HV or LV

[mikhe]

I have an application that requires a genset to support the load on a cement works. The mill has a 2.8 MW motor at 6.6kV. The site auxiliaries and other standing load is 1.5MW at 400V. This genset will run at base load and let the utility take any starting loads.

Should I generate at 6.6kV and transform down to support the site load or generate at 400V and transform up to support the mill motor? The gensets haven’t been speced yet but will be two or three 1.5 or 1.8 MW units.

 

[electricpete]

I would think the cost of 1.5MW 6.6kv generators would be lower than 1.5MW 400v generators.

There may be other factors.


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[mikhe]

Yes an LV alternator could be up to a third of the price of an MV or HV alternator. The question is , is there going to be a significant disadvatage in supplying the motor load through a transformer.

 

[DRWeig]

mikhe,

IMHO there should not be any significant performance difference as long as the transformer is sized correctly. Simply a matter of economics. I'd sketch it up both ways and see which one saves money.

Don't forget to post and let us know what you wind up doing!

Good on ya,

Old Dave

 

[alehman]

If you have multiple 400V generators on a common bus, be sure to evaluate the fault current carefully and factor in the cost of appropriately rated switchgear. At 6.6kV, generator fault current probably will not be an issue for the switchgear.

My experience is that a MV genset in this size range will be about 15% more costly than an equivalent 400V genset.

 

[IASMike]

I have seen it done both ways and do not believe it makes much difference.The one thing you might want to consider is the arc-flash safety requirements. It will be cheaper to provide safety equipment for a 400V system than a HV system.

 

[RRaghunath]

Three nos. 1.8MW generators at 400V - you will not be able to connect them and run in parallel due to 400V switchboard fault level limitations. Whereas, you need to run in parallel atleast two of the generators to meet the running kVA requirement of the motor through a 4MVA, 400V/6.9kV transformer

Since more load at 6.6kV level, you will be able to save on power losses if the generators are specified to be 6.6kV.

You could be saving on transformer cost as the required transformer could be rated 2MVA, 6.6/415V and not rated 4MVA, 400/6.9kV.

 

[alehman]

For packaged gensets, X" varies considerably among manufacturers and winding configurations. At 400V, I would guess the L-G may be somewhere in the 85kA range if the system is solidly grounded. With careful calculation, generator and breaker specification, it may be doable.

 

[ScottyUK]

A third option to consider could be to use the same practice used by large generating facilites, i.e. to directly couple the LV generator to the 400/6600 transformer via cable and place the switchgear on the HV side of the transformer. It is arguable whether the capital cost of the 400V genset with the transformer would be sufficiently lower than that of the MV generator. The MV generator installation costs will likely be significantly lower than the LV + transformer solution.


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