Means of starting AC motors in diesel powered grid

[zif63]

Hi All,
There is an old mine to be brought back to service. The electrical power will come from 4 x 1500 MVA diesel generators. On the low voltage side, there are many AC motors ranging from 0.75 HP to 200 HP, most of them in the range 15-75 HP.

Regarding 15-75 HP motors group: originally it was thought that all motors above 20 HP are to be started with soft starters. Due to electrical room size limitation, it would be hard to fit MCCs with all those soft starters.

Generator sizing program allows direct start most of motors under 75 HP (and have MCCs overall dimensions fit electrical room). Any thoughts about direct vs. soft start and their influence to diesel generators?

Thanks!

 

[Paulusgnome]

If you do indeed have 6000 MVA of generator capacity, then it would be somewhat surprising if DOL starting any of the motor sizes that you gave would cause any problems. Even a 200HP motor should not trouble generators of this size.

 

[waross]

700 KVA, should be enough capacity to start a 200 HP motor.
But first, find out what size the generators really are.
Four sets with two million horse-power diesel engines seems a little over the top.

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

 

[jraef]

LOL, yeah, more likely 4 x 1.5MVA or 1500kVA, but not 1500MVA (1,500,000 VA)!

Dirty ugly grossly over estimated rule-of-thumb is that you need 3-3.5X the HP as kVA for Across-the-Line (DOL) starting, 1.5-2X if you have a soft starter. But that is stand-alone. In your case, simply staggering the restarting of the mine will mitigate any potential voltage drop on the generators. I seriously doubt anyone would be so foolish as to flip one switch and start everything at the same time, even if it was a utility feed.

If it were me, I would not bother with soft starters under 150HP. At that point and up, there are other benefits to soft starting even if the generators CAN handle it.

"Will work for (the memory of) salami"

 

[zif63]

LOL ditto, I didn't see that M letter sneak instead of k. Anyway, thank you for responses!