HELP - Per unit impedance for Short Circuit calculation

[eestudent]

In the general formula for per unit impedance

Zp.u(old) = (Powerbase(new)/Powerbase(old)) x (Zp.u(old)) x ([V(old) / V(new)]^2)

if I use the MVA(scc) of utility as powerbase

For Zp.u.(utility)= Power Base(new)/ Power Base (old)
where in :
voltage is said constant, why ?
there is no Zp.u(old), why?

For Zp.u.(transformer)= Power Base(new)/ Power Base (old) x Zp.u(old)
where in :
voltage is said constant, why ?

 

[Mayimbe]

In the general formula for per unit impedance

Zp.u(old) = (Powerbase(new)/Powerbase(old)) x (Zp.u(old)) x ([V(old) / V(new)]^2)

its Zp.u(new) = (Powerbase(new)/Powerbase(old)) x (Zp.u(old)) x ([V(old) / V(new)]^2) (1)

For Zp.u.(utility)= Power Base(new)/ Power Base (old)
where in :
voltage is said constant, why ?
there is no Zp.u(old), why?

V(utility) usually is equal to V(base) so if you say:

Zp.u(old)=Powerbase(old)/V(base)^2 and
Zp.u(utility/new)=Powerbase(new)/V(utility)^2

since V(base)=V(utility) then,
Zp.u(utility)=Power Base(new)/ Power Base (old)

For Zp.u.(transformer)= Power Base(new)/ Power Base (old) x Zp.u(old)
where in :
voltage is said constant, why ?

same as before usually V(transformer)=V(base) in both sides. If it is not, say you move one tap in the transformer in the secundary side, then you have to use the equation (1) of above

 

[eestudent]

can't get it yet

can't get it yet

thanks for correcting Zpu(old) to Zpu(new)

Using the Utility's MVAscc as the commonbase you are right
V(utility) = V(base)

But if the transformer is connected in that Utility using the same
common base (Utility MVAscc) why the voltage of transformer
not been considered ?
Zp.u.new(transformer)= Power Base(new)/ Power Base (old) x Zp.u(old)

thank you !

 

[mivey]

But if the transformer is connected in that Utility using the same
common base (Utility MVAscc) why the voltage of transformer
not been considered ?

If I understand your question, Mayimbe has answered that already:

...
since V(base)=V(utility) then,
...
same as before usually V(transformer)=V(base)

In other words:
V(base)=V(utility)=V(transformer)