Ray, thank you very much for answer. As allways useful and very inspiring! Relaxing the power balance constraints to make the AC OPF feasible is a new idea for me. I always tried to do so by relaxing some inequality constraints, usually branch limits, sometimes the range of reactive power generated. It usually works fine. Usually, but not this time. I will try your idea. But now I think how to do this. Will it be enough to change the constraint violation tolerance (OPF_VIOLATION)? Or should I do something else?
Yes, when I said that idea with virtual generator sometimes works and sometimes not, I meant that OPF sometimes doesen't converge. I also think that is due to numerical problems. I tried different solvers, but without good results. As you mentioned the reason may be in to large costs of virtual generators (100 - 200 for real generators and 10000 for virtual ones). I remember that in my previous research concerning the unserved energy I also use the concept of virual generators and the effect was satisfying, but the range of costs was much more less. I will examine this issue. If these ideas fail I will think of iterations between AC OPF and DC OPF. Best regards Roman ----- Original Message ----- From: Ray Zimmerman To: MATPOWER discussion forum Sent: Thursday, April 11, 2013 3:02 PM Subject: Re: Branch limits as "soft" constraints For a case where branch limits make the AC OPF infeasible, this is a clever way to get a solution that is close to feasibility, however, it does so by relaxing the power balance constraints, not the flow limit constraints. I'm not sure that's what you want. When you say that sometimes it doesn't work, do you mean that the OPF doesn't converge? If so, it may be due to numerical issues stemming from a large range of costs (assuming you are using very large costs on these virtual generators). It's possible that a different solver or less extreme costs on these virtual generators would be more successful. Let me mention that just because you are using an AC OPF does not mean that you can't apply penalty costs to a linear approximation of the flows. If you iterate, you could improve the linear approximation of the flows at each iteration and probably get quite close to the solution you are looking for (penalties on violated AC flows). You could start by approximating the AC flows with the DC flows, assigning a penalty to violations of these linear approximations to the flows. Then you could linearize each line flow around the current operating point, reconstruct the violated flow penalties based on the new flows and re-run, repeating until you have convergence. Just an idea … -- Ray Zimmerman Senior Research Associate 419A Warren Hall, Cornell University, Ithaca, NY 14853 phone: (607) 255-9645 On Apr 10, 2013, at 3:11 AM, Roman Korab <[email protected]> wrote: Hi Ray, Thank you for your reply. In my current research I use AC OPF, so the problem is not easy. I tried to solve this issue by adding in every bus two virtual generators - the first one with positive and the second with negative generation, both with very high costs. In consequence virtual generators produce power only when there is no any other way to meet branch limits. But it sometimes works fine, and sometimes not. I wonder what can be the reason of such behaviour? Best regards Roman ----- Original Message ----- From: Ray Zimmerman To: MATPOWER discussion forum Sent: Tuesday, April 09, 2013 8:58 PM Subject: Re: Branch limits as "soft" constraints Hi Roman, In the current version of MATPOWER there is no easy way to do this for AC OPF problems, since the flows are non-linear functions of the optimization variables. It is possible to add user-defined costs on the DC model flows using the mechanism described in sections 5.3.1 and 6.1 of the User's Manual, since they are linear functions of optimization variables. -- Ray Zimmerman Senior Research Associate 419A Warren Hall, Cornell University, Ithaca, NY 14853 phone: (607) 255-9645 On Apr 9, 2013, at 11:26 AM, Roman Korab <[email protected]> wrote: Hello Matpower users! I wonder if is possible to treat the thermal branch limits as a "soft" constraints in Matpower OPF procedures? For example, when I make some OPF calculations in order to determine effectivness of cross-border real power flow control by using phase shifting transformers, for some settings of PST, OPF don't converge. The main reason is that some branches have not enough capacity to transmit more power. Is there any simple way to overcome mentioned problem, for example, by automatic treatment the thermal limits of such branches not as a strict constraints, but as constraints that can be exceeded? Best regards Roman
