Medium Voltage Fault Interrupter

[timm33333]

I have a question about sizing of 25kV SF6 type fault-interrupter. It receives power from utility on the line side, and supplies power to a 12/16/20 MVA transformer on the load side. The available sizes of fault-interrupter are 600A and 900A.

The maximum current on the transformer primary is 462A. If the detailed calculations are not available, then I think 900A would be better choice because 600A would be too small. What do you guys think about it? Thanks.

 

[Palletjack]

How often, and by how much will you be overloading this XF?
If it’s like many I have looked at it’s running at 50-75% loading on a day to day basis, maybe 80% at peak.
I would look more at the fault interrupting values.

 

[timm33333]

The transformer will be loaded about 65% on day to day basis, maximum 80%. The available interrupting ratings of the fault interrupter are 12.5kA and 25kA. The incoming fault current value from utility is not available yet, but I think it will be less than 12.5kA.

 

[davidbeach]

How much less? I certainly wouldn't install a 12.5kA device if the utility tells me that fault current is much more than about 8kA. And I find 8kA to be rather wimpy.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[timm33333]

I will check with the utility regarding the fault current. How about the continuous current rating, is 600A Ok, or should it be 900A?

 

[RRaghunath]

Regarding load current, the FI rating of 600A could be at 40degC ambient.
If design ambient temperature for your station happens to be > 40degC, you need to apply derating factor over 600A rating.
Don't forget to take it in to consideration while finalising the FI rating.