Inrush Current on 150KVA Tx...

[cfordyce]

I can't type today. my previous post stated 1500KVA TX but it is only a 150 KVA TX (250 FLA). Does that bring my inrush current down enough to make the 30A primary breaker feasable??

 

[electricpete]

For 600v 1 phase I calculate full load current is 150000/600=250.

For 600v 3 phase I calculate full load current is 150000/600/sqrt(3)=144.

Peak magneitizing inrush current can be many times this (8-10x)

Without have considered it carefully myself, I think you should have someone look carefully before you even attempt it.

any other comments from the others on the forum?

 

[peterb]

The 30A breaker will trip instantaneously at typically 10x rating, or 300A. Expected magnetizing inrush, as electricpete points out, is going to be around 10x transformer full load or 2500A. Your 30A breaker won't work.
Sounds as though you need to get a transformer more closely rated for your load, or size the primary breaker accordingly.

 

[dpc]

I agree with all of the above. A 30A breaker is much too small for a 600V 150kVA transformer primary. Probably something like a 200A breaker is what I would use, assuming a three-phase unit.

Keep in mind that if you increase breaker size, you also have to increase the feeder size between the breaker and the transformer.

 

[busbar]

That probably rules out the 1500kVA unit[!]

 

[MDom]

cfordyce,as you posted the grill has 133A 220V,it means
that current in the primary winding 600 V 1 phase will be
not less than 50 A , even if you don't consider inrush current, 30A CB is not suitable.
If you havn't the secondary winding protection of your
installation and the grill if dead connected to the seconadary winding, you doesn't have 8-10 times inrush
current during turn-on of TFo,but it may be still high.
The old manner to avoid inrush is to use the ballast resistance in the primary winding only during turn-on.
In your case, I think you need it around 30-50 Ohms to
reduce magnetizing current of idle Tfo by 4-5%.
It must be cheapper than moulded case CB 200 A rating
In this case you need CB as min 63 A with curve D designated
for Tfo and motor protection - instant. tripping 10-14 times.

 

[jbartos]

Suggestion to the original posting. Please, would you post the transformer primary voltage rating.
If 13.8kV then the 150000/13800=10.87A and 30A fuse could withstand it for this single-phase case and for the three phase case 150000/(sqrt3 x 13800)=6.27A, the 30A would be excessive.

 

[busbar]

Wouldn't the OP's initial spec of "150 KVA TX (250 FLA)" effectively obviate that?