GAS ENGINE DRIVEN INDUCTION GENERATOR

[freefallingbody]

Hi,

For captive power plants, a typical system consist of engine and alternator. I wonder if I can use an Induction generator coupled to the engine with proper giverning systems to deliver the electrical power, instead of using the alternator. I hope this will not have the usual headaches of synchronising the alternator with the grid. An Induction generator, I suppose, is always connected to the grid without the need of synchronisation.

Any thoughts on why this system is not popular? Any experience on engine driven induction generators and its governing control strategies?

Thanks in advance

A.V.Dinesh
Devki Energy Consultancy P. Ltd.,
Vadodara, India

 

[ScottyUK]

An induction machine requires an external source of excitation, i.e. the grid. If the induction machine is ot tied to the grid, it will produce no output. A synchronous machine can produce output irrespective of whether it is operating as a generator connected to a grid or as an islanded system. Clearly the induction machine is of no use as a standby generator to supply power when the grid is not available.

A synchronous machine can also be used to control the power factor of the overall site load, provide reactive power export to the grid if required, frequency correction in the event of a falling grid frequency, and so on. Most if not all these are difficult or impossible to accomplish with an induction machine, and most have financial benefit associated with them.


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I don't suffer from insanity. I enjoy it...

 

[freefallingbody]

SCOTTYUK,

The system here is not to be used as a standby generator, it is needed to top up the demand. The state electricity utility charges very high for exceeding the contracted demand and hence need for captive generation when a large load like an annealing furnace comes into operation.

Gas engine with alternator can produce 3.5 kWh/SM3, as per the catalogue. I hope the same can be achieved through induction generator also. The other points that you mentioned about synchronous generator is true, but not a primary requirement at this site.

Do you think, if I want to use this engine driven induction generator along with the grid, would be a good choice?

Thanks

A.V.Dinesh
Devki Energy Consultancy P. Ltd.,
Vadodara, India

 

[ScottyUK]

I'm honestly not an expert on induction machines in generation applications - virtually all my experience is on big synchronous machines. Hopefully someone who has greater knowledge of induction generators than I will be able to help you.

FWIW, I don't see any major reason in principle why your proposal would not be an acceptable means of peak lopping. The VAr consumption would be worth bearing in mind if your site also has a reactive power or PF penalty. If you use an embedded induction generator, the net power drawn from the utility will reduce, but the reactive power will increase. You could end up with a truly awful PF if you meet a substantial part of the power demand from your embedded source.


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I don't suffer from insanity. I enjoy it...

 

[waross]

freefallingbody
One of the reasons induction generators are not used much is bacause the cost of operating often excedes the cost of utility electricity. For peak shaving the induction generator may be a cost effective solution.
Power factor is a concern particularly in some areas of India where a rebate is offered for 100% power factor. You can address this with a capacitor bank that is switched on with the induction generator.
respectfully

 

[freefallingbody]

Waross,

Gas engine generators at 3.5 kWh/Sm3 of natural gas and with gas price of Rs 10/Sm3, one can generate electricity at Rs 2.9 per kWh. In India, the typical utility rates are Rs 4.0 /kWh. Hence, actually, it is cheaper to run gas engine. No so with liquid fuels. Do you forsee any other operating expenses?

As Scottyuk have also mentioned, p.f. can be an issue. But it can be corrected externally.

There was one thread posted in electric motors section recently on regenerative breaking. When the motor works as induction generator, use of VFD to control the speed of rotating magnetic field to maintain a slip required to deliver a given amount of power was pointed out. That appeared to me as an elegent solution, provided bidirectional power flow is possible in a VFD.

A.V.Dinesh
Devki Energy Consultancy P. Ltd.,
Vadodara, India

 

[waross]

I would forget the VFD. One of the advantages of induction generators is the simplicity.
Typical costs.
Filters and lube oil. Labour to change filters and lube oil. Wages of operator. Top-end rebuild at 6000 to 8000 hours.
Complete rebuild at 16000 hours. (For 1200 RPM diesels in the range of 350 kw to 2,000 kw.)
How large a machine are you considering? What is your normal kw demand? (In kw, not kwhr, to compare with the generator size.)
respectfully

 

[itsmoked]

You may have lots of other expenses if you actually believe what you said in your original post:
[green]"An Induction generator, I suppose, is always connected to the grid[/green][red] without the need of synchronisation.[/red]

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rbulsara]

I can't see any wisdom, specially in India, in installing all the engine and support systems (fuel, cooling etc) and then attaching to an induction generator which can not be used when really needed (or you wish you could use it), that is in case of a power outage.

Incremental cost to make it an alternator can't be so excessive. But benfit to be able to use as standby generator as well as for peak shaving are enormous, specially in India.

There is a reason why induction generators are not so common.

 

[waross]

freefallingbody;
rbulsara makes a lot of sense. I was going to make a similar suggestion. The controls and extra cost aren't that much and the're not that complex. For many years generators were synchronised manually with three light bulbs.
Once synchronized and connected to the line the generator stays in sync. If you have an old induction motor and a suitable gas engine in inventory that's another story, but if you are buying new or even used as a complete package consider synchronous.
respectfully

 

[freefallingbody]

I understand the reason for induction generator not popular, as it is not really a standby. I was only wondering when so many wind turbines are operating in India with induction generators, use of engine driven induction generators in demand saving may work out cheaper than engine-alternator combination.

Waross guessed it right. There is a 200 kW cummins gas engine available at site and a 300 HP, 415 volts induction motor.

I am having discussions with a few experts here. If it works out, I'll post you the details.




A.V.Dinesh
Devki Energy Consultancy P. Ltd.,
Vadodara, India

 

[ScottyUK]

Induction generators in wind turbines often have some complex power conversion to isolate the induction machine from the grid. They aren't a straightforward contactor connection by any means. There is a fair bit of information out there on the 'net about wind turbine grid connection techniques - it is quite a specialist area, given the highly variable nature of wind power.


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I don't suffer from insanity. I enjoy it...

 

[waross]

freefallingbody
I advise a reverse power relay or some other means to detect a grid failure and take the induction generator off line.
In the event of a grid failure the voltage and frequency output of the induction generator will be indeterminate.
The induction generator will contribute to your plants available fault current by the same amount as if it was a motor but the contribution will last longer. No problem, just be aware of it.
Engine control. Pretty simple. Any governor that will prevent the engine from going overspeed if the load is lost will work.
However, you may want to monitor the plant load and avoid backfeeding into the grid if your plant load is less than the generator output.
The only control of the generator is the setting of the governor. The grid will control frequency and voltage and the engine governor setting will determine kW output.
Good luck with your project.
respectfully