Thanks Jose, we’re looking into this.
Ray
> On Oct 23, 2015, at 8:48 AM, Jose Luis Marin <mari...@gridquant.com> wrote:
>
>
> Sorry, I think I made a mistake in my interpretation of what R1-2 means when
> CZ=3. I think it is just the losses in the actual short-circuit test, in
> which the applied voltage is very low, certainly NOT the nominal voltage. The
> voltage is applied until one gets the nominal current, I=1 in pu. Then the
> calculation of R in the Siemens paper I cited is correct.
>
> The corrected code would be then:
> R(cz3) = 1.0e-6 * ( R(cz3) ./ trans2(cz3,23) );
> 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).
>
>
> --
> Jose L. Marin
> Gridquant España SL
> Grupo AIA
>
>
>
> On Thu, Oct 22, 2015 at 2:52 PM, Jose Luis Marin <mari...@gridquant.com
> <mailto:mari...@gridquant.com>> wrote:
> 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/>
