Dear Ray and matpower users,
i'm writing my thesis about a data-converter for two power system
simulation softwares: PSSE@Netomac from Siemens and matpower. Here is
a problem with transformer modeling, i've read a lot in this archive
but still can't solve it, so i decide to post my question here.
My goal is to get the same or a simular power flow convergency using
the Newton's method after converting the net topology from one to the
other. However, I'm stucked when I try to convert a Netomac net into a
matpower case. Comparing the PF results of the 2 softwares, there is
always a big deviation (by bus voltage magnitude, bus voltage angle
and branch P/Q injection). More specifically, the bus voltages in pu
of matpower are all lower than those of Netomac, the deviation could
be up to 5%, like 0.977 instead of 0.997 in Netomac.
I simply grab all the bus/gen/branch parameters from Netomac except
for the transformer impedance, so I guess there might be a mistake
when the transformer model is built in matpower.
The transformers have 2-windings and no tap changer
The parameters I could get from Netomac are:
- Rated voltage of HV side winding (Un1)
- Nominal network voltage HV side (UB1? the value is equal to Un1)
- Rated voltage of LV side winding (Un2)
- Nominal network voltage LV side (UB2? the value is equal to Un2)
- Rated apparent power (equal to baseMVA)
- ur in % (may not be zero)
- uk in % (must be larger than ur)
- P0 and I0 are not given
- vector group YY0
in matpower a transformer is treated as a transmission line, where
r(p.u.) and x(p.u.) are needed for the power flow calculation. (b is
omitted in my case)
According to the formels:
z_pu = uk% / 100
x_pu = uk% *(Un/UB).^2 * (SB/Sn) / 100
r_pu = sqrt(z_pu.^2 - x_pu.^2) (here r_pu = ur%/ 100)
Since in my case Un/UB = 1, SB/Sn = 1, so x = uk%/100, r = ur%/100, b = 0
I set ratio = 1 and angle = 0, status = 1, angmin/max = +-360
other values are set to 0
Netomac can also export a .raw file for PSSE, the conversion above
will generate equivalent r and x values as the exported .raw file.
Theoretically, with the same net topology and P/Q accuracy, the PF
convergency of the two simulation tools should be almost the same.
Here is one more hint, as far as I know, the transformer impedance in
matpower is modeled at "to" side, while in Netomac it's modeled at
"HV" side (which is the "from" side), so there could be a difference
between the two models but i got no clue how to unify them.
Has anyone ever met this kind of problem before? Or the transformer is
converted in a right way, but there could be something wrong in other
parts?
I know this is a tough one, hopefully I could get some hints here.
Many thanks for all!
Best Regards
Shuo
