Simone – sorry for yet another email. But, I made a case in a commercial load flow tool for a 3-winding transformer. The particular tool accepts pair-wise transformer test data. I then converted the pair-wise data to T model by hand, and entered that as three 2-winding transformers into mpc.branch. I can verify that the results from both the commercial tool and MATPOWER were spot on for various conditions. In short, this verifies what I say below is actually the case.
Best regards, russ From: bounce-125723610-88411...@list.cornell.edu [mailto:bounce-125723610-88411...@list.cornell.edu] On Behalf Of Russ Patterson Sent: Monday, June 21, 2021 11:13 AM To: 'MATPOWER discussion forum' Subject: RE: Negative resistance Hi Simone, Actually, it looks like PSS/E just converts 3-winding pair-wise data to the T equivalent model (just like in the attachment I sent you in previous email). Then it just models as three 2-winding transformers (the node in the middle is fictitious but needed for the mathematical model). See page 2 here: https://static.dc.siemens.com/datapool/us/SmartGrid/docs/pti/2010July/PDFS/Modeling%20of%20Three%20Winding%20Voltage%20Regulating%20Transformers.pdf Here is a link to the MATPOWER documentation where you can find details on the psse2mpc() function: https://matpower.org/docs/ref/matpower5.1/menu5.1.html It calls another function (psse_convert_xfmr) which has these lines of code below: 0187 R1 = (R12+R31-R23) ./ 2; 0188 R2 = (R12+R23-R31) ./ 2; 0189 R3 = (R31+R23-R12) ./ 2; 0190 X1 = (X12+X31-X23) ./ 2; 0191 X2 = (X12+X23-X31) ./ 2; 0192 X3 = (X31+X23-X12) ./ 2; The above is simply the conversion from pair-wise test data to the T model. Which make it pretty clear that, aside from all the data structure manipulation and other “foggy” items), it is just building the T model and then making 3 two-winding transformers as the first link above describes. Best regards, russ From: bounce-125723529-88411...@list.cornell.edu [mailto:bounce-125723529-88411...@list.cornell.edu] On Behalf Of Russ Patterson Sent: Monday, June 21, 2021 10:49 AM To: 'MATPOWER discussion forum' Subject: RE: Negative resistance Hi Simone, I’m not certain I understand your question. The pair-wise test data include winding resistance and leakage inductance (I ignored resistance in my example for simplicity). This pair-wise test data does not account for the magnetizing branch if that is what you mean by RM. If you search the archive (https://www.mail-archive.com/matpower-l@cornell.edu/) on “3-winders” you will see a reply from Ray explaining that the psse2mpc() function is used to convert 3-winding transformers to multiple 2-winding transformers to put in mpc.branch. I don’t know what is “under the hood” on that function. I would be interested to see the math if you take the time to figure it out. You can’t just put the pair-wise data directly into mpc.branch for 3-winding transformers. In my experience, the only case in which that would work fine is when the transformer is balanced (H-X and H-Y pair impedances are equal, or very close) and the 2 generators are loaded the same (P,Q). Otherwise, you need to convert using psse2mpc(). Best regards, russ From: bounce-125721802-88411...@list.cornell.edu [mailto:bounce-125721802-88411...@list.cornell.edu] On Behalf Of Simone Fratton Sent: Sunday, June 20, 2021 5:36 AM To: MATPOWER discussion forum Subject: Re: Negative resistance Hello, thank you so much again for your answer to the problem of negative resistances. I still have doubts though. Considering precisely the T diagram for the 3-winding transformer (that of your attached) the possible negative resistance that I find myself is RM? How do I know the value of the other resistances? I will eventually only have 2 rows of mpc.branch used for the 3-winding transformer or am I wrong? Thank you so much in advance I hope I don't bother you too much Il giorno ven 18 giu 2021 alle ore 15:01 Russ Patterson <r...@relayman.org> ha scritto: Hi Simone, The negative branch is due to the conversion of pair-wise transformer test data to “T” model. See if the attached helps. It is not a “real” negative physical value – it is necessary for the mathematical model. Best regards, russ From: bounce-125719454-88411...@list.cornell.edu [mailto:bounce-125719454-88411...@list.cornell.edu] On Behalf Of Simone Fratton Sent: Friday, June 18, 2021 5:17 AM To: MATPOWER discussion forum Subject: Re: Negative resistance Thank you very much for your answer, but I am not sure that I have yet understood why; especially if at the end it's about having a transformer with negative resistance. Il giorno ven 18 giu 2021 alle ore 11:08 Dirk Van Hertem <dirk.vanher...@ieee.org> ha scritto: Hello, Negative resistances normally correspond to equivalent branches... Dirk On 18/06/2021 11:04, Simone Fratton wrote: Hello everyone, I wanted to ask you if anyone knows exactly why in some cases the resistance of mpc.branch is negative? Or at least if someone has any good ideas. For example, case3012wp has as many as 10 negative resistances (all refer to a transformer). Thank you all very much.
