Thanks Jose. I’ve added that to the manual as you suggested. I also agree with your suggestions for Chris. However, I’m curious about your [*] note. In the situation you describe, it’s just that the Newton method will diverge, correct? The original solution will still be a solution of the power flow equations won’t they?
Ray > On Apr 30, 2016, at 2:01 PM, Jose Luis Marin <mari...@gridquant.com> wrote: > > > Interesting, I didn't know that enforcing Q-limits also affects generators > when their bus-type is set to PQ. Ray, I suggest documenting this behavior > in the manual, probably at the end of the last paragraph in Section 4.1. > Something to the effect of "... Note also that this option affects generators > even if the bus they are attached to is already of type PQ." > > Going back to Chris's problem, I suggest you approach this as consisting of > two orthogonal issues: > Solving with Q-limits enforced vs. solving without > Solving with a given generator running as PQ-type vs. running as PV-type (by > manually switching it) > With regards to the first issue, I suggest to start by analyzing the behavior > of your case *without* enforcing Q limits: in particular, pay close attention > to the QG injections obtained *in the solution* for those gens operating as > PV-type (and also the specified QG for those operating as PQ), and *compare* > those to your specified [QMIN, QMAX] values. If you observe any large > violation, then solving this case with Q limits enforced will yield a *very* > different solution. > > Concerning the second issue, the main thing to check when manually switching > a given bus from PQ-type to PV-type, is that the gen setpoint VG has to be > set to the bus voltage VM obtained in the previous solution, otherwise your > next solution could be again very different. Conversely, when manually > switching a bus from PV-type to PQ-type [*], you have to specify a gen > injection QG equal to the value obtained in the previous solution. > > [*] As I mentioned before, the switch in this direction is not always > guaranteed to give you the same solution. If the setpoint VG is decreased > too much, or if the real power PG is increased too much, you can end up with > a case that is solvable with the bus running as PV, but unfeasible when the > bus is running as PQ (under the switching procedure described here). > Technically: this happens when you're running the PV generator at the > unstable branch of its Q-V curve. > > -- > Jose L. Marin > Grupo AIA > > > > On Fri, Apr 29, 2016 at 10:12 AM, Chris Prokop <christophprok...@gmail.com > <mailto:christophprok...@gmail.com>> wrote: > Hi, > > oh, I just realized that the Q-limits also hold on PQ-buses, hence the > generators are limited to it - I wouldn't have thought that. So without > Q-limits the results are the same - huch. > > Anyway, when changing the bus to a PV-bus I've got the problem that the Q of > the PV-buses (generators) are calculated way to high. At a first step runpf > makes the bus with highest gap between the Q-limit and the Q-result to a PQ > bus, in my case this is the reference bus -> the problem starts (even if I > increase the ref-Qmax to 1e20, Qmin to -1e20 - then the problem repeats after > converting the other 2 PV buses to PQ). > > (in a few hours I'll be without internet connection for 8 days so I wanna > apologize for not responding during the next days) > > Nice regards, > Chris > > Am 28.04.2016 um 23:48 schrieb Jose Luis Marin: >> >> Looking at those records I think I may have misunderstood what you're doing. >> I thought you were converting a given PQ bus (BUS_I=246) into PV, but the >> generator record you're showing is attached to BUS_I=1 instead. >> >> To be precise, I thought you were starting from (numbers made up for this >> example): >> >> bus_i type Pd Qd Gs Bs area Vm Va baseKV >> zone Vmax Vmin >> 246 1 15.37 1.12 0 0 3 1 0 20 >> 4 2 0.6; >> >> And, assuming that the solution for that starting case gave for instance >> V=1.0375 on BUS_I=246, then making these changes to the case: >> >> bus_i type Pd Qd Gs Bs area Vm Va baseKV >> zone Vmax Vmin >> 246 1 0.0 0.0 0 0 3 1 0 20 >> 4 2 0.6; >> >> plus adding this generator to the bus: >> bus Pg Qg Qmax Qmin Vg mBase >> status Pmax ... >> 246 -15.37 0 999.99 -999.99 1.0375 100 1 >> 1.3 ... >> >> Then, provided you are not enforcing MVAR limits (which may change a lot of >> things), you should obtain the same powerflow solution in this second case >> (with Qg near -1.12, in this example), >> >> >> Did I miss something? >> >> >> -- >> Jose L. Marin >> Grupo AIA >> >> >> >> On Thu, Apr 28, 2016 at 7:26 PM, Chris Prokop <christophprok...@gmail.com >> <mailto:christophprok...