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Selection of a generator for a plant.
2

Selection of a generator for a plant.

Selection of a generator for a plant.

(OP)
The company has a generator that is unable to start a big pump due to the high initial current. As such it was recommended to change the generator for a bigger size. As an electrical engineer what are the factors that need to be considered in selecting the appropriate generator for the plant. The generator must be able to drive other pumps (3 phase), building lightings and other equipments. What are the calculations need to be made (including formula from theory) for the total power required to arrive at the choice of the generator rating. (Consider the pump high initial current)Is it necessary to consider all the equipments being powered at same time. Thanks for support

RE: Selection of a generator for a plant.

The factors to consider are the voltage and frequency excursion caused by starting the pump, and the voltage and frequency excursion which your other loads can accept. With no other data to work from I'd go with 20% volt-drop and 10% frequency, which will cause noticeable dimming of lights and may upset other loads. If it is mainly motors and lighting then it will probably be ok - if you are running the company mainframe on the genny then it might not be so ok. Joking aside, if you have discharge lighting a dip to 80% may cause a lighting outage until the lamp can re-strike: a few minutes is an eternity if you are working in a hazardous environment and can't see where you are going. Contactors in the IEC world start to drop out - or at least aren't guaranteed to pull in - below about 85% voltage although in reality 80% is usually ok.

You will most likely have problems when the motor is a significant fraction of the set rating, and it will be worse if you have no other rotating loads already running. That's significant if you're trying to simulate what is going on.

Genset manufacturers provide software for the kind of task you're doing: SpecSizer is the one offered by Cat and it's a decent package and fairly straightforward to use, if a little conservative in my opinion. A cynic might say that is so Cat can sell you a bigger set. ponder Cummins used to have somethign similar although I can't remember what it is called.
  

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

RE: Selection of a generator for a plant.

2
The rule of thumb that has worked for me is the total three times the largest motor plus the sum of all the other loads. This may be in amperes or KVA.
A few months ago, just for fun, I ran the specs on a number of successful installations through the Cat software which reported that none of the installations was adequate.
My rule of thumb was developed published suggestions and verified with field experience a number of standby sets that had been spec'ed too small for the actual conditions. All were an exercise in ingenuity and load curtailment to handle the most possible load. Almost all the sets were replaced within a few years. I spec'ed the replacements based on 3 times largest plus other loads. All were successful installations. Most are still in service after about 15 years.
Cat software will give you a larger set, BUT it will be adequate and you will be able to CYA.
By the way, if these are standby or emergency sets it doesn't much matter, but if these will run a lot of hours, the greatest cost is often not the fuel consumption at full load, but the fuel needed to keep the engine running for many hours at quite light loading. when I was forced to run my home on diesel power for a few months I went a little under sized with a six KW set and curtailed the use of the clothes drier. I could have gone with an eight KW set but the six KW burned so much fuel during the many hours when the loading was below one KW that I was happy to curtail loading and save the money on fuel.
On the other hand it may be easier and cheaper to use a soft starter or VFD to start the pump on the existing generator.
This is an answer to the question on sizing a replacement set.
The answer to your problem may not be the same.
If:
1> No expansions to your load are anticipated.
and
2> The existing genset is at least two or three times the capacity of the pump with starting issues.
and
3> The genset rating exceeds the sum of the nameplate ratings of all connected equipment by 10% or so,
THEN you may consider a VFD.
If the pump may benefit from the availability to trim the speed with a VFD, and particularly if the process will benefit, you may be able to justify offloading part of the cost to someone elses budget.

 

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

RE: Selection of a generator for a plant.

(OP)
Thanks all for your quick reply. I need to submit an engineering report about how I arrived to the size of the new generator set. Basically it should be based on manual calculations from theory. Should I add all the power consumed individually by all the lightings, computers etc (for buildings) and the motors/pumps. Then add them all to arrive at what size is needed. Am enclosing the motor specs available. the FObunker one is drawing around 1600A initial current.This is why the existing generator one is not able to drive it. I would appreciate to have all the formula and calculations you would used (as per electrical engineering theory)to select the size of the motor. in this way i will have a concise report to present.
Many thanks for support to all.

RE: Selection of a generator for a plant.

This site is Engineering Tips, not engineering training.
I am not surprised that you can't start a 110 KW motor on a 100 KW generator.
Based on the inconsistent and incomplete data that you have provided it will be pointless to bother you with "electrical engineering theory".
If you want it that way, sign up for a short course that will cover:
1> Reactive current.
2> Synchronous reactance.
3> Transient reactance.
4> Subtransient reactance.
5> AVR response times  and Voltage droop.
6> Diesel engine response to  block loading.
7> Volts per Hertz protection or Under Frequency Roll Off. (As it affects terminal voltage during block overloading.)
8> Electric motors and typical phase angles of starting currents.
OR
Register on the Cat site and download the software.
OR
Hire an electrical engineer.

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

RE: Selection of a generator for a plant.

No wonder you are having trouble. That 5.9 Cummins is a good engine, but it is not going to start a load drawing 1600A.  

If I were in your shoes, my "engineering report" would be very concise. Something along the lines of "I contacted (generator vendor) and after a site survey, (name of vendor's representative)recommends and will warranty (set model, size) for this application.      

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