Hi Ray,
I believe you are correct – if you ensure that the off-nominal transformer is
added to the model with orientation in the branch data as coming FROM the
low-side TO the high-side it will build the model correctly. Below is a snip
from my notes PPT and I compare the textbook model (left-hand side) to the
MATPOWER model (right-hand side).
Entering the off-nominal transformer branch information like this (with the
low-side as the FROM bus) builds the exact correct model and the load flow
results exactly match those that are found by hand with the textbook approach.
The simple solution appears to require that off-nominal transformers be entered
in branch data with this orientation – otherwise incorrect results are to be
expected.
So far I have verified that PW, MATPOWER, PSS/E all agree and have the
incorrect result. Siemens PSS/CAPE load flow produces the correct result
as-is. Changing the orientation (as you suggested) in the MATPOWER branch
data for the off-nominal bank will result in it producing the correct result.
Best regards,
russ
From: [email protected]
[mailto:[email protected]] On Behalf Of Ray Daniel
Zimmerman
Sent: Wednesday, January 6, 2021 11:52 AM
To: MATPOWER-L
Subject: Re: circulating current (MVAR loss)
The term “pi-model” may be used to refer to different things, but I think what
I said is still correct. MATPOWER uses an ideal transformer (with possible
complex off-nominal taps) in series with a standard pi-model transmission line
that includes both a series impedance and shunt elements.
If the shunt elements are ignored, the resulting model (series impedance in
series with ideal transformer) can also be represented as a different pi-model.
But it matters whether the transformer is at the “from" bus and the impedance
at the “to” bus or vice versa. And I believe MATPOWER and the textbooks you are
using are identical, except for the convention used for this orientation.
So, if I am correct about that, simply swapping the orientation and inverting
the tap ratio in MATPOWER should result in identical results to your hand
calculations.
Ray
On Jan 4, 2021, at 2:21 PM, Russ Patterson <[email protected]> wrote:
I’ll take a look, thanks Ray. But, the pi-model’s I’m talking about are to
accommodate an off-nominal transformer. The pi- model you are talking about is
to accommodate the shunt terms which are not present in my case (bc). Two
different animals, right?
russ
From: <mailto:[email protected]>
[email protected] [
<mailto:[email protected]>
mailto:[email protected]] On Behalf Of Ray Daniel
Zimmerman
Sent: Monday, January 4, 2021 2:29 PM
To: MATPOWER-L
Subject: Re: circulating current (MVAR loss)
Actually, it looks to me like the difference is simply the convention of which
end of the branch has the impedance and which end has the ideal transformer. If
you flip the orientation of the branch in your MATPOWER case, so that it goes
from bus 2 to bus 1 and invert the tap ratio, I think the MATPOWER results
should match the other models exactly, right?
Ray
On Jan 4, 2021, at 12:32 PM, Ray Daniel Zimmerman < <mailto:[email protected]>
[email protected]> wrote:
Hi Russ,
I suspect the models may not be identical. Are they using the model shown in
the <https://matpower.org/docs/MATPOWER-manual-7.1.pdf> MATPOWER User’s Manual
in Fig 3-1 with an ideal transformer in series with a pi-model? If not, that
would explain the discrepancy.
I don’t have those books handy, so feel free to send me (off-list please) the
PDFs of the relevant pages, as it would be useful to confirm.
Thanks,
Ray
On Jan 4, 2021, at 11:19 AM, Russ Patterson < <mailto:[email protected]>
[email protected]> wrote:
Hi Ray,
I’ve chewed on this a while and it looks like the MATPOWER approach is
equivalent to the circuit below for the off-nominal transformer (#2). The
<image001.png> I show is based on <image002.png> for both transformers (#1 is
at nominal tap, #2 is at tap of 1.0502). The <image003.png> below is =
<image004.png>. This is the <image001.png> that MATPOWER builds. The
secondary bus voltage is then 0.976 pu.
