The quote from the manual that mentions the case with r = x = 0 is simply to make clear the definition of the tap ratio of the ideal transformer, which is only a piece of the branch model (see Fig 3-1 in the manual). A transformer is modeled as an ideal transformer in series with a pi-model line with series impedance and shunt capacitance.
In any case, zero impedance branches *will* cause numerical problems, since they result in NaNs or Infs in the Ybus matrix. -- Ray Zimmerman Senior Research Associate 419A Warren Hall, Cornell University, Ithaca, NY 14853 phone: (607) 255-9645 On Mar 9, 2012, at 10:09 AM, Jo Hoogslag wrote: > Dear all, > > I have question concerning the impedance of transformers. I was under the > impression that transformers were ideal and had impedance = 0. In the > Matpower User Manual is says that "if r = x = 0, tap = Vf/Vt" (page 79). > However, when I model transformers with a tap ratio, BR_R = 0 and BR_X = 0 it > doesn't converge (NaN). > > When I model the same transformers with BR_R = 0 and BR_X = 0.1 it works. > > I do not understand this, have I misunderstood the manual? > > Cheers, > Jo
