Simple calculation method to calculate Short circuit for busbar/transf

[katelec]

Could anyone please explain me the simple/easy/thumb rule method to calculate the short circuit current/fault level for Busbar/transformer?

 

[mpparent]

How about:

Isc = I(fla)/z

where:

Isc = 3 phase bolted fault current at the secondary of the transformer
I(fla) = Full load current of the transformer
z = impedance of the transformer

This is called the "infintite bus" method.

Mike

 

[weh3]

It is also a worst-case scenario, at least right at the secondary. If you have a large motor load, it is possible that they could increase the s/c value, especially near the motor.

William

 

[mpparent]

Make sure when you take the Z off of your transformer, you use the decimal equivalent. For example, if the nameplate says the Z = 5.75% (which is pretty standard), then you must use .0575 in the calculation I showed you above.

Mike

 

[tommom]

If you use Isc=Ifla/z, and you're designing the system (ie you're not looking at a nameplate), you should adjust z downward by 7.5%. In other words, min. imp for a 5.75% transformer would actually be 5.3%, unless the transformer was small (I'm not sure of the rating, but it's quite small, like 75 kva), in which case you use 10%.

 

[weh3]

To account for manufacturing tolerance in the determination of Z.

William

 

[jraef]

There is an excellent shareware program available at

http://www.lmphotonics.com

for about US$22 that will do this for you. It is provided by Marke, a regular contributor and VIP in this forum.

"Venditori de oleum-vipera non vigere excordis populi"