Correlation between generating capacity and motor rating for DOL start

[eletest]

We are planning a captive power generation of approximately 18 MW ( 3 x 6 MW gas engine driven generators). Can we go for a 1600 KW DOL start motor ? What is the correleation between the generator capacity and the maximum permissible rating of motor for DOL start ?Is there anything we can take care in the generator specification to ensure this ?

 

[waross]

There are two factors:
1. Starting current, highly reactive. This causes voltage dip. Mostly KVA loading.
2. Starting mechanical loading. This causes possible slowing or stalling of the prime mover. This is kW loading.
The old rule of thumb (Which correlates fairly well with more sophisticated sizing methods) is that generator capacity in KVA must be 3 times the motor KVA. If deep voltage dips may be allowed, the factor may be reduced to 2.5 times the motor KVA.
For a more accurate assessment of the voltage drop during motor starting, go to the Caterpillar site and search for the generator sizing software. This is often more conservative than the 3:1 rule of thumb.

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

 

[jraef]

To Bill's point on the lower size limitation based on allowable voltage drop, if the generator is feeding only that one motor, and you have dealt with control power loss issues, sometimes people allow the generator voltage to drop and bring the motor up to speed with the engine by letting the generator slow down. The trickery part is that left alone, your control system will shut down when you do that. So either you have a control system tht can take a very wide input range, or you have a UPS on it.

In general though, if you need to accelerate a 16MW motor from an 18MW generator capacity system, the only way that can possibly happen is by having a VFD of some sort on the motor, even if you do not need to vary the speed when running.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ScottyUK]

1.6MW motor.

 

[VTer]

You also have to consider AVR and Governor transient response times and capabilities for dips in voltage a frequency. In addition, not sure what the motor is driving but maybe consider a soft starter or a vfd for less stress on load, motor and the power system if the application allows for it. As Bill noted, mostly all major generator manufacturers have sizing software that will size the engine generator based on your loads and tolerances.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[jraef]

1.6MW motor.

D-oh!

Hey, what's a decimal point between friends, eh?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[eletest]

Thanks everyone for your replies.
Going by the '3 times KVA' rule , 6 MW generation capacity should be sufficient (not allowing for voltage drops)for starting the motor.However we are considering the option of soft starter.

 

[itsmoked]

I'd suggest you look at exactly what kind of voltage drop your 'captive' system would be ok with. Some places can stand large dips because "who cares if the lights dim briefly?". Other places running lots of complex equipment might have a very bad time with a big dip. Alternatively, swallowing a big dip, but putting everything of any concern on a UPS could be a much less expensive path to take. Perhaps a UPS would be advantageous in the big picture for other reasons besides the motor starting dip.

You probably need a complete system view rather than just the generator/motor starting view.

16MW

as bad as my last week missing; the "single phase motor" in the poster's first sentence.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mosaviasl2]

Hi
For generator sizing we should consider [KVA=KVA(runing)+(KVA)(lasest starting)] for various prosses conditions and chose the largest value.
A soulotion for large motor starting is capacitor bank usage.it improve reactive part of starting current .