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 <abhy...@mcs.anl.gov> wrote: > > > On Jul 23, 2013, at 9:42 AM, spyros gian <sp.g...@hotmail.com> 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 >> >>
