Dear Dr.Zimmerman Recently I've found MATPOWER really helpful and read a lot threads in the mailing list. I kind of overheard the existence of some codes that detect islanding and divide the system into several cases. Could you please send me these codes? I would really appreciate that. I am currently using MATGRAPH to detect islanding in contigencies but it brings troubles.
Also, I found that in OPF, when I want to clear the voltage limit of certain bus, I naturally assume that I could get that done by setting both the upper and lower boundaries to zero. However I found the result is that the voltage is limited to zero. Is there any way that I could skip the voltage limit of certain bus? Thank you! On Fri, Mar 2, 2012 at 2:57 PM, Hua Bowen <[email protected]> wrote: > 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 >> >> >> >> >>
