No, the constraint violation tolerance OPF_VIOLATION applies to violation of any constraints and is used as a termination criteria for many of the solvers. So, I don't think that is going to be a useful direction.
-- Ray Zimmerman Senior Research Associate 419A Warren Hall, Cornell University, Ithaca, NY 14853 phone: (607) 255-9645 On Apr 12, 2013, at 4:58 AM, Roman Korab <[email protected]> wrote: > 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 >> >> > >
