transformer fault current 2

[alanmitchell]

Can anyone give me the max fault current on the seconday star winding of a transformer.Details below.

HV 13.8V delta
LV 480V star
KVA 3125
5.72% inpedance
60 HZ

 

[RonShap]

3125000/.0572/480/1.73= 66,174A

 

[cuky2000]

The SC 66 kA as calculated by RonShap is based in the following assumptions:

a- Infinite source at the transformer primary side. In most calculations, this is a reasonable assumption that provides slight conservative value.

b- Fault is postulated at the transformer bushings or very close of them without any motor in operation.

c- If motor contribution is added the available Short Circuit should increase.

In absence of detail load information, practical assumptions are usually used to determine the design SC motor feedback contribution as follow:
For commercial buildings, assume one:
· 50% induction motor load
· Two times full load current of transformer.

For Industrial plant, assume one:
· 100% motor load.
· Motors types: 25% synchronous and 75% induction.
· Motor feedback contribution of four times full load current of transformer.

For precise calculation, a detail load and the SC damping effect of cable should be considered.

 

[GamezBeCJ]

A transformer design engineer also considers de asymetrical factor on the fault curret:
Iasm = Inom*sqrt(2)*(1+e^-pi*(R/X))/Zpu. My guess in your case is a R/X ratio of 0.1, so:
Inom=3758.8 then:
Iasm = 3758.8*2.447/0.0572 = 160,800.41 amps at least on the first cycle.
Forces in conductors, insulators and windings is proportional to this current, so you may want to consider this force in your mechanical design.

Carlos Gamez, P.E.
Industrial Consultant
Transformer Specialist