Hello Ray,
I think I may have found a bug in *psse2mpc*, which concerns the calculation of R,X for transformers when these are specified with *"Impedance Code" CZ=3*. The PSS/E docs decribing the RAW format specify that, when CZ=3, the values in the file are: - R1-2 is the transformer load loss in watts (to be precise: short-circuit test three-phase full-load winding copper losses, in Watts) - X1-2 is impedance magnitude in pu, on a specified base MVA (SBASE1-2) and winding base voltage (NOMV1) The conversion code is in lines L0107-L0108 for 2W transformers and L0180-L0185 for 3W transformers, in module psse_convert_xfmr <http://www.pserc.cornell.edu//matpower/docs/ref/matpower5.1/psse_convert_xfmr.html> . By my account, this is how the conversion should be done, conceptually (see if you agree): - The value X1-2 given in the file is actually the modulus of Z, i.e. the complex impedance of the "copper" part of the transformer. Use this to obtain the modulus of the intensity I in the short-circuit test (using V=NOMV1, i.e 1 in pu), and then from P=I^2 R one can obtain the per-unit resistance. The value P (in pu) is just the value R1-2 divided by SBASE1-2 * 1.0e+6. - Now that we've got R in pu, and we know |Z|, calculate X = sqrt ( |Z|^2 - R^2). - Up to here all quantities are pu on SBASE1-2 and NOMV1. Change to the pu of the system base and the bus terminals base voltages (these may be different than the transformers' sometimes). This last bit was correctly done in the code. But this is how it appears in the code now: 0107 X(cz23) = X(cz23) .* Zb(cz23) ./ Zbs(fbus(cz23)); 0108 R(cz3) = trans2(cz3,25).^2 ./ trans2(cz3,21) ./ Zbs(fbus(cz3)); The correction, if my conversion procedure above is correct, would be as follows (showing the code only for 2-winding cz3 transformers): R(cz3) = 1.0e-6 * ( R(cz3) ./ trans2(cz3,23) ) .* X(cz3).^2; X(cz3) = sqrt(X(cz3).^2 - R(cz3).^2); % now change to pu of system units (also with a possible correction due to nominal bus voltages) R(cz3) = R(cz3) .* Zb(cz3) ./ Zbs(fbus(cz3)); X(cz3) = X(cz3) .* Zb(cz3) ./ Zbs(fbus(cz3)); (and similarly for R,X of 3-winding transformers; note also that line 0107 has to stay for cz2 transformers). Now, keep in mind that there's a document by Siemens here <http://w3.usa.siemens.com/datapool/us/SmartGrid/docs/pti/2009July/PDFs/Modeling_of_two_winding_voltage_regulating_transformers.pdf> in which they work out the math of this conversion, but I think they get it wrong for R, because they are forgetting the factor |Z|^2. Unfortunately, I don't have PSS/E to test what the program really does internally, but my take is that if R1-2 and X1-2 are what the specs say they are, then the formulas I gave here are the correct ones. -- Jose L. Marin Gridquant España SL Grupo AIA <http://www.gridquant.com/>
