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 >>>> >>>> >>>> >>> >> >> > >
