Ray, Yes that makes sense to me, but i'm till struggling with the idea of this 'slack' for the real power, because if the generator info is not entered in that field then how does MATPOWER know the generator output?
thanks. shane. Quoting Ray Zimmerman <[email protected]>: > On Mar 22, 2007, at 4:32 PM, Prof. Jose Roberto Camacho wrote: > > > I didn't have the case9 here, but running a power-flow (a non- > > linear set of equations) you can > > leave your power generation at zero (active and reactive) in the > > slack bus (reference bus), in > > matpower you can have more than one. As an iterative process, on a > > slack bus (reference > > bus) you specify voltage (1 pu) and angle (zero), generator active > > and reactive powers in the > > slack bus will be function of the voltages obtained in other bars > > and the losses. In agreement > > with all the equations of your non-linear system describing the > > system. > > > > Run your case9 model, and look if the powers remains the same after > > convergence (at the > > end of the routine). > > Just one small additional clarification. There are two separate > concepts here that are often combined, and in fact they are combined > in the MATPOWER implementation of the power flow. > > (1) Voltage angle reference bus -- Since the power flow equations > essentially only deal with angle differences, a single arbitrary > reference is necessary in each "island" in the network to uniquely > solve for the remaining voltage angles. Normally, you would only want > to specify more than one angle reference if you have multiple > unconnected islands. > > (2) Real power slack -- When computing the power flow for an N-bus > system, where Q is specified at all load buses and V is specified at > the remaining buses and a reference angle is specified at the > reference bus, specifying P at every bus would leave us with 2N > equations [a] and 2N-1 unknowns [b]. A simple approach to solve this > is to allow P at a single bus to be a variable that takes up the > "slack". Conceptually, this could be any generator bus. I does not > have to be the same bus used for (1). It is also possible to allow P > to be free at multiple buses if we include additional equations > defining how the slack is to be distributed. Again this is > independent of (1). > > One more point, an optimal power flow formulation needs (1) but not > (2), and in MATPOWER's implementation the reference bus is exactly > that. The power flow problem, on the other hand needs both (1) and > (2), and in the MATPOWER implementation the reference bus serves as > both. > > > [a] P and Q balance at each bus > [b] V at load buses, Q at remaining buses and angle at all buses > except the reference > > > -- > Ray Zimmerman > Senior Research Associate > 428-B Phillips Hall, Cornell University, Ithaca, NY 14853 > phone: (607) 255-9645 > > > > >
