Dear Jose and Ray, thanks for your replies, I've tested another few things:
I deactivated the other two PV-buses, so there is only one reference bus, many PQ-buses and the problematic PQ/PV-bus. With the PQ (result is ok): - Slack inserts: -60 Mvar - Problematic bus: 0.118 Mvar, V=1.0335 pu With PV without limits (result is ok) - Slack inserts: -61 Mvar - Problematic bus: 0.851 Mvar, V=1.05 pu (as set as Vg) With PV with limits (not ok): - Slack inserts: +84 Mvar (145 Mvar difference) - Problematic bus: 0.7 Mvar, V=0.15 pu (generator has reached its upper limit, hence it was converted to PQ) But the result of V of the problematic bus should be somewhere in between 1.0335 and 1.05 p.u.. All other P and Q of the generators and buses remain constant within all variants. For me it looks like the result is in the unstable area of the QV-curve. @ Jose: when setting Vg=1.0335 than the result is ok again. The problem starts at the point the generator reaches it's Q-Limit and gets converted to PQ again. I don't understand why the result is in the unstable area of the QV-curve, because the reactive power is capacitive and not inductive, hence I wouldn't expect a stability Problem (I'm increasing the voltage). Interesting point: with the fast-decoupled solver the power flow with Q-limit converges to V=1.0465 p.u. (result is ok), with Gauss-Seidel it doesn't converge. Maybe there is a problem with Newton's method?! Thanks for your help/explanations, nice regards, Chris 2016-05-06 7:33 GMT+02:00 Jose Luis Marín <mari...@aia.es>: > > 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 >>>>>> >>>>>> >>>>>> >>>>> >>>> >>>> >>> >>> >> >> >
