Sizing a Diesel Generator for an electric motor with high starting current 8

[M.Barzegar]

Hi everybody. I want to size a stand-by diesel generator for a plant and I need a bit of help here
The final steady-state load is 750KVA, 630KW, 1100A. Derating Factor in the location is 12%.
My main concern is that this diesel generator should be able to start an electric motor that has a heavy starting current, even with a soft starter.
The starting current is 2220A and the starting time is 37 seconds. But after the motor started, it draws about 300A.
Considering that this motor should be started when the load on diesel is 500A already, the diesel generator should be able to provide (2220+500=2720 A) for 37 seconds.
Can I choose a 1MVA (1443A) diesel generator and hope that generator could be overloaded in starting part with no problem, or should I choose a 2MVA (2886A) diesel generator for this purpose?
Also, do I have to consider the derating factor (12%) and safety margin (10%) for the starting part? In this case, I have to choose a 2.5MVA for this plant.
Thanks

 

[LittleInch]

You should really buy two 1MVA generators and run the second one up before the motor starts, then kill it after wards.

Running any diesl or gas fired engine at <30% load for extended periods kills the engine - low efficiency, more coking up and less chance it will pick up when it needs to as the load increases, especially if the load increase suddenly.

That sort of massive spike may be difficult for the engines to pick up, even with two units.

I've looked before at having a large load bank connected prior to electric motor start to get the engines running at ~50%, then dropping it a split second before connecting the motor starter.

Otherwise you risk a brown out, trips on low frequency and everyone complaining that their equipment suffers due to that huge spike.

I don't think I've seen something like that with a soft start before - usually that gets max starting current down to 2-3 times running current, not nearly eight times. That is a difficult thing for a generating system to cope with.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[RRaghunath]

Firstly, check the mentioned starting current magnitude with Soft starter. 8xrated current can only be true without soft starter I think.
I have seen DG sets used with motors that have large starting current. The concept followed is
1) Rate the Diesel Engine for required kW output.
2) Rate the Alternator kVA and Xd' as required to limit the voltage drop during motor starting (with pre-existing load).
With the above approach, you may even go in for DOL starting (no need for soft starter) which will ensure higher torque is available for shorter starting time.

 

[M.Barzegar]

@LittleInch
Thank you for the reply

Running any diesel or gas fired engine at <30% load for extended periods kills the engine

This DG is going to be used only 10-12 times a year and each time about 2-3 hours. Is it still going to be a problem?

That sort of massive spike may be difficult for the engines to pick up, even with two units.

Because motor is connected to a soft starter, it is not a massive spike, it starts with low current then slowly increases to 2220A and then slowly comes to rated current. by the way, do you think a VFD could be a better choice here to limit starting current and limit KVAr required to start this motor?

I don't think I've seen something like that with a soft start before - usually that gets max starting current down to 2-3 times running current

Because this motor is connected to a fan and motor is designed oversized (330KW, 560A on nameplate) it has a large starting current (560*4=2220A) but after reaching its rated speed, it only draws 300A

My main question is: can diesel generators be overloaded in starting part of this motor with no problem or should I take into account the starting part and choose a very big DG because of the starting part?

 

[M.Barzegar]

@RRaghunath
Thanks for the reply

Firstly, check the mentioned starting current magnitude with Soft starter. 8xrated current can only be true without soft starter I think.

I started this motor yesterday, only to check the starting current. Because this motor is connected to a fan and motor is designed oversized (330KW, 560A on nameplate) it has a large starting current (560*4=2220A) but after reaching its rated speed, it only draws 300A

Rate the Alternator kVA and Xd' as required to limit the voltage drop during motor starting (with pre-existing load).

We have a 600KVAr capacitor bank on the bus which that motor is connected to. Because the main issue here is the starting current and the KVAr required for starting of this motor, can I switch on all the steps of this capacitor bank to provide 600KVAr for starting the motor and choose a smaller diesel generator? or capacitor banks are too slow to act and supply the KVAr that is needed in starting time?

 

[edison123]

Using a VFD, instead of a SS, for this 330 KW motor would keep the starting current low right up to the normal running current of 300 Amps so that you can use a DG small enough to meet the total plant KW demand (plus some margin, if you prefer). The VFD starting is clean and smooth right up to the rated speed with no voltage or current spikes.

Of course, with a VFD, 600 KVAR cap bank has to be disconnected. After the motor has hit the rated speed, you can bypass the VFD and connect direct to the DG with a much smaller 130 to 150 KVAR cap in parallel to compensate the motor no-load current. Or you can forget the PFC cap and run the motor continuously with VFD since your duty cycle is very small. You can even size the VFD to the actual motor load KW instead of the rated motor KW, if you don't foresee any increase in the connected load.

Muthu

http://www.edison.co.in

 

[Hoxton123]

Your question:

My main question is: can diesel generators be overloaded in starting part of this motor with no problem or should I take into account the starting part and choose a very big DG because of the starting part?

Diesel engines, and other RICE engines have a load acceptance (starting, running) that is less than rated power.

Engine and genset manufacturers often have programs which will assess the load acceptance capability.

Be careful that any frequency or voltage excursion does not exceed the capability of connected devices.

If the electric motor is the only load then you could assess conecting the motor to the generator at standstll, then starting them together.

 

[LionelHutz]

Well, a lot of that motor starting current is reactive so it does not load the prime mover until near full speed when the motor power factor improves and the prime mover suddenly gets loaded. However, an advanced soft-starter with power or torque control will be able to reduce the current as the power factor improves to keep the load on the prime mover more constant.

The alternator does need to be sized for the starting current.

A VFD would reduce the loading on the set considerably. A VFD is a power converter so it only draws the required real power from the source. Lots of people don't get it until they see it, but if that motor was unloaded then the VFD might only draw 10-20A from the source to spin the motor at full speed. Then, add the real power required to also spin the fan to this current.

The VFD will be an issue for that capacitor bank unless it is a de-tuned bank. That will require more investigation if you want to use a VFD.

 

[jraef]

The cap bank would likely be unnecessary with the biggest load on a VFD. In fact with some of the new Active Front End VFDs out there now, you can even run them in a slightly leading PF to make up for other smaller lagging PF loads on the same feed.

My “rule of thumb” on sizing generators for large motors is 3x the largest motor kW size for Across-the-line, 2x if there is a solid state soft starter, 1x if there is a VFD (plus any additional loads). It has never failed me.




" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[RRaghunath]

jraef,
Interesting and very useful Thumb rule you shared. Thanks.
I suppose the thumb rule is for selecting kVA rating of generator and selection of Diesel Engine kW rating can be based on total expected kW load!

 

[TugboatEng]

Negative. The generator kVA needs to exceed the largest motor motor rating by the factors listed above. This may not be reflective of total expected load.

An example, if you have a 200kW base load and a 100kW motor then you'll need a 300kW generator set to across the line start the motor.

If you have a 10kW base load and a 100kW motor you'll still need a 300kW set to start the motor.