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 <[email protected]> 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
