Oh, I forgot to consider real and reactive power output limits of generators! I thought they were considered in power flow.
The data are from power supply company and they are in a mess. I did some modification on the generator constrains to make them reasonable and it converged. Thank you! Your suggestions helped a lot :) On Fri, Mar 2, 2012 at 12:03 AM, Ray Zimmerman <[email protected]> wrote: > I was just thinking that setting the gencosts to zero might cause numerical > difficulties for some of the solvers. As far as the power flow solution > being a solution to the OPF problem, that is true only if all of the > voltages, flows and generator real and reactive dispatches are within limits > (which is not guaranteed by the power flow solver). > > -- > Ray Zimmerman > Senior Research Associate > 419A Warren Hall, Cornell University, Ithaca, NY 14853 > phone: (607) 255-9645 > > > > > On Mar 1, 2012, at 10:12 AM, Hua Bowen wrote: > > Thank you so much Dr.Zimmerman, for your really detailed reply. > > First of all, power flow with dispatchable loads works well. I mean, > the result is exactly the same with the normal loads situation. Also, > the reason I set the cost for all real generators to zero and the cost > for all dummy generators to 1$/MW is that I could get the amount of > load shedding directly from the objective function. For the case I am > solving now I expect no load shedding, but in reliability evaluation > process, if some outage of generators and/or transmission lines > happens, I could easily get the load shedding data I want. Is there > any danger caused by setting cost for all real generators to zero? > > I tried your suggestion, setting voltage limits to 0.9-1.1 (or more > relaxed ones) and setting the line limits to zero, also setting low > costs for the real generators and high value for dummy ones. > Unfortunately that they won't help. > > I didn't do any change to the system so I would expect that, given > that the power flow converges, then at least the OPF problem has a > solution which is the same result as the converged power flow. Am I > right? > > Sorry I didn't mention that the real-life system is a really big one > in Northwest China, which has buses of different base voltages, > varying from 6kV to 800kV. And in the converged power flow, the > voltages of some buses are as low as 0.74. (Most of the buses are > fine. And the one with the extremely low voltage is not a important > bus. I mean, not in the backbone network, and with a low base > voltage.) I think the case itself might not be a really good one. > I'll see whether I can do some simplification to the system, after all > I just want to know the reliability data for the high-voltage > transmission network. > > > > On Thu, Mar 1, 2012 at 9:33 PM, Ray Zimmerman <[email protected]> wrote: > > See responses below ... > > > On Mar 1, 2012, at 7:25 AM, Hua Bowen wrote: > > > Dear All, > > > I am dealing with reliability assessment of a real-life system. This > > system has 5370 nodes. I converted all the loads into dispatchable > > loads and the power flow converged. > > > I planned to use dispatchable load model to determine the amount of > > load shedding in different contingencies. So I set the cost for all > > real generators to zero and the cost for all dummy generators (used to > > model dispatchable loads) to 1$/MW. However, when I was running AC OPF > > on this system, using default solver, MATPOWER says that “Matrix is > > singular” and “Numerically failed”. I tweaked the all the voltage > > limits to 0.7 p.u. – 1.3 p.u., “Matrix is singular” message > > disappeared, but OPF still didn’t converge. > > > > If no load shedding is required and all generator costs are zero, then the > > optimization surface is very flat ... i.e. the objective function will be > > constant no matter what the generator cost. I normally use the actual > > generator costs for the real generators and some very high value, > > representing the value of lost load, for the dispatchable loads. > > > Also, I tried MINOPF, TRALM, and even DC OPF, it still won’t converge. > > > I tried to use the result of AC power flow as the initial value for AC > > OPF, MATPOWER says that "makeAvl: For a dispatchable load, PG and QG > > must be consistent with the power factor defined by PMIN and the Q > > limits." I wonder why? > > > > Hmmm ... well, the dispatchable load model was designed with only the OPF > > problem in mind, so it's probably not behaving as expected in the power flow > > problem. For the power flow, were you setting all of the loads at their > > nominal values (i.e. PG = PMIN)? > > > My guess is that for (at least some of) the buses with only dispatchable > > loads, the bus type was set to PV, which means that the power flow will > > change the QG in order to maintain the voltage setpoint, thus violating the > > constant power factor constraint. Setting the bus type to PQ for these buses > > should fix that. Unfortunately, for buses with both generators and > > dispatchable loads, the power flow computes the correct net QG, but then > > distributes it evenly across the "generators" (including dispatchable loads) > > at that bus. So it would also mess up the dispatchable load power factors. I > > suppose this could be corrected for manually after the fact, but that's not > > currently in the code (something for my to-do list). > > > Also, many of the solvers select their own starting point anyway. MINOPF is > > one of the few that attempts to use the starting point you provide. > > Unfortunately, 5370 buses is pretty big for MINOPF. > > > I checked the branch flow limits. They are quite reasonable. I set > > voltage limits to all zero. It didn’t help. > > > > I would use non-zero gen costs, keep the voltage limits at something > > reasonable (0.9-1.1) and try setting the line limits to zero (to disable > > them) to see if that allows it to converge. > > > I wonder what is wrong. Is my approach to this problem appropriate? > > Anything wrong with my setting of gencost? > > > > I think your approach is fine, except for the zero costs in gencost and the > > extreme relaxing of the voltage constraints. I think that trying to start > > with a power flow solution (that respects the constant power factor > > constraints of the dispatchable loads) is a good idea. Then have a look at > > the voltages, branch flows, reactive dispatches to see which ones (if any) > > are violating the OPF constraints. > > > By the way, I wonder whether MATPOWER allows multiple slack buses in > > AC OPF? In my case I tried both multiple and single slack bus. Won’t > > help. > > > > MATPOWER does allow multiple reference buses in the AC OPF. Keep in mind > > that for the OPF problem there is no concept of "slack", as in generation > > balance, only the concept of voltage angle reference. So, I would never use > > multiple reference buses unless the system is islanded, in which case you > > want a single reference bus in each island. > > > Hope this helps, > > > -- > > Ray Zimmerman > > Senior Research Associate > > 419A Warren Hall, Cornell University, Ithaca, NY 14853 > > phone: (607) 255-9645 > > > > >
