The comments were specifically as regards the OPF problem, *not* for the power flow problem. For the power flow problem, changing a PV to PQ bus switches from Vm fixed and Qg variable, to Qg fixed and Vm variable.
-- Ray Zimmerman Senior Research Associate B30 Warren Hall, Cornell University, Ithaca, NY 14853 phone: (607) 255-9645 On Jul 24, 2013, at 2:56 AM, Simon Schneider <[email protected]> wrote: > Dear Dr. Zimmerman, > > the "Bus types play no role" confused me a bit, so I tried declaring some > previous generator nodes as PQ buses. > It seems to me the only effect is, that the voltage now is not longer fixed > to the generator set point during a normal Power Flow calculation. > > Do you know if that's correct or has it any further consequences? > > thanks in advance > Simon > > > > >>> Ray Zimmerman <[email protected]> 23.07.2013 19:44 >>> > Shri is correct with some *very* minor tweaks the only bus type that matters > is the REF bus which determines the voltage reference for the system, and the > voltage angle at that bus is set to the corresponding value in the bus > matrix, which is usually set to 0, but need not be. > > And, yes, the OPF solvers in MATPOWER do find locally optimal solutions that > are not guaranteed to be globally optimal. Theoretically, MATPOWER could find > different solutions depending on the algorithm, starting point, algorithm > parameters, etc. However, in my experience, it has been very difficult to > find multiple local optima. The one example I have been able to confirm has > nearly identical objective values and active power dispatches, with some > differences in voltage profile and reactive dispatch in a few buses. > > My conjecture is that in most cases, especially for relatively small systems, > the solution found by MATPOWER is likely the global optimum or else something > extremely close to it. I hope to include in an upcoming version some > contributed code that will be able to confirm in some cases that a solution > is a indeed a global optimum. > > -- > Ray Zimmerman > Senior Research Associate > B30 Warren Hall, Cornell University, Ithaca, NY 14853 > phone: (607) 255-9645 > > > > > > On Jul 23, 2013, at 1:05 PM, Shri <[email protected]> wrote: > >> >> >> On Jul 23, 2013, at 9:42 AM, spyros gian <[email protected]> wrote: >> >>> Dear Dr Zimmerman, >>> >>> Running an OPF in matpower means that >>> >>> 1. Bus types play no role (eg slack, PV, PQ etc) >> Yes. >>> 2. All values for Real Power generation and reactive power generation are >>> unknown >> Yes. >>> 3. All values for bus_voltages and voltage phase angles in buses, are >>> unknown as well >> The voltage angle of the reference bus is fixed and set to 0. >>> 4. As a result, all values for real and reactive power flows are unknown. >> Yes. >>> 5. Losses are unknown. >> Yes. >>> >>> What is known : >>> 1. The resistance, reactance, admittance per unit / per conductor >>> 2. Values for Real and Reactive demand at each bus >>> 3. Limits on voltage magnitude , limits on real and reactive power >>> generation >>> 4. MVA limits on each line >>> 5. Fuel cost for each generator. >> Yes for all >>> >>> So my question is >>> a. Are the above correct for matpower ? >>> b. Since matpower uses a non-linear optimisation, is the result a local >>> minimum or a global minimum? >>> (for the case of a cost-minimization OPF) ? i.e. the values for >>> voltages, reactive powers etc, are >>> globally optimum or perhaps other optimum values for all the unknown >>> quantities exist ? >> I believe most of the optimization tools, such as fmincon in Matlab, find a >> local minimum. >> >> Shri >>> >>> Thank you, >>> Spyros Gian >>> >>> >
