transformer sequence impedances

[whycliffrussell]

I have a transformer (46kV to 4.16kV) with impedance z% of a delta wye, resistence grounded transformer with neutral grounding resistor zn.

The positive and negative sequence impedance = z%

Does the zero sequence impedance=z% + 3*zn% where zn% is zn represented on a per unit basis on the the transformer kVA and voltage...

additionally I'm assuming that you'de have to use the transformer kVA for the Sbase...what voltage do you use for the vbase?

 

[dfopiso]

hi,

the zero sequence impedance of a transformer is equal to the positive and the negative sequence impedance. its how the current is treated which is dependent on the transformer winding connection. as in your case, the wye-grounded side of the equivalent circuit is connected to the ground while the delta side is open circuited.

i noticed your formula to include 3*zn, you might be referring to the total impedance in computing the single line to ground fault current where the general formula is:

I_fault = 1 pu / (Z0 + Z1 + Z2 + 3*Z_fault).