Sorry for the confusion, Ray: where I said, *"...solvable with the bus running as PV, but unfeasible when the bus is running as PQ"*, I meant to say:
"*a valid powerflow solution* with the bus running as PV, but an *unstable* one when the bus is running as PQ" (it is one of the multiple low-voltage solutions of the full mathematical problem--these solutions are unstable form the point of view of real operations) So yes, in that situation the solution obtained with the bus type being PV is (of course) still a mathematical solution to the same problem, regardless of the bus type being reinterpreted as PQ. But the point is that, only if you look at the problem in this second case (bus type switched to PQ) it is then possible to see that the solution is a low voltage one, and therefore not valid. One way to detect this is to look at the slope dQ / dV of the Q-V curve, because this situation corresponds to being "on the wrong side" of the curve. It may happen either if the setpoint V was too low, or if the P output is too high. What the Newton method may do in that case is hard to say, since N-R does not "see" the kind of instability that we are talking about here. It may converge to this unstable solution, or to the operationally correct one, or diverge. It all depends on the complex landscape of the respective basins of attraction for each solution, which as you know is fractal. But unless the case is right at the bifurcation point (here that would be the minimum of the Q-V curve), the basin of attraction around this unstable solution will be finite, so there's a good chance that if you provide the original solution (obtained when the bus was type PV) and use it as the seed to the new problem (bus type now switched to PQ), then NR will converge to the same solution. You'd probably need to start from a flat start in order to find the high-voltage solution instead. -- Jose L. Marin Grupo AIA 2016-05-05 22:17 GMT+02:00 Ray Zimmerman <r...@cornell.edu>: > 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 > > 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 >> > 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 >>> >>> 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> 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>: >>>> >>>>> 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> >>>>> 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 >>>>> >>>>> >>>>> >>>> >>> >>> >> >> > >
