Change the base

[AAGR2013]

the formula to change the base is  Znew-pu = Zold-pu *(SBASE-NEW / SBASE-OLD) * (VBASE-OLD / VBASE-NEW)2 , but based on ZB = (VB)2/SB , the formula shall be upside down like Znew-pu = Zold-pu *(SBASE-OLD / SBASE-NEW) * (VBASE-NEW / VBASE-OLD)2

Right?

 

[cupojoe PE PMP]

The first formula is correct. Off hand I can't remember the derivation for that formula.

 

[Panchito]

The derivation is not too complicated. First convert your Z_old from P.U. to Ohms, by multiplying by the Z_base_old

Z_ohm = Z_old x Z_base_old

And now convert Z_ohm (in Ohms) to Z_new (in P.U) by dividing over Z_base_new:

Z_new = Z_old x Z_base_old / Z.base_new

We also know that

1)  Z_base_old = (V_base_old)^2 / S_base_old
and
2)  Z_base_new = (V_base_new)^2 / S_base_new

Now replace 1) and 2) in the expression for Z_new above, and we will get the first formula posted by AAGR2013.