<image005.png>
Below is what you get if you follow the approach that Gross1, Neunswander2, and
Kusic3 use (attached).
<image006.png>
Although the MATPOWER approach and Gross approach result in different
<image007.png> matrices, they result in nearly exactly the same secondary bus
voltage because the ratio of <image008.png> in both are nearly identical.
It seems to me that the <image001.png> created by MATPOWER may be wrong when
off-nominal taps are used. I say that with a large dose of humility because
surely I am missing something. But, it seems clear that either Gross or
MATPOWER is wrong here. What am I missing?
References (I can provide pdf of the pages if needed):
1 – “Power System Analysis”, 2ed, Charles A. Gross (p. 204)
2 – “Modern Power Systems”, John. R. Neunswander (p. 251)
3 – “Computer-Aided Power System Analysis”, George L. Kusic (p. 95)
Best regards,
Russ
From: <mailto:[email protected]>
[email protected] [
<mailto:[email protected]>
mailto:[email protected]] On Behalf Of Ray Daniel
Zimmerman
Sent: Monday, December 21, 2020 11:44 AM
To: MATPOWER-L
Subject: Re: circulating current (MVAR loss)
I suggest double-checking your calculations against the code in makeYbus.m,
which is pretty straightforward, and the model described in the
<https://matpower.org/docs/MATPOWER-manual-7.1.pdf> User’s Manual see Figure
3-1 and equation (3.2). Be sure to keep in mind the orientation of the taps in
the model.
Ray
On Dec 16, 2020, at 3:52 PM, Russ Patterson < <mailto:[email protected]>
[email protected]> wrote:
Carlos – thank you. Very helpful.
The YBus I get for my case is below. I expected Y(1,1) to equal the of this
sum: (1/j0.1) + (1/j0.09522) + (1/-j1.991) = j 19.9997 (negative sign is per
coder preference). Is attached (page 1) not how MATPOWER would modify the
bank #2 impedances before creating YBUS?
Yb =
Compressed Column Sparse (rows = 2, cols = 2, nnz = 4 [100%])
(1, 1) -> 0 - 19.0663i
(2, 1) -> 0 + 19.5217i
(1, 2) -> 0 + 19.5217i
(2, 2) -> 0 - 20i
Best regards,
russ
From: <mailto:[email protected]>
[email protected] [
<mailto:[email protected]>
mailto:[email protected]] On Behalf Of Carlos E
Murillo-Sanchez
Sent: Wednesday, December 16, 2020 4:12 PM
To: MATPOWER discussion forum
Subject: Re: circulating current (MVAR loss)
Russ Patterson wrote:
Hi - I am still trying to hand calculate the flow into branch 2 from bus 1 to
bus 2. I can’t get my results to match MATPOWER.
I get Q into the banks from bus 1 of,
Bank #1: 24.00 MVAR
Bank #2: -25.02 MVAr
Attached is my short calculation and the .m file. Is there a way to have
MATPOWER barf out the YBUS matrix?
>> help makeYbus
If buses are numbered consecutively starting from 1 in the bus table (see
ext2int if not), simply type:
>> mpc = loadcase('mycase');
>> [Yb, Yf, Yt] = makeYbus(mpc)
To get all the relevant current injections in the solved case, simply do
>> mpc = runpf(mpc);
>> define_constants;
>> V = mpc.bus(:, VM) .* exp(1i * mpc.bus(:, VA)*pi/180);
>> Ibus = Yb * V
>> Ifrom = Yf * V;
>> Ito = Yt * V;
>From there, compute power injections as
>> Sbusinj = V .* conj(Yb * V);
>> Sfrominj = V(mpc.branch(:, F_BUS)) .* conj(Yf * V);
>> Stoinj = V(mpc.branch(:, T_BUS)) .* conj(Yt * V);
carlos.
<power.pdf>
<image021.png><image007.png><image001.png><image012.png><image014.png><image022.png><image011.png><image005.png><image013.png><image020.png><image003.png><image019.png><snip.pdf>
