generator - thevenin impedance

[tonyflair]

I'm using some software to perform a fault study on a distribution system. When I input a substation (ie. generator) I have to specify the 3 phase fault current and and the short circuit ratio (X/R). From this, the program determines the positive and negative thevenin impedances....the zero sequence is left blank.

My question is how do I determine the correct zero sequence impedance ? If the configuratin is "star" is the zero component 0+jX (where jX is the same as the positive and negative sequence components)? When I do this, my calculated values and the ones the computer spits out are quite different.

Any help would be much appreciated

Thanks

 

[mersin33]

Generator zero and negative sequence impedance is not same as positive. You will have to look up in the generator data.

P.S. do not forget to consider the grounding resistance/impedance if any. Many programs would take this value as a separate input.

 

[rsherry]

I suspect the program you are using may have a default assumption that somewhere in the real system between your real generators and this substation where the generation is being modelled there will be a break in the zero sequence path and is therefore not including any zero sequence impedance path in the machine equivalent to represent this (an impedance of zero is often treated (not very logically) as an indicator of no zero sequence path).
Which program are you using ?
Your zero sequence path at distribution levels will usually be dominated by transformer earth paths (either Neutral earth impedances or earthing transformers) close to the busbar.