Why is LL bigger than 3ph fault

[wonderchi]

Hi,

I have just obtained some fault currents result from using SKM, it shows that both the SLG and LL(phase to phase) is much higher than the 3ph fault.

With SLG fault I can figure out that it is possible (due to low sequence impedance) but why is the phase to phase fault bigger than the 3ph fault?

Thanks

 

[wonderchi]

Correction...

it should be LLG (double phase to ground fault) not LL....

Why is LLG bigger than 3ph fault? Because of low zero sequence impedance from generators and transformers?

Thanks

 

[cuky2000]

Not necessarily LLG is greater than 3 phase fault. This is a function of the system sequence impedance at the fault location. See the enclose figure that may be used as a guide to determine maximum fault level.

http://cuky2000.250free.com/SC_Highest_Sort_Circuit.jpg

 

[stevenal]

Delta/wye transformers ahead of the fault location short circuit the upstream zero impedance network regardless of fault type. Any fault involving the zero sequence network will see a low zero sequence value.