and simulate it on ETAP (load flow and motor starting). At this point, looking for a genset rep for sizing purposes does not give me the luxury of time. We can deal with that later as we prepare the MR...right now, we are taking estimated approach to come up with a certain design budget.
I would appreciate any guidance on this, even if it is conservative.

On the other hand, if the generator application is for a dedicated MCC (no other power source)...will prime rated genset be ok or not?

What is the difference between prime rating and continuous rating? Is it in terms of duty and overload capability?

What is a typical overload capability for both engine and generator for a typical diesel genset?

Appreciate all the guidance and assistance I can possibly get. Thank you.

RE: Genset Size Estimation

Prime rated; Variable load.
Continous: for peak shaving or other uses where the load is a high continuous percentage of the set rating.
Standby: For use during power failures or outages.

Here is a typical spec for a genset model P800P1 / P900E1.
380-415V,50Hz
Prime rated    800.0 kVA  640.0 kW
Standby rated  900.0 kVA  720.0 kW

Prime
These ratings are applicable for supplying continuous electrical power (at variable load) in lieu of commercially purchased power.
There is no limitation to the annual hours of operation and this model can supply 10% overload power for 1 hour in 12 hours.

Standby
These ratings are applicable for supplying continuous electrical power (at variable load) in the event of a utility power failure. No overload is permitted on these ratings.  The alternator  on this model is peak continuous rated (as defined in ISO 8528-3).

The P800P1 is the prime model and P900E1 is the standby version.
Prime rated models are more likely to have an oil cooler and a larger lube oil sump.

Sizing. Noting the 10% overload factor, it was common to size a prime set at 125% of the calculated maximum load. This allowed continued operation as the set aged and lost power. You may be able to delay an overhaul for a year or two.

Sizing I would hesitate to de-rate a motor for generator sizing.
My quick and dirty calculation would be;
1: Motor FLA = 39.28A
2: Base load = 39.28A x 4 = 157.12A (plus any other loads)
3: Motor starting load. =  200% of largest motor. 39.28 x 2 = 78.56A
Total amps = 157.12A + 78.56 = 235.68
Basic rating = 235.68 X 3^0.5 x 4160/1000 = 1698 KVA
Adding the 1255 safety factor this gives us 2123 KVA
But if we are using an F.G. Wilson set it is already rated for 110% of nominal so 2133 KVA / 110% = 1939 KVA
You will not be well thought of if 18 months or 24 months from now, the diesel must have an overhaul before the fourth motor will start.
Beware of using less than the full rated current for the motors. In another thread we have an issue with a motor that worked well for a time, but production increased and the motor can no longer carry the full load imposed on it. The same creeping load increment may drive up your motor currents over time.
That is about as small as I would recommend based on the information given. You may get by with a smaller set.
Order the Permanent Magnet Generator option. It will give you better motor starting and better voltage response to block loading.
A heads up, when comparing sets, check the fuel consumption of each at the intended load level. If the larger set can support the load more efficiently, the fuel saving may more than repay the added original expense of the larger unit.
A second heads up, when comparing fuel consumption from different manufacturers, check the Specific Gravity of the fuel used for testing. I once reviewed some fuel consumption figures and found one manufacturer to be 10% better across the board. It looked really god, but then I read the fine print. They were using 10% heavier fuel than the other manufacturers. When the consumption was adjusted for the SG of the fuel, the fuel consumption was the same as the others.

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

RE: Genset Size Estimation

(OP)
Thanks Bill for your patience and very informative response.

I noticed that for the maximum loading, you considered all the motors' FLA (4 motors) plus 200% of the largest motor load (for starting).

In my calculation, I considered 3 motors' FLA plus 600% largest motor load.

Hence, by comparison, you used 300% as motor starting.

My question is, if I have my LRA given and it is 600% of FLA, I am curious as to why you only considered 300% (equivalent comparison with my calculation).

It will be helpful for me since I remember reading another thread wherein the same factor was used by Marke.

RE: Genset Size Estimation

There are two issues when starting a motor on a gen set. Voltage drop and frequency drop.
Frequency drop is dependent on real power loading slowing the prime mover. Motor starting current is at a very low power factor and does not add as much real load to the prime mover as may be supposed. The real load is much less than the 600% current.
Voltage drop is not quite the same with a generator as it is with a transformer. A generator has a voltage regulator to compensate for voltage drop. The voltage dip when a motor is stated on a gen set is as much caused by a response lag in the voltage regulator as it is by I^2Z.
We accept a little more dip from a generator because it is so expensive to try to avoid any dip when a large block load hts a generator.
By the way, if there are no general lighting circuits or users that will be affected by  motor starting dips, I consider dropping my 300% allowance to 250%.
On the grid, the frequency doesn't chang.
On the grid motor starting voltage dp is deterined by the transformer impedance and the "stiffness" of the supply grid.
On a gen set, the frequency depends on the response time and the droop of the governor.
The voltage depends on the response time of the voltage regulator (and the time to build up the current in the revolving field to compensate for the added current.
  

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

RE: Genset Size Estimation

(OP)
Bill, can you confirm if my understanding is correct.

Based from your latest post, the reason why its only 300% instead of 600% for motor starting is because of the fact that the generator once the motor is started does not actually start it at its full voltage enough to draw 600%. Due to the response lag of the AVR, the motor may get started at a lower voltage and thus lowers the starting current at a certain percentage.

My concern is that the 600%rated kVA as starting kVA has a large kVAR content on it due to a low power factor. Hence, I was under the impression that the 600% will have larger effect on the exciter's capability or the kVA rating of the generator than the ekW rating of the engine.

RE: Genset Size Estimation

If I may add, generator size also depends on the starting scheme you are going to implement. Use of sofstarters/ VFDs should help you decide a smaller gen set. I guess you should worry more on the voltage drop level during starting. Auxiliary contactors could drop out if supplied with AC voltage.

RE: Genset Size Estimation

Maybe, maybe not.  Depends on how much the VFD or soft starter harmonics foul up the generator AVR.  Sometimes drives actually require more generator capacity.  

RE: Genset Size Estimation

As David says, power electronics can make life hard for the AVR and cause voltage instability. Three phase averaging types are typically specc'd for UPS and rectifier loads, and if rectifiers are the main or only loads then the alternator is sometimes oversized too. There's plenty of information around on sizing generators for UPS loads as standby power is one the most common generator applications. Certainly don't assume that a soft start or VFD will inherently make things better for the generator - it may, or may not.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Genset Size Estimation

Good FAQ, Rafiq. Thanks.

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

RE: Genset Size Estimation

(OP)
Thanks Rafiq for the FAQ...did answer my latest query for Bill.

Thanks for all the patience and assistance!!!

RE: Genset Size Estimation

nightfox:

You are quite welcome.

Bill:
Thank you for reading it over!

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