Since the branch impedances are on the "to" side of the branch, you are correct that you would use the voltage base from the "to" bus.
The formula isn't really a MATPOWER question, so you should be able to find the answer in a good power systems textbook, however it is very straightforward to derive yourself. Essentially we want to find the impedance base (Zbase) in terms of the power and voltage bases (Sbase and Vbase). Start with some definitions ... (1) V = Vpu * Vbase (2) S = Spu * Sbase (3) Z = Zpu * Zbase Then some simple relationships ... (4) V = I * Z (5) S = V * conj(I) Solving (4) for I and substituting in (5), then solving for Z, gives ... (6) Z = |V|^2 / conj(S) And we know the p.u. quantities have the same relationships … (7) Zpu = |Vpu|^2 / conj(Spu) Substituting (1)-(3) and then (7) into (6) and rearranging, gives … Zbase = Vbase^2 / Sable Hope this helps, -- Ray Zimmerman Senior Research Associate 419A Warren Hall, Cornell University, Ithaca, NY 14853 phone: (607) 255-9645 On May 13, 2013, at 3:44 AM, Petre Razusi <[email protected]> wrote: > Dear all, > > I have some things I don't quite understand regarding the modeling of > branches and the pu system. > If I'm not wrong, to transform the R,X and B parameters to pu one needs a > base impedance. Since the model used for the branch is a complex one, dealing > with both transformers and lines, I presume that this impedance is calculated > using the "to" bus voltage. Am I right? And if so, could you tell me what is > the formula used to compute the base impedance? > > Thanks, > > Petre
