Ray – I built the case in Siemens PSS/CAPE power flow module yesterday and it’s results match mine exactly for this case (same YBUS and same results). I remember years ago when CAPE was owned by Electrocon that they went through an exercise to get off-nominal taps modeled correctly in their short-circuit module. They reached out to me and I sent them Neunswander’s reference which they incorporated into their help documents in 2014 (all of the texts I’ve looked at – attached – result in the exact same model). If you don’t model this correctly it results in an error – but it is not a huge error. Plus, it only shows up if you have a parallel path to the off-nominal transformer. If you just have an off-nominal transformer feeding a bus by itself – no error to the system flows.
In my test case I’m calculating by hand that the system supplies 1.19 MVAr (and CAPE reports 1.20). MATPOWER and PW say 1.14. Not a huge error, but an error none-the-less. I’ll keep scratching at it. Just FYI russ From: [email protected] [mailto:[email protected]] On Behalf Of Russ Patterson Sent: Tuesday, January 5, 2021 11:07 AM To: 'MATPOWER discussion forum' Subject: RE: circulating current (MVAR loss) Ray – I just built and ran this same case in PowerWorld and it gives the exact same results as MATPOWER. Including identical YBUS. My problem is that I can’t reconcile the textbook approach to modeling the off-nominal tapped transformer in parallel with on-nominal tapped transformer with these results. I’ll keep pecking away at it – just wanted to let you know of the agreement in results with PW. Fyi, russ From: [email protected] [mailto:[email protected]] On Behalf Of Russ Patterson Sent: Monday, January 4, 2021 3:22 PM To: 'MATPOWER discussion forum' Subject: RE: circulating current (MVAR loss) 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: [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 <[email protected]> wrote: Hi Russ, I suspect the models may not be identical. Are they using the model shown in the MATPOWER User’s Manual <https://matpower.org/docs/MATPOWER-manual-7.1.pdf> 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 <[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>
