Hi guys, first of all thanks @Roman for your inputs.
I have found a way that delivers reliable convergence, but still has a slight weakness: I am using the "Interface Flow Limits" extension. This enables me to define all lines between country A and B as one interface, which gives the code more flexibility than when restricting each line individually. But I still have one issue here: In some areas, my interface flow limits are not met. This is not surprising, as the manual states "the actual AC interface flows will not necessarily be limited to the specified values, since it is a DC flow approximation that is used for the constraint." If my flows are slightly above my constraints, I'd be happy with that. But for example, I get 900 MW interface flow in a location where I limited it to 250 MW... that's just too far off. Bottom line: Is there a "quick fix" or "trick" to be able to constrain the interface flows more closely to the desired values? In that sense, to get more control over the first DC step of the procedure? Any inputs would be appreciated. Best Regards, Patrick ________________________________________ Von: bounce-117304343-69334...@list.cornell.edu [bounce-117304343-69334...@list.cornell.edu]" im Auftrag von "Roman Korab [roman.ko...@polsl.pl] Gesendet: Dienstag, 15. Juli 2014 08:24 An: MATPOWER discussion forum Betreff: Re: Convergence issues: OPF simulation of several interconnected countries Hi Patrick, I have some experience with solving OPF for European transmission network, especially for Central and Eastern Europe grids. In my case I was able to solve OPF problem only by using KNITRO optimization procedure (other procedures implemented in MATPOWER failed). During my studies I discovered that in most situations the capacity (line MVA rating) of the tie lines was sufficient, but congestions appeared in internal networks of individual countries. So, as Ray said, a good approach is to relax the MVA ratings of all lines gradually until you get convergence. The other approach, that I used during my studies with European network, was to start solving OPF without any branch limits and, after getting corvengence, gradually tighten constraints using previous results as a starting point to the next step. After getting convergence in some step you can also compare the MVA power flows that you obtained with realistic MVA rates of individual branches. In that way you can find branches with not enough capacity. Best regards Roman ----- Original Message ----- From: "Eser Patrick" <es...@student.ethz.ch> To: <MATPOWER-L@cornell.edu> Sent: Monday, July 14, 2014 3:34 PM Subject: Convergence issues: OPF simulation of several interconnected countries Dear MATPOWER Community, I am trying to solve a relatively large (>1000 buses) AC OPF problem. It consists of several individual countries (200-300 buses each). Please allow me to describe my procedures, to paint the full picture. 1. In a first study, I simulated all countries individually. The cross-border power flows to the neighboring countries is known to me from a database. I include these cross-border flows by implementing "pseudo-generators" of zero production cost or "pseudo-demand" at the borders. This procedure works nicely, I get convergence in almost every case I run. 2. Now I would like to simulate not only one country, but all my countries interconnected as one large system. I have "stitched" the transmission grids of the individual countries together in a realistic manner. If I run the OPF without further constraints, I receive converging simulations. But unfortunately, the flow between my countries is massively too high, probably due to price level differences between my countries. I would like to prevent these high cross-border flows, which leads to point 3. 3. I would like to limit the power flow (especially the real power flow) between my countries (so on individual branches in my full system), but am not able to get a converging simulation. I have played around extensively the MVA ratings of the lines (long and short term), but as soon as I impose realistic line ratings, I only get "infeasible solution" problems. As a workaround, I tried replacing the AC cross-border transmission lines with DC lines (and limiting real power flow in those DC lines). This does converge sometimes, but not always. I am using MIPS and/or IPOPT. I would massively appreciate any help in this regard. Some questions about the problem: What is the best ways to limit the power flow in an individual branch? Why are my simulations never converging, if I impose MVA ratings? Has anybody solved such a problem before? It seems quite straightforward, I am sure someone has already done something like this with MATPOWER. What would you suggest me to do? Should I rather use other solvers than MIPS or IPOPT? As I am not an expert of transmission system, what indicators should I be looking at to find out, where my simulations are going wrong? Thanks a lot for your comments. Best Regards, Patrick