@gmail.com>> wrote: >> Hi, >> >> @ Jose L. Marin: >> I used a generator in both cases, hence it should be the correct sign >> (should result in the same I guess). Anyway P was very small, around 1e-6 MW. >> >> I'm not sure if I've modelled something wrong: S_base is 100, my generator >> is at bus 1 (at the end zeros(1,12)): >> bus Pg Qg Qmax Qmin Vg mBase status >> Pmax ... >> 1 0.4 0 0.7 -0.45 1.04 100 >> 1 1.3 ... >> >> And the PQ-bus: >> bus_i type Pd Qd Gs Bs area Vm Va >> baseKV zone Vmax Vmin >> 246 1 2.64e-06 -30 0 0 3 1 0 >> 20 4 2 0.6; >> >> I've tried another thing: I changed the PQ-bus from -30 to +30 Mvar (e.g. >> mpc.bus(1, 4) = 0) and for another time I did the same with the generator >> from +30 to -30 Mvar (e.g. mpc.gen(1, 3)=30), the other part (gen vs. load) >> was always set to 0. The result was not (!) the same... hm? - I guess this >> is the reason for the problem. >> With the bus I get results from -8 Mvar up to 30 Mvar with v=0.635 to 1.46 >> whereas with the generator the voltage varies only within -1 Mvar and +1 >> Mvar (appr. 1.01 pu to appr. 1.04 pu), for other Q-injections the voltage >> doesn't change. >> >> I uploaded the plot under: http://de.tinypic.com/r/350pk02/9 >> <http://de.tinypic.com/r/350pk02/9> >> >> Do you have any idea why? >> >> Thanks for your help, nice regards, >> Chris >> >> >> >> >> Am 28.04.2016 um 17:41 schrieb Jose Luis Marin: >>> >>> Those are certainly some crazy Mvar injections! Just checking: are you >>> sure you reversed the signs of P properly when switching the type of that >>> bus from PQ (load) to PV (gen with neg real power), also taking care of >>> making the corresponding changes in the bus row and adding a new gen row? >>> >>> If you could share your case file I could try to give you a quick >>> diagnostic. >>> >>> -- >>> Jose L. Marin >>> Grupo AIA >>> >>> >>> >>> On Thu, Apr 28, 2016 at 5:19 PM, Chris Prokop <christophprok...@gmail.com >>> <mailto:christophprok...@gmail.com>> wrote: >>> Thanks for your responses. >>> >>> @ Ray Zimmerman: >>> I can't find the problem here. If I set Q=+0.7 Mvar (cap.), the voltage at >>> this bus is about 1.04 p.u., with Q=-0.7 Mvar (ind.) it decreases to 1.02 >>> p.u., the power flow converges as expected (+the result is as expected). >>> Only converting this bus into a PV-bus results in the problem mentioned >>> above (with or without limits). I don't get why the Newton Power Flow >>> returns for example 1e3 Mvar for this bus as a result, as only <1 Mvar >>> should have been enough reactive power. >>> Before solving the case the reactive power of the generators at the >>> PV-buses are (variable gen in runpf): >>> - Reference bus: 0 Mvar >>> - PV-Bus1: 0 Mvar >>> - PV-Bus2: 64.4 Mvar >>> - PV-Bus3 (the problematic one): 0.1 Mvar >>> after solving it (after pfsoln), the variable gen is filled with: >>> - Reference bus: 1e6 Mvar >>> - PV-Bus1: 2.7e4 Mvar >>> - PV-Bus2: 9.6e3 Mvar >>> - PV-Bus3 (the problematic one): -600 Mvar >>> whereas the next most reactive power intensive gen&bus has less than 5e2 >>> Mvar. I don't understand where the power comes from/goes to... >>> >>> @ Jose Luis Marin: >>> If I use the voltage from the PQ-calculation (e.g. 20.6668/20 p.u.) there >>> remains the same problem. Also without Q-limits I get the the problem... >>> Vg=1 p.u. actually works as a PV-bus, Vg=1.01 or Vg=1.02 etc. don't. >>> >>> >>> Nice regards, >>> Chris >>> >>> 2016-04-28 16:04 GMT+02:00 Ray Zimmerman <r...@cornell.edu >>> <mailto:r...@cornell.edu>>: >>> It sounds like the voltage at that bus may be very sensitive to the >>> reactive power injection. One thing you might try to get some idea of this >>> is to change that bus back to PQ with the reactive at the lower limit, then >>> try running a few cases with slightly perturbed values of the reactive >>> power at that generator and see how the voltage at the bus changes. >>> >>> Ray >>> >>> >>>> On Apr 28, 2016, at 8:40 AM, Chris Prokop <christophprok...@gmail.com >>>> <mailto:christophprok...@gmail.com>> wrote: >>>> >>>> Hi, >>>> >>>> I'm using Matpower (v5.0b1, but the same holds for v5.1) for a 220 kV/110 >>>> kV/20 kV-grid quite a while. The grid has 1 reference bus (220 kV), 2 >>>> PV-buses (220 kV) and >100 PQ-buses (110 kV & 20 kV). So far calculating >>>> the grid using runpf with Newton has never been a problem. >>>> >>>> Now I've tried to change a 20 kV PQ-bus to a PV-bus with the Q-limits +0.7 >>>> for Q_max and -0.45 for Q_min (considering mpopt = >>>> mpoption('pf.enforce_q_lims', 1)). If I set Vg of the generator to 1.05 I >>>> get the error: >>>> "All 4 remaining gens exceed their Q limits : INFEASIBLE PROBLEM" >>>> whereas when using 1.00 as Vg there is no problem (then the generator is >>>> at its lower Q-limit, hence converted to PQ). If the problem is infeasible >>>> the result of pfsoln in runpf are Q-values of all 4 PV generators (Slack+3 >>>> PV) that are out of their limits. Why is there such a big difference >>>> between the case V=1.05 and V=1.00? >>>> >>>> As a info: the Slack has a Q-Limit of +-10000, both of the already >>>> existing PV buses +720/-290, according to case_info the total generation >>>> is -300 MW+j10 Mvar, the total load 300 MW-j50 Mvar, but I've tried >>>> several scenarios which are no problem when using the PQ instead of the >>>> PV-bus (or the Vg=1). >>>> >>>> Does anybody experience a similar problem/has an idea how to fix it? >>>> >>>> Nice regards, >>>> Chris >>> >>> >>> >> >> > >